Sensory Systems Design by Owen Borville February 14, 2025 BIO 36
Senses provide information about the body and its environment. Humans have five special senses: olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. We also have general senses called somatosensation, which respond to stimuli like temperature, pain, pressure, and vibration.
Vestibular sensation is an organism's sense of spatial orientation and balance, gravity, and movement allowing organisms to stay upright.
Proprioception is the sense of body position and movement (position of bones, joints, and muscles).
Kinesthesia is the sense of limb position that is used to track limb movement. All of these senses are part of somatosensation.
All of these senses share a common function: to convert a stimulus (light, sound, position of body) into an electric signal in the nervous system. This process is called sensory transduction.
There are two types of cellular systems that perform sensory transduction. In one, a neuron works with a sensory receptor, a cell, or cell process that is specialized to engage with and detect a specific stimulus. Stimulation of the sensory receptor activates the associated afferent neuron, which carries the information about the stimulus to the central nervous system. In the second type of sensory transduction, a sensory nerve ending responds to a stimulus in the internal or external environment and this neuron constitutes the sensory receptor.
Reception
The first step in sensation is reception, which is the activation of sensory receptors by mechanical stimuli, such as deformation, chemicals, or temperature. The receptor will then respond to the stimuli. The region in space in which a given sensory receptor can respond to a stimulus is the receptor's receptive field, and the field can be small or large.
Transduction
The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This translation takes place at the sensory receptor, and the change in electric potential that is produced is called receptor potential.
A mechanoreceptor is a type of receptor that possesses specialized membranes that respond to pressure including compression or bending the dendrites opens gated ion channels in the plasma membrane of the sensory neuron, changing its electric potential. Neurons are depolarized with a positive change to the neuron's electrical potential. Receptor potentials are graded potentials because the magnitude of these graded receptor potentials varies with the strength of the stimulus. If the magnitude of the depolarization is enough for the membrane potential to reach the threshold, the neuron will release an action potential.
Sensory receptors are specialized for different senses according to the type of stimulus. Stimuli can be combined to enhance the strength of sense.
Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus. Therefore, action potentials transmitted over a sensory receptor's afferent axons encode one type of stimulus, and this segregation of senses is preserved in other sensory circuits. The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. A second way in which intensity is encoded is by the number of receptors activated.
Perception is an individual's interpretation of a sensation. Although perception relies on the activation of sensory receptors, perception happens not at the level of the sensory receptor, but at higher levels in the nervous system in the brain. The brain distinguishes sensory stimuli through a sensory pathway. Action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus. The neurons synapse with particular neurons in the brain or spinal chord. All sensory signals are transmitted through the central nervous system (except olfactory system) and are routed to the thalamus and to the appropriate region of the cortex. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex that processes a particular sense. Interpretation of sensory signals between individuals of the same species is similar.
Somatosensation is a sensory phenomenon that includes all sensation received from the skin and mucous membranes, as well as from the limbs and joints. Somatosensation is also known as the tactile sense, or the sense of touch. Somatosensation occurs all over the exterior of the body and at some interior locations. Receptor types embedded in the skin, mucous membranes, muscles, joints, internal organs, and cardiovascular system play a role in somatosensation.
The epidermis is the outer layer of the skin in mammals. The epidermis is relatively thin, composed of keratin-filled cells, and has no blood supply. The epidermis serves as barrier to water and to invasion by pathogens.
Below the epidermis is the dermis, a much thicker layer that contains blood vessels, sweat glands, hair follicles, lymph vessels, and lipid-secreting sebaceous glands. Below the epidermis and dermis is the subcutaneous tissue, or hypodermis, the fatty layer that contains blood vessels, connective tissue, and the axons of sensory neurons. The hypodermis holds about 50 percent of the body's fat and attaches the dermis to the bone and muscle, and supplies nerves and blood vessels to the dermis.
Somatosensory Receptors: Sensory receptors are classified into five categories: mechanoreceptors, thermoreceptors, proprioceptors, pain receptors (nociceptors), and chemoreceptors. These categories are based on the nature of stimuli each receptor class transduces. The sense of touch involves more than one kind of stimulus and more than one kind of receptor. Mechanoreceptors in the skin are described as encapsulated or unencapsulated (free nerve endings).
A free nerve ending is an unencapsulated dendrite of a sensory neuron. Free nerve endings are the most common nerve endings in skin and they extend into the middle of the epidermis. Free nerve endings are sensitive to painful stimuli, to hot and cold, and to light touch. They are slow to adjust to a stimulus and so are less sensitive to abrupt changes in stimulation.
Three classes of mechanoreceptors are: tactile, proprioceptors, and baroreceptors. Mechanoreceptors sense stimuli due to physical deformation of their plasma membranes. They contain mechanically gated ion channels whose gates open or close in response to pressure, touch, stretching, and sound.
There are four primary tactile mechanoreceptors in human skin: Merkel's disks, Meissner's corpuscles, Ruffini endings, and Pacinian corpuscles. Two of these are located toward the surface of the skin and two are located deeper. A fifth type of mechanoreceptor, Krause end bulbs, are only found in specialized regions and detect cold.
Merkel's disks are found in the upper layers of skin near the base of the epidermis, both in skin that has hair and on glabrous skin, the hairless skin found on the palms and fingers, the soles of feet, and the lips of humans and primates. Merkel's disks are densely distributed in the fingertips and lips. They are slow-adapting, encapsulated nerve endings, and they respond to light touch. Light touch (or discriminative touch), is a light pressure that allows the location of a stimulus to be pinpointed. The receptive fields of Merkel's disks are small with well-defined borders, which makes them sensitive to edges.
Meissner's corpuscles, also known as tactile corpuscles, are found in the upper dermis, but they project into the epidermis. These are found primarily in the glabrous skin on the fingertips and eyelids. They respond to fine touch and pressure, but they also respond to low-frequency vibration or flutter. They are rapidly adapting, fluid-filled, encapsulated neurons with small, well-defined borders and are responsive to fine details. Like Merkel's disks, Meissner's corpuscles are not as plentiful in the palms as they are in the fingertips.
Ruffini endings (bulbous corpuscles) are deeper in the epidermis near the base and are found in both glabrous and hairy skin. Ruffini endings are also slow-adapting, encapsulated mechanoreceptors that detect skin stretch and deformations within joints, so they provide valuable feedback for gripping objects and controlling finger position and movement. Ruffini endings also contribute to proprioception and kinesthesia. They also detect warmth and are located deeper in the skin than cold detectors.
Pacinian corpuscles are located deep in the dermis of both glabrous and hairy skin and are structurally similar to Meissner's corpuscles. Pacinian corpuscles are found in the bone periosteum, joint capsules, pancreas and other viscera, breast, and genitals. Pacinian corpuscles are rapidly adapting mechanoreceptors that sense deep transient (but not prolonged) pressure and high-frequency vibration. Pacinian receptors detect pressure and vibration by being compressed, stimulating their internal dendrites. There are fewer Pacinian corpuscles and Ruffini endings in skin than there are Merkel's disks and Meissner's corpuscles.
In proprioception, proprioceptive and kinesthetic signals travel through myelinated afferent neurons running from the spinal chord to the medulla. Neurons are not physically connected, but communicate via neurotransmitters secreted into synapses or "gaps" between communicating neurons. Once in the medulla, the neurons continue carrying the signals to the thalamus.
Muscle spindles are stretch receptors that detect the amount of stretch, or lengthening of muscles. Related structures are Golgi tendon organs, which are tension receptors that detect the force of muscle contraction. Proprioceptive and kinesthetic signals come from limbs. Unconscious proprioceptive signals run from the spinal chord to the cerebellum, the brain region that coordinates muscle contraction, rather than to the thalamus, like most other sensory information.
Baroreceptors detect pressure changes in an organ. They are found in the walls of the carotid artery and the aorta where they monitor blood pressure, and in the lungs where they detect the degree of lung expansion. Stretch receptors are found at various sites in the digestive and urinary systems.
In addition to these two types of deeper receptors, there are also hair receptors which are found on nerve endings that wrap around the base of hair follicles. Some hair receptors detect slow and rapid hair movement, skin deflection, and detection of stimuli that have not yet touched the skin.
The different types of receptors that work together with the human skin produce a very refined sense of touch. The nociceptive receptors that detect pain are located near the surface. Merkel's disks and Meissner's corpuscles are small, finely calibrated mechanoreceptors located in the upper layers and can precisely detect gentle touch. Large mechanoreceptors like Pacinian corpuscles and Ruffini endings are located in the lower layers and respond to deeper touch. Both primary somatosensory cortex and secondary cortical areas are responsible for the total product of stimuli transmitted from all of the mechanoreceptors.
The density and distribution of mechanoreceptors is not consistent over the entire body. In humans, touch receptors are less dense in skin covered with any type of hair and denser in glabrous skin (non-hairy skin). Receptors in glabrous skin are thicker and more sensitive than hairy skin.
Thermoreception: In addition to Krause end bulbs that detect cold and Ruffini endings that detect warmth, there are different types of cold receptors on some free nerve endings called thermoreceptors located on the dermis, skeletal muscles, liver, and hypothalamus, that are activated by different temperatures. Their pathways into the brain run from the spinal chord through the thalamus to the primary somatosensory cortex. Warm and cold sensory information travels through the nerves to the brain.
Nociception, or pain, is the neural processing of injurious stimuli in response to tissue damage. Pain is caused by injury or contact with heat that causes burn or contact with a corrosive chemical. Pain can also be caused by harmless stimuli that mimic the action of damaging stimuli, such as eating spicy pepper plants. Capsaichins are chemical proteins that cause peppers to taste spicy hot by activating warm receptors in the human mouth.
Nociception begins at the sensory receptors, but pain, and the perception of nociception does not begin until it is communicated with the brain. There are several nociceptive pathways to and through the brain. Most axons carrying nociceptive information into the brain from the spinal chord project to the thalamus (as do other sensory organs) and the neural signal undergoes final processing in the primary somatosensory cortex. One nociceptive pathway projects not to the thalamus but directly to the hypothalamus in the forebrain, which modulates the cardiovascular and neuroendocrine functions of the autonomic nervous system.
Taste and Smell
Taste, also called gustation, and smell, also called olfaction, are the most interconnected senses in that both involve molecules of the stimulus entering the body and bonding to receptors. Smell allows animals to sense the presence of food, other animals, or substances in the environment. Smell also allows animals to distinguish one smell from another.
Taste and odor stimuli are molecules taken in from the environment. Primary tastes detected by humans are sweet, sour, bitter, salty, and umami. The first four are familiar. Umami was identified in 1908 but was not accepted as a fundamentally distinguished taste until later. The taste of umami, or savoriness, is attributed to the taste of the amino acid L-glutamate, and this amino acid is commonly used to enhance the taste of foods.
All odors that we perceive are molecules in the air we breathe. If molecules are not released from a substance, there is no smell or odor. Also, if a human or animal does not have a receptor that recognizes a specific molecule, then that molecule has no smell. Humans have about 350 olfactory receptor subtypes that work in various combinations to allow sensing of some 10,000 different odors. Smaller animals, however, can have more receptors, as mice have about 1,300 olfactory receptor types, allowing them to sense more odors. Both odors and tastes involve molecules that stimulate specific chemoreceptors. Therefore, both senses of taste and smell work together to help humans and animals identify flavors of substances.
Odorants (odor molecules) enter the nose and dissolve in the olfactory epithelium, the mucosa at the back of the nasal cavity. The olfactory epithelium is a collection of specialized olfactory receptors in the back of the nasal cavity that spans an area of about 5 square centimeters in humans. An olfactory receptor, which is a dendrite of a specialized neuron, responds when it binds certain molecules inhaled from the environment by sending impulses directly to the olfactory bulb of the brain. Humans have about 12 million olfactory receptors distributed among hundreds of different receptor types that respond to different odors. Animals, however have more receptors: rabbits have (100 million), dogs (1 billion), bloodhound dogs (4 billion). The olfactory epithelium of bloodhounds are many times larger than that of humans.
Olfactory neurons are bipolar neurons (neurons with two processes extending from the cell body). Each neuron has a single dendrite buried in the olfactory epithelium, and extending from this dendrite are 5 to 20 receptor-laden, hair-like cilia that trap odorant molecules. The sensory receptors on the cilia are proteins, and it is the variations in their amino acid chains that make the receptors sensitive to different odorants. Each olfactory sensory neuron has only one type of receptor on its cilia, and the receptors are specialized to detect specific odorants, so the bipolar neurons themselves are specialized. When an odorant binds with a receptor that recognizes it, the sensory neuron associated with the receptor is stimulated. Olfactory stimulation is the only sensory information that directly reaches the cerebral cortex, while other sensations are sent through the thalamus.
Taste detection (gustation) is fairly similar to detecting an odor (olfaction), given that both taste and smell rely on chemical receptors being stimulated by certain molecules. The primary organ of taste is the taste bud, which is a cluster of gustatory receptors (taste cells) that are located within the bumps on the tongue called papillae (papilla). There are several structurally distinct papillae. Filiform papillae, which are located across the tongue, are tactile, providing friction that helps the tongue move substances, and contain no taste cells. In contrast, fungiform papillae, which are located mainly on the anterior two-thirds of the tongue, each contain one to eight taste buds and also have receptors for pressure and temperature.
Folate papillae are leaf-like papillae located in parallel folds along the edges and toward the back of the tongue. Folate papillae contain about 1,300 taste buds within their folds. Circumvallate papillae, which are wall-like papillae in the shape of an inverted "V" at the back of the tongue. Each of these papillae is surrounded by a groove and contains about 250 taste buds.
Each taste bud's taste cells are replaced every 10 to 14 days. These are elongated cells with hair-like processes called microvilli at the tips that extend into the taste bud pore. Food molecules or tastants are dissolved in saliva, and they bind with and stimulate the receptors on the microvilli. The receptors for tastants are located across the outer portion and front of the tongue, outside the middle area where the filiform papillae are most prominent.
In humans, there are five primary tastes, and each taste has only one corresponding type of receptor. Thus, like olfaction, each receptor is specific to its stimulus (tastant). Transduction of the five tastes happens through different mechanisms that reflect the molecular composition of the tastant. A salty tastant (containing NaCl) provides the sodium ions (Na+) that enter the taste neurons and excite them directly. Sour tastants are acids and belong to the thermoreceptor protein family. Binding of an acid or other sour-tasting molecule triggers a change in the ion channel and these increase hydrogen ion (H+) concentrations in the taste neurons, thus depolarizing them. Sweet, bitter, and umami tastants require a G-protein coupled receptor. These tastants bind to their respective receptors, thereby exciting the specialized neurons associated with them. The ability of taste and smell declines with age.
Olfactory neurons project from the olfactory epithelium to the olfactory bulb as thin, unmyelinated axons. The olfactory bulb is composed of neural clusters called glomeruli, and each glomerulus receives signals from one type of olfactory receptor, so each glomerulus is specific to one odorant. From glomeruli, olfactory signals travel directly to the olfactory cortex and then to the frontal cortex and the thalamus. This is a different path than most other sensory information, which is sent directly to the thalamus before ending up in the cortex. Olfactory signals also travel directly to the amygdala, thereafter reaching the hypothalamus, thalamus, and frontal cortex. The last structure that olfactory signals directly travel to is a cortical center in the temporal lobe structure important in spatial, autobiographical, declarative, and episodic memories. Olfaction is finally processed by areas of the brain that deal with memory, emotions, reproduction, and thought.
Taste neurons project from the taste cells in the tongue, esophagus, and palate to the medulla, in the brainstem. From the medulla, taste signals travel to the thalamus and then to the primary gustatory cortex. Information from different regions of the tongue is segregated in the medulla, thalamus, and cortex.
Hearing and Vestibular Sensation
Audition (sense of hearing) helps humans and animals detect danger, and communication with other animals. The vestibular system is a sensory system that helps maintain balance and spatial orientation, and detects head and body movement. The vestibular system is located in the inner ear and is made up of semicircular canals and otolith organs.
Auditory stimuli are sound waves that are mechanical pressure waves that move through a medium such as air or water. Sound waves cannot travel through a vacuum because there is no medium to travel through. The speed of sound waves varies based on altitude, temperature, and medium. At sea level and 20 degrees C, sound waves travel in the air at 343 meters per second.
Just like all waves, sound waves have four main characteristics: frequency, wavelength, period, and amplitude. Frequency is the number of waves per unit of time, and in sound is heard as pitch. The most common unit of sound frequency is hertz (Hz), or cycles per second. Many animals can detect frequencies higher than humans are able to detect (called ultrasound).
Amplitude is the dimension of a wave from peak to trough and in sound is heard as volume. The sound waves of louder sounds have greater amplitude than those of softer sounds. The volume of sound waves is measured in decibels (dB).
The reception of sound in mammals, or sound waves are collected by the external, cartilaginous part of the ear called the auricle, then travel through the auditory canal and cause vibration of the thin diaphragm called the tympanum or ear drum, the innermost part of the outer ear. Interior to the tympanum is the middle ear. The middle ear holds three small bones called the ossicles, which transfer energy from the moving tympanum to the inner ear. The three ossicles are the malleus (the hammer), incus (the anvil), and stapes (the stirrup). The three ossicles are unique to mammals, and each plays a role in hearing. The malleus attaches at three points to the interior surface of the tympanic membrane. The incus attaches the malleus to the stapes. In humans, the stapes is not long enough to reach tympanum. Without the malleus and incus, the vibrations of the tympanum would never reach the inner ear. These bones also function to collect force and amplify sounds. Many animals use the stapes of the middle ear to transmit vibrations to the inner ear.
The Transduction of Sound
Vibrating objects, such as vocal chords, create sound waves or pressure waves in the air. When these pressure waves reach the ear, the ear transduces this mechanical stimulus (pressure wave) into a nerve impulse (electrical signal) that the brain perceives as sound. The pressure waves strike the tympanum, causing it to vibrate. The mechanical energy from the moving tympanum transmits vibrations to the three bones of the middle ear. The stapes transmits the vibrations to a thin diaphragm called the oval window, which is the outermost structure of the inner ear. The structures of the inner ear are found in the labyrinth, a bony, hollow structure that is the most interior portion of the ear. Here, the energy from the sound wave is transferred from the stapes through the flexible oval window and to the fluid of the cochlea. The vibrations of the oval window create pressure waves in the fluid (perilymph) inside the cochlea. The cochlea is a whorled structure, like the shell of a snail, and it contains receptors for transduction of the mechanical wave into an electrical signal. Inside the cochlea, the basilar membrane is a mechanical analyzer that runs the length of the cochlea, curling toward the cochlea's center.
The mechanical properties of the basilar membrane change along its length, such that it is thicker, tauter, and narrower at the outside of the whorl (where the cochlea is the largest), and thinner, floppier, and broader toward the apex, or center, of the whorl (where the cochlea is smallest). Different regions of the basilar membrane vibrate according to the frequency of the sound wave conducted through the fluid in the cochlea. For these reasons, the fluid-filled cochlea detects different wave frequencies (pitches) at different regions of the membrane. When the sound waves in the cochlear fluid contact the basilar membrane, it flexes back and forth in a wave-like fashion. Above the basal membrane is the tectorial membrane.
The site of transduction is in the organ of Conti (spiral organ). It is composed of hair cells held in place above the basilar membrane like flowers projecting up from soil, with their exposed short, hair-like stereocilia contacting or embedded in the tectorial membrane above them. The inner hair cells are the primary auditory receptors and exist in a single row, about 3,500. The stereocilia from the inner hair cells extend into small dimples on the tectorial membrane's lower surface. The outer hair cells are arranged in three or four rows. They number about 12,000, and they function to fine tune incoming sound waves. The longer stereocilia that project from the outer hair cells actually attach to the tectorial membrane. All of the stereocilia are mechanoreceptors, and when bent by vibrations they respond by opening a gated ion channel. As a result, the hair cell membrane is depolarized, and a signal is transmitted to the chochlear nerve. Intensity (volume) of sound is determined by how many hair cells at a particular location are stimulated.
The hair cells are arranged on the basilar membrane in an orderly way. The basilar membrane vibrates in different regions, according to the frequency of the sound waves impinging on it. Likewise, the hair cells that lay above it are most sensitive to a specific frequency of sound waves. Hair cells can respond to a small range of similar frequencies, but they require stimulation of greater intensity to fire at frequencies outside their optimal range. The difference in response frequency between adjacent inner hair cells is about 0.2 percent.
Place theory, which is the model for how biologists think pitch detection works in the human ear, states that high frequency sounds selectively vibrate the basilar membrane of the inner ear near the entrance port (the oval window). Lower frequencies travel farther along the membrane before causing appreciable excitation of the membrane. The basic pitch-determining mechanism is based on the location along the membrane where the hair cells are stimulated. The place theory is the first step toward the understanding of pitch reception. Considering the extreme pitch sensitivity of the human ear, it is thought that there must be some auditory "sharpening" mechanism to enhance the pitch resolution.
When sound waves produce fluid waves inside the cochlea, the basilar membrane flexes, bending the stereocilia that attach to the tectorial membrane. Their bending results in action potentials in the hair cells, and auditory information travels along the neural endings of the bipolar neurons of the hair cells (collectively, the auditory nerve) to the brain. When the hairs bend, they release an excitatory neurotransmitter at a synapse with a sensory neuron, which then conducts action potentials to the central nervous system. The cochlear branch of the vestibulocochlear cranial nerve sends information on hearing. The auditory system is very redefined, and there is some modulation or "sharpening" built in. The brain can send signals back to the cochlea, resulting in a change of length in the outer hair cells, sharpening or dampening the hair cells' response to certain frequencies.
The inner hair cells are the most important for conveying auditory information to the brain. About 90 percent of the afferent neurons carry information from inner hair cells, with each hair cell synapsing with 10 or so neurons. Outer hair cells connect to only 10 percent of the afferent neurons, and each afferent neuron innervates many hair cells. The afferent, bipolar neurons that convey auditory information travel from the cochlea to the medulla, through the pons and midbrain in the brainstem, finally reaching the primary auditory cortex in the temporal lobe.
Vestibular system stimuli are the linear acceleration (gravity), angular acceleration and deceleration. Gravity, acceleration, and deceleration are detected by evaluating the inertia on receptive cells in the vestibular system. Gravity is detected through head position. Angular acceleration and deceleration are expressed through turning or tilting of the head.
The vestibular system has some similarities to the auditory system. It utilizes hair cells just like the auditory system, but it excites them in different ways. There are five vestibular receptor organs in the inner ear: the utricle, the saccule, and the three semicircular canals. Together, they make up what is known as the vestibular labyrinth. The utricle and saccule respond to acceleration in a straight line, such as gravity. The hair cells in the utricle and saccule lie below a gelatinous layer, with their stereocilia projecting into the gelatin. Embedded in this gelatin are calcium carbonate crystals. When the head is tilted, the crystals continue to be pulled straight down by gravity, but the new angle of the head causes the gelatin to shift, thereby bending the stereocilia. The bending of the stereocilia stimulates the neurons, and they signal to the brain that the head is tilted, allowing the maintenance of balance. It is the vestibular branch of the vestibulocochlear cranial nerve that deals with balance.
The fluid-filled semicircular canals are tubular loops set at oblique angles and they are arranged in three spatial planes. The base of each canal has a swelling that contains a cluster of hair cells. The hairs project into a gelatinous cap called the cupula and monitor angular acceleration and deceleration from rotation. These would be stimulated as the head is turned or falls forward. One canal lies horizontally, while the other two lie about 45 degree angles to the horizontal axis. When the brain processes input from all three canals together, it can detect angular acceleration or deceleration in three dimensions. When the head turns, the fluid in the canals shifts, thereby bending stereocilia and sending signals to the brain. Upon stopping of accelerating, decelerating, or moving, the movement of the fluid within the canals slows or stops. Therefore, the canals are sensitive to changes in velocity.
Hair cells from the utricle, saccule, and semicircular canals also communicate through bipolar neurons to the cochlear nucleus in the medulla. Cochlear neurons send descending projections to the spinal chord and ascending projections to the pons, thalamus, and cerebellum. Connections to the cerebellum are important for coordinated movements. There are also projections to the temporal cortex, which account for feelings of dizziness. Projections to autonomic nervous system areas in the brainstem account for motion sickness. Projections to the primary somatosensory cortex monitors subjective measurements of the external world and self-movement. Vestibular signals project to certain optic muscles to coordinate eye and head movements.
Vision is the ability to detect light patterns from the outside environment and interpret them into images. The importance of vision to humans is that about one-third of the human cerebral cortex is dedicated to analyzing and perceiving visual information.
As with auditory stimuli, light travels in waves. The compression of waves that compose sound must travel in a medium: a gas, liquid, or solid. In contrast, light is composed of electromagnetic waves and needs no medium as light can travel in a vacuum. The behavior of light can be discussed in terms of the behavior of waves and also in terms of the behavior of the fundamental unit of light: a packet of electromagnetic radiation called a photon. In the electromagnetic spectrum, only a small section contains visible light and the rest of the spectrum contains radiation that cannot be seen by humans because the frequency is either too low or too high to be visible.
Important variables of light include wavelength (which varies inversely with frequency), and manifests itself as hue or color. Light at the red end of the visible spectrum has longer wavelengths (and lower frequency) while light at the violet end of the spectrum has shorter wavelengths (and higher frequency). The wavelength of light is expressed in nanometers (nm) (one billionth of a meter). Visible light for humans ranges from 380 nm to 740 nm. Some animals, however, can detect wavelengths outside the human range.
Wave amplitude is perceived as luminous intensity or brightness of light. The standard unit of intensity of light is the candela, the intensity of one common candle.
Light waves travel at 299, 792 km per second in a vacuum and slower in air and water. The eye senses long wavelengths waves as red, medium wavelengths waves as green, and short wavelengths of waves as blue. White light is light that is perceived as white by humans, when all color receptors of the human eye are stimulated equally. The apparent color of an object is the color or colors of light that the object reflects while the other colors of light of the object are absorbed and not visible.
Anatomy of the Eye
The photoreceptive cells of the eye, where transduction of light to nervous impulses occurs, are located in the retina on the inner surface of the back of the eye. But light does not impinge on the retina unaltered. Rather, it passes through other layers that process it so that it can be interpreted by the retina. The cornea, the front transparent layer of the eye, and the crystalline lens, a transparent convex structure behind the cornea, both refract (bend) light to focus on the image of the retina. The iris, which is visible as the colored part of the eye, is a circular muscular ring lying between the lens and the cornea that regulates the amount of light entering the eye. In conditions of high ambient light, the iris contracts, reducing the size of the pupil at its center. In conditions of low light, the iris relaxes and the pupil enlarges.
The main function of the lens is to focus light on the retina and fovea centralis. The lens is dynamic, focusing and refocusing light as the eye rests on near and far objects in the visual field. The lens is operated by muscles that stretch it flat or allow it to thicken, changing the focal length of light coming through it to focus it sharply on the retina. With age comes the loss of the flexibility of the lens, and a form of farsightedness called presbyopia results. Presbyopia occurs because the image focuses behind the retina. Presbyopia is a deficit similar to a different type of farsightedness called hyperopia caused by an eyeball that is too short. For both defects, images in the distance are clear but images nearby are blurry. Myopia (nearsightedness) occurs when an eyeball is elongated and the image focus falls in front of the retina. In this case, images in the distance are blurry but images nearby are clear.
There are two types of photoreceptors in the retina: rods and cones, named for their general appearance. Rods are strongly photosensitive and are located in the outer edges of the retina. They detect dim light and are used primarily for peripheral and nighttime vision. Cones are weakly photosensitive and are located near the center of the retina. They respond to bright light, and their primary role is in daytime, color vision.
The fovea is the region in the center back of the eye that is responsible for acute vision. The fovea has a high density of cones. When examining an object close up, the eyes adjust so that the object's image falls on the fovea. However, when looking at far away objects, like stars in the night sky, or objects in dim light, the object can be better viewed by the peripheral vision because it is the rods at the edges of the retina that operate better in low light or far away objects. Near objects are better seen with cones at the center. In humans, cones far outnumber rods in the fovea.
Rods and cones are the site of transduction of light to a neural signal. Both rods and cones contain photopigments. In vertebrates, the main photopigment, rhodopsin, has two main parts: an opsin, which is a membrane protein (in the form of a cluster of alpha helices that span the membrane), and retinal, a molecule that absorbs light. When light hits a photoreceptor, it causes a shape change in the retinal, altering its structure from a bent (cis) form of the molecule to its linear (trans) isomer. This isomerization of retinal activates the rhodopsin, starting a cascade of events that ends with the closing of Na+ channels in the membrane of the photoreceptor. Thus, unlike most other sensory neurons (which become depolarized by exposure to a stimulus), visual receptors become hyperpolarized and thus driven away from threshold.
There are three types of cones (with different photopsins), and they differ in the wavelength to which they are most responsive. Some cones are maximally responsive to short light waves of 420 nm (S cones for short). Other cones respond to waves of 530 nm (M cones for medium). A third group of cones responds maximally to light of longer wavelengths of 560 nm (L cones for long). With only one type of cone, color vision would not be possible, and a two-cone (dichromatic) system has color vision limitations. Primates and humans use a three-cone or trichromatic system, resulting in full color vision.
The color human see or perceive is the ratio of activity of our three types of cones. The colors of the visual spectrum, running from long-wavelength light to short, are red (700 nm), orange (600 nm), yellow (565 nm), green (497 nm), blue (470 nm), indigo (450 nm), and violet (425 nm). Humans have a very sensitive perception of color and can distinguish about 500 levels of brightness, 200 different hues, and two million distinct colors.
Retinal processing occurs as visual signals leave the cones and rods, travel to the bipolar cells, and then to the ganglion cells. A large degree of processing of visual information occurs in the retina itself, before information is sent to the brain.
Photoreceptors in the retina continuously undergo tonic activity, where they are always slightly active even when not stimulated by light. In neurons that exhibit tonic activity, the absence of stimuli maintains a firing rate at a baseline. While some stimuli increase the firing rate from the baseline, and other stimuli decrease firing rate. In the absence of light, the bipolar neurons that connect rods and cones to the ganglion cells are continuously and actively inhibited by the rods and cones. Exposure of the retina to light hyperpolarizes the rods and cones and removes their inhibition of bipolar cells. The now active bipolar cells in turn stimulate the ganglion cells, which send action potentials along their axons (which leave the eye as the optic nerve). Thus the visual system relies on change in the retinal activity, rather than the absence or presence of activity, to encode visual signals for the brain. Sometimes horizontal cells carry signals from one rod or cone to other photoreceptors and to several bipolar cells. When a rod or cone stimulates a horizontal cell, the horizontal cell inhibits more distant photoreceptors and bipolar cells, creating lateral inhibition. This inhibition sharpens edges and enhances contrast in the images by making regions receiving light appear lighter and dark surroundings appear darker. Amacrine cells can distribute information from one bipolar cell to many ganglion cells.
Myelinated axons of ganglion cells make up the optic nerves. Within the nerves, different axons carry different qualities of the visual signal. Some axons constitute the magnocellular (big cell) pathway, which carries information about form, movement, depth, and differences in brightness. Other axons constitute the parvocellular (small cell) pathway, which carries information on color and fine detail. Some visual information projects directly back into the brain, while other information crosses to the opposite side of the brain. This crossing of optical pathways produces the distinctive optic chiasma (crossing) found at the base of the brain and allows us to coordinate information from both eyes.
Once in the brain, visual information is processed in several places, and its routes reflect the complexity and importance of visual information to humans and other animals. One route takes the signals to the thalamus, which serves as the routing station for all incoming sensory impulses except olfaction. In the thalamus, the magnocellular and parvocellular distinctions remain intact, and there are different layers of the thalamus dedicated to each. When visual signals leave the thalamus, they travel to the primary visual cortex at the rear of the brain. From the visual cortex, the visual signals travel in two directions. One stream that projects to the parietal lobe, in the side of the brain, carries magnocellular (where) information. A second stream projects to the temporal lobe and carries both magnocellular (where) and parvocellular (what) information.
Another important visual route is a pathway from the retina to the superior colliculus in the midbrain, where eye movements are coordinated and integrated with auditory information. Finally, there is the pathway from the retina to suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is a cluster of cells that is considered to be the body's internal clock, which controls our circadian (day-long) cycle. The SCN sends information to the pineal gland, which is important in sleep/wake patterns and annual cycles.
Senses provide information about the body and its environment. Humans have five special senses: olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. We also have general senses called somatosensation, which respond to stimuli like temperature, pain, pressure, and vibration.
Vestibular sensation is an organism's sense of spatial orientation and balance, gravity, and movement allowing organisms to stay upright.
Proprioception is the sense of body position and movement (position of bones, joints, and muscles).
Kinesthesia is the sense of limb position that is used to track limb movement. All of these senses are part of somatosensation.
All of these senses share a common function: to convert a stimulus (light, sound, position of body) into an electric signal in the nervous system. This process is called sensory transduction.
There are two types of cellular systems that perform sensory transduction. In one, a neuron works with a sensory receptor, a cell, or cell process that is specialized to engage with and detect a specific stimulus. Stimulation of the sensory receptor activates the associated afferent neuron, which carries the information about the stimulus to the central nervous system. In the second type of sensory transduction, a sensory nerve ending responds to a stimulus in the internal or external environment and this neuron constitutes the sensory receptor.
Reception
The first step in sensation is reception, which is the activation of sensory receptors by mechanical stimuli, such as deformation, chemicals, or temperature. The receptor will then respond to the stimuli. The region in space in which a given sensory receptor can respond to a stimulus is the receptor's receptive field, and the field can be small or large.
Transduction
The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This translation takes place at the sensory receptor, and the change in electric potential that is produced is called receptor potential.
A mechanoreceptor is a type of receptor that possesses specialized membranes that respond to pressure including compression or bending the dendrites opens gated ion channels in the plasma membrane of the sensory neuron, changing its electric potential. Neurons are depolarized with a positive change to the neuron's electrical potential. Receptor potentials are graded potentials because the magnitude of these graded receptor potentials varies with the strength of the stimulus. If the magnitude of the depolarization is enough for the membrane potential to reach the threshold, the neuron will release an action potential.
Sensory receptors are specialized for different senses according to the type of stimulus. Stimuli can be combined to enhance the strength of sense.
Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus. Therefore, action potentials transmitted over a sensory receptor's afferent axons encode one type of stimulus, and this segregation of senses is preserved in other sensory circuits. The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. A second way in which intensity is encoded is by the number of receptors activated.
Perception is an individual's interpretation of a sensation. Although perception relies on the activation of sensory receptors, perception happens not at the level of the sensory receptor, but at higher levels in the nervous system in the brain. The brain distinguishes sensory stimuli through a sensory pathway. Action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus. The neurons synapse with particular neurons in the brain or spinal chord. All sensory signals are transmitted through the central nervous system (except olfactory system) and are routed to the thalamus and to the appropriate region of the cortex. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex that processes a particular sense. Interpretation of sensory signals between individuals of the same species is similar.
Somatosensation is a sensory phenomenon that includes all sensation received from the skin and mucous membranes, as well as from the limbs and joints. Somatosensation is also known as the tactile sense, or the sense of touch. Somatosensation occurs all over the exterior of the body and at some interior locations. Receptor types embedded in the skin, mucous membranes, muscles, joints, internal organs, and cardiovascular system play a role in somatosensation.
The epidermis is the outer layer of the skin in mammals. The epidermis is relatively thin, composed of keratin-filled cells, and has no blood supply. The epidermis serves as barrier to water and to invasion by pathogens.
Below the epidermis is the dermis, a much thicker layer that contains blood vessels, sweat glands, hair follicles, lymph vessels, and lipid-secreting sebaceous glands. Below the epidermis and dermis is the subcutaneous tissue, or hypodermis, the fatty layer that contains blood vessels, connective tissue, and the axons of sensory neurons. The hypodermis holds about 50 percent of the body's fat and attaches the dermis to the bone and muscle, and supplies nerves and blood vessels to the dermis.
Somatosensory Receptors: Sensory receptors are classified into five categories: mechanoreceptors, thermoreceptors, proprioceptors, pain receptors (nociceptors), and chemoreceptors. These categories are based on the nature of stimuli each receptor class transduces. The sense of touch involves more than one kind of stimulus and more than one kind of receptor. Mechanoreceptors in the skin are described as encapsulated or unencapsulated (free nerve endings).
A free nerve ending is an unencapsulated dendrite of a sensory neuron. Free nerve endings are the most common nerve endings in skin and they extend into the middle of the epidermis. Free nerve endings are sensitive to painful stimuli, to hot and cold, and to light touch. They are slow to adjust to a stimulus and so are less sensitive to abrupt changes in stimulation.
Three classes of mechanoreceptors are: tactile, proprioceptors, and baroreceptors. Mechanoreceptors sense stimuli due to physical deformation of their plasma membranes. They contain mechanically gated ion channels whose gates open or close in response to pressure, touch, stretching, and sound.
There are four primary tactile mechanoreceptors in human skin: Merkel's disks, Meissner's corpuscles, Ruffini endings, and Pacinian corpuscles. Two of these are located toward the surface of the skin and two are located deeper. A fifth type of mechanoreceptor, Krause end bulbs, are only found in specialized regions and detect cold.
Merkel's disks are found in the upper layers of skin near the base of the epidermis, both in skin that has hair and on glabrous skin, the hairless skin found on the palms and fingers, the soles of feet, and the lips of humans and primates. Merkel's disks are densely distributed in the fingertips and lips. They are slow-adapting, encapsulated nerve endings, and they respond to light touch. Light touch (or discriminative touch), is a light pressure that allows the location of a stimulus to be pinpointed. The receptive fields of Merkel's disks are small with well-defined borders, which makes them sensitive to edges.
Meissner's corpuscles, also known as tactile corpuscles, are found in the upper dermis, but they project into the epidermis. These are found primarily in the glabrous skin on the fingertips and eyelids. They respond to fine touch and pressure, but they also respond to low-frequency vibration or flutter. They are rapidly adapting, fluid-filled, encapsulated neurons with small, well-defined borders and are responsive to fine details. Like Merkel's disks, Meissner's corpuscles are not as plentiful in the palms as they are in the fingertips.
Ruffini endings (bulbous corpuscles) are deeper in the epidermis near the base and are found in both glabrous and hairy skin. Ruffini endings are also slow-adapting, encapsulated mechanoreceptors that detect skin stretch and deformations within joints, so they provide valuable feedback for gripping objects and controlling finger position and movement. Ruffini endings also contribute to proprioception and kinesthesia. They also detect warmth and are located deeper in the skin than cold detectors.
Pacinian corpuscles are located deep in the dermis of both glabrous and hairy skin and are structurally similar to Meissner's corpuscles. Pacinian corpuscles are found in the bone periosteum, joint capsules, pancreas and other viscera, breast, and genitals. Pacinian corpuscles are rapidly adapting mechanoreceptors that sense deep transient (but not prolonged) pressure and high-frequency vibration. Pacinian receptors detect pressure and vibration by being compressed, stimulating their internal dendrites. There are fewer Pacinian corpuscles and Ruffini endings in skin than there are Merkel's disks and Meissner's corpuscles.
In proprioception, proprioceptive and kinesthetic signals travel through myelinated afferent neurons running from the spinal chord to the medulla. Neurons are not physically connected, but communicate via neurotransmitters secreted into synapses or "gaps" between communicating neurons. Once in the medulla, the neurons continue carrying the signals to the thalamus.
Muscle spindles are stretch receptors that detect the amount of stretch, or lengthening of muscles. Related structures are Golgi tendon organs, which are tension receptors that detect the force of muscle contraction. Proprioceptive and kinesthetic signals come from limbs. Unconscious proprioceptive signals run from the spinal chord to the cerebellum, the brain region that coordinates muscle contraction, rather than to the thalamus, like most other sensory information.
Baroreceptors detect pressure changes in an organ. They are found in the walls of the carotid artery and the aorta where they monitor blood pressure, and in the lungs where they detect the degree of lung expansion. Stretch receptors are found at various sites in the digestive and urinary systems.
In addition to these two types of deeper receptors, there are also hair receptors which are found on nerve endings that wrap around the base of hair follicles. Some hair receptors detect slow and rapid hair movement, skin deflection, and detection of stimuli that have not yet touched the skin.
The different types of receptors that work together with the human skin produce a very refined sense of touch. The nociceptive receptors that detect pain are located near the surface. Merkel's disks and Meissner's corpuscles are small, finely calibrated mechanoreceptors located in the upper layers and can precisely detect gentle touch. Large mechanoreceptors like Pacinian corpuscles and Ruffini endings are located in the lower layers and respond to deeper touch. Both primary somatosensory cortex and secondary cortical areas are responsible for the total product of stimuli transmitted from all of the mechanoreceptors.
The density and distribution of mechanoreceptors is not consistent over the entire body. In humans, touch receptors are less dense in skin covered with any type of hair and denser in glabrous skin (non-hairy skin). Receptors in glabrous skin are thicker and more sensitive than hairy skin.
Thermoreception: In addition to Krause end bulbs that detect cold and Ruffini endings that detect warmth, there are different types of cold receptors on some free nerve endings called thermoreceptors located on the dermis, skeletal muscles, liver, and hypothalamus, that are activated by different temperatures. Their pathways into the brain run from the spinal chord through the thalamus to the primary somatosensory cortex. Warm and cold sensory information travels through the nerves to the brain.
Nociception, or pain, is the neural processing of injurious stimuli in response to tissue damage. Pain is caused by injury or contact with heat that causes burn or contact with a corrosive chemical. Pain can also be caused by harmless stimuli that mimic the action of damaging stimuli, such as eating spicy pepper plants. Capsaichins are chemical proteins that cause peppers to taste spicy hot by activating warm receptors in the human mouth.
Nociception begins at the sensory receptors, but pain, and the perception of nociception does not begin until it is communicated with the brain. There are several nociceptive pathways to and through the brain. Most axons carrying nociceptive information into the brain from the spinal chord project to the thalamus (as do other sensory organs) and the neural signal undergoes final processing in the primary somatosensory cortex. One nociceptive pathway projects not to the thalamus but directly to the hypothalamus in the forebrain, which modulates the cardiovascular and neuroendocrine functions of the autonomic nervous system.
Taste and Smell
Taste, also called gustation, and smell, also called olfaction, are the most interconnected senses in that both involve molecules of the stimulus entering the body and bonding to receptors. Smell allows animals to sense the presence of food, other animals, or substances in the environment. Smell also allows animals to distinguish one smell from another.
Taste and odor stimuli are molecules taken in from the environment. Primary tastes detected by humans are sweet, sour, bitter, salty, and umami. The first four are familiar. Umami was identified in 1908 but was not accepted as a fundamentally distinguished taste until later. The taste of umami, or savoriness, is attributed to the taste of the amino acid L-glutamate, and this amino acid is commonly used to enhance the taste of foods.
All odors that we perceive are molecules in the air we breathe. If molecules are not released from a substance, there is no smell or odor. Also, if a human or animal does not have a receptor that recognizes a specific molecule, then that molecule has no smell. Humans have about 350 olfactory receptor subtypes that work in various combinations to allow sensing of some 10,000 different odors. Smaller animals, however, can have more receptors, as mice have about 1,300 olfactory receptor types, allowing them to sense more odors. Both odors and tastes involve molecules that stimulate specific chemoreceptors. Therefore, both senses of taste and smell work together to help humans and animals identify flavors of substances.
Odorants (odor molecules) enter the nose and dissolve in the olfactory epithelium, the mucosa at the back of the nasal cavity. The olfactory epithelium is a collection of specialized olfactory receptors in the back of the nasal cavity that spans an area of about 5 square centimeters in humans. An olfactory receptor, which is a dendrite of a specialized neuron, responds when it binds certain molecules inhaled from the environment by sending impulses directly to the olfactory bulb of the brain. Humans have about 12 million olfactory receptors distributed among hundreds of different receptor types that respond to different odors. Animals, however have more receptors: rabbits have (100 million), dogs (1 billion), bloodhound dogs (4 billion). The olfactory epithelium of bloodhounds are many times larger than that of humans.
Olfactory neurons are bipolar neurons (neurons with two processes extending from the cell body). Each neuron has a single dendrite buried in the olfactory epithelium, and extending from this dendrite are 5 to 20 receptor-laden, hair-like cilia that trap odorant molecules. The sensory receptors on the cilia are proteins, and it is the variations in their amino acid chains that make the receptors sensitive to different odorants. Each olfactory sensory neuron has only one type of receptor on its cilia, and the receptors are specialized to detect specific odorants, so the bipolar neurons themselves are specialized. When an odorant binds with a receptor that recognizes it, the sensory neuron associated with the receptor is stimulated. Olfactory stimulation is the only sensory information that directly reaches the cerebral cortex, while other sensations are sent through the thalamus.
Taste detection (gustation) is fairly similar to detecting an odor (olfaction), given that both taste and smell rely on chemical receptors being stimulated by certain molecules. The primary organ of taste is the taste bud, which is a cluster of gustatory receptors (taste cells) that are located within the bumps on the tongue called papillae (papilla). There are several structurally distinct papillae. Filiform papillae, which are located across the tongue, are tactile, providing friction that helps the tongue move substances, and contain no taste cells. In contrast, fungiform papillae, which are located mainly on the anterior two-thirds of the tongue, each contain one to eight taste buds and also have receptors for pressure and temperature.
Folate papillae are leaf-like papillae located in parallel folds along the edges and toward the back of the tongue. Folate papillae contain about 1,300 taste buds within their folds. Circumvallate papillae, which are wall-like papillae in the shape of an inverted "V" at the back of the tongue. Each of these papillae is surrounded by a groove and contains about 250 taste buds.
Each taste bud's taste cells are replaced every 10 to 14 days. These are elongated cells with hair-like processes called microvilli at the tips that extend into the taste bud pore. Food molecules or tastants are dissolved in saliva, and they bind with and stimulate the receptors on the microvilli. The receptors for tastants are located across the outer portion and front of the tongue, outside the middle area where the filiform papillae are most prominent.
In humans, there are five primary tastes, and each taste has only one corresponding type of receptor. Thus, like olfaction, each receptor is specific to its stimulus (tastant). Transduction of the five tastes happens through different mechanisms that reflect the molecular composition of the tastant. A salty tastant (containing NaCl) provides the sodium ions (Na+) that enter the taste neurons and excite them directly. Sour tastants are acids and belong to the thermoreceptor protein family. Binding of an acid or other sour-tasting molecule triggers a change in the ion channel and these increase hydrogen ion (H+) concentrations in the taste neurons, thus depolarizing them. Sweet, bitter, and umami tastants require a G-protein coupled receptor. These tastants bind to their respective receptors, thereby exciting the specialized neurons associated with them. The ability of taste and smell declines with age.
Olfactory neurons project from the olfactory epithelium to the olfactory bulb as thin, unmyelinated axons. The olfactory bulb is composed of neural clusters called glomeruli, and each glomerulus receives signals from one type of olfactory receptor, so each glomerulus is specific to one odorant. From glomeruli, olfactory signals travel directly to the olfactory cortex and then to the frontal cortex and the thalamus. This is a different path than most other sensory information, which is sent directly to the thalamus before ending up in the cortex. Olfactory signals also travel directly to the amygdala, thereafter reaching the hypothalamus, thalamus, and frontal cortex. The last structure that olfactory signals directly travel to is a cortical center in the temporal lobe structure important in spatial, autobiographical, declarative, and episodic memories. Olfaction is finally processed by areas of the brain that deal with memory, emotions, reproduction, and thought.
Taste neurons project from the taste cells in the tongue, esophagus, and palate to the medulla, in the brainstem. From the medulla, taste signals travel to the thalamus and then to the primary gustatory cortex. Information from different regions of the tongue is segregated in the medulla, thalamus, and cortex.
Hearing and Vestibular Sensation
Audition (sense of hearing) helps humans and animals detect danger, and communication with other animals. The vestibular system is a sensory system that helps maintain balance and spatial orientation, and detects head and body movement. The vestibular system is located in the inner ear and is made up of semicircular canals and otolith organs.
Auditory stimuli are sound waves that are mechanical pressure waves that move through a medium such as air or water. Sound waves cannot travel through a vacuum because there is no medium to travel through. The speed of sound waves varies based on altitude, temperature, and medium. At sea level and 20 degrees C, sound waves travel in the air at 343 meters per second.
Just like all waves, sound waves have four main characteristics: frequency, wavelength, period, and amplitude. Frequency is the number of waves per unit of time, and in sound is heard as pitch. The most common unit of sound frequency is hertz (Hz), or cycles per second. Many animals can detect frequencies higher than humans are able to detect (called ultrasound).
Amplitude is the dimension of a wave from peak to trough and in sound is heard as volume. The sound waves of louder sounds have greater amplitude than those of softer sounds. The volume of sound waves is measured in decibels (dB).
The reception of sound in mammals, or sound waves are collected by the external, cartilaginous part of the ear called the auricle, then travel through the auditory canal and cause vibration of the thin diaphragm called the tympanum or ear drum, the innermost part of the outer ear. Interior to the tympanum is the middle ear. The middle ear holds three small bones called the ossicles, which transfer energy from the moving tympanum to the inner ear. The three ossicles are the malleus (the hammer), incus (the anvil), and stapes (the stirrup). The three ossicles are unique to mammals, and each plays a role in hearing. The malleus attaches at three points to the interior surface of the tympanic membrane. The incus attaches the malleus to the stapes. In humans, the stapes is not long enough to reach tympanum. Without the malleus and incus, the vibrations of the tympanum would never reach the inner ear. These bones also function to collect force and amplify sounds. Many animals use the stapes of the middle ear to transmit vibrations to the inner ear.
The Transduction of Sound
Vibrating objects, such as vocal chords, create sound waves or pressure waves in the air. When these pressure waves reach the ear, the ear transduces this mechanical stimulus (pressure wave) into a nerve impulse (electrical signal) that the brain perceives as sound. The pressure waves strike the tympanum, causing it to vibrate. The mechanical energy from the moving tympanum transmits vibrations to the three bones of the middle ear. The stapes transmits the vibrations to a thin diaphragm called the oval window, which is the outermost structure of the inner ear. The structures of the inner ear are found in the labyrinth, a bony, hollow structure that is the most interior portion of the ear. Here, the energy from the sound wave is transferred from the stapes through the flexible oval window and to the fluid of the cochlea. The vibrations of the oval window create pressure waves in the fluid (perilymph) inside the cochlea. The cochlea is a whorled structure, like the shell of a snail, and it contains receptors for transduction of the mechanical wave into an electrical signal. Inside the cochlea, the basilar membrane is a mechanical analyzer that runs the length of the cochlea, curling toward the cochlea's center.
The mechanical properties of the basilar membrane change along its length, such that it is thicker, tauter, and narrower at the outside of the whorl (where the cochlea is the largest), and thinner, floppier, and broader toward the apex, or center, of the whorl (where the cochlea is smallest). Different regions of the basilar membrane vibrate according to the frequency of the sound wave conducted through the fluid in the cochlea. For these reasons, the fluid-filled cochlea detects different wave frequencies (pitches) at different regions of the membrane. When the sound waves in the cochlear fluid contact the basilar membrane, it flexes back and forth in a wave-like fashion. Above the basal membrane is the tectorial membrane.
The site of transduction is in the organ of Conti (spiral organ). It is composed of hair cells held in place above the basilar membrane like flowers projecting up from soil, with their exposed short, hair-like stereocilia contacting or embedded in the tectorial membrane above them. The inner hair cells are the primary auditory receptors and exist in a single row, about 3,500. The stereocilia from the inner hair cells extend into small dimples on the tectorial membrane's lower surface. The outer hair cells are arranged in three or four rows. They number about 12,000, and they function to fine tune incoming sound waves. The longer stereocilia that project from the outer hair cells actually attach to the tectorial membrane. All of the stereocilia are mechanoreceptors, and when bent by vibrations they respond by opening a gated ion channel. As a result, the hair cell membrane is depolarized, and a signal is transmitted to the chochlear nerve. Intensity (volume) of sound is determined by how many hair cells at a particular location are stimulated.
The hair cells are arranged on the basilar membrane in an orderly way. The basilar membrane vibrates in different regions, according to the frequency of the sound waves impinging on it. Likewise, the hair cells that lay above it are most sensitive to a specific frequency of sound waves. Hair cells can respond to a small range of similar frequencies, but they require stimulation of greater intensity to fire at frequencies outside their optimal range. The difference in response frequency between adjacent inner hair cells is about 0.2 percent.
Place theory, which is the model for how biologists think pitch detection works in the human ear, states that high frequency sounds selectively vibrate the basilar membrane of the inner ear near the entrance port (the oval window). Lower frequencies travel farther along the membrane before causing appreciable excitation of the membrane. The basic pitch-determining mechanism is based on the location along the membrane where the hair cells are stimulated. The place theory is the first step toward the understanding of pitch reception. Considering the extreme pitch sensitivity of the human ear, it is thought that there must be some auditory "sharpening" mechanism to enhance the pitch resolution.
When sound waves produce fluid waves inside the cochlea, the basilar membrane flexes, bending the stereocilia that attach to the tectorial membrane. Their bending results in action potentials in the hair cells, and auditory information travels along the neural endings of the bipolar neurons of the hair cells (collectively, the auditory nerve) to the brain. When the hairs bend, they release an excitatory neurotransmitter at a synapse with a sensory neuron, which then conducts action potentials to the central nervous system. The cochlear branch of the vestibulocochlear cranial nerve sends information on hearing. The auditory system is very redefined, and there is some modulation or "sharpening" built in. The brain can send signals back to the cochlea, resulting in a change of length in the outer hair cells, sharpening or dampening the hair cells' response to certain frequencies.
The inner hair cells are the most important for conveying auditory information to the brain. About 90 percent of the afferent neurons carry information from inner hair cells, with each hair cell synapsing with 10 or so neurons. Outer hair cells connect to only 10 percent of the afferent neurons, and each afferent neuron innervates many hair cells. The afferent, bipolar neurons that convey auditory information travel from the cochlea to the medulla, through the pons and midbrain in the brainstem, finally reaching the primary auditory cortex in the temporal lobe.
Vestibular system stimuli are the linear acceleration (gravity), angular acceleration and deceleration. Gravity, acceleration, and deceleration are detected by evaluating the inertia on receptive cells in the vestibular system. Gravity is detected through head position. Angular acceleration and deceleration are expressed through turning or tilting of the head.
The vestibular system has some similarities to the auditory system. It utilizes hair cells just like the auditory system, but it excites them in different ways. There are five vestibular receptor organs in the inner ear: the utricle, the saccule, and the three semicircular canals. Together, they make up what is known as the vestibular labyrinth. The utricle and saccule respond to acceleration in a straight line, such as gravity. The hair cells in the utricle and saccule lie below a gelatinous layer, with their stereocilia projecting into the gelatin. Embedded in this gelatin are calcium carbonate crystals. When the head is tilted, the crystals continue to be pulled straight down by gravity, but the new angle of the head causes the gelatin to shift, thereby bending the stereocilia. The bending of the stereocilia stimulates the neurons, and they signal to the brain that the head is tilted, allowing the maintenance of balance. It is the vestibular branch of the vestibulocochlear cranial nerve that deals with balance.
The fluid-filled semicircular canals are tubular loops set at oblique angles and they are arranged in three spatial planes. The base of each canal has a swelling that contains a cluster of hair cells. The hairs project into a gelatinous cap called the cupula and monitor angular acceleration and deceleration from rotation. These would be stimulated as the head is turned or falls forward. One canal lies horizontally, while the other two lie about 45 degree angles to the horizontal axis. When the brain processes input from all three canals together, it can detect angular acceleration or deceleration in three dimensions. When the head turns, the fluid in the canals shifts, thereby bending stereocilia and sending signals to the brain. Upon stopping of accelerating, decelerating, or moving, the movement of the fluid within the canals slows or stops. Therefore, the canals are sensitive to changes in velocity.
Hair cells from the utricle, saccule, and semicircular canals also communicate through bipolar neurons to the cochlear nucleus in the medulla. Cochlear neurons send descending projections to the spinal chord and ascending projections to the pons, thalamus, and cerebellum. Connections to the cerebellum are important for coordinated movements. There are also projections to the temporal cortex, which account for feelings of dizziness. Projections to autonomic nervous system areas in the brainstem account for motion sickness. Projections to the primary somatosensory cortex monitors subjective measurements of the external world and self-movement. Vestibular signals project to certain optic muscles to coordinate eye and head movements.
Vision is the ability to detect light patterns from the outside environment and interpret them into images. The importance of vision to humans is that about one-third of the human cerebral cortex is dedicated to analyzing and perceiving visual information.
As with auditory stimuli, light travels in waves. The compression of waves that compose sound must travel in a medium: a gas, liquid, or solid. In contrast, light is composed of electromagnetic waves and needs no medium as light can travel in a vacuum. The behavior of light can be discussed in terms of the behavior of waves and also in terms of the behavior of the fundamental unit of light: a packet of electromagnetic radiation called a photon. In the electromagnetic spectrum, only a small section contains visible light and the rest of the spectrum contains radiation that cannot be seen by humans because the frequency is either too low or too high to be visible.
Important variables of light include wavelength (which varies inversely with frequency), and manifests itself as hue or color. Light at the red end of the visible spectrum has longer wavelengths (and lower frequency) while light at the violet end of the spectrum has shorter wavelengths (and higher frequency). The wavelength of light is expressed in nanometers (nm) (one billionth of a meter). Visible light for humans ranges from 380 nm to 740 nm. Some animals, however, can detect wavelengths outside the human range.
Wave amplitude is perceived as luminous intensity or brightness of light. The standard unit of intensity of light is the candela, the intensity of one common candle.
Light waves travel at 299, 792 km per second in a vacuum and slower in air and water. The eye senses long wavelengths waves as red, medium wavelengths waves as green, and short wavelengths of waves as blue. White light is light that is perceived as white by humans, when all color receptors of the human eye are stimulated equally. The apparent color of an object is the color or colors of light that the object reflects while the other colors of light of the object are absorbed and not visible.
Anatomy of the Eye
The photoreceptive cells of the eye, where transduction of light to nervous impulses occurs, are located in the retina on the inner surface of the back of the eye. But light does not impinge on the retina unaltered. Rather, it passes through other layers that process it so that it can be interpreted by the retina. The cornea, the front transparent layer of the eye, and the crystalline lens, a transparent convex structure behind the cornea, both refract (bend) light to focus on the image of the retina. The iris, which is visible as the colored part of the eye, is a circular muscular ring lying between the lens and the cornea that regulates the amount of light entering the eye. In conditions of high ambient light, the iris contracts, reducing the size of the pupil at its center. In conditions of low light, the iris relaxes and the pupil enlarges.
The main function of the lens is to focus light on the retina and fovea centralis. The lens is dynamic, focusing and refocusing light as the eye rests on near and far objects in the visual field. The lens is operated by muscles that stretch it flat or allow it to thicken, changing the focal length of light coming through it to focus it sharply on the retina. With age comes the loss of the flexibility of the lens, and a form of farsightedness called presbyopia results. Presbyopia occurs because the image focuses behind the retina. Presbyopia is a deficit similar to a different type of farsightedness called hyperopia caused by an eyeball that is too short. For both defects, images in the distance are clear but images nearby are blurry. Myopia (nearsightedness) occurs when an eyeball is elongated and the image focus falls in front of the retina. In this case, images in the distance are blurry but images nearby are clear.
There are two types of photoreceptors in the retina: rods and cones, named for their general appearance. Rods are strongly photosensitive and are located in the outer edges of the retina. They detect dim light and are used primarily for peripheral and nighttime vision. Cones are weakly photosensitive and are located near the center of the retina. They respond to bright light, and their primary role is in daytime, color vision.
The fovea is the region in the center back of the eye that is responsible for acute vision. The fovea has a high density of cones. When examining an object close up, the eyes adjust so that the object's image falls on the fovea. However, when looking at far away objects, like stars in the night sky, or objects in dim light, the object can be better viewed by the peripheral vision because it is the rods at the edges of the retina that operate better in low light or far away objects. Near objects are better seen with cones at the center. In humans, cones far outnumber rods in the fovea.
Rods and cones are the site of transduction of light to a neural signal. Both rods and cones contain photopigments. In vertebrates, the main photopigment, rhodopsin, has two main parts: an opsin, which is a membrane protein (in the form of a cluster of alpha helices that span the membrane), and retinal, a molecule that absorbs light. When light hits a photoreceptor, it causes a shape change in the retinal, altering its structure from a bent (cis) form of the molecule to its linear (trans) isomer. This isomerization of retinal activates the rhodopsin, starting a cascade of events that ends with the closing of Na+ channels in the membrane of the photoreceptor. Thus, unlike most other sensory neurons (which become depolarized by exposure to a stimulus), visual receptors become hyperpolarized and thus driven away from threshold.
There are three types of cones (with different photopsins), and they differ in the wavelength to which they are most responsive. Some cones are maximally responsive to short light waves of 420 nm (S cones for short). Other cones respond to waves of 530 nm (M cones for medium). A third group of cones responds maximally to light of longer wavelengths of 560 nm (L cones for long). With only one type of cone, color vision would not be possible, and a two-cone (dichromatic) system has color vision limitations. Primates and humans use a three-cone or trichromatic system, resulting in full color vision.
The color human see or perceive is the ratio of activity of our three types of cones. The colors of the visual spectrum, running from long-wavelength light to short, are red (700 nm), orange (600 nm), yellow (565 nm), green (497 nm), blue (470 nm), indigo (450 nm), and violet (425 nm). Humans have a very sensitive perception of color and can distinguish about 500 levels of brightness, 200 different hues, and two million distinct colors.
Retinal processing occurs as visual signals leave the cones and rods, travel to the bipolar cells, and then to the ganglion cells. A large degree of processing of visual information occurs in the retina itself, before information is sent to the brain.
Photoreceptors in the retina continuously undergo tonic activity, where they are always slightly active even when not stimulated by light. In neurons that exhibit tonic activity, the absence of stimuli maintains a firing rate at a baseline. While some stimuli increase the firing rate from the baseline, and other stimuli decrease firing rate. In the absence of light, the bipolar neurons that connect rods and cones to the ganglion cells are continuously and actively inhibited by the rods and cones. Exposure of the retina to light hyperpolarizes the rods and cones and removes their inhibition of bipolar cells. The now active bipolar cells in turn stimulate the ganglion cells, which send action potentials along their axons (which leave the eye as the optic nerve). Thus the visual system relies on change in the retinal activity, rather than the absence or presence of activity, to encode visual signals for the brain. Sometimes horizontal cells carry signals from one rod or cone to other photoreceptors and to several bipolar cells. When a rod or cone stimulates a horizontal cell, the horizontal cell inhibits more distant photoreceptors and bipolar cells, creating lateral inhibition. This inhibition sharpens edges and enhances contrast in the images by making regions receiving light appear lighter and dark surroundings appear darker. Amacrine cells can distribute information from one bipolar cell to many ganglion cells.
Myelinated axons of ganglion cells make up the optic nerves. Within the nerves, different axons carry different qualities of the visual signal. Some axons constitute the magnocellular (big cell) pathway, which carries information about form, movement, depth, and differences in brightness. Other axons constitute the parvocellular (small cell) pathway, which carries information on color and fine detail. Some visual information projects directly back into the brain, while other information crosses to the opposite side of the brain. This crossing of optical pathways produces the distinctive optic chiasma (crossing) found at the base of the brain and allows us to coordinate information from both eyes.
Once in the brain, visual information is processed in several places, and its routes reflect the complexity and importance of visual information to humans and other animals. One route takes the signals to the thalamus, which serves as the routing station for all incoming sensory impulses except olfaction. In the thalamus, the magnocellular and parvocellular distinctions remain intact, and there are different layers of the thalamus dedicated to each. When visual signals leave the thalamus, they travel to the primary visual cortex at the rear of the brain. From the visual cortex, the visual signals travel in two directions. One stream that projects to the parietal lobe, in the side of the brain, carries magnocellular (where) information. A second stream projects to the temporal lobe and carries both magnocellular (where) and parvocellular (what) information.
Another important visual route is a pathway from the retina to the superior colliculus in the midbrain, where eye movements are coordinated and integrated with auditory information. Finally, there is the pathway from the retina to suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is a cluster of cells that is considered to be the body's internal clock, which controls our circadian (day-long) cycle. The SCN sends information to the pineal gland, which is important in sleep/wake patterns and annual cycles.
