Nervous System Design in Animals and Humans by Owen Borville February 2, 2025 BIO 35
The nervous system in animals and humans varies in structure. Vertebrate nervous systems are usually more centralized and specialized than invertebrate nervous systems. However, all nervous systems have a basic structure in common: a central nervous system (CNS) that contains a brain and spinal chord and a peripheral nervous system (PNS) that contains a system of nerves that extend throughout the body of the animal and made up of peripheral sensory and motor nerves. In invertebrates, the nerve chords are commonly located ventrally (frontal) while vertebrate spinal chords are located dorsally (back side).
The nervous system is made up of neurons, which are specialized cells that can receive and transmit chemical or electrical signals. The nervous system also has glial cells, or glia that provide support functions for the neurons by playing an information processing role that is complementary to neurons. Neurons transmit signals from one place to another in the body, while the glial cells play a supportive role.
The number of neurons in animals vary from 100,000 in small insects to hundreds of millions in larger animals to almost 100 billion in each human body. Despite the number differences, the nervous systems of animals operate in a similar way in controlling body behavior and neurons control many of the same behaviors while communicating with each other and other types of cells. Most neurons have the same cellular components, but their sizes and shapes may vary, along with their functions also varying.
Parts of a Neuron
Neuron cells have some of the same parts that other cells have like a cell body or soma, a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other parts. Neurons also have some unique parts, such as structures used for sending and receiving electrical signals from other cells (communication). Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. Some neurons have no dendrites and other neurons have multiple dendrites. Dendrites can have small protrusions called dendric spines, which further increase surface area for possible synaptic connections.
Once a signal is received by a dendrite, it travels to the cell body. The cell body contains a specialized structure, the axon hillock, that integrates signals from multiple synapses and serves as a junction between the cell body and an axon. An axon is a tube-like structure that propagates the integrated signal to specialized endings called axon terminals. These terminals in turn synapse on other neurons, muscle, or target organs. Chemicals released at axon terminals allow signals to be communicated to these other cells. Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons. Some axons are covered with myelin, which acts as an insulator to minimize dissipation of the electrical signal is it travels down the axon, greatly increasing the speed of conduction. This insulation is important as the axon from a human motor neuron can be as long as a meter from the base of the spine to the toes. The myelin sheath is not actually part of the neuron. Myelin is produced by glial cells. Along the axon there are periodic gaps in in the myelin sheath. These gaps are called nodes of Ranvier and are sites where the signal is "recharged" as it travels along the axon.
Single neurons do not act alone. Neural communication depends on the connections that neurons make with one another (and with other cells). Dendrites from a single neuron may receive synaptic contact from many other neurons, as many as 200,000 other neurons.
Types of Neurons
There are different types of neurons with different functions, and the functional role of a neuron is very dependent on its structure. However, there are four basic types of neuron classification. Unipolar, bipolar, multipolar, and pseudounipolar. Unipolar neurons have only one structure that extends away from the soma. These neurons are not found in vertebrates but are found in insects where they stimulate muscles or glands. A bipolar neuron has one axon and one dendrite extending from the soma, such as in the retina. Multipolar neurons are the most common type of neuron and each multipolar neuron has one axon and multiple dendrites. Multipolar neurons can be found in the central nervous system (brain and spinal chord). Pseudounipolar cells share similar characteristics with both unipolar and bipolar cells. A pseudounipolar cell has a single process that extends from the soma, like a unipolar cell, but this process later branches into two distinct structures, like a bipolar cell. Most sensory neurons are pseudounipolar and have an axon that branches into two extensions: one connected to dendrites that receive sensory information and another that transmits this information to the spinal chord.
Neurogenesis is the birth of new neurons and this phenomena was discovered recently and play an important role in learning in animals, as new neuron birth has been correlated with learning.
Glia
Even though glial cells are thought to have a supportive role in the nervous system, the number of glial cells outnumber neurons by a factor of ten. Neurons would be unable to function without the vital roles of the glial cells. Glia guide developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, and provide myelin sheaths around axons. Scientists have recently discovered that glia play a role in responding to nerve activity and modulating communication between nerve cells. When glia do not function properly, the result can be mutations in glia that cause brain tumors.
Types of Glia
There are several different types of glia with different functions. Astrocytes make contact with both capillaries and neurons in the CNS. They provide nutrients and other substances to neurons, regulate the concentrations of ions and chemicals in the extracellular fluid, and provide structural support for synapses. Astrocytes also form the blood-brain barrier, a structure that blocks entrance of toxic substances into the brain. Astrocytes, in particular, have been shown throughout calcium imaging experiments to become active in response to nerve activity, transmit calcium waves between astrocytes, and modulate the activity of surrounding synapses. Satellite glia provide nutrients and structural support for neurons in the PNS. Microglia scavenge and degrade dead cells and protect the brain from invading microorganisms. Oligodendrocytes form myelin sheaths around axons in the CNS. One axon can be myelinated by several oligodendrocytes, and one oligodendrocyte can provide myelin for multiple neurons. This is distinctive from the PNS where a single Schwann cell provides myelin for only one axon as the entire Schwann cell surrounds the axon. Radial glia serve as scaffolds for developing neurons as they migrate to their end destinations. Ependymal cells line fluid-filled ventricles of the brain and the central canal of the spinal chord. They are involved in the production of cerebrospinal fluid, which serves as a cushion for the brain, moves the fluid between the spinal chord and the brain, and is a component for the choroid plexus.
How Neurons Communicate
Neurons use electrical and chemical signals to communicate with other neurons to perform all functions of the nervous system. Neurons must be able to send and receive signals to communicate and for the nervous system to function. Each neuron has a charged cellular membrane (which is a voltage difference between the inside and the outside), and the charge of this membrane can change in response to neurotransmitter molecules released from other neurons to environmental stimuli.
The lipid bilayer that surrounds a neuron is impermeable to charged molecules and ions. To enter or exit the neuron, ions must pass through special proteins called ion channels that span the membrane. Ion channels have different configurations such as open, closed, and inactive. Some ion channels need to be activated in order to allow ions to pass through the cell. Ion channels are sensitive to the environment and can change their shape accordingly. Voltage-gated ion channels change their shape or structure in response to voltage changes. Voltage-gated ion channels also regulate the relative ion concentration inside and outside the cell. The difference in total charge between the inside and outside of the cell is called the membrane potential.
Resting Membrane Potential
A neuron at rest is negatively charged, so that the inside is more negatively charged than the outside and is called the resting membrane potential because it is caused by differences in concentrations of ions inside and outside the cell (usually around -70 mV). There are different concentrations of several ions inside and outside the cell. The difference in the number of positively-charged potassium ions (K+) inside and outside the cell dominates the resting membrane potential. When the membrane is at rest, K+ ions accumulate inside the cell due to the activity of the Na/K pump, driving both ions against their concentration gradient.
The resting membrane potential is essentially the electrical charge difference across the membrane of a neuron when it's not actively sending signals. This electrical charge is typically around -70 millivolts (mV), meaning the inside of the neuron is negatively charged relative to the outside.
Ion distribution occurs as neurons maintain a high concentration of potassium ions (K⁺) inside and a high concentration of sodium ions (Na⁺) outside.
Ion channels occur when special proteins in the neuron's membrane allow ions to move in and out. Potassium channels are more permeable at rest, allowing K⁺ to leak out.
The sodium-potassium pump actively transports Na⁺ out and K⁺ in, using energy from ATP, and the pump helps maintain the concentration gradients. As K⁺ leaks out, it leaves behind negatively charged molecules inside, creating a negative charge.
Electrochemical gradient allows the balance of ion movement and charge difference and results in the resting membrane potential.
This movement of ions across the neuron's membrane, ensures that the neuron is ready to make an action potential when needed.
Action Potential
An action potential is a brief and powerful electrical signal that travels down a neuron, allowing it to communicate with other cells. A summary of action potential:
In the resting state, the neuron starts at its resting membrane potential, around -70 mV. Threshold occurs as a stimulus causes the membrane potential to become less negative, reaching a critical threshold (around -55 mV).
Depolarization occurs when voltage-gated sodium (Na⁺) channels open, causing Na⁺ ions to rush into the neuron. This makes the inside of the cell more positive, rapidly shifting the membrane potential to around +30 mV.
Repolarization occurs when Na⁺ channels close, and voltage-gated potassium (K⁺) channels open. K⁺ ions exit the cell, restoring the negative membrane potential.
Hyperpolarization occurs when the membrane potential temporarily becomes more negative than the resting state, due to K⁺ channels closing slowly.
Return to the resting state occurs when the sodium-potassium pump and other ion channels restore the resting membrane potential. This rapid sequence of events allows neurons to send signals quickly and efficiently across long distances.
Myelin and the Propagation of Action Potential
Myelin has a crucial role in the functioning of the nervous system. The structure of myelin is a fatty, white substance that wraps around the axons of neurons, forming an insulating layer called the myelin sheath.
The function of myelin is to increase the speed at which electrical impulses (action potentials) travel along the axon and myelin accomplishes this by insulating the axon and preventing the electrical signals from dissipating. Myelin ensures that signals travel quickly and efficiently.
Myelin is not continuous along the axon and there are small gaps called Nodes of Ranvier, where the axon is exposed. These gaps are essential for the rapid transmission of signals, allowing the action potential to "jump" from one node to the next in a process called saltatory conduction.
In the central nervous system (brain and spinal cord), myelin is produced by specialized cells called oligodendrocytes. In the peripheral nervous system (nerves outside the brain and spinal cord), Schwann cells produce myelin.
Proper myelination is essential for the efficient functioning of the nervous system. Disorders that affect myelin, such as multiple sclerosis, can lead to severe neurological symptoms.
During action potential propagation, myelin has a very important role enhancing the process.
Myelin acts as an insulator, preventing the loss of electrical signals along the axon. This insulation ensures that the action potential can travel efficiently over long distances.
The presence of gaps, known as Nodes of Ranvier, is crucial. These nodes are points where the axon membrane is exposed, and they contain a high density of voltage-gated sodium channels. When an action potential reaches a node, the influx of sodium ions depolarizes the membrane, recharging the action potential.
Because of the myelin sheath, the action potential doesn't need to depolarize the entire length of the axon. Instead, the action potential "jumps" from one node of Ranvier to the next. This process, known as saltatory conduction, significantly speeds up the transmission of the electrical signal compared to unmyelinated axons.
Energy efficient signal transmission increases because the action potential only regenerates at the nodes of Ranvier, fewer ion channels need to be activated, and less energy is expended on ion exchange and the sodium-potassium pump.
Myelin enables faster and more efficient propagation of action potentials by insulating the axon, creating nodes of Ranvier for saltatory conduction, and conserving energy. This intelligent design that ensures rapid and reliable communication within the nervous system.
Synaptic Transmission is the process where one neuron communicates with another neuron and this process is fundamental to the nervous system. The process includes:
Action Potential Arrival: The action potential travels down the axon of the presynaptic neuron and reaches the axon terminal.
Calcium Influx: The depolarization of the axon terminal opens voltage-gated calcium (Ca²⁺) channels. Calcium ions rush into the terminal.
Neurotransmitter Release: The influx of Ca²⁺ causes synaptic vesicles, which contain neurotransmitters, to fuse with the presynaptic membrane. This fusion releases neurotransmitters into the synaptic cleft, the small gap between the presynaptic and postsynaptic neurons.
Neurotransmitter Binding: Neurotransmitters diffuse across the synaptic cleft and bind to specific receptor sites on the postsynaptic membrane.
Postsynaptic Response: Binding of neurotransmitters to receptors opens ion channels in the postsynaptic membrane, leading to a change in the membrane potential. This can either excite (depolarize) or inhibit (hyperpolarize) the postsynaptic neuron, depending on the type of neurotransmitter and receptors involved.
Termination: The neurotransmitter's action is terminated by one or more processes: reuptake into the presynaptic neuron, enzymatic degradation, or diffusion away from the synaptic cleft.
Chemical Synapse
A chemical synapse is a specialized junction through which neurons communicate with each other using chemical signals known as neurotransmitters. The chemical synapse process includes:
Presynaptic Neuron: The neuron sending the signal has an axon terminal that contains synaptic vesicles filled with neurotransmitters.
Action Potential Arrival: When an action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium (Ca²⁺) channels, allowing Ca²⁺ ions to enter the terminal.
Neurotransmitter Release: The influx of calcium causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft via exocytosis.
Synaptic Cleft: This tiny gap between the presynaptic and postsynaptic neurons is where the neurotransmitters diffuse across.
Receptor Binding: Neurotransmitters bind to specific receptors on the postsynaptic membrane. This binding can open or close ion channels, leading to changes in the postsynaptic neuron's membrane potential.
Postsynaptic Potential: Depending on the type of neurotransmitter and receptor, this can result in an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP).
Signal Termination: The action of neurotransmitters is terminated by reuptake into the presynaptic neuron, enzymatic degradation, or diffusion away from the synaptic cleft.
Electrical Synapse
Electrical synapses offer a direct and efficient way for neurons to communicate. Unlike chemical synapses, which use neurotransmitters to convey signals, electrical synapses rely on direct electrical connections. Electrical synapse processes include:
Gap Junctions: Electrical synapses are formed by gap junctions, which are specialized channels that connect the membranes of two adjacent neurons. These channels allow ions and small molecules to pass directly from one neuron to another.
Direct Transmission: Because ions can flow freely through gap junctions, electrical signals can be transmitted almost instantaneously. This allows for rapid and synchronized activity between connected neurons.
Bidirectional Communication: Electrical synapses allow signals to travel in both directions, providing a means for neurons to quickly share information and coordinate their activities.
Synchronization: Electrical synapses are particularly important in areas of the nervous system where synchronized firing of neurons is crucial, such as in certain regions of the brain and in the heart. While electrical synapses are less common than chemical synapses in the human nervous system, they play a vital role in ensuring fast and coordinated communication between neurons.
Signal Summation
Signal summation is a crucial process in neurons, determining whether an action potential will be generated. There are two types of summation: spatial summation and temporal summation.
Spatial Summation
Multiple Inputs: Occurs when multiple presynaptic neurons release neurotransmitters at different synapses on the same postsynaptic neuron.
Combined Effect: The combined excitatory postsynaptic potentials (EPSPs) from these multiple inputs can depolarize the postsynaptic membrane to the threshold, triggering an action potential. Integration: This allows the postsynaptic neuron to integrate signals from different sources.
Temporal Summation
Single Input: Occurs when a single presynaptic neuron releases neurotransmitters multiple times in quick succession at the same synapse. Cumulative Effect: If the time interval between successive releases is short enough, the EPSPs can add up, bringing the membrane potential to the threshold and triggering an action potential. Frequency: The frequency of neurotransmitter release plays a key role in temporal summation.
Combined Influence Summation
Excitatory and Inhibitory Inputs: Neurons receive both excitatory and inhibitory inputs. The summation process takes into account both types, determining the overall effect on the postsynaptic neuron's membrane potential. Decision Making: The integration of these signals at the axon hillock decides whether the neuron will fire an action potential.
In essence, signal summation allows neurons to process and integrate multiple incoming signals, enabling complex decision-making processes within the nervous system.
Synaptic Plasticity
Synaptic plasticity is a remarkable property of neurons enabling the brain to adapt, learn, and store memories. Synaptic plasticity refers to the ability of synapses (the connections between neurons) to strengthen or weaken over time, in response to increases or decreases in their activity. Summary of synaptic plasticity:
Types of Synaptic Plasticity Include:
Long-Term Potentiation (LTP): Definition: LTP is the long-lasting enhancement of synaptic strength following high-frequency stimulation of a synapse. Mechanism: It involves the increase in the number of neurotransmitter receptors on the postsynaptic membrane and changes in the presynaptic neuron's release of neurotransmitters. Role: LTP is widely considered one of the primary mechanisms underlying learning and memory.
Long-Term Depression (LTD): Definition: LTD is the long-lasting decrease in synaptic strength following low-frequency stimulation of a synapse. Mechanism: It involves the removal of neurotransmitter receptors from the postsynaptic membrane and changes in neurotransmitter release from the presynaptic neuron. Role: LTD is thought to play a role in forgetting, synaptic pruning, and maintaining synaptic balance.
Factors Influencing Synaptic Plasticity
Activity-Dependent: Synaptic plasticity is influenced by the activity of the neurons. Frequent use of a synapse can lead to its strengthening, while infrequent use can lead to weakening. Molecular Mechanisms: Various molecules, such as neurotransmitters, growth factors, and intracellular signaling pathways, are involved in modulating synaptic strength. Experience-Dependent: Environmental factors and experiences, such as learning new skills or exposure to enriched environments, can drive changes in synaptic plasticity.
Importance of Synaptic Plasticity
Learning and Memory: Synaptic plasticity is fundamental to the brain's ability to learn and form memories. It allows the brain to encode, store, and retrieve information. Adaptation: Synaptic plasticity enables the brain to adapt to new situations, recover from injuries, and develop new skills. Neurological Health: Abnormalities in synaptic plasticity are associated with various neurological disorders, such as Alzheimer's disease, autism, and schizophrenia. Synaptic plasticity highlights the intelligent design of the brain and nervous system.
The Central Nervous System
The central nervous system (CNS) contains the brain and spinal chord and is covered with three layers of protective coverings called meninges (membranes). The outermost layer is the dura mater (hard mother) and its function for this thick layer is to protect the brain and the spinal chord. The dura mater also contains vein-like structures carry blood from the brain back to the heart. The middle layer is the web-like arachnoid mater. The last layer is the pia mater (soft mother) which directly contacts and covers the brain and spinal chord like plastic wrap. The space between the arachnoid matter and pia mater is filled with cerebrospinal fluid (CSF). CSF is produced by a tissue called choroid plexus in fluid-filled compartments in the CNS called ventricles. The brain floats in CSF, which acts as a cushion and shock absorber and makes the brain neutrally buoyant. CSF also functions to circulate chemical substances throughout the brain and into the spinal chord.
When a ventricle is blocked, CSF builds up and creates swelling and the brain is pushed against the skull, causing hydrocephalus and can cause seizures, cognitive problems, and death if surgery is not done.
The Brain
The brain is the part of the central nervous system that is contained in the cranial cavity of the skull. The brain includes the cerebral cortex, limbic system, basal ganglia, thalamus, hypothalamus, and cerebellum. The brain can be sectioned in three ways to view internal structures: left to right, front to back, and top to bottom.
The Cerebral Cortex
The cerebral cortex is the outermost part of the brain and a thick piece of nervous system tissue which is folded into hills called gyri (gyrus) and valleys called sulci (sulcus). The cortex is made up of two hemispheres: right and left which are separated by a large sulcus. A thick fiber bundle called the corpus callosum (tough body) connects the two hemispheres and allows information to be passed from one side to the other.
Brain surgery sometimes requires the elimination of an entire hemisphere of the brain (epilepsy). Sometimes also the corpus callosum is cut to treat epilepsy.
Each hemisphere of the mammalian cerebral cortex can be broken down into four functionally and spatially defined lobes that have different functions: frontal, parietal, temporal, and occipital.
The frontal lobe is located at the front of the brain, over the eyes. This lobe contains the olfactory bulb, which processes smells. The frontal lobe also contains the the motor cortex, which is important for planning and implementing movement. Parts of the motor cortex associate with different muscle groups. Neurons in the frontal lobe also control cognitive functions like maintaining attention, speech, and decision-making. Frontal lobes also are involved in personality, socialization, and accessing risk.
The parietal lobe is located at the top of the brain. Neurons in the parietal lobe are involved in speech and reading. Two of the parietal lobe's main functions are processing somatosensation (touch sensations like pressure, pain, heat, cold) and proprioception (the sense of how parts of the body are oriented in space). The parietal lobe contains a somatosensory map of the body similar to the motor cortex.
The occipital lobe is located at the back of the brain and it is involved primarily in vision, including seeing, recognizing, and identifying the visual world.
The temporal lobe is located at the base of the brain by the ears and is primarily involved in processing and interpreting sounds. The temporal lobe contains the hippocampus, a structure that processes memory formation.
Basal ganglia (basal nuclei) are interconnected areas of the brain that have important roles in movement control and posture. Damage to the basal ganglia leads to motor impairments. The basal ganglia also regulate motivation.
The thalamus (inner chamber) behaves as a gateway to and from the cortex as it receives sensory and motor inputs from the body and also receives feedback from the cortex. This feedback mechanism can modulate conscious awareness of sensory and motor inputs depending on the attention and arousal state of the animal. The thalamus helps regulate consciousness, arousal, and sleep states.
The hypothalamus, located below the thalamus, controls the endocrine system by sending signals to the pituitary gland, a pea-sized endocrine gland that releases several different hormones that affect other glands as well as other cells. The hypothalamus regulates important behaviors that are controlled by these hormones. The hypothalamus acts as a thermostat for the body as it ensures key functions like food and water intake, energy expenditure, and body temperature are kept at appropriate levels. Neurons in the hypothalamus also regulate circadian rhythms or sleep cycles.
The limbic system is a connected set of structures that regulates emotion and behaviors related to fear and motivation. The limbic system has a role in memory formation and includes parts of the thalamus and hypothalamus as well as the hippocampus. An important structure in the limbic system is a temporal lobe structure called the amygdala. The two amygdala are important for the sensation of fear and recognizing fearful faces. The cingulate gyrus helps regulate emotions and pain.
The cerebellum is located at the base of the brain on top of the brainstem. The cerebellum controls balance and aids coordinating movement and learning new motor tasks.
The brainstem connects the rest of the brain to the spinal chord. The brainstem contains the midbrain, medulla oblongata, and the pons. Motor and sensory neurons extend through the brainstem allowing for the relay of signals between the brain and spinal chord. Ascending neural pathways cross in this section of the brain allowing the left hemisphere of the cerebrum to control the right side of the body and vise versa. The brainstem coordinates motor control signals sent from the brain to the body. The brainstem controls several important functions of the body including alertness, arousal, breathing, blood pressure, digestion, heart rate, swallowing, walking, and sensory and motor information integration.
The spinal chord connects to the brainstem and extends down the body through the spinal column. The spinal chord is a thick bundle of nerve tissue that carries information about the body to the brain and from the brain to the body. The spinal chord is contained within the bones of the vertebrate column but is able to communicate signals to and from the body through its connections with spinal nerves. Myelinated axons make up the white matter and neuron and glial cell bodies make up the gray matter. Gray matter is also composed of interneurons, which connect two neurons each located in different parts of the body. Axons and cell bodies in the dorsal spinal chord convey mostly sensory information from the body to the brain. Axons and cell bodies in the ventral spinal chord primarily transmit signals controlling movement from the brain to the body.
The spinal chord also controls motor reflexes which are quick, unconscious movements that involve local synaptic connections. Synapses with interneurons in the spinal column transmit information to the brain to convey what happened. Damage to the spinal chord can lead to paralysis.
The Peripheral Nervous System
The peripheral nervous system (PNS) is the connection between the central nervous system (CNS) and the rest of the body. While the central nervous system is the power source of the nervous system and creates the signals that control the functions of the body, the peripheral nervous system is the medium that allows these signals to be sent throughout the body, which enables the central nervous system to control the body. The PNS also allows signals from the body to be sent back to the CNS.
The PNS contains two parts: the autonomic nervous system which controls bodily functions without conscious control, and the sensory-somatic nervous system which transmits sensory information from the skin, muscles, and sensory organs to the CNS and sends motor commands from the CNS to the muscles.
The autonomic nervous system functions as the relay between the CNS and the internal organs. It controls the lungs, heart, smooth muscle, and exocrine and endocrine glands. The autonomic nervous system controls these organs largely without conscious control. Signaling to the target tissue usually involves two synapses: a pre-ganglionic neuron (originating in the CNS) synapses to a neuron in the ganglion that, in turn, synapses on the target organ. There are two divisions of the autonomic nervous system that often have opposing effects: the sympathetic nervous system and the parasympathetic nervous system.
The sympathetic nervous system is responsible for the fight or flight response that occurs when an animal encounters a dangerous situation. Functions controlled by the sympathetic nervous system include an accelerated heart rate and inhibited digestion. These functions help prepare the organism's body for the physical strain required to escape a dangerous situation.
Most preganglionic neurons in the sympathetic nervous system originate in the spinal chord. The axons of these neurons release acetylcholine on postganglionic neurons within sympathetic ganglia (the sympathetic ganglia form a chain that extends alongside the spinal chord). The acetylcholine activates the postganglionic neurons. Postganglionic neurons then release norepinephrine onto target organs. One preganglionic neuron synapses on multiple postganglionic neurons, amplifying the effect of the original synapse. The adrenal gland also releases norepinephrine (and epinephrine) into the bloodstream. The physiological effects of this norepinephrine release include dilating the trachea and bronchi, which eases breathing, increasing heart rate, and moving blood from the skin to the heart, muscles, and brain. The strength and speed of the sympathetic response helps the organism avoid danger and even remember danger.
The parasympathetic nervous system allows an animal to rest and digest (as opposed to being activated during stress like the sympathetic nervous system). Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral (lower) spinal chord. The axons of preganglionic neurons release acetylcholine on the postganglionic neurons, which are located near the target organs. Most postganglionic neurons release acetylcholine onto target organs, but some release nitric oxide.
The parasympathetic nervous system resets organ function after the sympathetic nervous system is activated. The effects of acetylcholine release on target organs include slowing of heart rate, lowered blood pressure, and stimulation of digestion.
Sensory-Somatic Nervous System
The sensory-somatic nervous system is made up of cranial and spinal nerves and contains both sensory and motor neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the CNS. Motor neurons transmit messages about desired movement from the CNS to the muscles to make them contract. Without its sensory-somatic nervous system, an animal would be unable to process any information about its environment (senses) and could not control motor movements. While the autonomic nervous system has two synapses between the CNS and the target organ, sensory and motor neurons only have one synapse between the CNS and the organ. Acetylcholine is the main neurotransmitter released at the synapses.
Humans have 12 cranial nerves, or nerves that emerge from or enter the skull (cranium), as opposed to the spinal nerves, which emerge from the vertebral column. Each cranial nerve is named (olfactory, optic...). Some cranial nerves transmit only sensory information. Other cranial nerves transmit almost solely motor information. Other cranial nerves contain a mix of sensory and motor fibers.
Spinal nerves transmit sensory and motor information between the spinal chord and the rest of the body. Each of the 31 spinal nerves in humans contain both sensory and motor axons. The sensory neuron cell bodies are grouped in structures called dorsal root ganglia. Each sensory neuron has one projection with a sensory receptor ending in skin, muscle, or sensory organs and another that synapses with a neuron in the dorsal spinal chord. Motor neurons have cell bodies in the ventral gray matter of the spinal chord that project to muscle through the ventral root. These neurons are usually stimulated by interneurons within the spinal chord but are sometimes directly stimulated by sensory neurons.
Nervous System Disorders
Neurodegenerative disorders are illnesses characterized by a loss of nervous system functioning that are usually caused by neuronal death. Examples are Huntington's disease, amyotrophic lateral sclerosis, Alzheimer's disease, dementia, and Parkinson's disease.
Alzheimer's disease is the most common cause of dementia in the elderly adults. Symptoms are memory loss, confusion, difficulty planning or doing tasks, poor judgement, and personality changes. Alzheimer's disease is caused by mutations in certain genes that produce proteins that build up in the brain and damage nerve cells. The mutations can be inherited.
Parkinson's disease is another neurodegenerative disease. Parkinson's disease causes the loss of dopamine neurons in the substantia nigra, a structure of the brain that regulates movement. Symptoms are tremor, slow movement, speech changes, balance and posture problems, and rigid muscles. Most causes of Parkinson's disease are unknown.
Neurodevelopmental Disorders
Autism spectrum disorder is a neurodevelopment disorder. Symptoms are impaired social skills, lack of empathy for others, repetitive motor behaviors, interest in particular subjects, and unusual language use. Causes of many forms of autism are unknown. Treatments are available for autism but there is no cure.
Attention deficit hyperactivity disorder (ADHD) symptoms include the inability to focus, difficulty functioning, impulsivity, hyperactivity. The cause is unknown.
Neurologists are physicians who specialize in the study of disorders of the nervous system.
Mental Illnesses are nervous system disorders that result in problems with thinking, mood, or relating with other people.
Schizophrenia is a serious disorder with symptoms of being unable to differentiate reality from imagination, inappropriate or uncontrolled emotional responses, inability to think, and social dysfunction. Some people with schizophrenia suffer from hallucinations, hear voices, or delusions.
Depression involves having a severely depressed mood for an extended period of time, such as weeks. Symptoms of depression are loss of enjoyment of daily activities, loss of appetite, loss of sleep, difficulty concentrating, feelings of worthlessness, and suicidal thoughts.
Other Neurological Disorders
Epilepsy is a disorder that causes recurring seizures of the body and can be a result of brain injury, genetics, disease, or other illnesses.
Stroke occurs when blood fails to reach a portion of the brain long enough to cause damage, including neuron death, as blood supplies necessary oxygen to the brain. Symptoms are headache, muscle weakness, paralysis, speech disturbances, sensory problems, memory loss, and confusion.
The nervous system in animals and humans varies in structure. Vertebrate nervous systems are usually more centralized and specialized than invertebrate nervous systems. However, all nervous systems have a basic structure in common: a central nervous system (CNS) that contains a brain and spinal chord and a peripheral nervous system (PNS) that contains a system of nerves that extend throughout the body of the animal and made up of peripheral sensory and motor nerves. In invertebrates, the nerve chords are commonly located ventrally (frontal) while vertebrate spinal chords are located dorsally (back side).
The nervous system is made up of neurons, which are specialized cells that can receive and transmit chemical or electrical signals. The nervous system also has glial cells, or glia that provide support functions for the neurons by playing an information processing role that is complementary to neurons. Neurons transmit signals from one place to another in the body, while the glial cells play a supportive role.
The number of neurons in animals vary from 100,000 in small insects to hundreds of millions in larger animals to almost 100 billion in each human body. Despite the number differences, the nervous systems of animals operate in a similar way in controlling body behavior and neurons control many of the same behaviors while communicating with each other and other types of cells. Most neurons have the same cellular components, but their sizes and shapes may vary, along with their functions also varying.
Parts of a Neuron
Neuron cells have some of the same parts that other cells have like a cell body or soma, a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other parts. Neurons also have some unique parts, such as structures used for sending and receiving electrical signals from other cells (communication). Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. Some neurons have no dendrites and other neurons have multiple dendrites. Dendrites can have small protrusions called dendric spines, which further increase surface area for possible synaptic connections.
Once a signal is received by a dendrite, it travels to the cell body. The cell body contains a specialized structure, the axon hillock, that integrates signals from multiple synapses and serves as a junction between the cell body and an axon. An axon is a tube-like structure that propagates the integrated signal to specialized endings called axon terminals. These terminals in turn synapse on other neurons, muscle, or target organs. Chemicals released at axon terminals allow signals to be communicated to these other cells. Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons. Some axons are covered with myelin, which acts as an insulator to minimize dissipation of the electrical signal is it travels down the axon, greatly increasing the speed of conduction. This insulation is important as the axon from a human motor neuron can be as long as a meter from the base of the spine to the toes. The myelin sheath is not actually part of the neuron. Myelin is produced by glial cells. Along the axon there are periodic gaps in in the myelin sheath. These gaps are called nodes of Ranvier and are sites where the signal is "recharged" as it travels along the axon.
Single neurons do not act alone. Neural communication depends on the connections that neurons make with one another (and with other cells). Dendrites from a single neuron may receive synaptic contact from many other neurons, as many as 200,000 other neurons.
Types of Neurons
There are different types of neurons with different functions, and the functional role of a neuron is very dependent on its structure. However, there are four basic types of neuron classification. Unipolar, bipolar, multipolar, and pseudounipolar. Unipolar neurons have only one structure that extends away from the soma. These neurons are not found in vertebrates but are found in insects where they stimulate muscles or glands. A bipolar neuron has one axon and one dendrite extending from the soma, such as in the retina. Multipolar neurons are the most common type of neuron and each multipolar neuron has one axon and multiple dendrites. Multipolar neurons can be found in the central nervous system (brain and spinal chord). Pseudounipolar cells share similar characteristics with both unipolar and bipolar cells. A pseudounipolar cell has a single process that extends from the soma, like a unipolar cell, but this process later branches into two distinct structures, like a bipolar cell. Most sensory neurons are pseudounipolar and have an axon that branches into two extensions: one connected to dendrites that receive sensory information and another that transmits this information to the spinal chord.
Neurogenesis is the birth of new neurons and this phenomena was discovered recently and play an important role in learning in animals, as new neuron birth has been correlated with learning.
Glia
Even though glial cells are thought to have a supportive role in the nervous system, the number of glial cells outnumber neurons by a factor of ten. Neurons would be unable to function without the vital roles of the glial cells. Glia guide developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, and provide myelin sheaths around axons. Scientists have recently discovered that glia play a role in responding to nerve activity and modulating communication between nerve cells. When glia do not function properly, the result can be mutations in glia that cause brain tumors.
Types of Glia
There are several different types of glia with different functions. Astrocytes make contact with both capillaries and neurons in the CNS. They provide nutrients and other substances to neurons, regulate the concentrations of ions and chemicals in the extracellular fluid, and provide structural support for synapses. Astrocytes also form the blood-brain barrier, a structure that blocks entrance of toxic substances into the brain. Astrocytes, in particular, have been shown throughout calcium imaging experiments to become active in response to nerve activity, transmit calcium waves between astrocytes, and modulate the activity of surrounding synapses. Satellite glia provide nutrients and structural support for neurons in the PNS. Microglia scavenge and degrade dead cells and protect the brain from invading microorganisms. Oligodendrocytes form myelin sheaths around axons in the CNS. One axon can be myelinated by several oligodendrocytes, and one oligodendrocyte can provide myelin for multiple neurons. This is distinctive from the PNS where a single Schwann cell provides myelin for only one axon as the entire Schwann cell surrounds the axon. Radial glia serve as scaffolds for developing neurons as they migrate to their end destinations. Ependymal cells line fluid-filled ventricles of the brain and the central canal of the spinal chord. They are involved in the production of cerebrospinal fluid, which serves as a cushion for the brain, moves the fluid between the spinal chord and the brain, and is a component for the choroid plexus.
How Neurons Communicate
Neurons use electrical and chemical signals to communicate with other neurons to perform all functions of the nervous system. Neurons must be able to send and receive signals to communicate and for the nervous system to function. Each neuron has a charged cellular membrane (which is a voltage difference between the inside and the outside), and the charge of this membrane can change in response to neurotransmitter molecules released from other neurons to environmental stimuli.
The lipid bilayer that surrounds a neuron is impermeable to charged molecules and ions. To enter or exit the neuron, ions must pass through special proteins called ion channels that span the membrane. Ion channels have different configurations such as open, closed, and inactive. Some ion channels need to be activated in order to allow ions to pass through the cell. Ion channels are sensitive to the environment and can change their shape accordingly. Voltage-gated ion channels change their shape or structure in response to voltage changes. Voltage-gated ion channels also regulate the relative ion concentration inside and outside the cell. The difference in total charge between the inside and outside of the cell is called the membrane potential.
Resting Membrane Potential
A neuron at rest is negatively charged, so that the inside is more negatively charged than the outside and is called the resting membrane potential because it is caused by differences in concentrations of ions inside and outside the cell (usually around -70 mV). There are different concentrations of several ions inside and outside the cell. The difference in the number of positively-charged potassium ions (K+) inside and outside the cell dominates the resting membrane potential. When the membrane is at rest, K+ ions accumulate inside the cell due to the activity of the Na/K pump, driving both ions against their concentration gradient.
The resting membrane potential is essentially the electrical charge difference across the membrane of a neuron when it's not actively sending signals. This electrical charge is typically around -70 millivolts (mV), meaning the inside of the neuron is negatively charged relative to the outside.
Ion distribution occurs as neurons maintain a high concentration of potassium ions (K⁺) inside and a high concentration of sodium ions (Na⁺) outside.
Ion channels occur when special proteins in the neuron's membrane allow ions to move in and out. Potassium channels are more permeable at rest, allowing K⁺ to leak out.
The sodium-potassium pump actively transports Na⁺ out and K⁺ in, using energy from ATP, and the pump helps maintain the concentration gradients. As K⁺ leaks out, it leaves behind negatively charged molecules inside, creating a negative charge.
Electrochemical gradient allows the balance of ion movement and charge difference and results in the resting membrane potential.
This movement of ions across the neuron's membrane, ensures that the neuron is ready to make an action potential when needed.
Action Potential
An action potential is a brief and powerful electrical signal that travels down a neuron, allowing it to communicate with other cells. A summary of action potential:
In the resting state, the neuron starts at its resting membrane potential, around -70 mV. Threshold occurs as a stimulus causes the membrane potential to become less negative, reaching a critical threshold (around -55 mV).
Depolarization occurs when voltage-gated sodium (Na⁺) channels open, causing Na⁺ ions to rush into the neuron. This makes the inside of the cell more positive, rapidly shifting the membrane potential to around +30 mV.
Repolarization occurs when Na⁺ channels close, and voltage-gated potassium (K⁺) channels open. K⁺ ions exit the cell, restoring the negative membrane potential.
Hyperpolarization occurs when the membrane potential temporarily becomes more negative than the resting state, due to K⁺ channels closing slowly.
Return to the resting state occurs when the sodium-potassium pump and other ion channels restore the resting membrane potential. This rapid sequence of events allows neurons to send signals quickly and efficiently across long distances.
Myelin and the Propagation of Action Potential
Myelin has a crucial role in the functioning of the nervous system. The structure of myelin is a fatty, white substance that wraps around the axons of neurons, forming an insulating layer called the myelin sheath.
The function of myelin is to increase the speed at which electrical impulses (action potentials) travel along the axon and myelin accomplishes this by insulating the axon and preventing the electrical signals from dissipating. Myelin ensures that signals travel quickly and efficiently.
Myelin is not continuous along the axon and there are small gaps called Nodes of Ranvier, where the axon is exposed. These gaps are essential for the rapid transmission of signals, allowing the action potential to "jump" from one node to the next in a process called saltatory conduction.
In the central nervous system (brain and spinal cord), myelin is produced by specialized cells called oligodendrocytes. In the peripheral nervous system (nerves outside the brain and spinal cord), Schwann cells produce myelin.
Proper myelination is essential for the efficient functioning of the nervous system. Disorders that affect myelin, such as multiple sclerosis, can lead to severe neurological symptoms.
During action potential propagation, myelin has a very important role enhancing the process.
Myelin acts as an insulator, preventing the loss of electrical signals along the axon. This insulation ensures that the action potential can travel efficiently over long distances.
The presence of gaps, known as Nodes of Ranvier, is crucial. These nodes are points where the axon membrane is exposed, and they contain a high density of voltage-gated sodium channels. When an action potential reaches a node, the influx of sodium ions depolarizes the membrane, recharging the action potential.
Because of the myelin sheath, the action potential doesn't need to depolarize the entire length of the axon. Instead, the action potential "jumps" from one node of Ranvier to the next. This process, known as saltatory conduction, significantly speeds up the transmission of the electrical signal compared to unmyelinated axons.
Energy efficient signal transmission increases because the action potential only regenerates at the nodes of Ranvier, fewer ion channels need to be activated, and less energy is expended on ion exchange and the sodium-potassium pump.
Myelin enables faster and more efficient propagation of action potentials by insulating the axon, creating nodes of Ranvier for saltatory conduction, and conserving energy. This intelligent design that ensures rapid and reliable communication within the nervous system.
Synaptic Transmission is the process where one neuron communicates with another neuron and this process is fundamental to the nervous system. The process includes:
Action Potential Arrival: The action potential travels down the axon of the presynaptic neuron and reaches the axon terminal.
Calcium Influx: The depolarization of the axon terminal opens voltage-gated calcium (Ca²⁺) channels. Calcium ions rush into the terminal.
Neurotransmitter Release: The influx of Ca²⁺ causes synaptic vesicles, which contain neurotransmitters, to fuse with the presynaptic membrane. This fusion releases neurotransmitters into the synaptic cleft, the small gap between the presynaptic and postsynaptic neurons.
Neurotransmitter Binding: Neurotransmitters diffuse across the synaptic cleft and bind to specific receptor sites on the postsynaptic membrane.
Postsynaptic Response: Binding of neurotransmitters to receptors opens ion channels in the postsynaptic membrane, leading to a change in the membrane potential. This can either excite (depolarize) or inhibit (hyperpolarize) the postsynaptic neuron, depending on the type of neurotransmitter and receptors involved.
Termination: The neurotransmitter's action is terminated by one or more processes: reuptake into the presynaptic neuron, enzymatic degradation, or diffusion away from the synaptic cleft.
Chemical Synapse
A chemical synapse is a specialized junction through which neurons communicate with each other using chemical signals known as neurotransmitters. The chemical synapse process includes:
Presynaptic Neuron: The neuron sending the signal has an axon terminal that contains synaptic vesicles filled with neurotransmitters.
Action Potential Arrival: When an action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium (Ca²⁺) channels, allowing Ca²⁺ ions to enter the terminal.
Neurotransmitter Release: The influx of calcium causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft via exocytosis.
Synaptic Cleft: This tiny gap between the presynaptic and postsynaptic neurons is where the neurotransmitters diffuse across.
Receptor Binding: Neurotransmitters bind to specific receptors on the postsynaptic membrane. This binding can open or close ion channels, leading to changes in the postsynaptic neuron's membrane potential.
Postsynaptic Potential: Depending on the type of neurotransmitter and receptor, this can result in an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP).
Signal Termination: The action of neurotransmitters is terminated by reuptake into the presynaptic neuron, enzymatic degradation, or diffusion away from the synaptic cleft.
Electrical Synapse
Electrical synapses offer a direct and efficient way for neurons to communicate. Unlike chemical synapses, which use neurotransmitters to convey signals, electrical synapses rely on direct electrical connections. Electrical synapse processes include:
Gap Junctions: Electrical synapses are formed by gap junctions, which are specialized channels that connect the membranes of two adjacent neurons. These channels allow ions and small molecules to pass directly from one neuron to another.
Direct Transmission: Because ions can flow freely through gap junctions, electrical signals can be transmitted almost instantaneously. This allows for rapid and synchronized activity between connected neurons.
Bidirectional Communication: Electrical synapses allow signals to travel in both directions, providing a means for neurons to quickly share information and coordinate their activities.
Synchronization: Electrical synapses are particularly important in areas of the nervous system where synchronized firing of neurons is crucial, such as in certain regions of the brain and in the heart. While electrical synapses are less common than chemical synapses in the human nervous system, they play a vital role in ensuring fast and coordinated communication between neurons.
Signal Summation
Signal summation is a crucial process in neurons, determining whether an action potential will be generated. There are two types of summation: spatial summation and temporal summation.
Spatial Summation
Multiple Inputs: Occurs when multiple presynaptic neurons release neurotransmitters at different synapses on the same postsynaptic neuron.
Combined Effect: The combined excitatory postsynaptic potentials (EPSPs) from these multiple inputs can depolarize the postsynaptic membrane to the threshold, triggering an action potential. Integration: This allows the postsynaptic neuron to integrate signals from different sources.
Temporal Summation
Single Input: Occurs when a single presynaptic neuron releases neurotransmitters multiple times in quick succession at the same synapse. Cumulative Effect: If the time interval between successive releases is short enough, the EPSPs can add up, bringing the membrane potential to the threshold and triggering an action potential. Frequency: The frequency of neurotransmitter release plays a key role in temporal summation.
Combined Influence Summation
Excitatory and Inhibitory Inputs: Neurons receive both excitatory and inhibitory inputs. The summation process takes into account both types, determining the overall effect on the postsynaptic neuron's membrane potential. Decision Making: The integration of these signals at the axon hillock decides whether the neuron will fire an action potential.
In essence, signal summation allows neurons to process and integrate multiple incoming signals, enabling complex decision-making processes within the nervous system.
Synaptic Plasticity
Synaptic plasticity is a remarkable property of neurons enabling the brain to adapt, learn, and store memories. Synaptic plasticity refers to the ability of synapses (the connections between neurons) to strengthen or weaken over time, in response to increases or decreases in their activity. Summary of synaptic plasticity:
Types of Synaptic Plasticity Include:
Long-Term Potentiation (LTP): Definition: LTP is the long-lasting enhancement of synaptic strength following high-frequency stimulation of a synapse. Mechanism: It involves the increase in the number of neurotransmitter receptors on the postsynaptic membrane and changes in the presynaptic neuron's release of neurotransmitters. Role: LTP is widely considered one of the primary mechanisms underlying learning and memory.
Long-Term Depression (LTD): Definition: LTD is the long-lasting decrease in synaptic strength following low-frequency stimulation of a synapse. Mechanism: It involves the removal of neurotransmitter receptors from the postsynaptic membrane and changes in neurotransmitter release from the presynaptic neuron. Role: LTD is thought to play a role in forgetting, synaptic pruning, and maintaining synaptic balance.
Factors Influencing Synaptic Plasticity
Activity-Dependent: Synaptic plasticity is influenced by the activity of the neurons. Frequent use of a synapse can lead to its strengthening, while infrequent use can lead to weakening. Molecular Mechanisms: Various molecules, such as neurotransmitters, growth factors, and intracellular signaling pathways, are involved in modulating synaptic strength. Experience-Dependent: Environmental factors and experiences, such as learning new skills or exposure to enriched environments, can drive changes in synaptic plasticity.
Importance of Synaptic Plasticity
Learning and Memory: Synaptic plasticity is fundamental to the brain's ability to learn and form memories. It allows the brain to encode, store, and retrieve information. Adaptation: Synaptic plasticity enables the brain to adapt to new situations, recover from injuries, and develop new skills. Neurological Health: Abnormalities in synaptic plasticity are associated with various neurological disorders, such as Alzheimer's disease, autism, and schizophrenia. Synaptic plasticity highlights the intelligent design of the brain and nervous system.
The Central Nervous System
The central nervous system (CNS) contains the brain and spinal chord and is covered with three layers of protective coverings called meninges (membranes). The outermost layer is the dura mater (hard mother) and its function for this thick layer is to protect the brain and the spinal chord. The dura mater also contains vein-like structures carry blood from the brain back to the heart. The middle layer is the web-like arachnoid mater. The last layer is the pia mater (soft mother) which directly contacts and covers the brain and spinal chord like plastic wrap. The space between the arachnoid matter and pia mater is filled with cerebrospinal fluid (CSF). CSF is produced by a tissue called choroid plexus in fluid-filled compartments in the CNS called ventricles. The brain floats in CSF, which acts as a cushion and shock absorber and makes the brain neutrally buoyant. CSF also functions to circulate chemical substances throughout the brain and into the spinal chord.
When a ventricle is blocked, CSF builds up and creates swelling and the brain is pushed against the skull, causing hydrocephalus and can cause seizures, cognitive problems, and death if surgery is not done.
The Brain
The brain is the part of the central nervous system that is contained in the cranial cavity of the skull. The brain includes the cerebral cortex, limbic system, basal ganglia, thalamus, hypothalamus, and cerebellum. The brain can be sectioned in three ways to view internal structures: left to right, front to back, and top to bottom.
The Cerebral Cortex
The cerebral cortex is the outermost part of the brain and a thick piece of nervous system tissue which is folded into hills called gyri (gyrus) and valleys called sulci (sulcus). The cortex is made up of two hemispheres: right and left which are separated by a large sulcus. A thick fiber bundle called the corpus callosum (tough body) connects the two hemispheres and allows information to be passed from one side to the other.
Brain surgery sometimes requires the elimination of an entire hemisphere of the brain (epilepsy). Sometimes also the corpus callosum is cut to treat epilepsy.
Each hemisphere of the mammalian cerebral cortex can be broken down into four functionally and spatially defined lobes that have different functions: frontal, parietal, temporal, and occipital.
The frontal lobe is located at the front of the brain, over the eyes. This lobe contains the olfactory bulb, which processes smells. The frontal lobe also contains the the motor cortex, which is important for planning and implementing movement. Parts of the motor cortex associate with different muscle groups. Neurons in the frontal lobe also control cognitive functions like maintaining attention, speech, and decision-making. Frontal lobes also are involved in personality, socialization, and accessing risk.
The parietal lobe is located at the top of the brain. Neurons in the parietal lobe are involved in speech and reading. Two of the parietal lobe's main functions are processing somatosensation (touch sensations like pressure, pain, heat, cold) and proprioception (the sense of how parts of the body are oriented in space). The parietal lobe contains a somatosensory map of the body similar to the motor cortex.
The occipital lobe is located at the back of the brain and it is involved primarily in vision, including seeing, recognizing, and identifying the visual world.
The temporal lobe is located at the base of the brain by the ears and is primarily involved in processing and interpreting sounds. The temporal lobe contains the hippocampus, a structure that processes memory formation.
Basal ganglia (basal nuclei) are interconnected areas of the brain that have important roles in movement control and posture. Damage to the basal ganglia leads to motor impairments. The basal ganglia also regulate motivation.
The thalamus (inner chamber) behaves as a gateway to and from the cortex as it receives sensory and motor inputs from the body and also receives feedback from the cortex. This feedback mechanism can modulate conscious awareness of sensory and motor inputs depending on the attention and arousal state of the animal. The thalamus helps regulate consciousness, arousal, and sleep states.
The hypothalamus, located below the thalamus, controls the endocrine system by sending signals to the pituitary gland, a pea-sized endocrine gland that releases several different hormones that affect other glands as well as other cells. The hypothalamus regulates important behaviors that are controlled by these hormones. The hypothalamus acts as a thermostat for the body as it ensures key functions like food and water intake, energy expenditure, and body temperature are kept at appropriate levels. Neurons in the hypothalamus also regulate circadian rhythms or sleep cycles.
The limbic system is a connected set of structures that regulates emotion and behaviors related to fear and motivation. The limbic system has a role in memory formation and includes parts of the thalamus and hypothalamus as well as the hippocampus. An important structure in the limbic system is a temporal lobe structure called the amygdala. The two amygdala are important for the sensation of fear and recognizing fearful faces. The cingulate gyrus helps regulate emotions and pain.
The cerebellum is located at the base of the brain on top of the brainstem. The cerebellum controls balance and aids coordinating movement and learning new motor tasks.
The brainstem connects the rest of the brain to the spinal chord. The brainstem contains the midbrain, medulla oblongata, and the pons. Motor and sensory neurons extend through the brainstem allowing for the relay of signals between the brain and spinal chord. Ascending neural pathways cross in this section of the brain allowing the left hemisphere of the cerebrum to control the right side of the body and vise versa. The brainstem coordinates motor control signals sent from the brain to the body. The brainstem controls several important functions of the body including alertness, arousal, breathing, blood pressure, digestion, heart rate, swallowing, walking, and sensory and motor information integration.
The spinal chord connects to the brainstem and extends down the body through the spinal column. The spinal chord is a thick bundle of nerve tissue that carries information about the body to the brain and from the brain to the body. The spinal chord is contained within the bones of the vertebrate column but is able to communicate signals to and from the body through its connections with spinal nerves. Myelinated axons make up the white matter and neuron and glial cell bodies make up the gray matter. Gray matter is also composed of interneurons, which connect two neurons each located in different parts of the body. Axons and cell bodies in the dorsal spinal chord convey mostly sensory information from the body to the brain. Axons and cell bodies in the ventral spinal chord primarily transmit signals controlling movement from the brain to the body.
The spinal chord also controls motor reflexes which are quick, unconscious movements that involve local synaptic connections. Synapses with interneurons in the spinal column transmit information to the brain to convey what happened. Damage to the spinal chord can lead to paralysis.
The Peripheral Nervous System
The peripheral nervous system (PNS) is the connection between the central nervous system (CNS) and the rest of the body. While the central nervous system is the power source of the nervous system and creates the signals that control the functions of the body, the peripheral nervous system is the medium that allows these signals to be sent throughout the body, which enables the central nervous system to control the body. The PNS also allows signals from the body to be sent back to the CNS.
The PNS contains two parts: the autonomic nervous system which controls bodily functions without conscious control, and the sensory-somatic nervous system which transmits sensory information from the skin, muscles, and sensory organs to the CNS and sends motor commands from the CNS to the muscles.
The autonomic nervous system functions as the relay between the CNS and the internal organs. It controls the lungs, heart, smooth muscle, and exocrine and endocrine glands. The autonomic nervous system controls these organs largely without conscious control. Signaling to the target tissue usually involves two synapses: a pre-ganglionic neuron (originating in the CNS) synapses to a neuron in the ganglion that, in turn, synapses on the target organ. There are two divisions of the autonomic nervous system that often have opposing effects: the sympathetic nervous system and the parasympathetic nervous system.
The sympathetic nervous system is responsible for the fight or flight response that occurs when an animal encounters a dangerous situation. Functions controlled by the sympathetic nervous system include an accelerated heart rate and inhibited digestion. These functions help prepare the organism's body for the physical strain required to escape a dangerous situation.
Most preganglionic neurons in the sympathetic nervous system originate in the spinal chord. The axons of these neurons release acetylcholine on postganglionic neurons within sympathetic ganglia (the sympathetic ganglia form a chain that extends alongside the spinal chord). The acetylcholine activates the postganglionic neurons. Postganglionic neurons then release norepinephrine onto target organs. One preganglionic neuron synapses on multiple postganglionic neurons, amplifying the effect of the original synapse. The adrenal gland also releases norepinephrine (and epinephrine) into the bloodstream. The physiological effects of this norepinephrine release include dilating the trachea and bronchi, which eases breathing, increasing heart rate, and moving blood from the skin to the heart, muscles, and brain. The strength and speed of the sympathetic response helps the organism avoid danger and even remember danger.
The parasympathetic nervous system allows an animal to rest and digest (as opposed to being activated during stress like the sympathetic nervous system). Parasympathetic preganglionic neurons have cell bodies located in the brainstem and in the sacral (lower) spinal chord. The axons of preganglionic neurons release acetylcholine on the postganglionic neurons, which are located near the target organs. Most postganglionic neurons release acetylcholine onto target organs, but some release nitric oxide.
The parasympathetic nervous system resets organ function after the sympathetic nervous system is activated. The effects of acetylcholine release on target organs include slowing of heart rate, lowered blood pressure, and stimulation of digestion.
Sensory-Somatic Nervous System
The sensory-somatic nervous system is made up of cranial and spinal nerves and contains both sensory and motor neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the CNS. Motor neurons transmit messages about desired movement from the CNS to the muscles to make them contract. Without its sensory-somatic nervous system, an animal would be unable to process any information about its environment (senses) and could not control motor movements. While the autonomic nervous system has two synapses between the CNS and the target organ, sensory and motor neurons only have one synapse between the CNS and the organ. Acetylcholine is the main neurotransmitter released at the synapses.
Humans have 12 cranial nerves, or nerves that emerge from or enter the skull (cranium), as opposed to the spinal nerves, which emerge from the vertebral column. Each cranial nerve is named (olfactory, optic...). Some cranial nerves transmit only sensory information. Other cranial nerves transmit almost solely motor information. Other cranial nerves contain a mix of sensory and motor fibers.
Spinal nerves transmit sensory and motor information between the spinal chord and the rest of the body. Each of the 31 spinal nerves in humans contain both sensory and motor axons. The sensory neuron cell bodies are grouped in structures called dorsal root ganglia. Each sensory neuron has one projection with a sensory receptor ending in skin, muscle, or sensory organs and another that synapses with a neuron in the dorsal spinal chord. Motor neurons have cell bodies in the ventral gray matter of the spinal chord that project to muscle through the ventral root. These neurons are usually stimulated by interneurons within the spinal chord but are sometimes directly stimulated by sensory neurons.
Nervous System Disorders
Neurodegenerative disorders are illnesses characterized by a loss of nervous system functioning that are usually caused by neuronal death. Examples are Huntington's disease, amyotrophic lateral sclerosis, Alzheimer's disease, dementia, and Parkinson's disease.
Alzheimer's disease is the most common cause of dementia in the elderly adults. Symptoms are memory loss, confusion, difficulty planning or doing tasks, poor judgement, and personality changes. Alzheimer's disease is caused by mutations in certain genes that produce proteins that build up in the brain and damage nerve cells. The mutations can be inherited.
Parkinson's disease is another neurodegenerative disease. Parkinson's disease causes the loss of dopamine neurons in the substantia nigra, a structure of the brain that regulates movement. Symptoms are tremor, slow movement, speech changes, balance and posture problems, and rigid muscles. Most causes of Parkinson's disease are unknown.
Neurodevelopmental Disorders
Autism spectrum disorder is a neurodevelopment disorder. Symptoms are impaired social skills, lack of empathy for others, repetitive motor behaviors, interest in particular subjects, and unusual language use. Causes of many forms of autism are unknown. Treatments are available for autism but there is no cure.
Attention deficit hyperactivity disorder (ADHD) symptoms include the inability to focus, difficulty functioning, impulsivity, hyperactivity. The cause is unknown.
Neurologists are physicians who specialize in the study of disorders of the nervous system.
Mental Illnesses are nervous system disorders that result in problems with thinking, mood, or relating with other people.
Schizophrenia is a serious disorder with symptoms of being unable to differentiate reality from imagination, inappropriate or uncontrolled emotional responses, inability to think, and social dysfunction. Some people with schizophrenia suffer from hallucinations, hear voices, or delusions.
Depression involves having a severely depressed mood for an extended period of time, such as weeks. Symptoms of depression are loss of enjoyment of daily activities, loss of appetite, loss of sleep, difficulty concentrating, feelings of worthlessness, and suicidal thoughts.
Other Neurological Disorders
Epilepsy is a disorder that causes recurring seizures of the body and can be a result of brain injury, genetics, disease, or other illnesses.
Stroke occurs when blood fails to reach a portion of the brain long enough to cause damage, including neuron death, as blood supplies necessary oxygen to the brain. Symptoms are headache, muscle weakness, paralysis, speech disturbances, sensory problems, memory loss, and confusion.