The nervous system
The nervous system senses the body's internal and external environments, integrates what it senses and controls the body's response through signals that neurons pass to one another and to muscles and glands. It is made of neurons and the glial cells that support them, and it comprises the brain and spinal cord, the nerves outside them, and an autonomic system that runs the functions a person is not routinely aware of.
What the nervous system does
At the most fundamental level the human nervous system communicates. It relays information between a source and a target that can be anywhere from a few micrometres to nearly a metre apart. It integrates information from many sources at once and responds by generating appropriate signals. The primary functions of the nervous system include sensing the internal and external environments, integrating sensory inputs, coordinating motor outputs, and regulating or controlling peripheral systems. Integration and coordination occur within the brain and spinal cord. The nervous system controls many bodily functions and influences others, so knowledge of it aids medical communicators who work in fields such as endocrinology, cardiovascular medicine and gastroenterology. Much is known about neuron function and sensory systems, but much more remains for research to uncover.
Neurons and glial cells
Neural tissue, the substance of which the nerves and nerve centres are composed, is made up of neurons and neuroglia. The neuron is the basic structural and functional unit of the nervous system. It is a nerve cell specialised to conduct electrical signals, called action potentials, and to carry information as electrochemical impulses to other cells and from one area of the body to another. More than 100 billion neurons of various form and function are estimated to be in the human body. Glial cells are the other main type of cell in the nervous system. They primarily support the function of neurons by making the myelin sheath that covers the outer surface of neuron axons. Glial cells also deliver nutrients to neurons and scavenge dead neuron sections. Neuroglia, the accessory cells of the nervous system, isolate the neurons, provide a supporting framework for the tissue, and nourish and protect neurons, primarily by acting as phagocytes.
In general, a neuron is composed of a cell body, or soma, which contains the nucleus and other essential organelles; a long axon, which transmits the electrical signal away from the cell body toward the next neuron in the circuit; and fingerlike dendrites, which accept a signal from neighbouring neurons and pass it to the cell body. A glial cell also has a cell body with many extensions, called processes, but those processes are not categorised as axons or dendrites.
The soma, also called the perikaryon, contains the typical cellular organelles but no centrosome, and neurons cannot undergo mitosis. The axon begins at a specialised portion of the soma, and its cytoplasm, the axoplasm, contains an abundance of neurofilaments and neurotubules as well as other organelles. The axon may or may not have collateral branches. At its distal tip the axon gives rise to finer terminal branches, the telodendria, also written teleodendria. Each ends in a swelling called the synaptic knob, at a synapse, where one cell communicates with another.
The divisions of the nervous system
Neurons innervate most locations in the body and are involved in a wide variety of physiologic functions. The brain and spinal cord compose the central nervous system (CNS) and are probably the most obvious anatomical components of the nervous system. The peripheral nervous system (PNS) includes all of the neural tissue outside the CNS. The sensory nervous system comprises the eyes for vision, the ears for hearing and balance, the nose and tongue for the chemical senses, and the skin receptors for pain, heat and touch. Nerves that innervate skeletal muscles are part of the motor nervous system. Less obvious, but no less important, is the autonomic nervous system, which controls involuntary functions such as breathing, heart rate, digestion and sweating.
Neural circuits
Neurons do not act in isolation. Even the most basic function takes the coordinated communication of several neurons, and most neural functions in the body are carried out by such coordinated action. That coordinated action is achieved by a neural circuit, the assembly of neurons required to carry out a particular function. Within a circuit, afferent neurons carry neural signals toward the central nervous system, efferent neurons carry them away from it, and interneurons form local connections between two other neurons. Sensory neurons bring information about the external or internal environments to the brain and spinal cord. Motor neurons, which are efferent neurons, carry commands from the brain and spinal cord that control or modify the activity of peripheral effectors. Interneurons, or association neurons, lie between sensory and motor neurons inside the brain and spinal cord and pass information from sensory neurons to motor neurons.
The action potential
Neurons communicate by relaying an action potential, an electrical signal that originates at the cell body of a neuron, propagates along the axon and allows the neuron to communicate with neighbouring neurons or effector cells. An action potential arises when channels in the neuron's cell membrane open and let ions, electrically charged atoms or molecules, flow into or out of the neuron. That ion flow changes the electrical properties of the neuron. If the change is large enough, an action potential can be triggered at the cell body and travel along the axon. Most neurons in humans are myelinated, covered with a myelin sheath from glial cells, so action potentials propagate rapidly. Loss of myelin and myelin dysfunction cause clinically significant impairment within the nervous system. When the myelin is not functional, action potentials slow considerably or do not propagate at all. The action potential is the means of communication across most of the nervous system, but its functional result varies tremendously, as the specialised subsystems show.
The synapse and neurotransmitters
Once an action potential has propagated the length of the axon, it reaches the presynaptic terminal of the neuron. Neural signals can pass from one neuron to another at the synapse, the junction of the axon of one neuron with the dendrite of a neighbouring neuron. A synapse is a tiny gap, between two neurons or between a neuron and a muscle cell, across which chemicals called neurotransmitters transmit nerve impulses. At a synapse the activity of one neuron affects the membrane characteristics of another cell. Communication normally runs from the presynaptic neuron to the postsynaptic neuron or effector cell, and the presynaptic and postsynaptic membranes are separated by a narrow synaptic cleft.
Most neurons release neurotransmitters, which are molecules that enable communication with neighbouring neurons. A neurotransmitter is a chemical that a neuron releases from its axon into the synapse, where it travels across the synaptic cleft and can bind to receptors on other neurons, exciting or inhibiting the target cell. When an action potential reaches the presynaptic terminal of a chemical synapse, a vesicle containing neurotransmitter fuses with the cell membrane facing the synapse. Once the vesicle has fused, neurotransmitter is released into the synaptic cleft, the space between two adjoining neurons. The neurotransmitter binds to specific receptors in the postsynaptic terminal, causing some ion flow in the postsynaptic neuron and acting as a "switch" for the postsynaptic cell. The switch signal can perform many tasks, including initiating an action potential in the postsynaptic neuron, causing the contraction of a skeletal muscle, or inhibiting neurotransmitter release. Not all neurotransmitter released by the presynaptic neuron is bound by the postsynaptic neuron. Most of the remaining neurotransmitter is degraded by local enzymes or reabsorbed by the presynaptic neuron to be used for later cell-to-cell communication.
Norepinephrine, dopamine, γ-aminobutyric acid (GABA), glycine and serotonin are neurotransmitters found in the brain and spinal cord. GABA is an inhibitory neurotransmitter that decreases the excitability of neurons. So if more GABA binds at the postsynaptic neuron, less cell-to-cell communication takes place, slowing or calming many functions.
The central nervous system
The central nervous system acts as the communication hub of the body: neural signals arrive, are interpreted by the CNS and, if necessary, other neural signals are dispatched to relevant areas of the body. Within one functional area of the CNS, such as auditory perception, several distinct yet interconnected structures carry out different roles to process the incoming neural signals. Grey matter is tissue in the CNS that is mostly composed of cell bodies and is grey in colour. White matter is tissue in the CNS that is mostly composed of myelinated nerve fibres and is white in colour. A tract is a group of nerve fibres passing from one part of the CNS to another.
The meninges are three connective tissue membranes, the dura mater, the arachnoid and the pia mater, that enclose and protect the brain and spinal cord. They cover the brain and provide the space for the formation and circulation of cerebrospinal fluid. Cerebrospinal fluid is continuously formed and circulated through the CNS, bringing nutrients to tissues and taking waste products away from them.
The spinal cord
The adult spinal cord measures approximately 45 cm, almost 18 inches, in length and is generally the diameter of a standard lead pencil. It extends to the level of the first or second lumbar vertebrae. Like the brain, the spinal cord is surrounded by meninges, and cerebrospinal fluid circulates through the space between the meninges.
One of the most important functions of the spinal cord is to serve as a relay station for neural signals passing between the brain and peripheral neural structures. However, neural circuits that terminate in the spinal cord can also execute functions such as normal walking movements and motor reflexes to withdraw from painful stimuli without the influence of neural signals from the brain. In cross-section, the organisation of the spinal cord is preserved throughout its length, with white matter around the outside and grey matter on the inside. The grey matter is made up of the cell bodies of neurons, roughly resembles a butterfly in cross-section, and surrounds the central canal. Other landmarks of the cross-section are the anterior median fissure and the posterior median sulcus. Within the grey matter, the dorsal, or posterior, horn carries the cell bodies of somatosensory neurons, such as those for touch and pain, relaying information from the periphery. The ventral, or anterior, horn is composed of motor neurons that send signals out to muscles. The lateral horn comprises autonomic neurons that connect to internal organs, also described as visceral motor neurons. The lateral horns are found only in the thoracic, upper lumbar and sacral regions of the spinal cord.
The white matter of the spinal cord is primarily composed of neuron axons, largely those of neurons within the spinal cord, and it is divided into columns and tracts organised by function. Ascending tracts carry sensory information toward the brain, and descending tracts convey commands to motor neurons in the spinal cord. The dorsal columns carry somatosensory information toward the brain. The lateral columns contain nerves that transmit signals from the cortex of the brain to the spinal motor neurons. The ventral columns carry neural signals in both directions. Pain and temperature information is carried from the periphery to the brain, and motor information is carried from several brain structures to the spinal cord motor neurons.
Spinal and cranial nerves
Signals enter and exit the spinal cord through the roots of spinal nerves, which pass through openings between adjacent vertebrae. The dorsal and ventral roots of the spinal nerves merge just outside the spine to form a whole spinal nerve that contains sensory, motor and autonomic neurons. Thirty-one pairs of spinal nerves originate from the spinal cord and provide a two-way communication system, motor and sensory, between the spinal cord and parts of the arms, legs, neck and trunk. Spinal nerves do not have individual names, but they are grouped according to the level of the spinal cord from which they originate, and each nerve is numbered in sequence: 8 pairs of cervical nerves (C1–C8), 12 pairs of thoracic nerves (T1–T12), 5 pairs of lumbar nerves (L1–L5), 5 pairs of sacral nerves (S1–S5) and 1 pair of coccygeal nerves, which is not numbered. Twelve pairs of cranial nerves originate in the brain and extend into the body without passing through the spinal cord. Each cranial nerve is named for its appearance or function and numbered according to its order of appearance along the axis of the brain.
The brain
The adult brain contains almost 98% of the neural tissue in the body. It consists of tracts of white matter and grey matter in superficial areas of the cerebral cortex. The brain communicates with the rest of the body over tracts from the spinal cord and over the cranial nerves. The brain is not solid: it contains a series of interconnected chambers, the ventricles, which are continuous with the central canal of the spinal cord.
The adult brain has five major divisions: the telencephalon (cerebrum), the diencephalon (pineal gland and hypothalamus), the mesencephalon (midbrain), the metencephalon (cerebellum and pons) and the myelencephalon (medulla). The diencephalon provides switching and relay centres for the integration of voluntary and involuntary activities. The pineal gland sits on the roof of the diencephalon. The hypothalamus, another part of the diencephalon, contains integrative centres important to the control of autonomic and endocrine functions and to sensations of thirst and hunger.
The brainstem
The brainstem is the physical juncture between the brain and the spinal cord and is located at the base of the skull. It is made up of the medulla, the pons and the midbrain, each of which plays a different role in the CNS. Brainstem functions can be broadly categorised into three groups: information throughway, cranial nerve functions and vital function control. The medulla is the section of the brainstem closest to the spinal cord, and it regulates vital physiologic functions such as breathing rate, heart rate and blood vessel diameter. It is also a crossing point for many axons passing between the spinal cord and the opposite side of the brain. All communication between the brain and the spinal cord involves the medulla as a relay stop or a thoroughfare. It also contains reflex centres, including the cardiac, vasomotor and respiratory rhythmicity centres. The pons is involved in motor control and in level of consciousness, including sleep, and it acts as a major relay point for information passing between multiple brain centres. It contains nuclei associated with several cranial nerves and two centres that control breathing. The midbrain serves as a throughway for motor nerve axons travelling from the cerebral cortex to the spinal cord. It contains the superior and inferior colliculi, collections of neurons that play critical roles in vision and hearing respectively. It also receives a variety of sensory inputs and is responsible for adjusting muscle tone and posture. The brainstem also contains the axons and cell bodies of many head and neck neurons that are involved in sensory and motor functions. Most of the cranial motor and sensory nerves enter the brainstem at the pons, including the motor nerves that control facial expressions, eye movements and chewing. In this sense the brainstem can be viewed as an extension of the spinal cord, which houses the axons and cell bodies of sensory and motor neurons originating below the neck.
The limbic system
The limbic system has frequently been described as the "emotional brain" because of the large part it plays in originating and regulating emotions and behaviour. It is not as anatomically discrete as other sections of the brain, being organised more by functional relationships and neural connections. So reports differ on which structures it includes. If the brain is pictured as a sphere, the limbic system lies toward its middle. Parts of the limbic system are contained in the cerebrum. The hypothalamus, a very small structure, controls functions such as body temperature, water regulation and circadian rhythm. It can also influence endocrine hormone release from the pituitary gland, and it is involved in regulating sexual satisfaction, fear and aggression. The amygdala, another major portion of the limbic system, is largely a behaviour awareness area, and it coordinates behaviour and autonomic responses to the external environment. Early research indicated that the amygdala is involved with emotional responses to fear and anxiety. More recent studies have linked the amygdala to attention and to a general ability to define a stimulus and evoke an appropriate response. The amygdala also helps form memories of emotional events. The hippocampus is best known for its role in learning and memory, although it also plays a role in understanding the body's spatial position. It consolidates short-term memories of experienced events into long-term memories. When a lesion occurs in the hippocampus, a person cannot form new long-term memories but can usually access memories that were consolidated before the lesion formed. The hippocampus also aids spatial memory and navigation, allowing people to remember where they have been or how to get somewhere.
The cerebellum
The cerebellum, meaning "little brain", is a spherical structure that is estimated to contain 50% of the total neurons in the brain. It plays a major role in coordinating movement, but it cannot initiate motor function by itself. The cerebellum controls important postural reflexes, refines voluntary and involuntary movements of the body, and monitors all sensory and motor information reaching the brain. Fast movements, such as running and typing, are smoothly executed under the control of the cerebellum. The cerebellum influences the timing of coordinated movements and makes corrections to monitored motor signals. It can do so because it receives motor information about what should happen and sensory information about what is happening, and compares the two inputs to see whether they agree. When muscle loads change, the cerebellum compensates by changing the force of muscle contraction. It also relays information to the motor cortex to plan the next movement in a sequence so that the movement is smoothly executed. Motor learning happens in the cerebellum, both learning new skills and learning from previous movement errors.
The cerebral cortex
The cerebral cortex is the outer shell of the brain, covering approximately its top three-quarters if the brain is pictured as a sphere. It contains the primary motor cortex and the primary sensory cortex. Its surface is not smooth. One account names the folds gyri and the grooves sulci. Another calls the folds themselves sulci and places more than two-thirds of the cortical surface within them. The cerebral cortex is often referred to as grey matter because of the grey colour of the tissue, which is largely due to the presence of neuron cell bodies. The functional portion of the cortex is approximately 2 to 4 mm thick in humans, and most of it is composed of six morphologically distinct layers of neurons. The cortex is organised into right and left hemispheres, which are divided by a longitudinal fissure. The cerebral cortex is often classified by physiologic function, into an auditory cortex responsible for perceiving sound, a somatosensory cortex responsible for touch perception, a visual cortex responsible for seeing, and a motor cortex responsible for executing voluntary movements. Each cortical section is subdivided. The primary cortical area is involved in the direct perception of a stimulus or in executing a movement. The secondary, or associated, cortical area is tasked with interpreting the signals delivered to the primary sensory cortex or, in the motor cortex, with selecting the voluntary movements to execute.
In the somatosensory cortex, body areas that are more sensitive to touch, such as the fingers and lips, compose a larger portion of the brain than body areas that are less touch sensitive. The two hemispheres of the somatosensory cortex receive information almost exclusively from the opposite side, so touch information from the left hand is transmitted to the right somatosensory cortex. Neurons in the visual cortex are highly activated by high-contrast light–dark edges in images, with different neuron locations responding preferentially to different orientation angles of the edge. Stereo vision aids depth perception and is achieved by mixing the input from both eyes into each hemisphere of the visual cortex. Like the somatosensory and visual cortices, the auditory cortex contains a topographical map, with different acoustic frequencies at different cortical locations. Some areas of the auditory cortex are suited to processing frequencies found in speech, whereas others are specialised to extract information about variations in loudness or timing. The motor cortex receives inputs from the spinal cord and brainstem on the opposite side of the body. Its topographical organisation largely mirrors that of the somatosensory cortex, with a larger representation for areas with fine movements, such as the hands and face. The motor cortex appears to execute one functional movement by a neural circuit, rather than each area of the cortex corresponding directly to a single muscle.
Besides the well-defined sensory and motor cortices, the cortex has many association areas that receive and process information from other cortical and subcortical structures. Areas of the cerebral cortex other than the primary motor and sensory cortices are integrative centres that receive information from many different associated areas, and these centres control the most complex mental activities. The association cortices process complex information, compare information from different sensory cortices, extract related information and plan any necessary responses. For instance, Wernicke's area is associated with language comprehension, whereas a separate area, Broca's area, is associated with speech and language expression. Other cortical areas are involved to a lesser degree in language skills. Association area functions have been determined mostly through lesion and deficit studies in humans and animals. The parietal association cortex is concerned with body awareness and with external stimuli that act on the body. The temporal association cortex aids in facial and complex image recognition and is thought to participate in long-term memory. The frontal association cortex, also known as the prefrontal area, is largely responsible for personality expression and for executive functions such as judgment, reasoning and working memory.
Plasticity
Plasticity is change in synapses and neural circuits due to changes in the neural environment or inputs. The cortex is not a static entity: it has plasticity, the ability to change and develop in response to its environment and inputs. The cortex can be modified in response to an increase or a decrease in sensory or motor system inputs. For instance, when a lesion on the retina causes focal vision loss, the corresponding location on the topographic map of the visual cortex regains function with inputs from areas neighbouring the retinal lesion. Similar changes following lesions have been seen in other sensory cortices.
Skeletal muscle and the motor unit
Skeletal muscles are the large muscles most commonly associated with movement. They are composed of individual muscle fibres, attach to bones and facilitate joint movement. Before a joint moves, a muscle fibre receives a neural signal from the alpha motor neuron that innervates it. That neural signal causes contraction of the skeletal muscle, which typically causes movement of a joint. Each muscle fibre is innervated by one alpha motor neuron, but each alpha motor neuron innervates many muscle fibres. So a signal originating from one alpha motor neuron produces the coordinated contraction of many muscle fibres. An alpha motor neuron and its associated muscle fibres are considered one motor unit. The force with which a particular muscle contracts is variable and regulated by the number of motor units that are recruited.
For most movements, a pair of muscles evokes opposite actions on a joint. When the flexor muscle contracts the joint closes, and when the extensor muscle contracts the joint opens. To achieve any particular joint movement, one muscle must contract and the other must lengthen in a coordinated fashion.
Reflexes and spinal motor circuits
Reflex movements are automatic movements that occur in response to certain stimuli. The leg extension reflex, for instance, occurs when the knee is tapped with a small rubber reflex hammer. The tap stretches the extensor muscle, and a muscle stretch receptor in that muscle sends a signal through a sensory neuron to the spinal cord. In the spinal cord the sensory neuron activates a motor neuron innervating the stretched extensor muscle, causing the extensor muscle to contract. At the same time, the sensory neuron's signal inactivates the flexor muscle in the back of the leg, causing the flexor muscle to relax.
Many usual body movements involve groups of sensory and motor neurons that form circuits through the spinal cord. Research in paralysed animals has shown that walking movements can be completely restored by controlled stimulation in the spinal cord alone. Several "higher" areas of motor control in the CNS are involved in coordinating, adjusting and deciding about movements.
The autonomic nervous system
Many body functions a person is not routinely aware of are controlled by the autonomic nervous system. The neurons of the autonomic system innervate gland cells, cardiac muscle and smooth muscle to regulate physiologic functions such as sweating, blood pressure and heart rate. The autonomic nervous system has three divisions: the sympathetic system, the parasympathetic system and the enteric system. Sensory signals are relayed through autonomic neurons to a higher processing centre in the brain, which interprets the signals and sends a corresponding neural signal to either increase or decrease a certain function. The sympathetic and parasympathetic systems act in opposition to each other. The autonomic nervous system is always functioning, and most of the time a balance is achieved between the sympathetic and parasympathetic divisions in their influence on bodily functions. At certain times of the day and for certain functions, one division has more influence than the other.
The phrase "fight or flight" is often used to summarise the function of the sympathetic nervous system. The sympathetic system acts to maximise the resources used by the body when the body is presented with a stressful or threatening situation. For instance, blood vessels constrict in less critical organs such as the skin and the gastrointestinal tract to make more blood available in the muscles for exertion. The sympathetic system also increases heart rate and heart contraction force and relaxes and opens the lung airways. At the same time, less survival-critical functions such as digestion decrease. Most sympathetic neurons release norepinephrine, which along with its close relative epinephrine can bind to two types of receptor on receptor organs, alpha and beta adrenergic receptors. The neurons that control the sympathetic system arise primarily from the spinal cord and from ganglia, masses of neuron cell bodies, that lie outside the spinal cord. Many of the functions the sympathetic system influences can be activated individually and in a graded fashion. During exercise, for instance, the heart rate and breathing rate increase, but the pupils do not dilate and the hair on the skin does not stand on end.
The parasympathetic system counterbalances the sympathetic system and puts the body into a "rest and digest" mode to conserve energy. When the parasympathetic system is exerting influence, salivation and the secretion of digestive enzymes increase, and heart rate and airway diameter decrease. The neurons that transmit information for the parasympathetic system mostly originate in the brainstem, although elimination functions are influenced by neurons that originate in the lowest, sacral portion of the spinal cord. Almost all parasympathetic neurons release acetylcholine, which can bind to two types of receptor on the effector organs, muscarinic and nicotinic receptors. Humans can voluntarily control some of the parasympathetic impulses they receive, most notably urination and defecation. Sympathetic impulses, in contrast, are under very little voluntary control. Certain functions are influenced by the sympathetic system but not by the parasympathetic system, including sweating, blood vessel dilation and the release of epinephrine by the adrenal medulla.
The third and least well-known division of the autonomic nervous system is the enteric system, a local nervous system for the digestive system. The digestive system is under sympathetic and parasympathetic control, but the enteric system is influential in how food is processed and can act independently of the sympathetic or parasympathetic system. The enteric system is composed of various networks of neurons located within the walls of the digestive tract. The enteric neurons allow digestive functions to continue without sympathetic or parasympathetic signals. They respond to changes in the local environment to control gastrointestinal motility, fluid retention, electrolyte exchange and digestive secretions. The enteric system has many connections with the central nervous system, so that there is both local and global control of the digestive process.