Nervous System Anatomy and Physiology

The nervous system is the body's command and control network. Know how it is organized, how a nerve impulse fires, and which structures do what, because every neuro assessment, stroke workup, and spinal precaution you run on the floor traces back to this anatomy.

Functions of the Nervous System

The nervous system has three overlapping jobs, plus its higher roles.

1. Monitoring changes. Millions of sensory receptors track changes inside and outside the body. These changes are stimuli; the information gathered is sensory input.
2. Integration. It processes and interprets the sensory input and decides what to do at each moment.
3. Motor output. It acts on that decision by activating muscles or glands (effectors).
4. Mental activity. The brain is the center of consciousness, thinking, and memory.
5. Homeostasis. By detecting, interpreting, and responding to internal and external change, it stimulates or inhibits other systems to hold a constant internal environment.

Anatomy of the Nervous System

The nervous system does not regulate homeostasis alone. The endocrine system is the second major regulator.

Organization of the Nervous System

There is only one nervous system, but its complexity forces us to study it in pieces. We divide it by structure (structural classification) or by activity (functional classification).

Structural Classification

The structural classification covers all nervous system organs and has two subdivisions: the central nervous system and the peripheral nervous system.

• Central nervous system (CNS). The brain and spinal cord. They occupy the dorsal body cavity and act as the integrating and command centers.
• Peripheral nervous system (PNS). Everything outside the CNS, mainly the nerves extending from the brain and spinal cord.

Functional Classification

The functional classification concerns only PNS structures.

• Sensory (afferent) division. Nerves that carry impulses to the CNS from sensory receptors throughout the body.
• Somatic sensory fibers. Deliver impulses from skin, skeletal muscles, and joints.
• Visceral sensory fibers. Carry impulses from the visceral organs.
• Motor (efferent) division. Carries impulses from the CNS to effector organs (muscles and glands). It has two subdivisions: the somatic nervous system and the autonomic nervous system.
• Somatic nervous system. Lets us consciously (voluntarily) control skeletal muscle.
• Autonomic nervous system. Regulates automatic (involuntary) events. This involuntary system has two parts, the sympathetic and parasympathetic, which usually produce opposite effects.

Nervous Tissue: Structure and Function

Nervous tissue is built from two cell types: supporting cells and neurons.

Supporting Cells

In the CNS, supporting cells are lumped together as neuroglia, literally "nerve glue."

• Neuroglia. Many cell types that support, insulate, and protect the delicate neurons. Each glial type (also called glia or glial cells) has its own job.
• Astrocytes. Abundant star-shaped cells that account for nearly half of neural tissue. They form a living barrier between capillaries and neurons, controlling exchange and shielding neurons from harmful blood-borne substances.
• Microglia. Spiderlike phagocytes that clear debris, including dead brain cells and bacteria.
• Ependymal cells. Line the central cavities of the brain and spinal cord. Their beating cilia circulate the cerebrospinal fluid that cushions the CNS.
• Oligodendrocytes. Wrap their flat extensions tightly around nerve fibers to form fatty insulating myelin sheaths.
• Schwann cells. Form the myelin sheaths around nerve fibers in the PNS.
• Satellite cells. Act as protective, cushioning cells.

Neurons

Neurons (nerve cells) are specialized to transmit nerve impulses from one part of the body to another.

• Cell body. The metabolic center of the neuron. It holds a transparent nucleus with a conspicuous nucleolus; rough ER (Nissl substance) and neurofibrils are abundant here.
• Processes. The armlike fibers range from microscopic to 3 to 4 feet long. Dendrites carry incoming messages toward the cell body; axons generate nerve impulses and conduct them away from the cell body.
• Axon hillock. A neuron may have hundreds of dendrites but only one axon, which arises from a conelike region of the cell body, the axon hillock.
• Axon terminals. Contain hundreds of tiny vesicles (membranous sacs) holding neurotransmitters.
• Synaptic cleft. The tiny gap separating each axon terminal from the next neuron.
• Myelin sheaths. Most long fibers are covered by a whitish, fatty myelin with a waxy look. Myelin protects and insulates the fiber and speeds impulse transmission.
• Nodes of Ranvier. Because the sheath is built from many individual Schwann cells, it has gaps called nodes of Ranvier.

Classification. Neurons are grouped by function or by structure.

• Functional classification. Groups neurons by the direction the impulse travels relative to the CNS: sensory, motor, and association neurons.
• Sensory (afferent) neurons. Carry impulses from sensory receptors to the CNS and keep us informed about what is happening inside and outside the body.
• Motor (efferent) neurons. Carry impulses from the CNS to the viscera, muscles, and glands.
• Interneurons (association neurons). Connect motor and sensory neurons in neural pathways.
• Structural classification. Based on the number of processes extending from the cell body.
• Multipolar neuron. Several processes. Because all motor and association neurons are multipolar, this is the most common type.
• Bipolar neurons. Two processes (one axon, one dendrite). Rare in adults, found mainly as receptor cells in some special sense organs.
• Unipolar neurons. A single short process that divides almost immediately into proximal (central) and distal (peripheral) processes.

Central Nervous System

During embryonic development the CNS first appears as a simple neural tube running down the dorsal median plane of the embryo.

Brain

The brain is the largest, most complex mass of nervous tissue in the body. It is discussed in four major regions: cerebral hemispheres, diencephalon, brain stem, and cerebellum.

Cerebral Hemispheres

The paired cerebral hemispheres (collectively the cerebrum) are the most superior part of the brain and together are larger than the other three regions combined.

• Gyri. Elevated ridges of tissue covering the surface, separated by shallow grooves called sulci.
• Fissures. Deeper grooves that separate large regions. The hemispheres are divided by one deep longitudinal fissure.
• Lobes. Other fissures and sulci divide each hemisphere into lobes, named for the cranial bones over them.
• Regions of each hemisphere. A superficial cortex of gray matter, internal white matter, and the basal nuclei.
• Cerebral cortex. Seat of speech, memory, logical and emotional response, consciousness, sensory interpretation, and voluntary movement.
• Parietal lobe. The primary somatic sensory area sits posterior to the central sulcus. Impulses from the body's sensory receptors are localized and interpreted here.
• Occipital lobe. Holds the visual area in its posterior part.
• Temporal lobe. Holds the auditory area bordering the lateral sulcus and the olfactory area deep inside.
• Frontal lobe. Holds the primary motor area anterior to the central sulcus, which controls voluntary skeletal muscle movement.
• Pyramidal tract. Axons of these motor neurons form the major voluntary motor tract, the corticospinal (pyramidal) tract, which descends to the cord.
• Broca's area. A cortical area at the base of the precentral gyrus that drives the ability to speak.
• Speech area. Located at the junction of the temporal, parietal, and occipital lobes; lets us sound out words.
• Cerebral white matter. Fiber tracts carrying impulses to, from, and within the cortex.
• Corpus callosum. A large fiber tract connecting the hemispheres; such tracts are called commissures.
• Fiber tracts. Association tracts connect areas within a hemisphere; projection tracts connect the cerebrum with lower CNS centers.
• Basal nuclei (basal ganglia). Islands of gray matter deep in the white matter. They help regulate voluntary motor activity by modifying instructions the primary motor cortex sends to skeletal muscle.

Diencephalon

The diencephalon (interbrain) sits atop the brain stem, enclosed by the cerebral hemispheres.

• Thalamus. Encloses the shallow third ventricle and relays sensory impulses passing up to the sensory cortex.
• Hypothalamus. Forms the floor of the diencephalon. A key autonomic center, it regulates body temperature, water balance, and metabolism; drives many emotions as part of the limbic system ("emotional-visceral brain"); regulates the pituitary gland; and produces two hormones of its own.
• Mammillary bodies. Reflex centers for olfaction (smell) that bulge from the floor of the hypothalamus, posterior to the pituitary.
• Epithalamus. Forms the roof of the third ventricle. Key parts are the pineal body (endocrine) and the choroid plexus of the third ventricle, which forms cerebrospinal fluid.

Brain Stem

The brain stem is about thumb-thick and roughly 3 inches long.

• Structures. The midbrain, pons, and medulla oblongata.
• Midbrain. Extends from the mammillary bodies to the pons. Built from two bulging fiber tracts, the cerebral peduncles, which carry descending and ascending impulses.
• Corpora quadrigemina. Four dorsal rounded protrusions that serve as reflex centers for vision and hearing.
• Pons. A rounded bulge just below the midbrain, mostly fiber tracts, with nuclei that help control breathing.
• Medulla oblongata. The most inferior part of the brain stem. Its nuclei regulate vital visceral activity, including heart rate, blood pressure, breathing, swallowing, and vomiting.
• Reticular formation. A diffuse mass of gray matter running the length of the brain stem, involved in motor control of the visceral organs. Its reticular activating system (RAS) governs consciousness and the sleep-wake cycle.

Cerebellum

The large, cauliflower-like cerebellum projects dorsally from under the occipital lobe.

• Structure. Like the cerebrum, it has two hemispheres, a convoluted surface, an outer gray-matter cortex, and inner white matter.
• Function. Provides precise timing for skeletal muscle activity and controls balance and equilibrium.
• Input. Receives fibers from the inner-ear equilibrium apparatus, the eye, and proprioceptors of the skeletal muscles and tendons.

Protection of the Central Nervous System

Nervous tissue is soft and delicate, and neurons are irreplaceable, so the body protects the brain and spinal cord with bone (skull and vertebral column), membranes (the meninges), and a watery cushion (cerebrospinal fluid).

Meninges

Three connective tissue membranes cover and protect the CNS.

• Dura mater. The tough, leathery outer layer, double-layered around the brain. One layer attaches to the inner skull as the periosteum (periosteal layer); the other, the meningeal layer, covers the brain and continues as the spinal dura mater.
• Falx cerebri. An inward fold of the inner dural membrane that anchors the brain to the cranial cavity.
• Tentorium cerebelli. Separates the cerebellum from the cerebrum.
• Arachnoid mater. The weblike middle layer. Its threadlike extensions span the subarachnoid space to reach the innermost membrane.
• Pia mater. The delicate innermost layer, clinging tightly to every fold of the brain and spinal cord.

Cerebrospinal Fluid

Cerebrospinal fluid (CSF) is a watery broth similar to blood plasma, from which it forms.

• Contents. Compared with plasma, CSF has less protein and more vitamin C and glucose.
• Choroid plexus. CSF forms continuously from blood at the choroid plexuses, clusters of capillaries hanging from the roof of each brain ventricle.
• Function. Forms a watery cushion that protects the fragile nervous tissue from blows and trauma.
• Normal volume. CSF forms and drains at a constant rate, holding normal pressure and a volume of 150 ml (about half a cup).
• Lumbar tap. A CSF sample is taken by lumbar (spinal) tap. Because withdrawal drops CSF pressure, the patient stays horizontal (lying down) for 6 to 12 hours afterward to prevent a severe spinal headache.

The Blood-Brain Barrier

No other organ depends so completely on a constant internal environment, so the blood-brain barrier guards it.

• Function. Separates neurons from blood-borne substances using the least permeable capillaries in the body.
• Allowed. Of water-soluble substances, only water, glucose, and essential amino acids cross easily.
• Blocked. Metabolic wastes such as toxins, urea, proteins, and most drugs are kept out of brain tissue.
• Fat-soluble substances. The barrier is nearly useless against fats, respiratory gases, and other fat-soluble molecules, which diffuse easily through plasma membranes.

Spinal Cord

The cylindrical spinal cord is a glistening white continuation of the brain stem.

• Length. Approximately 17 inches (42 cm) long.
• Major function. Provides a two-way conduction pathway to and from the brain and serves as a major reflex center (spinal reflexes complete here).
• Location. Enclosed in the vertebral column, it runs from the foramen magnum of the skull to the first or second lumbar vertebra, ending just below the ribs.
• Meninges. Like the brain, it is cushioned by the meninges, which extend well beyond the cord's end in the vertebral canal.
• Spinal nerves. In humans, 31 pairs of spinal nerves arise from the cord and exit the vertebral column to serve nearby body areas.
• Cauda equina. The collection of spinal nerves at the inferior end of the vertebral canal, named for its resemblance to a horse's tail.

Gray Matter of the Spinal Cord and Spinal Roots

In cross-section, the gray matter looks like a butterfly or the letter H.

• Projections. The two posterior projections are the dorsal (posterior) horns; the two anterior projections are the ventral (anterior) horns.
• Central canal. The gray matter surrounds the central canal, which holds CSF.
• Dorsal root ganglion. Holds the cell bodies of sensory neurons whose fibers enter the cord by the dorsal root. Damage to the dorsal root or its ganglion causes loss of sensation from the area served.
• Dorsal horns. Contain interneurons.
• Ventral horns. Contain cell bodies of somatic motor neurons, which send their axons out the ventral root.
• Spinal nerves. The dorsal and ventral roots fuse to form the spinal nerves.

White Matter of the Spinal Cord

The white matter is built from myelinated fiber tracts, some running to higher centers, some descending from the brain, and some crossing from one side of the cord to the other.

• Regions. The white matter on each side splits into three columns, the dorsal, lateral, and ventral columns. Each holds fiber tracts of axons sharing a destination and function.
• Sensory (afferent) tracts. Carry sensory impulses to the brain.
• Motor (efferent) tracts. Carry impulses from the brain to skeletal muscle.

Peripheral Nervous System

The PNS consists of nerves and scattered groups of neuronal cell bodies (ganglia) outside the CNS.

Structure of a Nerve

A nerve is a bundle of neuron fibers found outside the CNS.

• Endoneurium. A delicate connective tissue sheath around each fiber.
• Perineurium. A coarser wrapping that binds groups of fibers into bundles (fascicles).
• Epineurium. A tough fibrous sheath binding all fascicles into the cordlike nerve.
• Mixed nerves. Carry both sensory and motor fibers.
• Sensory (afferent) nerves. Carry impulses toward the CNS only.
• Motor (efferent) nerves. Carry motor fibers only.

Cranial Nerves

The 12 pairs of cranial nerves mainly serve the head and neck.

• Olfactory (I). Fibers from olfactory receptors in the nasal mucosa synapse with the olfactory bulbs. Purely sensory, for smell.
• Optic (II). Fibers from the retina form the optic nerve. Purely sensory, for vision.
• Oculomotor (III). Runs from the midbrain to the eye. Supplies motor fibers to four of the six muscles (superior, inferior, and medial rectus, and inferior oblique) that move the eyeball, plus the eyelid and the internal eye muscles controlling lens shape and pupil size.
• Trochlear (IV). Runs from the midbrain to the eye. Supplies one external eye muscle (superior oblique).
• Trigeminal (V). Emerges from the pons in three divisions to the face. Carries sensory impulses from the skin of the face and mucosa of the nose and mouth, plus motor fibers to the chewing muscles.
• Abducens (VI). Leaves the pons to the eye. Supplies the lateral rectus muscle, which rolls the eye laterally.
• Facial (VII). Leaves the pons to the face. Activates the muscles of facial expression and the lacrimal and salivary glands, and carries taste from the anterior tongue.
• Vestibulocochlear (VIII). Runs from the inner-ear equilibrium and hearing receptors to the brain stem. Purely sensory: the vestibular branch carries balance, the cochlear branch carries hearing.
• Glossopharyngeal (IX). Emerges from the medulla to the throat. Supplies motor fibers to the pharynx for swallowing and saliva production, and carries taste from the posterior tongue and pressure data from the carotid artery.
• Vagus (X). Emerges from the medulla into the thorax and abdomen. Carries sensory and motor impulses to the pharynx, larynx, and abdominal and thoracic viscera. Most fibers are parasympathetic, promoting digestion and helping regulate the heart.
• Accessory (XI). Arises from the medulla and superior spinal cord to the neck and back. Mostly motor, activating the sternocleidomastoid and trapezius muscles.
• Hypoglossal (XII). Runs from the medulla to the tongue. Motor fibers control tongue movement; sensory fibers carry impulses from the tongue.

Spinal Nerves and Nerve Plexuses

The 31 pairs of human spinal nerves form from the combined ventral and dorsal roots of the spinal cord.

• Rami. Almost immediately, each spinal nerve splits into dorsal and ventral rami, making the spinal nerve itself only about 1/2 inch long. The rami carry both sensory and motor fibers.
• Dorsal rami. The smaller dorsal rami serve the skin and muscles of the posterior body trunk.
• Ventral rami. The ventral rami of T1 through T12 form the intercostal nerves, which supply the muscles between the ribs and the skin and muscles of the anterior and lateral trunk.
• Cervical plexus. Arises from C1-C5. The phrenic nerve is its key branch, serving the diaphragm plus the skin and muscles of the shoulder and neck.
• Brachial plexus. The axillary nerve serves the deltoid and skin of the shoulder and superior thorax; the radial nerve serves the triceps and forearm extensors and the skin of the posterior upper limb; the median nerve serves the forearm flexors and skin and some hand muscles; the musculocutaneous nerve serves the arm flexors and the skin of the lateral forearm; the ulnar nerve serves some forearm flexors, the wrist and many hand muscles, and the skin of the hand.
• Lumbar plexus. The femoral nerve serves the lower abdomen, anterior and medial thigh muscles, and the skin of the anteromedial leg and thigh; the obturator nerve serves the medial-thigh adductors, small hip muscles, and the skin of the medial thigh and hip joint.
• Sacral plexus. The sciatic nerve (the largest nerve in the body) serves the lower trunk and posterior thigh and splits into the common fibular and tibial nerves. The common fibular nerve serves the lateral leg and foot; the tibial nerve serves the posterior leg and foot; the superior and inferior gluteal nerves serve the gluteal muscles of the hip.

Autonomic Nervous System

The autonomic nervous system (ANS) is the motor subdivision of the PNS that runs body activity automatically.

• Composition. Specialized neurons that regulate cardiac muscle, smooth muscle, and glands.
• Function. Signals constantly flood in from the visceral organs, and the autonomic nerves adjust output to best support body activity.
• Divisions. Two arms, the sympathetic and parasympathetic divisions.

Anatomy of the Parasympathetic Division

The parasympathetic division lets the body unwind and conserve energy.

• Preganglionic neurons. Located in brain nuclei of several cranial nerves (III, VII, IX, and X, with the vagus the most important) and in the S2 through S4 levels of the spinal cord.
• Craniosacral division. Another name for this division. Cranial-region neurons send axons out in cranial nerves to serve the head and neck organs.
• Pelvic splanchnic nerves. In the sacral region, preganglionic axons leave the cord and form the pelvic splanchnic (pelvic) nerves, which travel to the pelvic cavity.

Anatomy of the Sympathetic Division

The sympathetic division mobilizes the body in extreme situations. It is also called the thoracolumbar division because its preganglionic neurons sit in the gray matter of the spinal cord from T1 through L2.

• Ramus communicans. Preganglionic axons leave the cord in the ventral root, enter the spinal nerve, then pass through a ramus communicans (a small communicating branch) into a sympathetic chain ganglion.
• Sympathetic chain. The sympathetic trunk runs along the vertebral column on each side.
• Splanchnic nerves. At the ganglion, an axon may synapse with the second neuron at the same or a different level, or pass through without synapsing and join the splanchnic nerves.
• Collateral ganglion. The splanchnic nerves run to the viscera and synapse with the ganglionic neuron in a collateral ganglion anterior to the vertebral column.

Physiology of the Nervous System

Nervous system physiology comes down to how impulses fire and travel.

Nerve Impulse

Neurons have two key functional properties: irritability, the ability to respond to a stimulus and convert it into a nerve impulse, and conductivity, the ability to transmit that impulse to other neurons, muscles, or glands.

• Resting membrane. The plasma membrane of a resting neuron is polarized: fewer positive ions on the inner face than the outer face. As long as the inside stays more negative than the outside, the neuron stays inactive.
• Action potential trigger. Most neurons are excited by neurotransmitters released by other neurons. Whatever the stimulus, the result is the same: the membrane's permeability changes briefly.
• Depolarization. An inward rush of sodium ions flips the membrane's polarity at that site.
• Graded potential. Locally the inside turns more positive and the outside less positive.
• Nerve impulse. If the stimulus is strong enough, local depolarization triggers a long-distance signal, the action potential (nerve impulse). It is all-or-none: it either propagates over the entire axon or not at all. It never travels partway and never dies out with distance the way a graded potential does.
• Repolarization. Positive ions flow out of the cell, restoring the resting polarized state. Until repolarization occurs, the neuron cannot conduct another impulse.
• Saltatory conduction. Myelinated fibers conduct faster because the impulse jumps from node to node, since no current can cross the axon membrane where fatty myelin insulates it.

The Nerve Impulse Pathway

Step by step:

1. Resting conditions. The outer membrane face is slightly positive, the inner slightly negative. The chief extracellular ion is sodium; the chief intracellular ion is potassium. The membrane is relatively permeable to both.
2. Stimulus and local depolarization. A stimulus changes the permeability of a patch of membrane, sodium ions rush in, and the polarity reverses at that site (inside more positive, outside more negative).
3. Action potential generation. If the stimulus is strong enough, depolarization fully reverses membrane polarity and an action potential is initiated.
4. Propagation. Depolarization of the first patch shifts permeability in the adjacent membrane, repeating the process, so the action potential propagates rapidly along the whole membrane.
5. Repolarization. Potassium ions diffuse out as permeability changes again, restoring the negative inside and positive outside. Repolarization runs in the same direction as depolarization.

Communication of Neurons at Synapses

Events at the synapse, in order:

1. Arrival. The action potential reaches the axon terminal.
2. Fusion. A vesicle fuses with the plasma membrane.
3. Release. Neurotransmitter is released into the synaptic cleft.
4. Binding. The neurotransmitter binds a receptor on the receiving neuron.
5. Opening. The ion channel opens.
6. Closing. Once the neurotransmitter is broken down and cleared, the ion channel closes.

Autonomic Functioning

Organs served by the autonomic nervous system receive fibers from both divisions.

• Antagonistic effect. When both divisions serve one organ, they produce opposite effects, mainly because their postganglionic axons release different transmitters.
• Cholinergic fibers. Parasympathetic fibers release acetylcholine.
• Adrenergic fibers. Sympathetic postganglionic fibers release norepinephrine.
• Preganglionic axons. Both divisions release acetylcholine at the preganglionic level.

Sympathetic Division

The sympathetic division is the fight-or-flight system.

• Signs of activation. A pounding heart; rapid, deep breathing; cold, sweaty skin; a prickly scalp; and dilated pupils.
• Effects. It raises heart rate, blood pressure, and blood glucose, dilates the bronchioles, and drives many other changes that help the body handle a stressor.
• Duration. Effects continue for several minutes until the liver destroys the released hormones.
• Function. Sets up the best conditions to meet a threat, whether the right response is to run, see better, or think more clearly.

Parasympathetic Division

The parasympathetic division dominates when the body is at rest and unthreatened.

• Function. The rest-and-digest system promotes normal digestion, elimination of feces and urine, and energy conservation, largely by lowering demand on the cardiovascular system.
• Relaxed state. Blood pressure and heart and respiratory rates sit at normal levels, the digestive tract is actively working, and the skin is warm (no need to divert blood to skeletal muscle or vital organs).
• Eyes. Pupils constrict to protect the retinas from excess light, and the lenses are set for close vision.