The nervous system

All body systems would be immobilized without the nervous system, which receives and correlates information from inside and outside the body and reacts by sending signals to appropriate muscles and glands so that these produce coordinated responses. The body’s nerve network has been likened to a telephone system, with the billions of elongated, interconnected cells called neurons serving as its wires.

Neurons form two great associated groups: the central and peripheral nervous systems.

The nervous system has three main parts. Central: the brain, cranial nerves, and spinal cord. Peripheral: sensory and motor nerves leading to and from the spinal cord. Autonomic: the sympathetic trunk and branches, and parasympathetic fibers.

Neurons and nerves

Neurons, or nerve cells, form the active units of the nervous system, although they are heavily outnumbered by glial (“glue”) cells that help to supply the neurons with nourishment, support, and insulation. Neurons may be grouped broadly into one of three categories, according to the jobs they do: sensory neurons (affer-ents) bring signals to the central nervous system from sensory receptors; motor neurons (effectors, or efferents) send signals out to muscles and glands; and interneurons (also called association neurons) serve as intermediaries.

Nerve cells are uniquely structured to communicate with one another. A typical neuron has three structural elements: a compact body, many short, branching “threads” called dendrites, and one very long “thread” called an axon. Dendrites receive signals from nearby neurons, and the axon hands them to another neuron muscle or gland. Naked dendrites and cell bodies color the gray matter of the brain and spinal cord; white matter consists of axons sheathed by a white, fatty, insulating substance known as myelin. Bundles of myelin-coated axons make up nerve fibers.
Unlike the electrical signals passed along a telephone wire, nervous signals travel by an electrochemical relay system. Inactive neurons tend to contain more potassium than sodium ions, while outside their cell membranes are more sodium than potassium ions. When a nearby neuron is stimulated, chemicals called neurotransmitters burst from sacs in knobs at its axon ends, leap the gap or synapse to the inactive neuron, and fill receptor sites in its cell membranes. This makes the cell lose some potassium ions and take in some sodium ions so that the cell’s electrical charge is changed at the affected site. That charge flows through the cell at up to 220 miles (354 kilometers) per hour, which is very fast (but slower than an electrical impulse).

While excitatory neurotransmitters make neurons fire off signals, inhibitory ones tend to block them. A single brain cell may receive thousands of simultaneous signals. How it reacts depends largely on how many “fire” and “don’t fire” signals it receives. Individual spinal and peripheral neurons act like brain cells, processing and responding to the information reaching them.

Central nervous system

The command center of the body consists of the spinal cord and a swollen and highly differentiated outgrowth of it, the brain.

The spinal cord forms a cylinder of nervous tissue some 16-20 inches (40-50 centimeters) long in an adult, extending from the brain stem down through the bony arches of the vertebrae. The cord bulges at the points where nerves branch off to the arms and legs. It is buffered by three membranes called meninges that continue upward to enclose the brain. Beneath the meninges, both brain and spinal cord are bathed by cerebrospinal fluid, which acts partly as a shock absorber, as well as serving to bring nourishment to the nervous tissue and protect it from infection.

If you slice through the column of the cord you notice an outer layer of white matter-myelinated afferent and efferent nerve fibers, respectively, which transmit signals up and down the cord. Inside this layer is gray matter, the arrangement of which resembles butterfly’s wings in cross section. The “upper” wings, pointing toward the back of the body, contain afferent neurons that receive signals from outside the cord. The “lower” wings, pointing toward the front of the body, contain efferents, controlling muscles and glands.

Spinal sensory and motor pathways may or may not pass through the brain. In a spinal reflex (left), a sensory signal enters by the dorsal root and synapses with motor fibers within the spinal cord. These then stimulate muscular action through the ventral motor root. In a conscious action (right), the signal travels to and from the brain.

Peripheral nervous system

Nerves originating in or linked with the central nervous system branch out through the body. This outer, or peripheral nervous system, has two overlapping components: the somatic (“of the body”) and autonomic (“self-regulating”) systems.

Somatic motor nerves supply striated (skeletal) muscle. Somatic sensory nerves supply sensory receptors in skin, tongue, nostrils, eyes, joints, and muscles.

Twelve pairs of cranial nerves arising from the brain supply ears, eyes, nose, facial skin and muscles, the tongue, jaw and neck muscles, and various internal organs. Thirty-one pairs of spinal nerves sprout from the spinal cord, supplying limbs and trunk. Sensory nerve fibers enter from the back, motor nerve fibers leave from the front, both passing in or out through gaps between vertebrae. Although there are approximately as many gaps as spinal nerves, the spinal cord ends high above the bottom of the spine, so the nerves of the lowest nerve roots must travel some distance inside the spine before they reach their exit holes.

Autonomic nervous system

Part of the peripheral nervous system works automatically, controlling the smooth muscle of internal organs and some glandular secretions. It normally functions outside conscious, willed control.

The autonomic nervous system has sympathetic and parasympathetic divisions, which operate to counteract each other the sympathetic system generally having an excitatory effect, the parasympathetic system the reverse.

The sympathetic system releases chemical neurotransmitters to stimulate heart, lungs, and other organs. Nerves of the sympathetic system form two cords parallel with the spinal cord and linked to it along its length, but not at its ends. Pealike swellings in both of these cords, called ganglia, contain nerve-cell bodies.

The parasympathetic system, arising from both ends of the spinal cord, releases neurotransmitters that act mostly on the same organs as those affected by the sympathetic system, but in ways that slow them down. Under stress, the sympathetic system dominates; at rest, the parasympathetic system takes control.

A spinal neuron (far left) consists of a cell body, with a nucleus, several threadlike dendrites linking with other neurons, and a longer fiber that carries the nervous impulse. In a motor neuron, this long fiber is covered by a fatty sheath and ends in a motor end plate.
Motor end plates (left), shown here at the same magnification as the spinal neuron (far left), attach to and stimulate individual muscle fibers. Electrochemical changes occur in the neurons that stimulate muscle function.