Central Nervous System

The brain and spinal cord are the organs of the CNS. Because they are so vitally important, the brain and spinal cord, located in the dorsal body cavity, are encased in bone for protection. The brain is in the cranial vault, and the spinal cord is in the vertebral canal of the vertebral column. Although considered to be two separate organs, the brain and spinal cord are continuous at the foramen magnum.

Peripheral Nervous System

The organs of the PNS are the nerves and ganglia. Nerves are bundles of nerve fibers, much as muscles are bundles of muscle fibers. Cranial nerves (12 pairs) and spinal nerves (31 pairs) extend from the CNS to peripheral organs, such as muscles and glands. Ganglia are collections, or small knots, of nerve cell bodies outside the CNS.

The PNS is further subdivided into an afferent (sensory) division and an efferent (motor) division. The afferent or sensory division transmits impulses from peripheral organs to the CNS. The efferent or motor division transmits impulses from the CNS out to the peripheral organs to cause an effect or action.

Finally, the efferent or motor division is again subdivided into the somatic nervous system and the autonomic nervous system (ANS). The somatic nervous system, also called the somatomotor or somatic efferent nervous system, supplies motor impulses to the skeletal muscles. Because these nerves permit conscious control of the skeletal muscles, the somatic nervous system is sometimes called the voluntary nervous system. The ANS, also called the visceral efferent nervous system, supplies motor impulses to cardiac muscle, smooth muscle, and glandular epithelium. It is further subdivided into sympathetic and parasympathetic divisions. Because the ANS regulates involuntary or automatic functions, it is sometimes called the involuntary nervous system.

Neurons

Neurons, or nerve cells, carry out the functions of the nervous system by conducting nerve impulses. They are highly specialized and amitotic. This means that if a neuron is destroyed, it cannot be replaced because neurons do not undergo mitosis.

Each neuron has three basic parts:

Figure 9-2 illustrates a typical neuron. The main part of the neuron is the cell body or soma. In many ways the cell body is similar to other types of cells. It has a nucleus with at least one nucleolus and contains many of the typical cytoplasmic organelles. It lacks centrioles, however. Because centrioles function in cell division, the fact that neurons lack these organelles is consistent with the amitotic nature of the cell.

Dendrites and axons are cytoplasmic extensions, or processes, that project from the cell body. They are sometimes referred to as fibers. Dendrites are usually, but not always, short and branching, which increases their surface area to receive signals from other neurons. The number of dendrites on a neuron varies. They are called afferent processes because they transmit impulses to the neuron cell body. Only one axon projects from each cell body. It is usually elongated, and because it carries impulses away from the cell body, it is called an efferent process.

An axon may have infrequent branches called axon collaterals. Axons and axon collaterals terminate in many short branches or telodendria (tell-oh-DEN-dree-ah). The distal ends of the telodendria are slightly enlarged to form synaptic bulbs. Many axons are surrounded by a segmented, white, fatty substance called myelin (MY-eh-lin) or the myelin sheath. Myelinated fibers make up the white matter in the CNS, whereas cell bodies and unmyelinated fibers make up the gray matter. The unmyelinated regions between the myelin segments are called the nodes of Ranvier (nodes of ron-vee-AY). In the PNS the myelin is produced by Schwann cells. The cytoplasm, nucleus, and outer cell membrane of the Schwann cell form a tight covering around the myelin and around the axon itself at the nodes of Ranvier. This covering is the neurilemma (noo-rih-LEM-mah), which plays an important role in the regeneration of nerve fibers. In the CNS, oligodendrocytes (ah-lee-go-DEN-droh-sites) produce myelin, but there is no neurilemma, which is why fibers within the CNS do not regenerate. The structure of an axon and its coverings is illustrated in Figure 9-2.

Functionally, neurons are classified as afferent, efferent, or interneurons (association neurons) according to the direction in which they transmit impulses relative to the CNS (Table 9-1). Afferent, or sensory, neurons carry impulses from peripheral sense receptors to the CNS. They usually have long dendrites and relatively short axons. Efferent, or motor, neurons transmit impulses from the CNS to effector organs, such as muscles and glands. Efferent neurons usually have short dendrites and long axons. Interneurons, or association neurons, are located entirely within the CNS, where they form the connecting link between the afferent and efferent neurons. They have short dendrites and may have either a short or a long axon.

Neuroglia

Neuroglia cells do not conduct nerve impulses; instead, they support, nourish, and protect the neurons. They are far more numerous than neurons and, unlike neurons, are capable of mitosis.

Resting Membrane

A resting membrane is the cell membrane of a nonconducting, or resting, neuron. The membrane is impermeable to the passive diffusion of sodium (Na⁺) and potassium (K⁺) ions. An active transport mechanism, the sodium-potassium pump, maintains a difference in concentration of these ions on the two sides of the membrane. Sodium ions are concentrated in the extracellular fluid, whereas the potassium ions are inside the cell. The intracellular fluid also contains proteins and other negatively charged ions. The result is a polarized membrane with more positive charges outside the cell and more negative charges inside the cell. This difference in charges on the two sides of the resting membrane is the resting membrane potential. Electrical measurements show the resting membrane potential to be about −70 millivolts (mV), which means that the inside of the membrane is 70 mV less positive (more negative) than the outside.

Stimulation of a Neuron

A stimulus is a physical, chemical, or electrical event that alters the permeability of the neuron cell membrane. This allows sodium ions to move inside the cell, then potassium ions move to the outside. This ionic movement briefly changes the polarization of the membrane.

This response to a stimulus—namely, depolarization, reverse polarization, and repolarization—is called the action potential. Electrical measurements show the action potential to peak at approximately +30 mV (Figure 9-3). At the conclusion of the action potential, the sodium-potassium pump actively transports sodium ions out of the cell and potassium ions into the cell to completely restore resting conditions.

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