LEARNING OBJECTIVE: Identify the components and function of a neuron, recall the process of impulse transmission, and identify the components and functions of the central and peripheral nervous systems.
The activities of the widely diverse cells, tissues, and organs of the body must be monitored, regulated, and coordinated to effectively support human life. The interaction of the nervous and endocrine systems provides the needed control.
The nervous system is specifically adapted to the rapid transmission of impulses from one area of the body to another. On the other hand, the endocrine system, working at a far slower pace, maintains body metabolism at a fairly constant level.
In this section, you will study the neuron, the basic functional unit of the nervous system. Also, you will study the components and functions of the different divisions ofthe nervous system. The nervous system is divided into two major groups, the central nervous system (CNS) and the peripheral nervous system (PNS). Another division of the nervous system is the autonomic nervous system (ANS), which is further subdivided into the sympathetic and parasympathetic nervous systems.
The structure and functional unit of the nervous system is the nerve cell, or neuron, which can be classified into three types. The first is the sensory neuron, which conveys sensory impulses inward from the receptors. The second is the motor neuron, which carries command impulses from a central area to the responding muscles or organs. The third type is the interneuron, which links the sensory neurons to the motor neurons.
The neuron is composed of dendrites, a cyton, and an axon (fig. 1-42). The dendrites are thin receptive branches, and vary greatly in size, shape, and number with different types of neurons. They serve as receptors, conveying impulses toward the cyton. The cyton is the cell body containing the nucleus. The single, thin extension of the cell outward from the cyton is called the axon. It conducts impulses away from the cyton to its terminal branches, which transmit the impulses to the dendrites of the next neuron.
Large axons of the peripheral nerves are commonly enclosed in a sheath, called neurilemma, composed of Schwann cells (fig. 1-42). Schwann cells wrap around the axon and act as an electrical insulator.
Figure 1-42.—The neuron and its parts.
The membranes of the Schwann cell are composed largely of a lipid-protein called myelin, which forms a myein sheath on the outside of an axon. The myelin sheath has gaps between adjacent Schwann cells called nodes of Ranvier. Nerve cells without Schwann cells also lack myelin and neurilemma sheaths.
When dendrites receive a sufficiently strong stimulus, a short and rapid change in electrical charge, or polarity, ofthe neuron is triggered. Sodium ions rush through the plasma membrane into the cell, potassium ions leave, and an electrical impulse is formed, which is conducted toward the cyton. The cyton receives the impulse and transmits it to the terminal filaments ofthe axon. At this point a chemical transmitter such as acetylcholine is released into the synapse, a space between the axon of the activated nerve and the dendrite receptors of another neuron. This chemical transmitter activates the next nerve. In this manner, the impulse is passed from neuron to neuron down the nerve line to a central area at approximately the speed of a bullet.
Almost immediately after being activated, the chemical transmitter in the synapse is neutralized by the enzyme acetylcholinesterase, and the first neuron returns to its normal state by pumping out the sodium ions and drawing potassium ions back in through the plasma membrane. When these actions are completed, the nerve is ready to be triggered again. A particularly strong stimulus will cause the nerve to fire in rapid succession, or will trigger many other neurons, thus giving a feeling of intensity to the perceived sensation.
A nerve is a cordlike bundle of nerve fibers held together with connective tissue. Each nerve fiber is an extension of a neuron. Nerves that conduct impulses into the brain or the spinal cord are called sensory nerves, and those that carry impulses to muscles and glands are termed motor nerves. Most nerves, however, include both sensory and motor fibers, and they are called mixed nerves.
CENTRAL NERVOUS SYSTEM
The central nervous system (CNS) consists of the brain and spinal cord. The brain is almost entirely enclosed in the skull, but it is connected with the spinal cord, which lies in the canal formed by the vertebral column.
The brain has two main divisions, the cerebrum and the cerebellum. The cerebrum is the largest and most superiorly situated portion of the brain. It occupies most of the cranial cavity. The outer surface is called the cortex. This portion of the brain is also called “gray matter” because the nerve fibers are unmyelinated (not covered by a myelin sheath), causing them to appear gray. Beneath this layer is the medulla, often called the white matter of the brain because the nerves are myelinated (covered with a myelin sheath), giving them their white appearance.
CEREBRUM.—The cortex of the cerebrum is irregular in shape. It bends on itself in folds called convolutions, which are separated from each other by grooves, also known as fissures. The deep sagittal cleft, a longitudinal fissure, divides the cerebrum into two hemispheres. Other fissures further subdivide the cerebrum into lobes, each of which serves a localized, specific brain function (fig. 1-43). For example, the frontal lobe is associated with the higher mental processes such as memory, the parietal lobe is concerned primarily with general sensations, the occipital lobe is related to the sense of sight, and the temporal lobe is concerned with hearing.
CEREBELLUM.—The cerebellum is situated posteriorly to the brain stem (which is made up of the pons, mid-brain, and medulla oblongata) and inferior to the occipital lobe. The cerebellum is concerned chiefly with bringing balance, harmony, and coordination to the motions initiated by the cerebrum.
PONS AND MEDULLA OBLONGATA.— Two smaller divisions of the brain vital to life are the pons and the medulla oblongata. Together, the pons and medulla form the brain stem (fig. 1-43). The pons consists chiefly of a mass of white fibers connecting the other three parts of the brain (the cerebrum, cerebellum, and medulla oblongata).
The medulla oblongata is the inferior portion of the brain, the last division before the beginning of the spinal cord. It connects to the spinal cord at the upper level of the first cervical vertebra (C-1). In the medulla oblongata are the centers for the control of heart action, breathing, circulation, and other vital processes such as blood pressure.
MENINGES.—The outer surface ofthe brain and spinal cord is covered with three layers of membranes called the meninges. The dura mater is the strong outer layer; the arachnoid membrane is the delicate middle layer; and the pia mater is the vascular inner-most layer that adheres to the surface ofthe brain and spinal cord. Inflammation of the meninges is called meningitis. The type of meningitis contracted depends upon whether the brain, spinal cord, or both are affected, as well as whether it is caused by viruses, bacteria, protozoa, yeasts, or fungi.
Figure 1-43.—Functional areas of the brain.
CEREBROSPINAL FLUID.—Cerebrospinal fluid is formed by a plexus, or network, of blood vessels in the central ventricles of the brain. It is a clear, watery solution similar to blood plasma. The total quantity of spinal fluid bathing the spinal cord is about 75 ml. This fluid is constantly being produced and reabsorbed. It circulates over the surface of the brain and spinal cord and serves as a protective cushion as well as a means of exchange for nutrients and waste materials.
The spinal cord is continuous with the medulla oblongata and extends from the foramen magnum, through the atlas, to the lower border of the first lumbar vertebra, where it tapers to a point. The spinal cord is surrounded by the bony walls of the vertebral canal (fig. 1-44). Ensheathed in the three protective meninges and surrounded by fatty tissue and blood vessels, the cord does not completely fill the vertebral canal, nor does it extend the full length of it. The nerveroots serving the lumbar and sacral regions must pass some distance down the canal before making their exit. The sympathetic trunk contains the paravertebral ganglia (sing. ganglion), knotlike masses of nerve cell bodies (fig. 1-44).
A cross section of the spinal cord shows white and gray matter (fig. 1-45). The outer white matter is composed of bundles of myelinated nerve fibers arranged in functionally specialized tracts. It establishes motor communication between the brain and the body parts. The inner gray unmyelinated matter is shaped roughly like the letter H. It establishes sensory communication between the brain and the spinal nerves, conducting sensory impulses from the body parts.
Figure 1-44.—Spinal cord.
Figure 1-45.—Cross section of the spinal cord and reflex arc—arrows and numbers show impulse pathway.
The spinal cord may be thought of as an electric cable containing many wires (nerves) that connect parts of the body with each other and with the brain. Sensations received by a sensory nerve are brought to the spinal cord, and the impulse is transferred either to the brain or to a motor nerve. The majority of impulses go to the brain for action. However, a system exists for quickly handling emergency situations. It is called the reflex arc.
If you touch a hot stove, you must remove your hand from the heat source immediately or the skin will burn very quickly. But the passage of a sense impulse to the brain and back again to a motor nerve takes too much time. The reflex arc responds instantaneously to emergency situations like the one just described. The sensation of heat travels to the spinal cord on a sensory nerve. When the sensation reaches the spinal cord, it is picked up by an interneuron in the gray matter. This reception then triggers the appropriate nerve to stimulate a muscle reflex drawing the hand away. An illustrated example of the reflex arc is shown in figure 1-45.
The reflex arc works well in simple situations requiring no action of the brain. Consider, however, what action is involved if the individual touching the stove pulls back and, in so doing, loses balance and has to grab a chair to regain stability. Then the entire spinal cord is involved. Additional impulses must travel to the brain, then down to the muscles of the legs and arms to enable the individual to maintain balance and to hold on to a steadying object. While all this activity is going on, the stimulus is relayed through the sympathetic autonomic nerve fibers to the adrenal glands, causing adrenalin to flow, which stimulates heart action. The stimulus then moves to the brain, making the individual conscious of pain. In this example, the spinal cord has functioned not only as a center for spinal relaxes, but also as a conduction pathway for other areas of the spinal cord to the autonomic nervous system and to the brain.
PERIPHERAL NERVOUS SYSTEM
The peripheral nervous system (PNS) consists of the nerves that branch out from the CNS and connect it to the other parts of the body. The PNS includes 12 pairs of cranial nerves and 31 pairs of spinal nerves. Cranial and spinal nerves carry both voluntary and involuntary impulses.
The 12 pairs of cranial nerves are sensory, motor, or mixed (sensory and motor). Table 1-3 shows the 12 cranial nerves and parts of the body they service.
Table 1-3.—Cranial Nerves
|Olfactory||Sense of smell.|
|Oculomotor||Eye movement, size of pupil, and eye focus.|
|Trigeminal||Sensations of head and face and chewing movements.|
|Abducens||Abduction of eye (muscles that turn eye outward).|
|Facial||Facial expressions, secretion of saliva, and sense of taste.|
|Acoustic||Sense of hearing and balance or equilibrium sense.|
|Glossopharyngeal||Taste and other sensations of the tongue, swallowing movements, secretion of saliva.|
|Vagus||Sensations of movement (e.g.,
decrease in heart rate, increase in peristalsis, and
contracting of muscles for voice production).
|Accessory||Shoulder movements, turning movements of the head, and voice production.|
There are 31 pairs of spinal nerves that originate from the spinal cord. Although spinal nerves are not named individually, they are grouped according to the level from which they arise, and each nerve is numbered in sequence. Thus, there are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves. See figure 1-46.
Spinal nerves (mixed) send fibers to sensory surfaces and muscles of the trunk and extremities. Nerve fibers are also sent to involuntary smooth muscles and glands of the gastrointestinal tract, urogenital system, and cardiovascular system.
Figure 1-46.—Spinal nerves.
AUTONOMIC NERVOUS SYSTEM
The autonomic nervous system (ANS) is the portion of the PNS that functions independently, automatically, and continuously, without conscious effort. It helps to regulate the smooth muscles, cardiac muscle, digestive tube, blood vessels, sweat and digestive glands, and certain endocrine glands. The autonomic nervous system is not directly under the control of the brain but usually works in harmony with the nerves that are under the brain's control. The autonomic nervous system includes two subdivisions (the sympathetic and parasympathetic nervous systems) that act together.
The sympathetic nervous system's primary concern is to prepare the body for energy-expending, stressful, or emergency situations. On the other hand, the parasympathetic nervous system is most active under routine, restful situations. The parasympathetic system also counterbalances the effects of the sympathetic system, and restores the body to a resting state. For example, during an emergency the body's heart and respiration rate increases. After the emergency, the parasympathetic system will decrease heart and respiration rate to normal. The sympathetic and parasympathetic systems counterbalance each other to preserve a harmonious balance of body functions and activities.