The Autonomic Nervous System (ANS)
The autonomic (visceral) nervous system regulates the action of the glands, the smooth muscles of hollow organs and vessels, and the heart muscle. These actions are carried on automatically; whenever a change occurs that calls for a regulatory adjustment, it is made without conscious awareness. Most studies of the ANS concentrate on the motor (efferent) portion of the system. All autonomic pathways contain two motor neurons connecting the spinal cord with the effector organ. The two neurons synapse in ganglia that serve as relay stations along the way. The first neuron, the preganglionic neuron, extends from the spinal cord to the ganglion. The second neuron, the postganglionic neuron, travels from the ganglion to the effector. This differs from the voluntary (somatic) nervous system, in which each motor nerve fiber extends all the way from the spinal cord to the skeletal muscle with no intervening synapse. Some of the autonomic fibers are within the spinal nerves; some are within the cranial nerves.
Divisions of the Autonomic Nervous System
The motor neurons of the ANS are arranged in a distinct pattern, which has led to their separation for study purposes into sympathetic and parasympathetic divisions (Fig. 5-16).
Figure 5-16 Autonomic nervous system. The diagram shows only one side of the body for each division.
Sympathetic Nervous System.
The sympathetic motor neurons originate in the spinal cord with cell bodies in the thoracic and lumbar regions, the thoracolumbar area. These preganglionic fibers arise from the spinal cord at the level of the first thoracic spinal nerve down to the level of the second lumbar spinal nerve. From this part of the cord, nerve fibers extend to ganglia where they synapse with postganglionic neurons, the fibers of which extend to the glands and involuntary muscle tissues. Many of the sympathetic ganglia form the sympathetic chains, two cordlike strands of ganglia that extend along either side of the spinal column from the lower neck to the upper abdominal region. (Note that Figure 5-16 shows only one side for each division of the ANS.) In addition, the nerves that supply the organs of the abdominal and pelvic cavities synapse in three single collateral ganglia farther from the spinal cord. These are the:
* Celiac ganglion, which sends fibers mainly to the digestive organs;
* Superior mesenteric ganglion, which sends fibers to the large and small intestines;
* Inferior mesenteric ganglion, which sends fibers to the distal large intestine and organs of the urinary and reproductive systems The postganglionic neurons of the sympathetic system, with few exceptions, act on their effectors by releasing the neurotransmitter epinephrine (adrenaline) and the related compound norepinephrine (noradrenaline). This system is therefore described as adrenergic, which means “activated by adrenaline.”
Parasympathetic Nervous System The parasympathetic motor pathways begin in the craniosacral areas, with fibers arising from cell bodies in the brainstem (midbrain and medulla) and the lower (sacral) part of the spinal cord. From these centers, the first fibers extend to autonomic ganglia that are usually located near or within the walls of the effector organs and are called terminal ganglia. The pathways then continue along postganglionic neurons that stimulate the involuntary tissues. The neurons of the parasympathetic system release the neurotransmitter acetylcholine, leading to the description of this system as cholinergic (activated by acetylcholine).
Functions of the Autonomic Nervous System
Most organs are supplied by both sympathetic and parasympathetic fibers, and the two systems generally have opposite effects. The sympathetic part of the ANS tends to act as an accelerator for those organs needed to meet a stressful situation. It promotes what is called the fight-or-flight response because in the most primitive terms, the person must decide to stay and “fight it out” with the enemy or to run away from danger. If you think of what happens to a person who is frightened or angry, you can easily remember the effects of impulses from the sympathetic nervous system:
* Increase in the rate and force of heart contractions.
* Increase in blood pressure due partly to the more effective heartbeat and partly to constriction of small arteries in the skin and the internal organs.
* Dilation of blood vessels to skeletal muscles, bringing more blood to these tissues.
* Dilation of the bronchial tubes to allow more oxygen to enter.
* Stimulation of the central portion of the adrenal gland. This produces hormones, including epinephrine, that prepare the body to meet emergency situations in many ways. The sympathetic nerves and hormones from the adrenal gland reinforce each other.
* Increase in basal metabolic rate.
* Dilation of the pupil and decrease in focusing ability (for near objects).
The sympathetic system also acts as a brake on those systems not directly involved in the response to stress, such as the urinary and digestive systems. If you try to eat while you are angry, you may note that your saliva is thick and so small in amount that you can swallow only with difficulty. Under these circumstances, when food does reach the stomach, it seems to stay there longer than usual. The parasympathetic part of the ANS normally acts as a balance for the sympathetic system once a crisis has passed. The parasympathetic system brings about constriction of the pupils, slowing of the heart rate, and constriction of the bronchial tubes. It also stimulates the formation and release of urine and activity of the digestive tract. Saliva, for example, flows more easily and profusely, and its quantity and fluidity increase. Most organs of the body receive both sympathetic and parasympathetic stimulation, the effects of the two systems on a given organ generally being opposite.
Table 5. Effects of the Sympathetic and Parasympathetic Systems on Selected Organs
Table 5 shows some of the actions of these two systems.