Stimulation of Discrete Organs in Some Instances and Mass Stimulation in Other Instances by the Sympathetic and Parasympathetic Systems
Sympathetic System Often Responds by Mass Discharge. Inmany instances, almost all portions of the sympathetic nervous system discharge simultaneously as a com-plete unit, a phenomenon called mass discharge. This frequently occurs when the hypothalamus is activated by fright or fear or severe pain. The result is a wide-spread reaction throughout the body called the alarm or stress response, which we shall discuss shortly.
At other times, activation occurs in isolated portions of the sympathetic nervous system. The most impor-tant of these are the following: (1) During the process of heat regulation, the sympathetics control sweating and blood flow in the skin without affecting other organs innervated by the sympathetics. (2) Many “local reflexes” involving sensory afferent fibers travel centrally in the peripheral nerves to the sympathetic ganglia and spinal cord and cause highly localized reflex responses. For instance, heating a local skin area causes local vasodilation and enhanced local sweating, whereas cooling causes opposite effects. (3) Many of the sympathetic reflexes that control gastrointestinal functions operate by way of nerve pathways that do not even enter the spinal cord, merely passing from the gut mainly to the paravertebral ganglia, and then back to the gut through sympathetic nerves to control motor or secretory activity.
Parasympathetic System Usually Causes Specific Localized Responses. In contrast to the common mass dischargeresponse of the sympathetic system, control functions by the parasympathetic system are much more likely to be highly specific. For instance, parasympathetic cardiovascular reflexes usually act only on the heart to increase or decrease its rate of beating. Likewise, other parasympathetic reflexes cause secretion mainly by the mouth glands, or in other instances secretion is mainly by the stomach glands. Finally, the rectal emp-tying reflex does not affect other parts of the bowel to a major extent.
Yet there is often association between closely allied parasympathetic functions. For instance, although sali-vary secretion can occur independently of gastric secretion, these two also often occur together, and pancreatic secretion frequently occurs at the same time. Also, the rectal emptying reflex often initiates a urinary bladder emptying reflex, resulting in simulta-neous emptying of both the bladder and the rectum. Conversely, the bladder emptying reflex can help ini-tiate rectal emptying.
When large portions of the sympathetic nervous system discharge at the same time—that is, a mass dis-charge—this increases in many ways the ability of thebody to perform vigorous muscle activity. Let us sum-marize these ways:
1. Increased arterial pressure
2. Increased blood flow to active muscles concurrent with decreased blood flow to organs such as the gastrointestinal tract and the kidneys that are not needed for rapid motor activity
3. Increased rates of cellular metabolism throughout the body
4. Increased blood glucose concentration
5. Increased glycolysis in the liver and in muscle
6. Increased muscle strength
7. Increased mental activity
8. Increased rate of blood coagulation
The sum of these effects permits a person to perform far more strenuous physical activity than would otherwise be possible. Because either mental orphysical stress can excite the sympathetic system, it isfrequently said that the purpose of the sympathetic system is to provide extra activation of the body in states of stress: this is called the sympathetic stressresponse.
The sympathetic system is especially strongly acti-vated in many emotional states. For instance, in the state of rage, which is elicited to a great extent by stim-ulating the hypothalamus, signals are transmitted downward through the reticular formation of the brain stem and into the spinal cord to cause massive sym-pathetic discharge; most aforementioned sympathetic events ensue immediately. This is called the sympa-thetic alarm reaction. It is also called the fight or flightreaction because an animal in this state decides almostinstantly whether to stand and fight or to run. In either event, the sympathetic alarm reaction makes the animal’s subsequent activities vigorous.
Many neuronal areas in the brain stem reticular sub-stance and along the course of the tractus solitarius of the medulla, pons, and mesencephalon, as well as in
many special nuclei (Figure 60–5), control different autonomic functions such as arterial pressure, heart rate, glandular secretion in the gastrointestinal tract, gastrointestinal peristalsis, and degree of contraction of the urinary bladder. Control of each of these is dis-cussed at appropriate points in this text. Suffice it to point out here that the most important factors con-trolled in the brain stem are arterial pressure, heart rate, and respiratory rate. Indeed, transection of the brainstem above the midpontine level allows basal control of arterial pressure to continue as before but prevents its modulation by higher nervous centers such as the hypothalamus. Conversely, transection immediately below the medulla causes the arterial pressure to fall to less than one-half normal.
Closely associated with the cardiovascular regula-tory centers in the brain stem are the medullary and pontine centers for regulation of respiration. Although this is not consid-ered to be an autonomic function, it is one of the invol-untary functions of the body.
Control of Brain Stem Autonomic Centers by Higher Areas.
Signals from the hypothalamus and even from the cerebrum can affect the activities of almost all the brain stem autonomic control centers. For instance, stimulation in appropriate areas mainly of the poste-rior hypothalamus can activate the medullary cardio-vascular control centers strongly enough to increase arterial pressure to more than twice normal. Likewise, other hypothalamic centers control body temperature, increase or decrease salivation and gastrointes-tinal activity, and cause bladder emptying. To some extent, therefore, the autonomic centers in the brain stem act as relay stations for control activities initiated at higher levels of the brain, especially in the hypothalamus.
It is pointed out also that many of our behavioral responses are mediated through (1) the hypothalamus, (2) the reticular areas of the brain stem, and (3) the autonomic nervous system. Indeed, some higher areas of the brain can alter function of the whole autonomic nervous system or of portions of it strongly enough to cause severe autonomic-induced disease such as peptic ulcer of the stomach or duodenum, constipation, heart palpitation, or even heart attack.
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