Patterns Of Neurotransmission

Neural input to various organs via the postganglionic neurons of the sympathetic and parasympathetic divisions is varied in two general ways. The behavior of many of the individual peripheral autonomic pathways can be altered in concert such that several organs and organ systems are regulated simultaneously to meet specific physiologic demands placed on the body. The reaction of the overall autonomic nervous system to imposed demands is often referred to as mass action or the fight-orflight mechanism. Alternatively, the neural firing pattern of individual postganglionic pathways to specific organs can be varied independently such that the behavior of a single effector or group of effectors can be altered without producing changes in the behavior of other organs.

The fight-or-flight mechanism occurs in individuals acutely concerned for their physical safety, as in the case of someone about to be physically attacked. The autonomic system immediately and simultaneously increases heart rate, decreases blood flow to organs not needed for rapid body movement, increases blood flow to many skeletal muscles, and increases blood glucose. A similar set of responses occurs when someone is in a state of rage. In the fight-or-flight mode of operation, there is a generalized increase in the firing rate of sympathetic neurons and a generalized decrease in the firing rate of parasympathetic neurons.

FIGURE 7 Synthesis, release, and degradation of ACh at a bouton of a parasympathetic postganglionic axon.

Individuals do not, however, spend most of their time in the fight-or-flight state. During the more mundane physical and emotional states associated with typical daily events, the firing rates of various sympathetic pathways may increase, whereas the firing rates of other sympathetic pathways decrease. A similar lack of homogeneity is usually associated with the activity of parasympathetic neurons. Furthermore, most of the neural circuitry of the autonomic nervous system is organized to produce specific reflexes. For example, the smell of food initiates the secretion of digestive juices even before food enters the mouth. Low blood pressure in the carotid arteries causes a decrease in the firing rate of baroreceptors located in these arteries, which in turn causes appropriate changes to occur in the cardiovascular system to increase carotid arterial pressure and, hence, maintain blood supply to the brain. These are only two examples of the many autonomic reflexes that function to regulate the behavior of organ systems.

Autonomic regulation of organ function is usually modulatory and not initiatory. The heart, for example, pumps blood in the absence of sympathetic or para-sympathetic inputs; thus, autonomic inputs do not initiate cardiac contractile activity but only modulate the existing autogenic capability of the heart. This modulation is accomplished by the parasympathetic and sympathetic postganglionic fibers maintaining a variable, but omnipresent, discharge of action potentials referred to as tonic discharge or, simply, tone. An increase in sympathetic tone to the heart will, for example, increase heart rate. Caution, however, is warranted when one encounters or uses the phrase sympathetic tone; unless the context in which the phrase is used is clear, it is often impossible to determine if one means tone along all efferent pathways of the sympathetic nervous system or only along specific pathways to one or more effectors.

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