The Autonomic Nervous System

The autonomic nervous system is primarily an effector system that innervates smooth musculature, heart muscle, and exocrine glands. It is a visceral and largely involuntary motor system. Anatomical principles underlying the organization of both somatic motor and autonomic nervous systems are similar (Fig. 4) and the two systems function in parallel to adjust the body to environmental changes. Nevertheless, the two systems differ in several ways. Within the autonomic nervous system, two subsystems, the sympathetic and the parasympathetic, have long been distinguished by means of anatomical, chemical, and functional criteria. The sympathetic is the most extensive of the two systems. Its preganglionic motor neurons are located in the spinal cord, where they occupy a region called the latteral horn of the spinal gray matter. The preganglionic fibers employ the neurotransmitter acetylcholine, whereas the postganglionic fibers often must travel a substantial distance and employ the neurotransmitter norepinephrine. The sympathetic division of the autonomic nervous system promotes the organism's ability to expend energy (Hess called it ergotropic) and governs an endocrine gland, the adrenal medulla, considered to be a modified auto-nomic ganglion. It is indeed a universal mobilizing mechanism, valuable in emergencies, with postgan-glionic ramifications throughout the visceral realm.

In contrast, Hess described the parasympathetic nervous system as trophotropic to signify that it promotes the restitution of the organism. The para-sympathetic nervous system in fact antagonizes the sympathetic's effects. The preganglionic motor neurons of the parasympathetic nervous system are in the brain stem and in a short stretch of the spinal cord near its caudal tip. Like the preganglionic motor neurons of the sympathetic nervous system, they employ acetyl-choline as their transmitter, but the postganglionic transmitter of the parasympathetic is also acetylcho-line. Their axons are long because the ganglia to which they project lie near the tissues of the viscera and sometimes even inside them (Fig. 4). The resilence of autonomic control is in good accordance with what is known about the conduction lines descending from the hypothalamus. Indeed, the hypothalamus emits axons

Figure 4 Three different motor innervations. (a) In the somatic motor pattern, a motor neuron from the spinal cord or in the brain stem animates striated muscles directly. In the visceral motor pattern, a two-neuron chain is required. (b) The sympathetic nervous system stations a ''preganglionic'' visceral motor neuron in the spinal cord. (c) The parasympathetic nervous system employs a two-neuron pathway. The first neuron is situated in the brain stem or toward the bottom of the spinal cord, and the ganglion is close or even inside the

Figure 4 Three different motor innervations. (a) In the somatic motor pattern, a motor neuron from the spinal cord or in the brain stem animates striated muscles directly. In the visceral motor pattern, a two-neuron chain is required. (b) The sympathetic nervous system stations a ''preganglionic'' visceral motor neuron in the spinal cord. (c) The parasympathetic nervous system employs a two-neuron pathway. The first neuron is situated in the brain stem or toward the bottom of the spinal cord, and the ganglion is close or even inside the that descend toward both sympathetic and parasympathetic preganglionic visceral motor neurons, thus regulating the viscera. Hence, the hypothalamus may function as the so-called head ganglion of the auto-nomic nervous system that mediates conventional reflexes involving neural inputs and outputs. Fibers passing directly from the hypothalamus to the lateral horn of the spinal cord's gray matter, where the preganglionic motor neurons of the sympathetic nervous system are situated, have recently been found. However, these fibers seem to constitute a small minority of hypothalamic efferents; the hypothalamus has nothing similar to a pyramidal tract to carry its descending outputs. Instead, it appears in large measure to project no further than the midbrain, where neurons of the reticular formation take over. It is noteworthy that pathways descending to autonomic motor neurons are interrupted at numerous levels, at which further instructions can enter the descending lines.

Most of the regions of the brain that influence the autonomic nervous system's output (e.g., the cerebral cortex, the hippocampus, the entorhinal cortex, parts of the thalamus, basal ganglia, cerebellum, and the reticular formation) produce their actions by way of the hypothalamus, which integrates the information it receives from these structures into a coherent pattern of autonomic responses. The hypothalamus controls the output of the autonomic nervous system in two ways. The first one is direct and consists of projections to nuclei in the brain stem and the spinal cord that act on preganglionic autonomic neurons to control respiration, heart rate, temperature and blood pressure. Thus, stimulation of the lateral hypothalamus leads to general sympathetic activation (increase in blood presure, piloerection, etc.). Second, the hypothalamus that governs the autonomic nervous system by controlling the endocrine system, which releases hormones that influence autonomic functions is discussed in the following section.

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