5.3The Autonomic Nervous System
The part of the nervous system that participates in maintaining homeostasis is the autonomic nervous system (Fig. 5-5). Autonomic nerves control organs that perform involuntary movements and were given their name because their actions are relatively not controlled by the brain. The autonomic nervous system is composed of the sympathetic and parasympathetic nervous systems, and almost all peripheral organs are controlled by both these systems. Often, the two functions of the autonomic nervous system are competitive, and the functions of each organ are regulated by the balance between them.
The Sympathetic Nervous System
The cells of the sympathetic neurons exit via the thoracic and lumbar areas of the spinal cord, and some form synapses in the sympathetic nerve trunk (Fig. 5-5). In addition, some cells pass through without synapsing in the sympathetic nerve trunk and are distributed to various organs supplied by the sympathetic ganglia that are in the vicinity of the abdominal viscera.
Synapses at the ends of the primary neurons are mediated by nicotinic receptors activated by acetylcholine. Transmissions across the synapses in the secondary neurons of functional organs are mediated by adrenergic receptors activated by catecholamines (mainly noradrenaline). The adrenergic receptors in synapses are identified as α and β receptors, but several different types of each of these receptors are known. To increase activity when the body is active or stimulated, the sympathetic nerves are activated as a response to fluctuations in the external environment and stress. For example, in the heart, the heart rate and cardiac output are elevated by sympathetic activation in order to elevate blood pressure, providing the body with the ability to adapt to environmental changes. Moreover, secretion of various hormones is regulated in the central and peripheral nervous systems. For example, when the sympathetic nerves are activated by stress, they promote secretion of the adrenocorticotropic hormone (ACTH) from the pituitary gland and adrenaline from the adrenal medulla.
The Parasympathetic Nervous System
The cells of the parasympathetic nervous system neurons exit via the cranial nerve nuclei (the nerve nuclei of the oculomotor, facial, glossopharyngeal, and vagus nerves) and transmit neurons by forming synapses at the ganglia either within or adjacent to the functional organs. In the neurons found in the ganglia and the regulated organs, acetylcholine is also used as a neurotransmitter; however, at the peripheral ends of the primary neurons, information is transmitted through nicotinic receptors and at the peripheral ends of the secondary neurons, it is transmitted through muscarinic receptors. The parasympathetic nervous system is very active at rest and is often inhibited by changes in the external environment or stress. Therefore, its coordination with the endocrine system is somewhat less significant than that with the sympathetic system. When the parasympathetic system is activated, blood pressure is lowered by reduction in the heart rate and cardiac volume.