The autonomic nervous system

The autonomic nervous system is by definition that part of the nervous system that innervates smooth muscle, cardiac muscle and glands. It is thus a motor system. Perception arising fium the viscera involves pathways similar to those arising from the body surface and skeletal muscle. Thus there are visceral afferent fibres that pass from the viscera to the central nervous stem. Such impulses then ascend the spinal cord to the thalamus and are thence relayed to the post-central gyrus of the brain (or sensory cortex). Visceral refiex arcs use visceral afferent fibres to convey impulses to the cord, but the efferent limb of such a visceral reflex is the autonomic nervous system. Although visceral reflexes are under higher central control, it is usually impossible to bring them under the control of the will. 

It should be noted, as an apparent exception, that preganglionic fibres pass through the sympathetic chain, and witk-out synapse, innervate the adrenal medulla. The cells of the latter may, however, be considered as modified postganghonic tissue. 

The physiology of the autonomic nervous system. Most viscera an innervated by both divisions of the autonomic nervous ygtem. In a sense they are antagonistic and as such are con-oemed with important homeostatic mechanisms. 

Some of the actions of the two systems are as follows. It is interesting to compare this list with the known physiological actions of D.F.P. as detailed on p. 71. Fig. 5 will also make the relationship clearer to the non-biologist. 

Alimentary canal sympathetic dilates, parasympathetic constricts, especially the rectum (see effect of D.F.P. in treatment of post-operative paralytic ileus, p. 196). Furthermore, parasympathetic stimulates glandular secretion. 

The parasympatheHc nervous system. Here the postganglionic fibres are always short. The preganglionic fibres are long and puB almost up to the muscle or gland to be innervated. The pre-ganglkniic fibres arise firom the brain stem (cranial nerves VII, IX and X), fix m the tectal region (cranial nerve III which applies the eye, p. 38), and from the sacral region. 

---

A special feature of the iris is its autonomic innervation. Sympathetic activation widens the aperture of the iris whereas impulses from the parasympa thetic nervous system decrease the aperture size. Therefore adrenergic agonists and anticholinergic compounds both increase the aperture of the iris, i.e., cause mydriasis, and antiadrenergic and cholinergic agonists decrease it, i.e., cause miosis. The iris can thus be considered an excellent mirror reflecting the balance of the autonomic nervous system in the body. " ... 

Parasympathetic nervous system. That portion of the autonomic nervous system that utilizes acetylcholine as the neurotransmitter at the neuro-effector junctions. 

The baroreceptor reflex is a central reflex mechanism, which reduces heart rate following an increase in blood pressure. Each change in blood pressure is sensed by baroreceptors in the carotid arteries, which activate the autonomic nervous system to alter heart rate and thereby readjust blood pressure. 

It is well established that the autonomic nervous system shows marked plasticity. The expression of cotransmitters and receptors shows dramatic changes during development and ageing, in nerves that remain... 

The PNS is further divided into the somatic nervous system and the autonomic nervous system. The somatic branch of the PNS is concerned witii sensation and voluntary movement. The sensory part of the somatic nervous system sends messages to the brain concerning die internal and external environment, such as sensations of heat, pain, cold, and pressure The voluntary part of die somatic nervous system is concerned witii die voluntary movement of skeletal muscles, such as walking, chewing food, or writing a letter. 

The autonomic branch of die PNS is concerned witii functions essential to the survival of the organism. Functional activity of the autonomic nervous system... 

TABLE 22-1 Action of the Autonomic Nervous System on Body Organs and Structures... 

The PNS is a part of the autonomic nervous system (see Chap. 22). It helps conserve body energy and is partly responsible for activities such as slowing the heart rate, digesting food, and eliminating body wastes. 

Smooth muscle cell activity is in general under neural control. Thus, the many transmitters of the autonomic nervous system are paired with receptors on the smooth muscle cell membrane. One of the current questions about smooth muscle function is What intracellular processes are the different transmitters modulating in the smooth muscle cells, in addition to their effects on the contractile state ... 

Ocular Effects. One study reported that seven children exposed to methyl parathion by inhalation, oral, and possibly dermal routes exhibited pinpoint pupils (miosis) (Dean et al. 1984). This effect is a consequence of the effects on the autonomic nervous system. No other studies were located regarding ocular effects in humans or animals after inhalation exposure to methyl parathion. 

Gastrointestinal Effects. Case reports indicate that acute inhalation exposure to trichloroethylene results in nausea and vomiting (Buxton and Hayward 1967 Clearfield 1970 David et al. 1989 DeFalque 1961 Gutch et al. 1965 Milby 1968). Anorexia, nausea, vomiting, and intolerance to fatty foods have also been reported after chronic occupational exposure to trichloroethylene (El Ghawabi et al. 1973 Schattner and Malnick 1990 Smith 1966). Trichloroethylene-induced efiects on the autonomic nervous system may contribute to these effects (Grandjean et al. 1955). Some of the people exposed to trichloroethylene and other chlorinated... 

Enteric nerves control intestinal smooth muscle action and are connected to the brain by the autonomic nervous system. IBS is thought to result from dysregulation of this brain-gut axis. The enteric nervous system is composed of two gan-glionated plexuses that control gut innervation the submucous plexus (Meissner s plexus) and the myenteric plexus (Auerbach s plexus). The enteric nervous system and the central nervous system (CNS) are interconnected and interdependent. A number of neurochemicals mediate their function, including serotonin (5-hydroxytryptamine or 5-HT), acetylcholine, substance P, and nitric oxide, among others. 

Both the G- and V-agents have the same physiological action on humans. They are potent inhibitors of the enzyme acetylcholinesterase (AChE), which is required for the function of many nerves and muscles in nearly every multicellular animal. Normally, AChE prevents the accumulation of acetylcholine after its release in the nervous system. Acetylcholine plays a vital role in stimulating voluntary muscles and nerve endings of the autonomic nervous system and many structures within the CNS. Thus, nerve agents that are cholinesterase inhibitors permit acetylcholine to accumulate at those sites, mimicking the effects of a massive release of acetylcholine. The major effects will be on skeletal muscles, parasympathetic end organs, and the CNS. 

PAM chloride - 2-pralidoxime chloride. See oxime, parasympathetic nervous system -the part of the autonomic nervous system that decreases pupil size, heart rate, and blood pressure and increases functions, such as secretion of saliva, tears, and perspiration. 

Dopamine is a catecholamine neurotransmitter in the CNS and at some ganglia in the autonomic nervous system. To date, three main types of receptors have been found D1( D2, and D3. The main dopaminergic systems in the brain are the nigro-neostriatal... 

---