The somatic motor nervous system

Neurohumoral transmitters are chemicals that facilitate the transmission of nerve impulses across nerve synapses and neuroeffector junctions. Acetylcholine is a neurohumoral transmitter that is present in the peripheral autonomic nervous system, in the somatic motor nervous system, and in some portions of the central nervous system. 

The somatic motor nervous system or voluntary nervous system consists of nerve libers that irmervate skeletal muscle motor end-plates. 

Lefkowitz RJ, Hoffman BB, Taylor P. 1996. Neurotransmission The autonomic and somatic motor nervous systems. In Hardman JG, Limbird EE, eds. Goodman Gilman s the pharmacological basis of therapeutics. New York, NY McGraw-Hill, 105-139. 

Hoffman, B.B., Lefkowitz, R.J., and Taylor, P., Neurotransmission the autonomic and somatic motor nervous systems, in Goodman and Gilman s The Pharmacological Basis of Therapeutics, 9th ed., Hardman, J.G. and Limbird, L.E., Eds., McGraw-Hill, New York, 1996, chap. 6. 

Hoffman, B. B. and Taylor, P. Neurohumoral transmission the autonomic and somatic motor nervous systems. In J. G. Hardman and L. E. Limbird (eds.), Goodman Gilman s Pharmacological Basics of Therapeutics, 10th edn. New York Macmillan, pp. 115-154, 2001. 

LeiVowii/. R. J.. Hoffman. B. B., and Taylor. P.. Neunitran-smission The autonomic and somatic motor nervous. systems. In Hardman. J. G... iiul Limbird. I-. E. (eds.). The Phumuicological Basis of 1 hcrapcutics. lOlh ed. New York. McGraw-Hill. 2001. p. II5... 

NEUROTRANSMISSION THE AUTONOMIC AND SOMATIC MOTOR NERVOUS SYSTEMS... 

CHAPTER 6 Neurotransmission The Autonomic and Somatic Motor Nervous Systems 87... 

The efferent portion of the peripheral nervous system can be further divided into two major functional subdivisions, the somatic and autonomic systems (see Figure 3.1). The somatic efferents are involved in voluntarily controlled functions such as contraction of the skeletal muscles in locomotion. The autonomic system functions involuntarily to regulate the everyday needs and requirements of the body without the conscious participation of the mind. It is composed primarily of visceral motor (efferent) neurons that innervate smooth muscle of the viscera, cardiac muscle, vasculature and the exocrine glands. 

As with other organophosphorothioate agents, the toxicity of methyl parathion is due to inhibition of acetylcholinesterase by the active metabolite (i.e., methyl paraoxon), resulting in stimulation of the central nervous system, the parasympathetic nervous system, and the somatic motor nerves. 

Figure 6-1. Schematic diagram comparing some features of the parasympathetic and sympathetic divisions of the autonomic nervous system with the somatic motor system. Parasympathetic ganglia are not shown as discrete structures because most of them are diffusely distributed in the walls of the organs innervated. ACh, acetylcholine Epi, epinephrine NE, norepinephrine, D, dopamine N, nicotinic M, muscarinic a, p, alpha and beta adrenoceptors D, dopamine, receptors. (Reproduced, with permission, from Katzung BG [editor] Basic Clinical Pharmacology, 8th ed. McGraw-Hill, 2001.)...

Acetylcholine is the endogenous neurotransmitter of the parasympathetic nervous system. The parasympathetic nerve fibers are found in both the autonomic and central nervous systems. These fibers are classified into those that are stimulated by muscarine and those that are stimulated by nicotine. Nicotine, an alkaloid Uom Nicotiana tabacum, stimulates preganglionic fibers in both the parasympathetic and sympathetic systems as well as the somatic motor fibers of the skeletal system. Muscarine, an alkaloid from the poisonous mushroom Amanita muscaria, stimulates postganglionic parasympathetic fibers with receptors found on autonomic effector... 

Loewi s discovery led to a chain reaction which resulted in the rapid spread of the principle of chemical transmission to all the peripheral synapses in the autonomic nervous system, and to the discovery of chemical transmission from the somatic motor nerve to the skeletal muscles and from one neuron to another, first in the autonomic and then in the central nervous system (studies of Samoilov, the classical experiments of the Dale and Cannon groups, the investigations of Ecdes, and many others). Finally, the chemical theory was confirmed by modem experiments using the latest methods electron microscopy, histochemical methods, the microelectrode technique, and others ... 

Cholinergic neurotransmission ChEs terminate cholinergic transmission in the central nervous system (CNS), in NMJs and in the autonomic system (the parasympathetic system, somatic motor nerves and pre-ganglionic sympathetic nerves). A few sensory cells and the NMJ in nematodes also include ChEs. 

The afferent division carries sensory information toward the CNS and the efferent division carries motor information away from the CNS toward the effector tissues (muscles and glands). The efferent division is further divided into two components (1) the somatic nervous system, which consists of motor neurons that innervate skeletal muscle and (2) the autonomic nervous system that innervates cardiac muscle, smooth muscle, and glands. 

Skeletal muscle is neurogenic and requires stimulation from the somatic nervous system to initiate contraction. Because no electrical communication takes place between these cells, each muscle fiber is innervated by a branch of an alpha motor neuron. Cardiac muscle, however, is myogenic, or self-excitatory this muscle spontaneously depolarizes to threshold and generates action potentials without external stimulation. The region of the heart with the fastest rate of inherent depolarization initiates the heart beat and determines the heart rhythm. In normal hearts, this "pacemaker region is the sinoatrial node. 

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