Somatic motor system

Pharmacologically, carbofuran inhibits cholinesterase, resulting in stimulation of the central, parasympathetic, and somatic motor systems. Sensitive biochemical tests have been developed to measure cholinesterase inhibition in avian and mammalian brain and plasma samples and are useful in the forensic assessment of carbamate exposure in human and wildlife pesticide incidents (Bal-lantyne and Marrs Hunt and Hooper 1993). Acute toxic clinical effects resulting from carbofuran exposure in animals and humans appear to be completely reversible and have been successfully treated with atropine sulfate. However, treatment should occur as soon as possible after exposure because acute carbofuran toxicosis can be fatal younger age groups of various species are more susceptible than adults (Finlayson et al. 1979). Carbofuran labels indicate that application is forbidden to streams, lakes, or ponds. In addition, manufacturers have stated that carbofuran is poisonous if swallowed, inhaled, or absorbed through the skin. Users are cautioned not to breathe carbofuran dust, fumes, or spray mist and treated areas should be avoided for at least 2 days (Anonymous 1971). Three points are emphasized at this juncture. First, some carbofuran degradation... 

In addition to the somatic motor system that operates the voluntary (striated) muscles via the pyramidal tract, there is the autonomic system, which controls the involuntary (smooth) muscles, glands, heartbeat, blood pressure, and body temperature. This system has its origins in both the cerebral cortex and... 

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.)...

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. 

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. 

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. 

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. 

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. 

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. 

Pharmacology Cyclobenzaprine, structurally related to the tricyclic antidepressants (TCAs), relieves skeletal muscle spasm of local origin without interfering with muscle function. It is ineffective in muscle spasm caused by CNS disease. The net effect is a reduction of tonic somatic motor activity, influencing both gamma and alpha motor systems. 

The nervous system is divided into two parts the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists of all afferent (sensory) neurons, which carry nerve impulses into the CNS from sensory end organs in peripheral tissues, and all efferent (motor) neurons, which carry nerve impulses from the CNS to effector cells in peripheral tissues. The peripheral efferent system is further divided into the somatic nervous system and the autonomic nervous system. The effector cells innervated by the somatic nervous system are skeletal muscle cells. The autonomic nervous system innervates three types of effector cells (1) smooth muscle, (2) cardiac muscle, and (3) exocrine glands. While the somatic nervous system can function on a reflex basis, voluntary control of skeletal muscle is of primary importance. In contrast, in the autonomic nervous system voluntary control can be exerted, but reflex control is paramount. 

Anatomical differences between the peripheral somatic and autonomic nervous systems have led to their classification as separate divisions of the nervous system. These differences are shown in Figure 9.1. The axon of a somatic motor neuron leaves the CNS and travels without interruption to the innervated effector cell. In contrast, two neurons are required to connect the CNS and a visceral effector cell of the autonomic nervous system. The first neuron in this sequence is called the preganglionic neuron. The second neuron, whose cell body is within the ganglion, travels to the visceral effector cell it is called the postganglionic neuron. 

Lateral efferents of the MPOA innervate septum, amygdala, other hypothalamic regions, ventral tegmental area (VTA) of the midbrain, and midbrain central gray matter (Numan, 1986). Studies have shown that the MPOA-VTA-basal ganglia (BG) circuit is believed to be of importance in maternal behavior because the BG is a major component of the extrapyramidal system (EPS) (Numan, 1986). Perhaps MPOA neurons relevant to maternal behavior via the BG to EPS promote the somatic-motor processes underlying maternal responsiveness (Numan, 1986). 

Nicotinic receptors are part of a transmembrane polypeptide whose subunits form cation-selective ion channels (see Figure 2-9). These receptors are located on plasma membranes of postganglionic cells in all autonomic ganglia, of muscles innervated by somatic motor fibers, and of some central nervous system neurons (see Figure 6-1). 

The somatic nervous system is composed of sensory afferents and motor efferents and serves to perceive external states and to modulate appropriate body responses. The autonomic nervous system (ANS), together with the endocrine system, controls the milieu interieur. It adjusts internal organ functions to the changing needs of the organism. The ANS operates largely autonomously, beyond voluntary control, at the subconscious level. Its central components reside in the hypothalamus, brain stem, and spinal cord. The ANS has sympathetic and parasympathetic branches. Both are made up of afferent, mainly in the vagus nerve, and efferent fibers. 

The efferent somatic nervous system differs from the autonomic system in that a single myelinated motor neuron, originating in the CNS, travels directly to skeletal muscle without the mediation of ganglia. As noted earlier, the somatic nervous system is under voluntary control, whereas the autonomic is an involuntary system. 

Skeletal muscle twitching is due to effects at the skeletal neuromuscular junction, which is innervated by the somatic nervous system, via motor nerves. The anticholinesterase prolongs and intensifies the actions of released acetylcholine at the junction, causing fasciculation (strong, jerky contractions) of skeletal muscle. Normally at the skeletal neuromuscular junction, the released acetylcholine is rapidly hydrolysed by cholinesterases to choline and acetate. This allows repolarization of the muscle membrane to occur following initial stimulation. In the presence of anticholinesterases the acetylcholine remains at the junction for a very prolonged period and produces repeated twitching of the muscle fibres via nicotinic receptors. 

The somatic nervous system controls skeletal muscle movement through motor neurons. Alpha motor neurons extend from the spinal cord and terminate on indi-... 

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... 

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