The somatic nervous system is responsible for our conscious perception of the environment and for our voluntary responses to that perception by means of skeletal muscles. However, this misses an important point: somatic refers to a functional division, whereas peripheral refers to an anatomic division. The somatic nervous system is traditionally considered a division within the peripheral nervous system. Explain the stimulus-response motor pathway.Describe regions of the central nervous system that contribute to somatic functions.Distinguish between general and special senses.Name the modalities and submodalities of the sensory systems.Describe the components of the somatic nervous system.Sensory neurons are activated by a stimulus, which is sent to the central nervous system, and a motor response is sent out to the skeletal muscles that control this movement.Īfter studying this chapter, you will be able to: However they are discovered, they should permit more selective targeting of the therapeutic end point with fewer unwanted effects.Figure 14.1 Too Hot to Touch When high temperature is sensed in the skin, a reflexive withdrawal is initiated by the muscles of the arm. Such drugs will be found by using, for example, high-throughput screening assays or molecular modeling techniques-or even by serendipity. The SNS and ANS together provide a mechanistic framework for understanding the effects (good and bad) of drugs.Įlucidation of additional roles for neurotransmitters and identification of other receptor subtypes will likely lead to development of more selective drugs. Hence, antagonism of one system results in enhanced activity of the other. Parasympathetic activity is increased by drugs that mimic or enhance the action of ACh. Sympathetic activity is increased by drugs that mimic or enhance the action of norepinephrine. The nerves of the ANS innervate the organs of the body and can be further classified into sympathetic and parasympathetic subdivisions. The cholinergic receptors at these skeletal neuromuscular junctions are sufficiently different structurally (3-dimensional shape) from those at other sites to allow drugs to be designed to bind to only this type (nicotinic) of cholinergic receptor. Because all skeletal neuromuscular junctions contain ACh as the neurotransmitter, ACh and its receptors are targets for drugs intended to modify skeletal muscle function. The nerves of the SNS innervate skeletal muscles, and drugs that act on this system thus affect skeletal muscle function such as tone (eg, muscle relaxants given before surgery). It is convenient for the understanding of drug action to subclassify the peripheral nervous system (PNS) into 2 components: the somatic nervous system (SNS) and the autonomic nervous system (ANS). In most cases, the actual action of the drug-and even much of its unwanted action-is predictable on the basis of the anatomy and physiology of the nervous system. The integrated arrangement of the nervous system and the special distribution of neurotransmitter receptors allow for a targeted drug effect. The major neurotransmitters in the periphery are acetylcholine (ACh) and norepinephrine, and drugs can be designed either to mimic or to inhibit their actions. The fidelity is ensured by neurotransmitter-specific receptors located on the postsynaptic cell membrane.īecause an action potential, or the change in membrane potential occurring in excitable tissue during excitation, relies on a chemical process (ion flux across the membrane) and the transmission across synapses is primarily chemical, exogenously administered chemicals or drugs can modify physiologic processes mediated by the nervous system. Accuracy of signal transmission requires that the postsynaptic cell reliably receive the intended message from the presynaptic cell. The endogenous chemical substances that transmit these signals are termed neurotransmitters. Between neurons or between neurons and target cells are gaps termed synapses across which the signal is transmitted chemically rather than electrically (with some exceptions). Information is transmitted by electrical conduction along axons of neurons to (via afferent nerves) and from (via efferent nerves) the central nervous system (CNS). The nervous system functions as a major communication system within the body.
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