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This pathway presents simplified anatomy and physiology and illustrates certain major synapses and receptor systems of the efferent peripheral nervous system (i.e., both autonomic and somatic). Prototype drugs which influence these systems are also presented and may also be viewed in the drug list. Some of the prototype drugs are also employed in the virtual lab.
The efferent autonomic nervous system is divided into sympathetic and parasympathetic outflows based on the origins in the CNS and spinal cord. The sympathetic outflow arises in the thorax and lumbar spinal cord, and the parasympathetic outflow arises in both cranial and sacral parts of the CNS/spinal cord.
The outflow of the somatic nervous system is broad. A significant anatomical distinction between somatic an autonomic fibers is that the cell body of the final efferent neuron of the somatic system is located in the ventral horn of the spinal cord (i.e., within the CNS). For autonomic fibers, the final neuronal cell body is located outside the CNS. The neuron from the CNS is a preganglionic neuron; the final neuron innervating the target organ is a postganglionic neuron.
Typically, parasympathetic preganglionic fibers are long and synapse in rather diffuse ganglia usually located in or near the innervated organ. Usually there are a small number of postganglionic fibers innervated by preganglionic fibers. Thus, this is a system set up for discrete regulation of organ function and conservation of resources (feed or breed). Typically, sympathetic preganglionic fibers are short and synapse in paravertebral ganglia. The typical postganglionic sympathetic fiber is long and sends its endings into organs throughout the body from the paravertebral ganglia. Usually there are a large number of postganglionic fibers inner bated by preganglionic fibers. Thus, this is a system set up for amplification of outflow and widespread discharge during times of stress, fear, anger, etc. (fight or flight).
The adrenal medulla is an important and unique part of the SNS. It is useful to think of the adrenal medulla as a population of postganglionic sympathetic fibers that did not sent axons to any particular organ. Instead, they are set up to release hormones rather than neurohormones or neurotransmitters.
With the exception of the postganglionic sympathetic fibers (and the adrenal medulla), acetylcholine (ACh) is at autonomic ganglia and neuroeffector junctions, as well as the somatic neuromuscular junction (NMJ).
Norepinephrine (NE), one of a family of compounds called catecholamines, is the neurotransmitter at the neuroeffector junctions of postganglionic sympathetic fibers (with some exceptions we will note). One exception is the adrenal medulla. While they do not directly synapse with cells of effector organs, the cells of the adrenal medulla function very much as postganglionic sympathetic cells. That is, they release preformed catecholamines from intraneuronal vesicles by exocytosis when activated by the neurotransmitter of the preganglionic sympathetic fiber (ACh). However, the catecholamines released from the adrenal medulla enter the blood stream and so act as hormones rather than neurotransmitters. The catecholamines released from the adrenal medulla consist of a mixture of about 80% epinephrine (EPI) and 20% NE.
Another exception to the general rule that postganglionic sympathetic fibers release NE is the fibers that innervate sweat glands. Although they are anatomically sympathetic (the preganglionic fibers emerge from the thoracolumbar region of the spinal cord), the fibers are functionally cholinergic (that is, they release ACh at the neuroeffector junction of sweat glands). Because different drugs affect cholinergic and adrenergic transmission, the effects of drugs on sweat glands do not always match effects on other sympathetically innervated organs.
Familiarity with this diagram and the transmitters, responses, receptors, agonists and antagonists at each different site will be useful in your quest.
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