Postganglionic fibers

Synapses between the autonomic postganglionic neuron and effector tissue — the neuroeffector junction — differ greatly from the neuron-to-neuron synapses discussed previously in Chapter 5 (see Table 9.1). The postganglionic fibers in the ANS do not terminate in a single swelling like the synaptic knob, nor do they synapse directly with the cells of a tissue. Instead, the axon terminals branch and contain multiple swellings called varicosities that lie across the surface of the tissue. When the neuron is stimulated, these varicosities release neurotransmitter over a large surface area of the effector tissue. This diffuse release of the neurotransmitter affects many tissue cells simultaneously. Furthermore, cardiac muscle and most smooth muscle have gap junctions between cells. These specialized intercellular communications... 

Long cholinergic preganglionic fibers short cholinergic postganglionic fibers... 

Because the terminal ganglia are located within the innervated tissue, there is typically little divergence in the parasympathetic system compared to the sympathetic system. In many organs, the ratio of preganglionic fibers to postganglionic fibers is 1 1. Therefore, the effects of the parasympathetic system tend to be more discrete and localized, with only specific tissues stimulated at any given moment, compared to the sympathetic system in which a more diffuse discharge is possible. 

The two most common neurotransmitters released by neurons of the ANS are acetylcholine (Ach) and norepinephrine (NE). Several distinguishing features of these neurotransmitters are summarized in Table 9.3. Nerve fibers that release acetylcholine are referred to as cholinergic fibers and include all preganglionic fibers of the ANS — sympathetic and parasympathetic systems all postganglionic fibers of the parasympathetic system and sympathetic postganglionic... 

Since autonomic postganglionic fibers lack a myelin sheath, they are particularly susceptible to blockade by local anesthetics. As a result, vasodilation ensues in the anesthetized region, because sympathetically driven vasomotor tone decreases. This local vasodilation is undesirable (see below). 

Sympathetic arc involved in blood pressure regulation and sites where drugs may act to influence the system. A. Receptors on effector cell. 6. Adrenergic varicosity. C. Nicotinic receptors (postganglionic fibers). D. Brainstem nuclei. NTS, nucleus of the tractus solitarii VMC, vasomotor center ACh, acetylcholine NE, norepinephrine a, a-adrenoceptors (3, 13-adrenoceptors P2, P2-purinoceptors ATR adenosine triphosphate. 

Suspension. A preparation of a finely divided drug intended to be incorporated in some suitable liquid vehicle before it is used, or already incorporated in such a vehicle. Sympathomimetic. Mimicking the effects of impulses conveyed by adrenergic postganglionic fibers of the sympathetic nervous system. 

Schematic diagram comparing some anatomic and neurotransmitter features of autonomic and somatic motor nerves. Only the primary transmitter substances are shown. Parasympathetic ganglia are not shown because most are in or near the wall of the organ innervated. Cholinergic nerves are shown in blue noradrenergic in red and dopaminergic in green. Note that some sympathetic postganglionic fibers release acetylcholine or dopamine rather than norepinephrine. The adrenal medulla, a modified sympathetic ganglion, receives sympathetic preganglionic fibers and releases epinephrine and norepinephrine into the blood. ACh, acetylcholine D, dopamine Epi, epinephrine M, muscarinic receptors N, nicotinic receptors NE, norepinephrine.

A patient with a preganglionic lesion, on the other hand, shows a normal response to both drugs, since the postganglionic fibers and their catecholamine stores remain intact in this situation. 

As in the sympathetic division, parasympathetic preganglionic neurons synapse in the periphery with a postganglionic fiber. This synapse usually takes place in a terminal ganglion that is located directly in the organ or tissue supplied by the postganglionic neuron. Consequently, the parasympathetic ganglia are usually embedded directly in the innervated organ or tissue. 

As indicated in Figure 18-1, the transmitter at the preganglionic-postganglionic synapse in both divisions is acetylcholine, as is the transmitter at the parasympathetic postganglionic-effector cell synapse. The transmitter at the sympathetic postganglionic-effector cell synapse is usually norepinephrine. A small number of sympathetic postganglionic fibers, however, also use acetylcholine as their neurotransmitter. 

Target tissues receive two (sympathetic and parasympathetic) efferent neurons The neurons between the CNS and the organ are myelinated preganglionic fibers and unmyelinated postganglionic fibers... 

If the lesion of Homer s syndrome is postganglionic, indirectly acting sympathomimetics (eg, cocaine, hydroxyamphetamine) will not dilate the abnormally constricted pupil—because catecholamines have been lost from the nerve endings in the iris. In contrast, the pupil will dilate in response to phenylephrine, which acts directly on the receptors on the smooth muscle of the iris. A patient with a preganglionic lesion, on the other hand, will show a normal response to both drugs, since the postganglionic fibers and their catecholamine stores remain intact in this situation. 

The most well known of the naturally occurring phenethylamine derivatives (Table I) are the transmitters of the sympathetic nervous system, epinephrine, norepinephrine, and dopamine. All these compounds are 3,4-dioxygenated in the aromatic nucleus and are collectively known as the catecholamines. Norepinephrine is the transmitter of most sympathetic postganglionic fibers, dopamine is the predominant transmitter of the mammalian extrapyramidal system and of several mesocortical and mesolimbic neuronal pathways, and epinephrine is the major hormone of the adrenal medulla (363). The literature that has accumulated on the action of these compounds in higher animals is enormous. Metanephrine and normetanephrine are known from animals as deactivated metabolites of epinephrine and norepinephrine that result from the action of the enzyme catechol O-methyltransferase (364). 

Parasympathetic neurons The parasympathetic preganglionic fibers arise from the cranial and sacral areas of the spinal cord and synapse in ganglia near or on the effector organs. In both the sympathetic and parasympathetic systems, postganglionic fibers extend from the ganglia to effector organs. 

Summary of the neurotransmitters released and the types of receptors found within the autonomic and somatic nervous systems. [Note This schematic diagram does not show that the parasympathetic ganglia are close to or on the surface of the effector organs and that the postganglionic fibers are usually shorter than the preganglionic fibers.]... 

D. The postganglionic fibers of the parasympathetic division are long, compared to those of the sympathetic nervous system. 

Correct answer = C. The parasympathetic system maintains essential bodily functions, such as vision, movement of food, and urination. It uses acetylcholine, not norepinephrine, as a neurotransmitter, and discharges as discrete fibers that are activated separately. The postganglionic fibers of the parasympathetic system are short compared to the sympathetic division. The adrenal medulla is under control of the sympathetic system. 

The preganglionic fibers terminating in the adrenal medulla, the autonomic ganglia (both parasympathetic and sympathetic), and the postganglionic fibers of the parasympathetic division use acetylcholine as a neurotransmitter (Figure 4.2). Cholinergic neurons innervate voluntary muscles of the somatic system and are also found in the CNS. 

Binding to the receptor leads to a biological response within the cell such as the initiation of a nerve impulse in a postganglionic fiber or activation of specific enzymes in effector cells as mediated by second messenger molecules (see p. 33 and below). 

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