Visceral afferents

Although not strictly part of the autonomic nervous system, the axons of sensory neurones often run with the autonomic nerves. Their cell bodies are in the dorsal root ganglia, and they carry information from sense organs in

Fig. 2.20 A comparison of the structures of adrenal medullary enterochromaffin cells, SIF cells and sympathetic neurones. (A) Adrenal medullary chromaffin cell. Note afferent synapse (a), many dense cored vesicles and close connection to fenestrated capillary (f ). Transmitter is released into the blood stream. Adjacent chromaffin cells (c) are held together through attachment plaques (p). (B) SIF cell (small granule containing cell).

SP synapse

Colon

Fig. 2.21 Organisation of substance P (SP) containing visceral afferent from the colon. The afferent fibre is a bipolar neurone with its cell body in the dorsal root ganglion (DRG). The sensory nerve terminals are in the wall of the colon, and before leaving the gut wall, send collaterals to blood vessels. The axon leaves the gut and enters the inferior mesenteric ganglion (IMG) via a colonic nerve fascicle (CN). Again it gives off collaterals which innervate cells in the IMG. The main axon then runs in the lumbar splanchnic nerve (LSN) to the lumbar sympathetic ganglion (LSG) of the paravertebral chain and so to the dorsal root ganglion. The central branch enters the dorsal horn of the spinal cord (DH) and synapses with cells in the substantia gelatinosa (sg). (imn) intermesenteric nerve. (HNN) hypogastric nerves. (VH) ventral horn. Adapted from Matthews & Cuello (1982).

SP synapse

Colon

Fig. 2.21 Organisation of substance P (SP) containing visceral afferent from the colon. The afferent fibre is a bipolar neurone with its cell body in the dorsal root ganglion (DRG). The sensory nerve terminals are in the wall of the colon, and before leaving the gut wall, send collaterals to blood vessels. The axon leaves the gut and enters the inferior mesenteric ganglion (IMG) via a colonic nerve fascicle (CN). Again it gives off collaterals which innervate cells in the IMG. The main axon then runs in the lumbar splanchnic nerve (LSN) to the lumbar sympathetic ganglion (LSG) of the paravertebral chain and so to the dorsal root ganglion. The central branch enters the dorsal horn of the spinal cord (DH) and synapses with cells in the substantia gelatinosa (sg). (imn) intermesenteric nerve. (HNN) hypogastric nerves. (VH) ventral horn. Adapted from Matthews & Cuello (1982).

Fig. 2.20 Continued

This has properties intermediate between the other two types, and releases transmitter both to the blood stream and through efferent synapses (e) with local neurones. (C) Principal sympathetic neurone. This cell has afferent synapses on dendrites (d) and dendritic spines, and releases transmitter from varicose nerve endings (y). From Matthews & Raisman (1969).

Fig. 2.22 Drawing of substance P containing varicose nerve terminals innervating ganglion cells in the inferior mesenteric ganglion of the rat. Adapted from Matthews & Cuello (1982).

Fig. 2.22 Drawing of substance P containing varicose nerve terminals innervating ganglion cells in the inferior mesenteric ganglion of the rat. Adapted from Matthews & Cuello (1982).

the viscera and can trigger both somatic and visceral reflexes. The sensory fibres are normally small myelinated fibres (classed as AS fibres), or are unmyelinated (C fibres).

A subpopulation of the C fibres containing peptides, such as substance P and calcitonin gene-related peptide (CGRP), are now thought to have a dual sensory and motor function. Activation of these nerves results not only in propagation of action potentials to the central nervous system and nociceptive responses, but also action potential invasion of collaterals, which may release peptides that have a motor or secretory function in the viscera. Branches of these fibres may also have actions on neurones in the prevertebral and paravertebral ganglia (see Figs. 2.21 & 2.22). Capsaicin, the 'hot' component of peppers in the capsicum family, can interact with nerve terminals in this group, and activate them. The burning sensation that results from eating peppers is due to non-selective activation of these nociceptive fibres. At high doses the drug can kill the nerve cells, and if capsaicin is applied systemically (under anaesthesia) to neonatal rats, they grow up into adults that do not respond to painful stimuli. Capsaicin application has been used to study the role of these sensorimotor fibres in autonomic reflexes.

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