Parasympathetic nerve fibres

The concept of chemical transmission in the nervous system arose in the early years of the century when it was discovered that the functioning of the autonomic nervous system was largely dependent on the secretion of acetylcholine and noradrenaline from the parasympathetic and sympathetic nerves respectively. The physiologist Sherrington proposed that nerve cells communicated with one another, and with any other type of adjacent cell, by liberating the neurotransmitter into the space, or synapse, in the immediate vicinity of the nerve ending. He believed that transmission across the synaptic cleft was unidirectional and, unlike conduction down the nerve fibre, was delayed by some milliseconds because of the time it took the transmitter to diffuse across the synapse and activate a specific neurotransmitter receptor on the cell membrane. 

The detrusor, whose smooth muscle fibres comprise the body of the bladder, is innervated mainly by parasympathetic nerves which are excitatory and cause the muscle to contract. The internal sphincter, a concentration of smooth muscle at the bladder neck, is well developed only in the male and its principal hmction is to prevent retrograde flow of semen during ejaculation. It is rich in aj-adrenoceptors, activation of which causes contraction. There is an abvmdant supply of oestrogen receptors in the distal two-thirds of the female urethral epithelium which degenerates after the menopause causing loss of urinary control. 

The naturally mediated inhibition of insulin secretion in response to hypoglycaemia is probably elicited after activation of cerebral glucosecep-tors (Rohner-Jeanrenaud et al., 1983) and through central noradrenergic pathways (McCaleb and Myers, 1982). The nerve fibres that enter the pancreas are mixed nerves (i.e. sympathetic and parasympathetic) derived from the splanchnic area and the N. vagus. These nerves generally accompany the superior pancreatic-duodenal vessels. Autonomic nerves have their terminals in close proximity to A- and B-cells (Ahren et al., 1986). 

Evidence that acetylcholine is a chemical mediator of synaptic transmission elsewhere in the central nervous system is circumstantial but convincing. Acetylcholine is the transmitter substance from motor nerves, preganglionic autonomic fibres and the postganglionic fibres of the parasympathetic system to their respective effector cells. In all these fibres, the activity of choline acetyltransferase (choline acetylase), the enzyme responsible for the synthesis of acetylcholine, is high. Conversely, its activity is negligibly low in sensory nerve fibres and transmission from these fibres is demonstrably non-cholinergic in nature . It is reasonable, therefore, to... 

The parasympathetic nervous system. Here the postganglionic fibres are always short. The preganglionic fibres are long and pus almost up to the muscle or gland to be innervated. The preganglionic fibres arise from the brain stem (cranial nerves VII, IX and X), from the tectal region (cranial nerve III which supplies the eye, p. 38), and from the sacral region. 

Similar considerations would apply at the synapse between the preganglionic fibre and the nerve cell of the postganglionic fibre of the sympathetic and parasympathetic systems and curare is the blocking agent (fig. 7 A). 

The pupil is supplied with constrictor fibres from the parasympathetic system (via the Illrd or oculomotor nerve and the ciliary ganglion) and with dilator fibres from the sympathetic system (the upper thoracic nerves to the sympathetic and to the inferior and superior cervical ganglia fig. 8). 

Somatic nerves originate in the CNS and terminate at the neuromuscular junction where acetylcholine is the transmitter. Nerves of the autonomic system also use acetylcholine as the neurotransmitter at the end of the preganglionic fibres within the ganglia. With few exceptions, the postganglionic sympathetic fibres secrete noradrenaline (norepinephrine) whilst postganglionic parasympathetic fibres secrete acetylcholine. 

Fig. 7.3 summarizes the anatomical features of the autonomic nervous system. This differs from the voluntary system (exemplified by the neuro-niuscular junction in Fig. 7.1) in that one extra control point exists for each fibre, namely an extra synapse (situated in a ganglion). In sympathetic fibres, these ganglia are not far from the spinal cord in parasympathetic fibres, the ganglia are near the end-organ, i.e. the one which the nerve is to influence. As... 

Acetylcholine is the natural transmitter at several different kinds of synaptic site, e.g. (a) the somatic nerve voluntary muscle junctions, (b) the ganglionic synapses which are nerve — nerve junctions in the autonomic system, (c) such postganglionic nerve-endings as are parasympathetic. In addition it has some transmitting duties in the central nervous system, e.g. between spinal cord root fibres and Renshaw cells. 

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