The inhibitory and motor innervation of the anococcygeus muscle

(1) The subject of this thesis was the investigation of the dual innervation of the anococcygeus muscle. The rat anococcygeus had previously been shown in vitro to have a motor adrenergic innervation and also an inhibitory nerve response whose transmitter was unknown. (2) The object of the present study was threefold - (a) To determine whether this inhibitory response was due to a nerve pathway distinct from the motor innervation and with a separate spinal origin and if so whether this pathway had a ganglion synapse and so could be considered as part of the autonomic nervous system. (b) To compare the pharmacological properties of the anococcygeus muscle and vas deferens and to determine whether the rate of depletion of noradrenaline by reserpine in these two tissues was affected by nerve stimulation. (c) To compare the properties of the anococcygeus muscles from the cat and the rat in order to find whether the dual innervation found in the rat was represented in this further species and if so whether this comparison would throw any more light on the nature of the inhibitory response. (3) Using a pithed rat preparation permitting selective stimulation of the autonomic spinal outflows, it was shown that the inhibitory pathway to the anococcygeus muscles arose from the spinal canal, that it was interrupted by a ganglion synapse and that the spinal origin of its preganglionic nerves was L5 - S2 as opposed to T10 - L3 for the preganglionic nerves in the motor pathway. (4) Using this same preparation, the pharmacological properties of the motor nerves to the anococcygeus were examined in situ and compared with those of the vas deferens. This comparison demonstrated that the pharmacological properties of the anococcygeus motor innervation were those of a classical adrenergic innervation whereas the vas deferens showed responses which were in themselves complex and showed unconventional responses to drugs. A hypothesis is suggested to explain this unconventional nature of the vas deferens response. (5) An analysis of the dose dependence and time course of the depletion of tissue noradrenaline by reserpine showed that the rat anococcygeus and vas deferens were depleted to a similar extent and at a similar rate and that this was slower than that found in the heart. Increase in sympathetic nerve activity by spinal stimulation in pithed rats significantly increased the noradrenaline depletion in both anococcygeus and vas deferens. From this it is suggested that nerve impulse traffic may be an important factor in determining the rate of depletion of noradrenaline by reserpine and in the vas deferens may explain the apparent resistance to depletion. (6) The cat anococcygeus muscle was investigated in vitro and shown to possess a dual innervation similar to that in the rat. Due to the presence of intrinsic tone, both motor and inhibitory nerve responses could be demonstrated in the absence of blocking drugs and their interaction studied. The pharmacological properties of the cat anococcygeus were similar to those of the rat except that several substances relaxed the cat muscle which contracted the rat including acetylcholine, isoprenaline, prostaglandins and ATP. These substances were therefore assessed as possible inhibitory transmitters but further analysis with blocking drugs suggested that the relaxations produced by these drugs were different from that produced by the inhibitory nerves. The inhibitory effect of acetylcholine on the cat muscle was particularly interesting since it inhibited motor nerve responses as well as relaxing the muscle. Several substances not normally associated with release of noradrenaline from nerves, including guanethidine, cocaine LSD and 5HT produced indirect sympathomimetic effects in both species. (7) It is concluded that the anococcygeus muscle receives a dual innervation consisting of a motor adrenergic pathway originating from the lower thoracic and upper lumbar cord and a separate inhibitory pathway with its preganglionic fibres originating from the lower lumbar and upper sacral region of the vertebral column. This dual innervation is found in both the rat and cat anococcygeus but in neither species does the inhibitory pathway appear to be adrenergic, cholinergic or purinergic and the transmitter remains unknown.