Control of the external sphincter of the anus in the cat

Abstract

There is of necessity co-ordination between the activity of the caudal region of the large gut and the behaviour of the sphincters which control the anal canal. There are, however, two sphincters. The internal anal sphincter of smooth muscle receives its motor supply through the hypogastric nerves from the sympathetic outflow and its inhibitory supply through the pelvic nerves from the parasympathetic outflow. The external anal sphincter of striped muscle is controlled by the somatic pudendal nerves. In this case relaxation can only be due to a reduction in frequency of existing motor impulses in the pudendal nerves. Since the two sphincters are superimposed, one upon the other, it is difficult to disentangle the roles played by these two in the control of the anal canal. Garry (1933aGarry ( , 1934 gives diagrams of the anatomical arrangement, with the nerve supply, both in the cat and in man. Floyd & Walls (1953) have an accurate drawing of the actual relationships in man. The nerve supply to the distal portion of the colon and to the anal sphincters of the cat is shown schematically in Fig. 1 of this paper. Garry (1933b) was able to show by mechanical recording in the cat that movement of an object within the distal colon, when the pelvic and pudendal nerves were both intact, led to dilatation of the anal canal. Such dilatation could still be evoked after section of both pudendal nerves had paralysed the external anal sphincter. Thus stimulation of the colon must lead to inhibition of the internal anal sphincter by impulses travelling in the pelvic nerves. But there was no actual proof for inhibition of the external anal sphincter when the colon was stimulated. Such inhibition had to be assumed, otherwise dilatation of the anal canal could not have taken place when the pudendal nerves were intact. This belief that the two sphincters relaxed simultaneously was further supported by the observations of Barrington (1921, 1931) on the reciprocal behaviour of the detrusor muscle of the urinary bladder and the striped

Segmental and descending control of the external urethral and anal sphincters in the cat.

“…The existence of monosynaptic reflex connexions of external urethral and external anal sphincter motoneurones suggests that a monosynaptic reflex, similar to that present in other somatic muscles, contributes to the stretch-evoked contraction of the sphincter muscles following their distension (Bishop et al 1956;Garry et al 1959). A possible functional role for such segmental reflex connexions, for instance of the cat external urethral sphincter, could be to aid in preventing the emptying of the bladder.…”

“…Increases in e.m.g. activity and contraction of the sphincter muscles and associated striated perineal muscles can be observed when the sphincters are distended (Evans, 1936; Langworthy, Kolb & Lewis, 1940;Bishop, Garry, Roberts & Todd, 1956;Garry, Roberts & Todd, 1959). The question arises whether there is a monosynaptic contribution to these stretch evoked responses, as is the case for the hind limb muscles (Lloyd, 1943;Eccles, Eccles & Lundberg, 1957a, b).…”

“…The sphincter muscles are striated, red, tonic, slow-twitch type muscle fibres (Evans, 1936;Langworthy et al 1940;Bishop et al 1956;Garry & Garven, 1957).…”

Two groups of spinal interneurones that respond to stimulation of the abdominal viscera of the cat

“…The reflexes that are initiated by visceral distension or contraction and are retained after a low lumbar spinal transaction include the sphincteric reflexes and reflexes affecting the somatic musculature (Bishop, Garry, Roberts & Todd, 1956;Garry, Roberts & Todd, 1959;McPherson, 1966;Evans & McPherson, 1958). The latencies of pelvic-pudendal nerve reflexes are the shortest of all reflexes evoked from pelvic nerve afferents (Bradley & Teague, 1977;Floyd, McMahon & Morrison, 1979), and hypogastric afferents appear not to have any action on pudendal nerve efferents (unpublished observations).…”

Cerebral cortical representation of external anal sphincter contraction: effect of effort

“…This sphincter is comprised of deep, superficial, and subcutaneous striated muscle groups located around the terminus of the gastrointestinal tract (47). Innervation to this sphincter is provided by the somatic fibers of the second, third, and fourth sacral routes through the pudendal nerves (3,27). Considerable information exists about the peripheral and reflex control of the external anal sphincter (6,18,26,35,41).…”