Anatomy
- Motor activity of the colon must ultimately propel its contents in the anal direction, and dehydrate contents
- High amplitude propagated contractions;
- Low amplitude propagated contractions and changes in tone all achieve this
- Colonic motor patterns are primarily activated by local reflexes and set independently by the Enteric Nervous system;
- Independent motor activity is organised by the myenteric plexus between circular and longitudinal bowel muscle
- Interplay between the ENS, autonomic and central nervous system is required
Extrinsic neural control of colonic motility
- Afferent
- Sensory nerves convey information from the bowel to the spine and higher centres
- Activation of spine reflexes like the recto colonic reflex (rectal distension and increase in proximal colonic motility and is central for urge)
- Efferent
- Sympathetic innervation
- T11–L2
- Stops defecation
- Rectal and anal canal relaxation
- Internal sphincter contraction
- Somatic control
- Pudendal nerve
- External anal sphincter consists of striated muscle and is therefore under voluntary control.
- Parasympathetic innervation
- With rectal distension of faeces there is a cortical awareness of the need to defecate, causing a reduction in sympathetic tone, relaxation of the internal anal sphincter and pelvic floor.
- This is followed by the voluntary opening of the external sphincter and subsequent parasympathetic activity resulting in peristaltic contraction of the rectum and evacuation of faeces
Evacuation of the bowels
- Arrival of faeces → stretch within the wall of the rectum → activating mechanoreceptors → Via S2-4 →
- Brainstem → thalamus → insula → knowledge of need to defecate
- Recto colonic reflex →
- Reduction in sympathetic tone
- Relaxation of the internal anal sphincter and pelvic floor.
Dysfunction
- Supra-pontine
- Many cortical and subcortical diseases impair the normal inhibition of the IAS and EAS that is needed for voluntary defecation
- The paracentral lobule in NPH is damaged this causes loss of inhibition of bladder and bowel voiding
- Spinal Cord
- Loss of connection between higher centres and sacral cord → inability of the IAS (internal anal sphincter ) to relax
- Loss of stretch sensation → faeces overload
- Loss of interplay between ENS and CNS, reduced colonic motility
- Overflow incontinence
- Sub-sacral
- Lesions of the cauda equina often cause a loss of EAS control and loss of rectal sensation (via pudenal nerve)
- This results in overflow with EAS weakness resulting in incontinence