Musculus sphincter ani externus
A ring of skeletal muscle surrounding the anus providing voluntary control of defecation. Loss of function is a key indicator of cauda equina or sacral nerve damage.
Origin: Anococcygeal ligament and skin surrounding the anus
Insertion: Circular fibers encircling the anal canal; blends with bulbospongiosus ventrally
Action: Voluntary closure of the anus; maintains fecal continence
Innervation: Caudal rectal nerve (pudendal nerve branch, S3-S5)
Polyneuritis equi is an immune-mediated demyelinating disease of the cauda equina and cranial nerves (especially trigeminal and facial). Granulomatous inflammation of nerve roots. Associated with EHV-
Polyneuritis equi is an immune-mediated demyelinating disease of the cauda equina and cranial nerves (especially trigeminal and facial). Granulomatous inflammation of nerve roots. Associated with EHV-1 and P2 myelin protein. Signs include: Tail paralysis; perineal anesthesia; fecal and urinary incontinence; hindlimb weakness.; Loss of anal tone; fecal incontinence; urinary incontinence; flaccid tail; reduced perineal sensation; often from EHV-1, trauma, or polyneuritis equi.; Progressive tail and anal sphincter paralysis; fecal retention; perineal hypalgesia; rubbing tail base; if cranial nerves involved: facial paralysis, trigeminal dysfunction, dysphagia; Flaccid, dilated anus; fecal staining of the hindquarters; often accompanied by tail paralysis, urinary incontinence, and perineal anesthesia.; Flaccid, hanging tail with no voluntary movement; urine scalding of hindlimbs; fecal retention or incontinence; reduced anal tone; often from trauma, EHV-1, or polyneuritis equi.
See the External Anal Sphincter on our interactive 3D horse model. Open 3D Model