Human anal reflexes.

By perianal electrical stimulation and EMG recording from the external anal sphincter the anal reflex was constantly present in normal subjects. The latency decreased within certain limits with increasing stimulation to an average minimum latency of 50 ms (SD 10.5). There was no difference between the minimum latency in normal subjects and patients with suprasegmental lesions of the CNS. The latency may be prolonged in patients with lesion of the reflex arc. By stimulation over the posterior tibial nerve behind the medial malleolus a reflex reaction could be picked up constantly from the anal sphincter in normal subjects. This reflex had a longer latency but a lower threshold than the reflex reaction from the tibialis anterior muscle. The average minimum latency from the anal sphincter was 93 ms (SD 21.1) and from the tibialis anterior muscle 64 ms (SD 7.9). In the absence of the anal reflex it may be possible to localise the defect to the afferent or efferent parts of the reflex by using types of stimulation. Preliminary studies of spinal shock revealed a perianally elicited anal reflex in all cases, but also a response to peripheral stimulation in some of the cases, more frequently found in the anal sphincter than in the tibialis anterior muscle.