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Related Experiment Videos

Electrically evoked activity in the human external anal sphincter.

A L Wright, N S Williams, J S Gibson

    The British Journal of Surgery
    |January 1, 1985
    PubMed
    Summary

    Short latency electromyographic (EMG) responses are not spinal reflexes but direct nerve activation. Long latency responses, representing the true anal reflex, require a functional spinal cord.

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    Area of Science:

    • Neuroscience
    • Physiology
    • Electromyography

    Background:

    • The anal reflex, measured by electromyographic (EMG) responses to perianal stimulation, is crucial for continence.
    • Delayed EMG responses in fecal incontinence are often attributed to external anal sphincter denervation.

    Purpose of the Study:

    • To differentiate between spinal reflex and direct nerve activation for short latency EMG responses.
    • To clarify the neurophysiological basis of the true anal reflex and its implications for diagnosing pudendal neuropathy.

    Main Methods:

    • Studied electrically evoked EMG responses in normal subjects.
    • Compared responses before and during spinal anesthesia (n=8).
    • Examined responses before and during competitive neuromuscular blockade (n=4).

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    Main Results:

    • Short latency responses remained unchanged during spinal anesthesia, indicating direct alpha-motoneuronal terminal activation.
    • Long latency responses (>40 ms) were abolished by spinal anesthesia, confirming their spinal reflex nature.
    • Both short and long latency responses were abolished by neuromuscular blockade.

    Conclusions:

    • Short latency EMG responses do not represent a spinal reflex but direct activation of alpha-motoneuronal terminal branches.
    • Delayed responses in incontinent patients do not necessarily indicate pudendal neuropathy.
    • Long latency responses are the true anal reflex, requiring a functional sacral spinal cord; their variable latency in healthy individuals limits their diagnostic utility for neuropathy.