Neurology | Spinal Cord: Stretch Reflex | Muscle Spindle

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Ninja Nerds!
Join us for this lecture, where Professor Zach Murphy will discuss the spinal cord stretch reflex and the role of the muscle spindle in detecting changes in muscle length. We begin by breaking down the anatomy of the muscle spindle, including its intrafusal fibers, sensory afferents (Ia and II fibers), and gamma motor neuron innervation. The discussion explains how these specialized sensory receptors detect stretch and send signals to the spinal cord to maintain muscle tone and posture.

We then walk through the physiology of the stretch reflex arc, highlighting the monosynaptic connection between Ia afferents and alpha motor neurons in the ventral horn, which results in rapid contraction of the stretched muscle. The importance of reciprocal inhibition is also emphasized, showing how inhibitory interneurons prevent contraction of the antagonist muscle to allow smooth, coordinated movement.

Finally, we connect the stretch reflex to clinical applications, including how it is tested during neurological exams, what hyperreflexia or hyporeflexia may indicate about upper or lower motor neuron lesions, and how gamma motor neuron activity fine-tunes muscle spindle sensitivity during voluntary movement.

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