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Understanding the Knee-Jerk Reflex
Sep 23, 2024
2-Minute Neuroscience: The Knee-Jerk Reflex
Introduction
The knee-jerk reflex, also known as the patellar reflex, is a simple reflex arc.
It helps maintain posture and balance.
Often tested in medical exams by tapping the patellar tendon.
Abnormal reflexes may indicate nervous system issues.
Mechanism of the Knee-Jerk Reflex
Reflex Arc:
Occurs at the spinal cord level, not involving the brain directly.
Brain receives information post-movement initiation.
Process:
Stretching of the patellar tendon activates muscle spindles in the quadriceps.
Muscle spindles stimulate sensory neurons.
Sensory neurons travel to the spinal cord and synapse with motor neurons.
Motor neurons cause quadriceps contraction, resulting in leg movement.
Characteristics
Monosynaptic Reflex:
Direct synapse between sensory and motor neurons without intermediaries.
Involves numerous neurons despite simplified diagrams.
Hundreds of sensory neurons activate ~50 motor neurons each.
Inhibitory Interneurons:
Sensory neurons stimulate interneurons that inhibit opposing muscles (e.g., hamstring).
Ensures unopposed action of the quadriceps muscle.
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