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.