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.