2-Minute Neuroscience: The Knee-Jerk Reflex

Introduction

• Topic: Knee-jerk reflex (also known as the patellar reflex)
• Purpose: To explain the mechanism and significance of the knee-jerk reflex.

What is the Knee-Jerk Reflex?

• A simple reflex arc essential for maintaining posture and balance.
• Commonly tested during a doctor's visit using a small hammer to tap the patellar tendon.
• A normal response is the automatic kicking out of the lower leg.
  • Lack of response or excessive response can indicate a nervous system disorder.

Mechanism of the Reflex

• Location: Occurs at the level of the spinal cord, not involving the brain directly.
  • The brain receives information about the reflex after it occurs.
• Process:
  • The patellar tendon stretch is detected by muscle spindles in the quadriceps muscle.
  • Muscle spindles activate sensory neurons.
  • Sensory neurons travel to the spinal cord and synapse with motor neurons.
  • Motor neurons trigger immediate contraction of the quadriceps muscle, leading to leg movement.

Characteristics of the Reflex

• Type: Monosynaptic reflex (direct connection between sensory and motor neurons).
  • Often simplified in diagrams to one sensory and one motor neuron.
  • In actuality, involves multiple neurons.

Neural Interactions

• Activation:
  • Stretching the quadriceps muscle activates several hundred sensory neurons.
  • Each sensory neuron contacts about 50 motor neurons.
• Inhibition:
  • Sensory neurons stimulate interneurons that inhibit motor neurons of opposing muscles (e.g., hamstrings).
  • Inhibition leads to unopposed action of the quadriceps muscle.