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Understanding the Knee-Jerk Reflex Mechanism
Oct 14, 2024
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.
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