Chapter 11: Reflexes - Knee Jerk Reflex - Video

Overview

This lecture reviews the knee-jerk (patellar) reflex as an example of a simple reflex arc that helps maintain posture and balance.

The knee-jerk reflex (patellar reflex) is tested by tapping the patellar tendon below the kneecap.
Tapping the tendon stretches the quadriceps muscle, triggering stretch receptors called muscle spindles.
Muscle spindles activate sensory neurons that carry signals to the spinal cord.
Sensory neurons synapse directly with motor neurons in the spinal cord.
Motor neurons immediately contract the quadriceps muscle, causing the lower leg to kick outward.
The reflex occurs at the spinal cord level, without initial brain involvement.
The brain is informed of the movement only after it is initiated.
Abnormal responses can indicate nervous system disorders or damage.

The knee-jerk reflex is monosynaptic, involving direct synapse between sensory and motor neurons.
In reality, hundreds of sensory neurons and about 50 motor neurons are involved in the reflex.
Sensory neurons also activate inhibitory interneurons that prevent contraction of opposing muscles, like the hamstring.
This inhibition ensures that the quadriceps muscle contraction is unopposed.

Knee-jerk reflex (Patellar reflex) — automatic extension of the lower leg in response to tapping the patellar tendon.
Patellar tendon — tendon just below the kneecap that is tapped during the reflex test.
Muscle spindle — stretch receptor in the muscle that detects changes in muscle length.
Sensory neuron — neuron that transmits sensory information from muscle spindles to the spinal cord.
Motor neuron — neuron that sends signals from the spinal cord to muscles to produce movement.
Monosynaptic reflex — reflex involving a single synapse between sensory and motor neurons.
Interneuron — neuron that connects other neurons; here, it inhibits motor neurons to opposing muscles.