Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less.
In this installment I will discuss the knee-jerk reflex.
The knee-jerk reflex, also known as the patellar reflex, is a well-known example of a simple
reflex arc that allows us to maintain posture and balance.
Most of us have had our knee-jerk reflex tested at a doctor’s visit when the doctor taps
the tendon just below your kneecap, which is the patellar tendon, with a small hammer.
This causes your lower leg to automatically kick outward.
If this doesn’t happen, or if it happens excessively, it can be an indication of a
disorder or of damage to the nervous system.
The knee-jerk reflex is a simple reflex arc that occurs at the level of the spinal cord;
in other words, the associated movement occurs without the involvement of the brain; the
brain receives information about the movement after it has been initiated.
When the patellar tendon of the quadriceps muscle is stretched, the stretch is detected
by stretch receptors known as muscle spindles that are found in the quadriceps muscle.The
muscle spindles stimulate sensory neurons that travel to the spinal cord, where they
synapse with motor neurons that control the contraction of the quadriceps muscle.
These motor neurons cause immediate contraction of the quadriceps muscle to produce movement
of the leg.
The knee-jerk reflex is considered a monosynaptic reflex because it involves direct connections
between sensory neurons and motor neurons, without any neurons in between.
Although the reflex is often simplified in diagrams to show only one sensory and one
motor neuron, in reality the reflex involves many neurons; stretching the quadriceps muscle
activates several hundred sensory neurons, each of which makes contact with around 50
Additionally, the sensory neurons traveling from the muscle spindle stimulate interneurons
that inhibit the activity of motor neurons that supply opposing muscles like the hamstring
The stimulation of these inhibitory interneurons causes the action of the quadriceps muscle
to be unopposed.