2-Minute Neuroscience: Knee-jerk Reflex Video Lecture | A-Z Guide to Neuroscience - Personal Learning

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
motor neurons.
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
muscle.
The stimulation of these inhibitory interneurons
causes the action of the quadriceps muscle
to be unopposed.