Reciprocal inhibition in the patellar reflex
In the patellar reflex (the knee jerk), an inhibitory interneurone, a short spinal neurone that switches other neurones off, quietens the hamstrings so they do not oppose the quadriceps.
Remember
The sensory fibre excites the quadriceps directly and, through an inhibitory interneurone, inhibits the hamstrings. This is called reciprocal inhibition: the opposing muscle is switched off whenever its partner is switched on.
Tapping the patellar tendon with a hammer stretches the quadriceps, and its muscle spindles (stretch sensors inside the muscle) fire. A sensory fibre carries the signal to the spinal cord and excites the quadriceps motor neurones (the neurones that drive the muscle) directly. This route has only one synapse, so it is monosynaptic, and the quadriceps contract and straighten the leg.
A branch of the same sensory fibre also activates an inhibitory interneurone. This cell inhibits the motor neurones of the hamstrings. Because the inhibitory branch passes through two synapses, it is disynaptic, and a pathway with more than one synapse is called polysynaptic. The effect is reciprocal inhibition.
In the knee jerk, the quadriceps is the agonist, a muscle that moves a joint, and the hamstrings are the antagonist, a muscle that moves it the other way. Without the inhibition, both groups would contract together (co-contraction), and the knee would stiffen rather than extend. The sensory fibre does not directly excite both muscles. It excites the quadriceps pathway and recruits inhibition of the hamstrings.
The loop is completed in the spinal cord, so the reflex works without the brain.
Key facts
- Excitation
- the sensory fibre excites the quadriceps motor neurones through one synapse, so this part is monosynaptic.
- Inhibition
- a branch of the same fibre recruits an inhibitory interneurone that inhibits the hamstring motor neurones, so this part is disynaptic.
- Purpose
- the opposing hamstrings are quietened, so the quadriceps can straighten the knee without a fight.
- Speed and coordination
- the direct route to the quadriceps provides the speed, while the extra pathway provides coordination.
- Level
- the knee jerk mainly tests the L3 to L4 spinal segments (levels of the spinal cord), especially L4.
Exam traps
- The sensory fibre does not directly excite both the quadriceps and the hamstrings, because they would then contract together and the knee could not extend effectively.
- Voluntary control is not the reason for the interneurone: the cortex can modify spinal circuits, but the reflex does not need the cerebral cortex.
- Recurrent inhibition by Renshaw cells (inhibitory interneurones excited by branches of a motor neurone's own axon) is a feedback brake on the same motor neurone, so it does not relax the opposing muscle.
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