Skip to content
Robotic Spine Universe

Pathways

Stretch reflex arc

Keeping a muscle at its length (the knee jerk)

Stretching a muscle activates its spindle receptor; the receptor’s axon enters the cord and makes the same muscle contract directly, while a collateral inhibits the motor neuron of the opposing muscle. The reflex helps keep muscles at a constant length; the knee jerk is the usual example.

Drawn for the left side of the body. Schematic route through the model’s landmarks; the pulse speed is illustrative.

Route

served sideother sideCortexDeep brainMidbrainPonsMedullaSpinal cordBody12345

Step by step

  1. Muscle spindle (stretch receptor) · served side
  2. Dorsal root into the cord · served side
  3. Motor neuron of the same muscle · served side · synapse
  4. Collateral inhibits the antagonist’s motor neuron · served side · synapse
  5. The same muscle contracts · served side

Schematic Filled circles: synapses. X: the pathway crosses the midline. Numbers match the list.

What the sources say

“In this reflex, when a skeletal muscle is stretched, a muscle spindle receptor is activated.” Betts et al. 2022, §14.3
“The axon from this receptor structure will cause direct contraction of the muscle.” Betts et al. 2022, §14.3
“A collateral of the muscle spindle fiber will also inhibit the motor neuron of the antagonist muscles.” Betts et al. 2022, §14.3
“The reflex helps to maintain muscles at a constant length.” Betts et al. 2022, §14.3
“A common example of this reflex is the knee jerk that is elicited by a rubber hammer struck against the patellar ligament in a physical exam.” Betts et al. 2022, §14.3

Sources: Betts et al. 2022, §14.3