why does the knee jerk reflex still work for people who have spinal cord injuries?
The knee jerk reflex, also known as the patellar reflex, is a simple reflex that occurs when the patellar tendon below the knee is tapped. This reflex causes the quadriceps muscle at the front of the thigh to contract, resulting in the leg kicking forward.
In people with spinal cord injuries, the knee jerk reflex can still work because it is a spinal reflex rather than a brain-controlled voluntary movement. The reflex arc responsible for the knee jerk involves a sensory neuron that senses the stretch of the patellar tendon, and a motor neuron that triggers the contraction of the quadriceps muscle.
Normally, the sensory neuron sends a signal to the spinal cord, where it synapses with the motor neuron, which then sends a signal to the muscle. However, in individuals with spinal cord injuries, the communication between the brain and the spinal cord is disrupted. The spinal cord itself contains the circuitry necessary for the reflex, allowing it to still occur even if the brain's control is impaired.
This is why someone with a spinal cord injury can still exhibit the knee jerk reflex. Despite the loss of voluntary motor control from the brain, the basic reflex pathway in the spinal cord remains intact and functional.