why does the knee jerk reflex work for someone with spinal cord injuries?

The patellar reflex or knee-jerk (American spelling knee reflex) is a stretch reflex which tests the L2, L3, and L4 segments of the spinal cord.

As long as the injury is above those segments, the knee jerk will be evident.

The knee jerk reflex, also known as the patellar reflex, is a reflex action that occurs when the patellar tendon is tapped, resulting in the contraction of the quadriceps muscles in the thigh and extension of the leg. This reflex is controlled by a simple neural circuit called the monosynaptic reflex arc, which involves the sensory neuron, the motor neuron, and a single synapse between them.

In individuals with spinal cord injuries, the communication between the brain and the spinal cord may be disrupted or impaired. However, the knee jerk reflex can still work because it is a monosynaptic reflex arc that does not require input from the brain. The sensory neuron responsible for this reflex is directly connected to the motor neuron in the spinal cord, bypassing the higher brain centers.

Here's a step-by-step breakdown of how the knee jerk reflex works:

1. When the patellar tendon is tapped, it stretches the muscle spindles in the quadriceps muscle.
2. The stretch of these muscle spindles activates specialized sensory neurons called proprioceptors, which detect changes in muscle length and tension.
3. The proprioceptors transmit sensory information to the spinal cord through the sensory neuron.
4. In the spinal cord, the sensory neuron makes a direct synapse with the motor neuron responsible for the quadriceps muscle.
5. The sensory neuron's signal triggers the motor neuron, which in turn sends a message to the quadriceps muscle to contract.
6. This rapid contraction of the quadriceps muscle causes the leg to kick out.

Since the knee jerk reflex bypasses the brain, it can still occur in individuals with spinal cord injuries as long as the neural connections involved in the reflex arc remain intact. However, it's important to note that the knee jerk reflex is just one of many reflexes and functions that may or may not be affected by a spinal cord injury.