The Knee jerk Reflex

The Knee jerk Reflex

Spinal cord reflexes represent the most basic of motor responses. These reflexes are carried out entirely within the spinal cord and are modified by inputs from higher brain centers to generate complex movements.

Why does your leg kick out when the doctor taps just below your knee? Doctors often test the knee jerk, or patellar reflex, to look for potential neurological problems. Lightly tapping your patellar tendon just below the kneecap stretches the femoral nerve located in your thigh, which in turn causes your thigh muscle (quadriceps) to contract and the lower leg to extend. When struck, impulses travel along a pathway in the dorsal root ganglion, a bundle of nerves in the L4 level of the spinal cord. Reflex actions are performed independently of the brain. This allows them to happen almost instantaneously in about 50 milliseconds in  the case of the knee-jerk reflex. This reflex helps you to maintain balance and posture when you walk, without having to think about every step you take.

The knee-jerk reflex is a spinal reflex activated by tapping the patellar tendon below the knee. This tendon then stretches the muscle spindles, generating sensory impulse to the spinal cord. Alpha motor neurons in the spinal cord cause a brief, rapid contraction of the quadriceps femoris, which causes the leg to extend.

Tapping the Achilles tendon behind the ankle and just above the heel activates plantar flexion of the foot. This reflex response is like walking tip-toed or standing on your toes.