Knee Jerk - QS Study
QS Study

Tapping on patellar tendon is semiflexed knee causes contraction of the quadriceps muscle and forward jerking of the leg is called knee-jerk. It is a sudden instinctive reflex kick caused by a blow on the tendon just under the knee. Normally this causes the quadriceps muscle to contract and bring the lower leg forward.

Type: Unconditioned deep reflex, functionally stretch reflex.

Stimuli: Tapping over the patellar tendon of the semiflexed knee.

Response: Forward jerking of the leg.

Receptor: Muscle spindle of quadriceps.

Afferent limb: Femoral nerve.

Center: 1, 2, 3, 4 segments of spinal cord.

Enema lamb: Femoral nerve,

Effectors: Extra fusal muscle fiber of quadriceps.

Pathway of Knee Jerk

  1. Tapping of patellar tendon;
  2. Stretches the quadriceps muscle spindle;
  3. Stimulation of afferent (femoral) nerve;
  4. Impulse goes to the spinal cord (L – 2, 3, 4) via dorsal root ganglia;
  5. Excitation of a motor neuron;
  6. Motor nerve (femoral) to quadriceps muscles;
  7. Contraction of quadriceps and relaxation of flexor muscles;
  8. Causes jerking forward of the leg.

Fig.: Pathway of Knee Jerk

Clinical importance of knee-jerk

  1. Abolished in lower motor neuron lesions;
  2. Exaggerated is an upper motor neuron lesion;
  3. Pendular in certain cerebellar lesions;
  4. Knee clonus present in upper motor neuron lesions.

Tapping on patellar tendon is semiflexed knee causes contraction of the quadriceps muscle and forward jerking of the leg is called knee-jerk. It is a sudden instinctive reflex kick caused by a blow on the tendon just under the knee. Normally this causes the quadriceps muscle to contract and bring the lower leg forward.