VPB 121: Veterinary Physiology - II (2+1)

KNEE-JERK REFLEX

• Strech of quadriceps muscle results when patellar tendon is struck with a blunt object. (the muscle spindle is stretched).
• Action potential from muscle spindle go to lumbar spinal cord and causes excitation of alpha motor neurons of quadriceps which causes contraction of the muscle and extension of knee joint.(muscle stretch reflex).
• Action potential passing on the gamma motor neurons causes shortening of polar region of intra fusal fibres and not the equatorial region.Hence the equatorial region stretches and initiates contraction.
• CNS initiates contraction of extra fusal fibres reflexely by gamma motor neurons viz gamma-loop. Gamma-loop has gamma motor neurons, intra fusal fibres (including middle receptor), sensory nerve of the spindle which forms excitatory mono synaptic synapse and alpha-motor neurons back to the same muscle and extra fusal fibres

Co-activation

• Each time when CNS transmits signals to alpha-motor neurons, the Gamma-motor neurons are also stimulated.This causes the contraction of both extra fusal and intra fusal fibres around the same time.
• This mechanism of simultaneous contraction check the length of the muscle spindle receptor and to maintain proper body movements without jerks.