In the knee jerk reflex, receptor and effector respectively are

In the knee jerk reflex, the muscle spindle acts as the receptor, and the effector is the quadriceps muscle group. When the quadriceps muscle is stretched, the muscle spindle detects the change in length and triggers a reflexive contraction of the quadriceps, causing the leg to extend. This reflex helps test the spinal cord's sensory and motor pathways.

Muscle spindle-Afferent neuron-CNS-Efferent neuron-Motor end plate