There are five steps in a knee-jerk reflex arc as follows:

1. A tap on the patellar tendon (tendon attached to the kneecap) is sensed by stretch receptors in the muscle (muscle spindles).

2. The muscle spindle produces a pattern of nerve impulses that are conveyed along a sensory (afferent) nerve fiber, past its cell body in the dorsal root ganglion, and to its termination on a motor neuron.

3. The nerve terminal releases neurotransmitters onto the motor neurons, which generate excitatory, postsynaptic potentials in the motor neurons’ dendrites and cell bodies.

4. The motor neuron generates action potentials, leading to the release of acetylcholine from its terminals on muscles.

5. The muscle responds to the acetylcholine by depolarizing and contracting.