Reflex hammer (the stimulus)Its tap on the ligament is the stimulus. The tap gives the quadriceps a quick stretch.
Patellar ligament (patellar tendon)The band below the kneecap, where the tap lands. A pull on it stretches the quadriceps.
Quadriceps muscleThe muscle that straightens the knee. The tap stretches it, and then it contracts.
Muscle spindle (the receptor)The stretch receptor inside the muscle. It senses the stretch and starts the signal.
Sensory neuron (afferent neuron)It carries the signal from the spindle to the spinal cord. Its cell body lies outside the cord.
SynapseThe one synapse of the arc. Here the sensory neuron meets the motor neuron directly.
Motor neuron to the quadricepsIt carries the signal back to the quadriceps. The muscle contracts.
Kick of the leg (the response)The lower leg swings forward as the knee straightens. That is the knee jerk.
The side branch 3
Inhibitory interneuronA side branch of the sensory neuron excites it. It quiets the motor neuron to the hamstrings.
Motor neuron to the hamstringsIt serves the opposing muscle. The interneuron keeps it quiet during the kick.
Hamstring musclesThe muscles at the back of the thigh. They relax as the quadriceps contracts.
How this list was compiled: Every name and step checked on 2026-10-03 in two of these. (1) Wikipedia, English edition, Patellar reflex, Stretch reflex, Reciprocal inhibition and Patellar tendon, text only. (2) Neuroscience Online, McGovern Medical School at UTHealth Houston, Section 3, Chapter 2, Spinal Reflexes and Descending Motor Pathways, https://nba.uth.tmc.edu/neuroscience/m/s3/chapter02.html. (3) Kenhub, Reflex arc (its part on the patellar reflex), https://www.kenhub.com/en/library/anatomy/reflex-arc. (4) TeachMeAnatomy, The Femoral Nerve and The Sciatic Nerve, for what the two muscles do and where they lie, and The Knee Joint, for the patellar ligament, https://teachmeanatomy.info/lower-limb/nerves/femoral-nerve/ and https://teachmeanatomy.info/lower-limb/joints/knee-joint/. Hints and detail lines are in our own words. Licence: own work
The drawing: Drawn for this site on 2026-10-03 from plain geometry; nothing traced. A plan, not to scale. Licence: own work
How it works
The patellar reflex, or knee jerk, is a monosynaptic reflex arc: one sensory neuron, one synapse, one motor neuron. The drawing shows a seated leg from the side and the spinal cord cut across.
Read the full guide
Follow the signal around the arc
The tap of a reflex hammer (1) on the patellar ligament (2) stretches the quadriceps (3). A muscle spindle (4) senses the stretch, and a sensory neuron (5) carries the signal to the spinal cord. There it meets a motor neuron at a single synapse (6). The motor neuron (7) makes the quadriceps contract, and the leg kicks (8).
Then the side branch
A branch of the sensory neuron excites an inhibitory interneuron (9). It quiets the motor neuron to the hamstrings (10), so the hamstring muscles (11) relax while the quadriceps contracts.
Monosynaptic counts the synapses on the path to the quadriceps: one.
The interneuron belongs to the side branch, not to the path that makes the kick.
Questions and answers
What are the parts of the patellar reflex arc?
A receptor, the muscle spindle in the quadriceps; a sensory neuron; one synapse in the spinal cord; a motor neuron; and the effector, the quadriceps itself. A side branch through an inhibitory interneuron relaxes the hamstrings.
Why is the knee-jerk reflex called monosynaptic?
Because the sensory neuron synapses directly on the motor neuron of the quadriceps, with no interneuron between them. There is only one synapse on that path.
What is the receptor in the patellar reflex?
The muscle spindle, a stretch receptor inside the quadriceps muscle. The tap on the patellar ligament stretches the muscle, and the spindle senses that stretch.
Why do the hamstrings relax during the knee jerk?
A branch of the sensory neuron excites an inhibitory interneuron in the spinal cord. It keeps the motor neuron to the hamstrings from firing, so the opposing muscle does not work against the kick. This is called reciprocal inhibition.
Can I practice here for “drag the labels onto the diagram to identify the parts of a monosynaptic reflex arc”?
Yes, for practice. That sentence is the instruction of a labeling exercise. The drawing here is our own and not a copy of any course figure, so your figure may ask for other names.