Spinal Reflex: The Patellar Reflex

Spinal Reflex: The Patellar Reflex

Here is a diagram of the components of a spinal reflex. Note that even though the patellar reflex is a monosynaptic reflex, we have included an association neuron in the diagram to illustrate a bisynaptic reflex arc for instructional purposes.


A spinal reflex is one in which the decision to react is made at the level of the spinal cord, allowing extremely rapid reaction without awaiting the participation of the brain. Protective reactions and postural adjustments are typical examples of this kind of spinal reflex.

Typically it will involve a sequence of neurons consisting of a sensory neuron, an internuncial or association neuron, and a motor neuron for a bisynaptic reflex arc. Monosynaptic arcs consist of only a sensory and a morot neuron. We will explore the best know of spinal reflexes, a stretch reflex called the patellar reflex, a monosynaptic reflex, which occurs when the patellar ligament is tapped.

Illustrate the reflex pathway and participating neurons as shown in the illustration above.

1) Muscle spindles (transducers in quadriceps) are stretched (golgi tendon organs may detect increased tension, usually only when active contraction occurs)

2) sensory impulses are carried on dendrites up along a spinal nerve to the unipolar sensory neuron (ganglion cell) in the dorsal root ganglia.

3) IF BISYNAPTIC: the ganglion cell relays the impulse out along its axon into posterior gray horn of spinal cord, where it synapses with an association or internuncial neuron.

4) The internuncial neuron sends an impulse along its axon to a motor neuron in the anterior gray horn.

5) The motor neuron sends an impulse along its axon out through the ventral root, through the spinal nerve to quadriceps femoris. At the motor endplate, acetylcholine is released, causing contraction of the muscle. The leg kicks…


1) Construct a data table to record the patellar reflex of the right versus the left leg for you and your bench partner.

2) Have your partner sit on edge of table or with legs crossed so that leg swings freely.

3) Locate the tibial tuberosity and the lower edge of patella.

4) With a percussion hammer, strike with the pointed end in the center of the soft space between these two hard landmarks.

5) Experiment with striking in various locations to see where the most pronounced reaction is elicited. Repeat for the other leg.

6) Describe the reflexive reactions of the R versus the L leg of your benchmates, record in the data table. Compare yours with your benchmates.