Monosynaptic vs Polysynaptic Reflex Arcs

[rtmsf95]

So it states in my book that the patellar tendon scenario is an example of a monosynaptic reflex arc. It then defines a monosynaptic reflex arc as a reflex involving only two neurons and one synapse.

However, there are actually 3 neurons and 3 synapses going on. When you hit the patellar tendon, one sensory neuron will synapse with a motor neuron that will cause contraction of the quadriceps.

Another sensory neuron will synapse with an inhibitory interneuron, which in turn, will synapse with a separate motor neuron that will cause relaxation of the hamstring muscles.

Basically, when you kick up your leg in response, your quads need to be contracted but your hamstrings have to be relaxed. Why, then, is this a monosynaptic reflex arc? Shouldn't it be a polysynaptic reflex arc, since multiple synapses are involved?

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[RegalDan]

There is only one synapse in the spinal cord that is related to the Patellar Reflex arc. Remember that it is the homonymous muscle that produces the reflex, not the heteronymous muscle (the hamstrings in this case), so, even though there is an interneuron present, it is not technically a part of the reflex but instead part of the process called "reciprocal inhibition". This means that while there are three synapses in the spinal cord, only one is part of the actual reflex.

 

[rtmsf95]

Excellently explained. Makes perfect sense now. Thank you!

 

[leonardoson]

As what was explained to me, monosynaptic reflex arc requires no processing in cns. Just instant reaction. Polysnaptic reflex would be like touching a hot stove (sensory), processing of heat (cns), move hand away (motor output)