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MCAT Question A Day - 6/22/13 - Answer!

Which statement is true about all spinal reflexes?

A.  No synapses are necessary for the response to occur.
B.  No motor neurons are necessary for the response to occur.
C.  The brain does not have to receive or send information for the response to occur.
D.  No interneurons are necessary for the response to occur.

The correct answer is (C). Spinal reflexes do not involve the brain. In a stretch reflex such as the patellar reflex, a sensory neuron carries information into the spinal cord from stretch receptors in the knee. The sensory neuron forms a synapse with a motor neuron that then leaves the cord to form a neuromuscular junction back in the muscle that extends the knee. Although this spinal reflex does not involve an interneuron (choice D), a withdrawal reflex (pulling the hand away from a fire) is a 3-neuron circuit (sensory n., interneuron, motor n.).