What is a monosynaptic reflex and give an example.

• A. A reflex arc with a single synapse between a sensory and a motor neuron; e.g., knee-jerk (patellar) reflex
• B. A reflex arc with multiple interneurons; e.g., swallowing
• C. A reflex that requires brain input; e.g., blink reflex
• D. A reflex using two synapses; e.g., gag reflex

Answer: (A)

A monosynaptic reflex has a single synapse between a sensory neuron and a motor neuron in the spinal cord, so the response is very fast. The knee-jerk reflex is the classic example: tapping the patellar tendon stretches the quadriceps, activating a muscle spindle. The sensory neuron from that spindle makes a direct synaptic connection with an alpha motor neuron in the ventral horn, which then signals the quadriceps to contract. There are no interneurons involved, so only one synapse relays the message.

Other options describe more complex circuits. Swallowing involves multiple interneurons and brainstem pathways, so it’s polysynaptic. The gag reflex also uses several synapses and brainstem circuits. The blink reflex typically relies on brainstem pathways and can involve multiple synapses or higher processing, not a single direct connection.