What is the defining feature of a monosynaptic reflex?

The defining feature of a monosynaptic reflex is that it consists of one sensory neuron and one motor neuron. This direct connection allows the reflex arc to function with minimal delay, enabling a quick response to a stimulus. In this type of reflex, the sensory neuron transmits the signal directly to the motor neuron, which then initiates the appropriate motor response without involving any interneurons.

Monosynaptic reflexes are typically exemplified by the knee-jerk reflex (patellar reflex), where the tapping of the patellar tendon stretches the quadriceps muscle, activating the sensory neuron that directly synapses with the motor neuron controlling that muscle.

The presence of multiple motor neurons or interneurons is characteristic of polysynaptic reflexes, which involve more complex pathways and allow for more nuanced motor responses. Additionally, while spinal reflexes can encompass monosynaptic reflexes, not all reflexes are monosynaptic. Therefore, the specificity of having just one sensory and one motor neuron is what clearly defines a monosynaptic reflex.