What type of reflex arc consists of only two neurons, one sensory and one motor?

The correct answer is the monosynaptic reflex arc, which is characterized by its simplicity, consisting of just two neurons: one sensory neuron that receives and transmits information from a sensory receptor, and one motor neuron that carries the response signal to an effector (like a muscle). This type of reflex arc involves a single synapse between these two neurons, allowing for a rapid response to stimuli.

The classic example of a monosynaptic reflex is the knee-jerk reflex, where tapping the patellar tendon stretches sensory receptors in the muscle, triggering a quick contraction of the muscle without the need for higher brain involvement. This reflex arc is essential for immediate responses to certain stimuli without the delay that would come from processing by the central nervous system.

Other types of reflex arcs, such as polysynaptic reflex arcs, involve additional interneurons and multiple synapses, causing delays in the response. The autonomic reflex arc pertains to involuntary bodily functions, often involving organs and glands, while the notion of a neurotransmitter reflex arc is not a recognized term in the study of reflexes within the nervous system. Thus, the simplicity and direct nature of the monosynaptic reflex arc are why it is accurately identified as consisting of only two neurons.