Final answer:
A reflex arc that has one sensory neuron synapsing only with one motor neuron is called a monosynaptic reflex.
Step-by-step explanation:
A reflex arc that involves only one sensory neuron synapsing with one motor neuron is known as a monosynaptic reflex, a simple and fast circuit enabling quick responses such as the knee-j*rk reaction. This type of reflex is exemplified in simple reflexes such as the knee-j*rk reflex often tested by doctors during physical examinations.
Monosynaptic reflexes are fast because they involve a direct, local synaptic connection between the sensory neuron that receives the stimulus and the motor neuron that triggers the muscle response without the need for an interneuron.
The spinal cord controls these motor reflexes and can execute these quick, unconscious movements, like automatically removing a hand from a hot object, without the direct involvement of the brain. The efficiency of reflex arcs is crucial, and it is achieved through minimal synaptic delay since the signal does not travel to the brain but is instead processed at the level of the spinal cord, enabling the rapid response required by reflex actions.