Final answer:
The afferent branch of the reflex arc receives a signal from a receptor and transmits it to the central nervous system, where it is then relayed by an interneuron to the efferent branch, triggering a muscle response.
Step-by-step explanation:
The part of the reflex arc that receives a signal from a receptor is known as the afferent branch. This branch is responsible for transmitting the sensory input from the receptor along the sensory neurons to the central nervous system (CNS). Within the CNS, typically the spinal cord, the message is then passed to an interneuron. The interneuron will then relay the signal to the efferent branch, which carries the impulse to the muscle, triggering a response such as a muscle contraction. This reflex action is a fundamental component of the body's response to stimuli, operating without direct involvement of the brain, thereby providing a rapid reaction to potentially harmful events like touching a hot pot. Reflex arcs play a crucial role in maintaining bodily functions and responding swiftly to environmental changes.