Which of the following happens 'first' within an animal response?

a. motor neurons carry directions from interneurons to muscle cells.
b. interneurons receive and process information about the stimulus.
c. sensory neurons pass information about the stimulus to other nerve cells.
d. sensory neurons sense a stimulus.

The correct answer is option (D) Sensory neurons sense a stimulus.

Reflex action is the simplest kind of response to a stimulus by the body. The nerve pathway traveled by the response is called the reflex arc.

In a reflex arc, the receptors in the skin detect the stimulus. The sensory neurons then sense the stimulus and pass it to the relay neurons located in the spinal cord of the central nervous system in the form of electrical impulses. These relay neurons connect the sensory neurons to the motor neurons and process the information about the stimulus. The motor neurons then send the impulses to the effector (part of the body that responds) exhibiting a response to the external stimulus.

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