Final answer:
An effector is a muscle fiber or gland cell that responds to efferent impulses, such as contracting in response to neurotransmitters or secreting hormones. These responses are fundamental to the functioning of the autonomic nervous system and the regulation of bodily functions.
Step-by-step explanation:
The blank in the given question should be filled with the term Effector. An effector is a muscle fiber or gland cell that responds to efferent impulses by contracting or secreting. In biology, effectors are essential components of the nervous system's response mechanisms. The efferent branch of the nervous system sends signals, often from the brain or spinal cord, to the effectors, which then carry out the response. For example, when facing a fight-or-flight situation, the adrenal medulla releases epinephrine, which acts as a signaling molecule that prepares the body for action. In muscle tissues, such as smooth muscle that is under involuntary control and moves internal organs, the end-plate of a muscle fiber has receptors for the neurotransmitter acetylcholine. This receptor binding triggers a cascade of events that leads to the contraction of the muscle fiber.
Understanding how muscle fibers and gland cells act as effectors in response to efferent stimuli helps us comprehend the functionality of the autonomic nervous system and the role of chemical substances, like hormones in regulation of bodily functions.