a physiologic term denoting that part of the brainstem reticular formation that plays a central role in the organism's bodily and behavorial alertness; it extends as a diffusely organized neural apparatus through the central region of the brainstem into the subthalamus and the intralaminar nuclei of the thalamus; by its ascending connections it affects the function of the cerebral cortex in the sense of behavioral responsiveness; its descending (reticulospinal) connections transmit its activating influence on bodily posture and reflex mechanisms (e.g., muscle tonus), in part by way of the gamma motor neurons.reticular formation. SYN: nonspecific system.