Integration of somatic and visceral inputs by the brainstem: functional considerations.

Studies of the brainstem have traditionally either focused on mechanisms for the control of limb or axial muscles or for the maintenance of homeostasis (e.g., the circuitry for the regulation of respiration or blood pressure). However, recent studies have demonstrated that movement and changes in cardiorespiratory activity often are coordinated, indicating a linkage between the pathways that control somatic motor activity and those that regulate respiration or circulation. Neurons in regions such as nucleus tractus solitarius and the lateral medullary reticular formation that regulate circulation and respiration have been shown to receive somatic sensory signals (including those from the vestibular system) indicating body position in space. In addition, somatic motor control may be influenced by signals from visceral receptors that provide cues regarding body position in space. Some areas of the brainstem may also participate directly in both motor control and regulation of blood pressure and respiration. Thus, a simple division of the brainstem into "somatic" and "autonomic" regions is no longer possible.