Walsh & Hoyt: General Organization of the Autonomic Nervous System

Body functions that are regulated independently of voluntary activity using reflex mechanisms involving afferent nerve input, efferent nerve output, and central integrating nerve pathways are part of the autonomic nervous system. Although the activity of this system is essentially autonomous, at higher levels of the central nervous system (CNS) voluntary modulation is still possible. As early as 1875, Hughlings Jackson offered clinical, physiologic, and experimental evidence to show that the autonomic nervous system, like the somatic nervous system, is integrated at all levels of nervous activity and that autonomic and somatic activities are closely correlated. Segmental autonomic reflexes are coordinated in the spinal cord, but regulation of functions such as respiration, blood pressure, swallowing, and pupillary movement requires suprasegmental integration higher in the brain stem. The autonomic subsystems in the brain stem are, in turn, influenced by more complicated integrating systems in the hypothalamus. Finally, certain cortical areas, particularly the frontal cortex, govern many of the activities of the hypothalamus. Thus, coordination and integration of somatic and autonomic activities from the highest level of neurologic activity in cortex down to the spinal cord and peripheral nervous system (PNS) are used to attain fundamental adjustments of the organism to its environment.