After studying this chapter, the student should be able to:
Understand the differences between the central, peripheral, autonomic, and enteric nervous systems (ENS).
Understand the antagonism between the sympathetic and parasympathetic divisions of the autonomic nervous systems.
See how the neurotransmitters acetylcholine (ACh) and norepinephrine mediate autonomic nervous system control.
Understand the organization and projections of the cephalic portion of the autonomic nervous system.
See how the sympathetic and parasympathetic divisions of the autonomic nervous system operate at the spinal cord level.
Understand the role of the brainstem in autonomic nervous system processing.
See the crucial roles of the hypothalamus and pituitary gland in autonomic control.
Understand the organs controlled by the autonomic nervous system and notable disorders associated with autonomic nervous system dysfunction.
The nervous system consists of the central, peripheral, autonomic, and enteric systems. This chapter will discuss the autonomic and enteric nervous systems (ENS). The autonomic nervous system receives inputs from receptors in glands and cardiac and smooth muscle and sends motor commands to those areas. The enteric nervous system is the nervous system of digestion. Previously, the ENS was considered part of the autonomic nervous system, but it is now generally treated separately, occasionally being referred to as the “second brain.”
The autonomic nervous system consists of sympathetic and parasympathetic branches that generally have opposite effects on organs such as the heart and lungs. The sympathetic system activates the fight-or-flight response, whereas parasympathetic activity promotes homeostatic functions such as digestion and the immune system. A major output of the autonomic nervous system is the hypothalamus via the reticular formation.
OUTSIDE THE CENTRAL NERVOUS SYSTEM: THE ENTERIC & AUTONOMIC NERVOUS SYSTEMS
The brain is the locus of consciousness and consciously controlled behavior. The brain also interacts with an extended nervous system outside the brain that controls body metabolic processes such as respiration, heart rate, temperature, and digestion. This control is mediated by neural clusters in various parts of the body called ganglia, which exist even in invertebrates. Vertebrate ganglia are the control centers for the autonomic and enteric nervous systems. These systems interact with the central nervous system (CNS) in the brainstem and hypothalamus.
The Enteric Nervous System
Life requires the consumption of nutrients and water and the removal of waste products of digestion. The ENS mediates homeostasis by controlling digestion and influencing hunger and satiety. This control requires interaction with the autonomic nervous system and CNS. The more central systems set priorities for digestion versus other body functions. Figure 18–1 shows a block diagram of the neural control of digestion that illustrates the location of the ENS in the control scheme and the means and loci of interaction with other parts of the nervous system. Digestive control includes both neural projections and neurohumoral regulatory mechanisms.