CHAPTER 9: Autonomic Nervous System

KEY CONCEPTS

• The autonomic nervous system plays a central role in maintaining homeostasis and regulates almost every organ system in the body.

• The major functional divisions are the sympathetic and parasympathetic nervous systems. A third division, the enteric nervous system, is an intrinsic neural network that regulates gastrointestinal function.

• In most organs, the sympathetic and parasympathetic nervous systems produce functionally opposite effects and can be viewed in simple terms as physiologic antagonists.

• The sympathetic nervous system is activated in response to changes in the environment and produces a coordinated “fight-or-flight” response to a threat.

• The parasympathetic nervous system is continuously active, and coordinates the function of multiple organs in accord with the physiologic state of the organism, thereby facilitating such functions as digestion and excretion.

• Because of its importance to the physiology of the organism, the autonomic nervous system is a target for many pharmacologic interventions and is also responsible for the untoward effects of many medications and toxins.

• The peripheral autonomic nervous system (both sympathetic and parasympathetic divisions) consists of a preganglionic neuron in the brainstem or spinal cord that innervates postganglionic neurons in peripheral autonomic ganglia. Synaptic transmission in the autonomic ganglia is mediated by acetylcholine interacting with a nicotinic receptor that is pharmacologically distinct from receptors in the brain or at the neuromuscular junction. The postganglionic neurons innervate target organs throughout the body.

• Acetylcholine is the main neurotransmitter used by parasympathetic postganglionic neurons; its target receptors are muscarinic acetylcholine receptors.

• With a few exceptions, postganglionic sympathetic neurons release norepinephrine, which acts on α- and β-adrenergic receptors located on end organs.

INTRODUCTION

The autonomic nervous system (ANS) is a semiautonomous division of the nervous system that innervates virtually every organ in the body. Central control of autonomic function involves integration of afferent information and cortical input by brainstem centers and the hypothalamus. These structures control the overall activity of the ANS (autonomic tone). The peripheral ANS (or visceral system) serves to distribute autonomic efferents throughout the body and can also mediate simple autonomic reflexes independent of central control.

The overall function of the ANS is to maintain homeostasis in the body (ie, optimize conditions for survival) in the face of constantly changing environmental and activity demands. For example, the ANS adjusts blood pressure and heart rate to meet the circulatory needs of the body that can vary tremendously from supine sleep to vigorous exercise. The ANS also maintains a constant body temperature despite changing environmental conditions and metabolic activity. Under ordinary circumstances, the ANS functions independently of consciousness yet can be influenced to some degree by volition and emotion. Without autonomic innervation, most organ systems continue to function but cannot adapt to changing environmental and emotional conditions.

Because of its anatomic accessibility and its robust regulation of peripheral organ functions, the ANS was among the first components of the mammalian nervous system to be studied. Consequently, ...