Chapter 14: The Autonomic Nervous System

Unlike the somatic nervous system which directly controls skeletal muscles through single neurons, the autonomic nervous system operates through a two-neuron chain in which a preganglionic neuron originating in the central nervous system synapses with a postganglionic neuron located in an autonomic ganglion, which then innervates visceral effectors including smooth muscle, cardiac muscle, glands, and adipose tissue. The autonomic nervous system is subdivided into three functional divisions: the sympathetic division originating from thoracic and lumbar spinal segments, the parasympathetic division originating from the brainstem and sacral spinal cord, and the enteric nervous system which forms an independent neural network within the gastrointestinal tract. The sympathetic division prepares the body for acute stress through activation of fight-or-flight responses, increasing heart rate, blood pressure, and metabolic activity while mobilizing energy reserves and enhancing mental alertness. In contrast, the parasympathetic division promotes rest-and-digest functions, conserving energy through decreased heart rate, enhanced digestive secretions, and coordinated elimination of waste. These two divisions employ different neurotransmitter systems: sympathetic neurons release norepinephrine and epinephrine acting on adrenergic receptors, while parasympathetic neurons release acetylcholine binding to nicotinic and muscarinic cholinergic receptors. The chapter explores how most vital organs receive dual innervation from both divisions, allowing opposing effects to be precisely balanced through autonomic tone, a continuous baseline activity maintained even at rest. Autonomic control integrates reflex responses at multiple hierarchical levels, from short reflexes operating entirely within peripheral ganglia to long reflexes requiring central nervous system processing. Interoceptors including baroreceptors and chemoreceptors provide essential sensory feedback about internal conditions such as blood pressure, blood gas composition, and chemical gradients. The hypothalamus serves as the central command center coordinating autonomic function while integrating input from higher brain centers including the cerebral cortex and limbic system, thereby allowing emotional states and conscious thoughts to modulate autonomic responses.