Chapter 29: Introduction to the Autonomic Nervous System

The Autonomic Nervous System (ANS), frequently referred to as the involuntary or visceral nervous system, functions alongside the endocrine system to tightly regulate internal body processes and sustain homeostasis through continuous, minute-to-minute adjustments. This complex system integrates parts of the central and peripheral nervous systems, employing a unique two-neuron pathway where preganglionic neurons from the CNS synapse in groups of nerve cell bodies called ganglia before transmitting signals via postganglionic neurons to effector organs. The ANS is divided into two contrasting branches: the Sympathetic Nervous System (SNS) and the Parasympathetic Nervous System (PNS). The SNS, known as the “fight-or-flight” system or thoracolumbar division, prepares the body for stress by increasing cardiac output, heart rate, respiration, and blood flow to skeletal muscles, while diverting blood away from the gastrointestinal and urinary tracts, resulting in decreased digestion and urinary retention. SNS preganglionic fibers release acetylcholine (ACh), but its long postganglionic fibers release norepinephrine (NE) or epinephrine onto adrenergic receptors. These adrenergic receptors are classified as alpha1 (causing vasoconstriction and pupil dilation), alpha2 (modulating NE release), beta1 (increasing cardiac rate and force), beta2 (causing vasodilation and bronchial dilation), and beta3 (increasing lipolysis and metabolism). NE activity is terminated primarily through reuptake into the nerve terminal or metabolized by the enzymes monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT). Conversely, the PNS, or craniosacral division, mediates the “rest-and-digest” response, focusing on energy conservation, promoting digestion, increasing nutrient absorption, slowing heart rate, and facilitating the evacuation of waste products. The PNS uses both long preganglionic and short postganglionic axons that exclusively release ACh, defining it as the cholinergic system. Cholinergic receptors are categorized into muscarinic receptors (found in visceral organs like the heart, GI tract, and bladder) and nicotinic receptors (found in autonomic ganglia, the adrenal medulla, and the neuromuscular junction). ACh stimulation is rapidly deactivated by the enzyme acetylcholinesterase. The opposing yet balanced functions of the SNS and PNS allow the ANS to maintain precise control over the body’s internal environment.