Chapter 15: The Autonomic Nervous System and Visceral Reflexes

Vocabulary practice flashcards covering anatomical pathways, functional divisions, visceral reflexes, receptors, and neurotransmitters of the autonomic nervous system.

Autonomic nervous system (ANS)

A motor nervous system, also called the visceral motor system, that controls glands, cardiac muscle, and smooth muscle involuntarily.

Denervation hypersensitivity

The exaggerated responses of cardiac and smooth muscle that occur when their autonomic nerve supply is severed.

Visceral reflexes

Unconscious, automatic, stereotyped (unlearned) responses to stimulation involving visceral receptors and effectors.

Autonomic tone

The normal background rate of activity that represents the balance between the sympathetic and parasympathetic systems according to the body's changing needs.

Parasympathetic tone

The normal background autonomic rate that maintains smooth muscle tone in the intestines and holds the resting heart rate down to about $70\text{ to }80\,\text{beats per minute}$.

Sympathetic tone

The background autonomic rate that keeps most blood vessels partially constricted to maintain baseline blood pressure.

Sympathetic division

The autonomic division, also called the thoracolumbar division, that adapts the body for physical activity (the "fight-or-flight" response) by increasing heart rate, blood pressure, airflow, and blood glucose while reducing blood flow to the skin and digestive tract.

Parasympathetic division

The autonomic division, also called the craniosacral division, that calms body functions to reduce energy expenditure and facilitates bodily maintenance, waste elimination, and digestion ("resting and digesting").

Preganglionic fiber

The first neuron in an autonomic efferent pathway, which has its cell body in the brainstem or spinal cord and extends a myelinated axon to synapse in an autonomic ganglion.

Postganglionic fiber

The second neuron in an autonomic efferent pathway, whose cell body is located in a peripheral ganglion and whose unmyelinated axon extends to the target cell.

Varicosities

Swellings along the distal nerve endings of autonomic postganglionic axons that release neurotransmitters across target tissues.

Sympathetic chain ganglia

A longitudinal series of ganglia adjacent to both sides of the vertebral column extending from cervical to coccygeal levels (also called paravertebral ganglia) receiving preganglionic fibers from spinal cord levels $T1$ to $L2$.

White communicating ramus

A branch carrying small, myelinated preganglionic sympathetic fibers from the spinal nerve into a sympathetic chain ganglion.

Gray communicating ramus

A bridge through which unmyelinated postganglionic sympathetic fibers leave the sympathetic chain ganglion to return to the spinal nerve.

Splanchnic nerves

Sympathetic preganglionic fibers that pass through the sympathetic chain without synapsing and continue onward to reach collateral ganglia.

Abdominal aortic plexus

A network of nerve fibers wrapping around the abdominal aorta that contains the major collateral ganglia.

Celiac ganglion

A collateral ganglion innervated by sympathetic fibers from thoracic ganglia $5$ to $9$ or $10$, whose postganglionic fibers target the stomach, spleen, liver, pancreas, small intestine, and kidneys.

Superior mesenteric ganglion

A collateral ganglion innervated by sympathetic fibers from thoracic ganglia $9$ to $12$, supplying postganglionic innervation to the small intestine, colon, and kidneys.

Inferior mesenteric ganglion

A collateral ganglion innervated by sympathetic fibers from lumbar ganglia, sending postganglionic fibers to the rectum, urinary bladder, and reproductive organs.

Solar plexus

The collective anatomical term for the celiac and superior mesenteric ganglia, from which nerves radiate out like rays of the sun.

Neuronal divergence

A structural pattern predominant in the sympathetic division where each preganglionic cell branches and synapses on $10\text{ to }20$ postganglionic cells, producing widespread, generalized effects.

Adrenal medulla

The inner core of the adrenal gland, consisting of modified postganglionic sympathetic neurons without axons that secrete hormones into the bloodstream upon preganglionic stimulation.

Sympathoadrenal system

The collective physiological entity formed by the sympathetic nervous system and the adrenal medulla.

Catecholamines

A hormone mixture secreted into the bloodstream by the adrenal medulla, composed of $85\%$ epinephrine (adrenaline) and $15\%$ norepinephrine (noradrenaline).

Terminal ganglia

Ganglia of the parasympathetic division located in or adjacent to their target organs, characterized by long preganglionic and short postganglionic fibers.

Pelvic splanchnic nerves

Parasympathetic fibers arising from spinal segments $S2$ to $S4$ that exit the ventral roots to form the inferior hypogastric plexus and supply the distal colon, rectum, urinary bladder, and reproductive organs.

Cholinergic fibers

Any nerve fibers that secrete acetylcholine (ACh), which includes all autonomic preganglionic neurons and all parasympathetic postganglionic neurons.

Cholinergic receptors

Any receptors that bind acetylcholine (ACh), classified into muscarinic receptors and nicotinic receptors.

Muscarinic receptors

A class of cholinergic receptors located on all cardiac muscle, smooth muscle, and gland cells, which can mediate excitatory or inhibitory target responses depending on the receptor subclass.

Nicotinic receptors

A class of cholinergic receptors located on all autonomic postganglionic neurons, in the adrenal medulla, and at skeletal muscle neuromuscular junctions, producing excitatory effects whenever ACh binds.

Adrenergic fibers

Nerve fibers that synthesize and secrete norepinephrine (NE), which includes nearly all sympathetic postganglionic neurons.

Adrenergic receptors

Receptors that bind norepinephrine (NE), categorized into alpha-adrenergic and beta-adrenergic classes.

Alpha-adrenergic receptors

A class of adrenergic receptors (with subclasses $\alpha_1$ and $\alpha_2$) that usually promote excitatory effects and utilize distinct second-messenger systems.

Beta-adrenergic receptors

A class of adrenergic receptors (with subclasses $\beta_1$ and $\beta_2$) that usually promote inhibitory effects and act through cyclic AMP ($\text{cAMP}$) as a second messenger.