The Autonomic Nervous System (General Learning)

Comprehensive vocabulary flashcards covering anatomical divisions, neurotransmitter mechanisms, receptor pathways, physiological organ responses, and clinical pathophysiology of the autonomic nervous system.

Autonomic Nervous System (ANS)

The motor subdivision of the peripheral nervous system consisting of motor neurons that innervate smooth muscle, cardiac muscle, glands, and adipose tissue under subconscious control.

Somatic Nervous System (SNS)

The voluntary motor division of the peripheral nervous system characterized by a single heavily myelinated neuron extending from the CNS directly to skeletal muscle effectors.

Preganglionic Neuron

The first neuron in the two-neuron autonomic efferent chain, whose cell body resides in the central nervous system and whose lightly myelinated axon projects to an autonomic ganglion.

Postganglionic Neuron

The second neuron in the autonomic efferent pathway, possessing an unmyelinated axon extending from an autonomic ganglion to an involuntary effector organ.

Denervation Hypersensitivity

An exaggerated response and heightened receptor sensitivity of autonomic effector tissues resulting from the severance or loss of their autonomic nerve supply.

Sympathetic Division

The 'fight-or-flight' division of the autonomic nervous system that mobilizes body energy during 'E activities' (exercise, excitement, emergency, and embarrassment).

Parasympathetic Division

The 'rest-and-digest' division of the autonomic nervous system that conserves energy and mediates maintenance 'D activities' (digestion, defecation, and diuresis).

Parasympathetic Tone

The continuous basal autonomic influence that decelerates heart rate and maintains normal resting activity levels of the digestive and urinary tracts.

Sympathetic Tone (Vasomotor Tone)

The basal state of continuous, partial arterial smooth muscle contraction maintained by the sympathetic division to regulate vascular resistance and blood pressure.

Thoracolumbar Outflow

The anatomical origin of sympathetic preganglionic neurons emerging from thoracic and lumbar segments ($T_1$ through $L_2$ or $T_5$ through $L_3$) of the spinal cord.

Craniosacral Outflow

The anatomical origin of parasympathetic preganglionic fibers exiting the central nervous system through cranial nerves ($III$, $VII$, $IX$, $X$) and sacral spinal cord segments ($S_2-S_4$).

Paravertebral Ganglia

A longitudinal chain of sympathetic ganglia lying bilaterally adjacent to the vertebral column forming the sympathetic trunk.

Collateral (Prevertebral) Ganglia

Unpaired sympathetic ganglia located anterior to the abdominal aorta—including the celiac, superior mesenteric, and inferior mesenteric ganglia—innervated by splanchnic nerves.

Splanchnic Nerves

Autonomic nerve bundles formed by sympathetic preganglionic fibers that pass through the sympathetic trunk without synapsing, terminating instead in collateral ganglia or the adrenal medulla.

Divergence

An autonomic organizational feature where a single preganglionic axon branches and synapses with multiple postganglionic neurons situated at different spinal levels.

Convergence

An autonomic structural arrangement where multiple preganglionic neurons originating at different spinal levels synapse onto a single postganglionic neuron.

Adrenal Medulla

A modified sympathetic ganglion containing chromaffin cells (modified postganglionic neurons) that secrete epinephrine and norepinephrine directly into systemic circulation as hormones upon stimulation by preganglionic fibers.

Oculomotor Nerve (CN III)

The cranial nerve carrying parasympathetic fibers to the ciliary ganglion that innervate the eye to narrow the pupil (constriction) and focus the lens.

Facial Nerve (CN VII)

The cranial nerve supplying parasympathetic preganglionic innervation to the pterygopalatine and submandibular ganglia, controlling secretions of the lacrimal, nasal, submandibular, and sublingual glands.

Glossopharyngeal Nerve (CN IX)

The cranial nerve transmitting parasympathetic preganglionic fibers to the otic ganglion to stimulate salivation from the parotid salivary gland.

Vagus Nerve (CN X)

The principal parasympathetic nerve providing wide visceral distribution to cardiac, pulmonary, and esophageal plexuses as well as abdominal viscera down to the proximal half of the colon.

Choline Acetyltransferase (ChAT)

The enzyme localized within cholinergic presynaptic terminals responsible for the rapid synthesis of acetylcholine ($\text{ACh}$) from acetyl-CoA and choline.

Acetylcholinesterase (AChE)

The enzyme present in synaptic clefts that rapidly terminates cholinergic transmission by hydrolyzing acetylcholine into acetate and choline.

Nicotinic Receptors

Ligand-gated ion channels found on skeletal muscle ($N_m$), all postganglionic cell bodies ($N_n$), and adrenal chromaffin cells that produce fast, excitatory depolarizations in response to acetylcholine.

Muscarinic Receptors

G-protein coupled cholinergic receptors ($M_1-M_5$) located on all parasympathetic effector targets and sympathetic sweat glands that mediate slow, prolonged excitatory or inhibitory actions.

$M_2$ Muscarinic Receptor

A G-protein coupled receptor in the heart whose activation by acetylcholine opens $K^+$ channels, producing hyperpolarization and a slower heart rate.

$M_3$ Muscarinic Receptor

A G-protein coupled receptor on smooth muscle and secretory glands that triggers intracellular $Ca^{2+}$ elevation to cause smooth muscle contraction and glandular secretion.

Tyrosine Hydroxylase (TH)

The initial and rate-limiting enzyme in catecholamine biosynthesis that converts the amino acid tyrosine into L-DOPA.

Dopamine $\beta$-Hydroxylase (DBH)

The enzyme that catalyzes the hydroxylation of dopamine to form norepinephrine in adrenergic neurons.

Phenylethanolamine N-Methyltransferase (PNMT)

The adrenal medullary enzyme that methylates norepinephrine to synthesize epinephrine.

Monoamine Oxidase (MAO)

An intracellular mitochondrial enzyme that degrades catecholamines (such as norepinephrine and dopamine) following their reuptake into the presynaptic neuron.

Catechol-O-Methyltransferase (COMT)

An enzyme found in postjunctional cells and circulating tissues that inactivates catecholamines via O-methylation.

$\alpha_1$-Adrenergic Receptor

A $G_q$-coupled receptor activating the phospholipase C/$IP_3$/DAG pathway to elevate intracellular $Ca^{2+}$, causing vasoconstriction of blood vessels, contraction of GI sphincters, and dilation of the pupil.

$\alpha_2$-Adrenergic Receptor

A $G_i$-coupled presynaptic autoreceptor that inhibits adenylyl cyclase to diminish further norepinephrine exocytosis, while also decreasing insulin secretion in the pancreas.

$\beta_1$-Adrenergic Receptor

A $G_s$-coupled adrenergic receptor predominantly situated in cardiac myocytes and kidneys that increases heart rate, cardiac contractility, and stimulates renin secretion.

$\beta_2$-Adrenergic Receptor

A $G_s$-coupled adrenergic receptor located in bronchiolar smooth muscle and skeletal muscle vasculature that stimulates cAMP production to promote bronchodilation and vasodilation.

$\beta_3$-Adrenergic Receptor

A $G_s$-coupled adrenergic receptor found in adipose tissue and the urinary bladder that stimulates lipolysis and relaxes the detrusor muscle to prevent urination.

Albuterol

A selective $\beta_2$-adrenergic agonist that elevates intracellular cAMP to inhibit myosin light chain kinase (MLCK), causing relaxation of constricted bronchiolar smooth muscle in asthma.

Pupillary Dilator Muscle (Radial Muscle)

The radially oriented iris muscle containing $\alpha_1$-adrenergic receptors that contracts under sympathetic stimulation to dilate the pupil (mydriasis).

Hypothalamus

The master integration and control center of the autonomic nervous system ('the boss') coordinating visceral balance, body temperature, water retention, and neuroendocrine output.

Visceral Reflex

An unconscious, automatic, polysynaptic regulatory feedback loop mediated by the ANS consisting of a receptor, afferent sensory neuron, integration center, efferent motor neurons, and an internal effector organ.

Baroreceptor Reflex

A cardiovascular visceral reflex wherein increased blood pressure detected by arterial stretch receptors triggers afferent impulses via the glossopharyngeal nerve ($CN\,IX$) to the medulla, producing vagal ($CN\,X$) efferent parasympathetic discharge that slows heart rate.

Raynaud's Disease

An autonomic disorder caused by excessive sympathetic stimulation of $\alpha_1$ receptors in peripheral arterioles, leading to severe digital vasospasms, cyanosis, pallor, and coldness.

Orthostatic Hypotension

An autonomic disorder marked by an excessive, abnormal decrease in blood pressure upon assuming an upright posture due to impaired sympathetic vasoconstrictor tone.

Which motor division innervates skeletal muscle effectors through a single, heavily myelinated axon extending directly from the central nervous system?

Somatic nervous system (SNS)

What neurotransmitter is released by all autonomic preganglionic neurons in both the sympathetic and parasympathetic divisions?

Acetylcholine ($\text{ACh}$)

Which specific sympathetic target tissue is innervated by postganglionic cholinergic fibers releasing acetylcholine instead of norepinephrine?

Thermoregulatory sweat glands

What enzyme catalyzes the initial, rate-limiting conversion of tyrosine into L-DOPA in catecholamine biosynthesis?

Tyrosine hydroxylase (TH)

What adrenal medullary enzyme converts norepinephrine into epinephrine?

Phenylethanolamine N-methyltransferase (PNMT)

Which cholinergic receptor type is an intrinsic ligand-gated ion channel located on all autonomic postganglionic neurons and chromaffin cells?

Nicotinic neuronal receptor ($N_n$)

Which G-protein pathway is coupled to the $\alpha_1$-adrenergic receptor to mediate smooth muscle contraction and vasoconstriction?

$G_q$ pathway (stimulates phospholipase C to yield $IP_3$ and DAG, elevating intracellular $Ca^{2+}$)

What is the primary physiological function of presynaptic $\alpha_2$-adrenergic autoreceptors?

To inhibit adenylyl cyclase via $G_i$ and suppress further exocytosis of norepinephrine through negative feedback.

Which adrenergic receptor subtype mediates positive inotropic and chronotropic responses in the myocardium as well as renin secretion from the kidneys?

$\beta_1$-adrenergic receptor

Which adrenergic receptor subtype couples to $G_s$ to activate cAMP production and relax smooth muscle in the bronchioles and skeletal muscle vasculature?

$\beta_2$-adrenergic receptor

Which adrenergic receptor subtype is located on adipocytes to stimulate lipolysis and on the urinary bladder detrusor to prevent micturition?

$\beta_3$-adrenergic receptor

Which muscarinic receptor subtype acts via $G_i$ to open $K^+$ channels in the heart, resulting in hyperpolarization and reduced heart rate?

$M_2$ muscarinic receptor

Which muscarinic receptor subtype activates the $G_q$/phospholipase C cascade to drive digestive secretions and visceral smooth muscle contraction?

$M_3$ muscarinic receptor

What two enzymes are responsible for the metabolic inactivation of circulating and synaptic catecholamines?

Monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT)

Why is peripheral epinephrine classified as a hormone rather than a traditional neurotransmitter?

It is secreted by adrenal chromaffin cells into systemic circulation to act at distant target organs rather than across a local synaptic cleft.

What spinal cord segments give rise to the preganglionic fibers of the sympathetic thoracolumbar outflow?

$T_1$ through $L_2$ (or $T_5$ through $L_3$)

Which central and spinal regions constitute the anatomical outflow of the parasympathetic nervous system?

Craniosacral outflow: cranial nerves $III$, $VII$, $IX$, and $X$, plus sacral spinal cord segments $S_2-S_4$.

Which cranial nerve provides the majority of parasympathetic preganglionic innervation to thoracic and abdominal organs up to the mid-colon?

Vagus nerve (CN $X$)

Which cranial nerve carries parasympathetic preganglionic fibers to the ciliary ganglion to regulate pupil constriction and lens focusing?

Oculomotor nerve (CN $III$)

Which cranial nerve carries parasympathetic secretomotor fibers to the otic ganglion to stimulate the parotid salivary gland?

Glossopharyngeal nerve (CN $IX$)

Which cranial nerve directs parasympathetic stimulation to the pterygopalatine and submandibular ganglia for lacrimal and salivary secretion?

Facial nerve (CN $VII$)

What organizational feature of the sympathetic division occurs when one preganglionic axon branches to synapse with multiple postganglionic neurons across spinal levels?

Divergence

What structural circuit exists when multiple preganglionic axons from different spinal levels synapse upon a single postganglionic neuron?

Convergence

Which brain region serves as the primary master control and integration center ('the boss') for autonomic nervous system activity?

Hypothalamus

What clinical cardiovascular changes occur immediately after heart transplantation due to surgical autonomic denervation?

Elevated resting heart rate, lack of vagal tone, blunted acceleration during exercise, and loss of heart rate variability.

What tonic autonomic mechanism maintains continuous partial contraction of systemic arterioles to regulate resting blood pressure?

Sympathetic tone (vasomotor tone)

Which cardiovascular reflex activates vagal efferents to decelerate heart rate in response to elevated arterial wall stretch detected via CN $IX$?

Baroreceptor reflex

What autonomic disorder is characterized by intermittent, severe digital vasospasms and cyanosis caused by exaggerated sympathetic $\alpha_1$ activity?

Raynaud's disease

What clinical condition manifests as an excessive reduction in systemic arterial blood pressure upon standing due to insufficient sympathetic vasomotor response?

Orthostatic hypotension

Which iris muscle is stimulated by sympathetic $\alpha_1$ signaling to cause pupil dilation (mydriasis)?

Pupillary dilator muscle (radial muscle)

Which iris muscle contracts under parasympathetic muscarinic signaling to cause pupillary constriction (miosis)?

Pupillary sphincter muscle (circular muscle)

What pharmacological effect occurs when presynaptic $\alpha_2$ autoreceptors are blocked by an antagonist?

Norepinephrine exocytosis into the synaptic cleft increases due to loss of autoinhibitory negative feedback.

What selective $\beta_2$-adrenergic agonist is administered as an inhaler to relax bronchial smooth muscle in acute asthma exacerbations?

Albuterol