ANS condensed

What is the ANS, what does it control, and what regulates it?

Division of PNS supplying smooth muscle, cardiac muscle, and glands. Acts largely unconsciously, regulated by the hypothalamus. Primary mechanism for "fight or flight" (Sympathetic) and "rest and digest" (Parasympathetic). Also controls heart rate, digestion, pupillary response, urination, and reflex actions (coughing, swallowing).

What are the origins (outflows) of the Sympathetic and Parasympathetic divisions?

Sympathetic = Thoracolumbar outflow (T1-L2, intermediolateral cell column). Parasympathetic = Craniosacral outflow (CN III, VII, IX, X in brainstem + S2-S4 spinal cord).

Define GSE and GSA.

GSE (General Somatic Efferent): Motoneurons to striated muscle (from somites), heavily myelinated, single neuron from CNS. GSA (General Somatic Afferent): Sensory from somatopleural structures (pain, touch, temp) to CNS via DRG.

Define GVE and GVA.

GVE (General Visceral Efferent): Autonomic (sympathetic/parasympathetic) motor fibers to smooth muscle, cardiac muscle, glands. GVA (General Visceral Afferent): Sensory from visceral structures (splanchnopleure) for chemoreception, mechanoreception, and stretch.

What are the "Special" afferent and efferent components (only in cranial nerves)?

SVA = taste/smell. SSA = eye/ear (vision, hearing, balance). SVE (branchiomotor) = motor to striated muscles from pharyngeal arches (mastication, facial expression, pharynx/larynx).

Describe the two-neuron chain and myelination of the autonomic motor pathway.

Preganglionic neuron (cell body in CNS) → synapses in peripheral ganglion → Postganglionic neuron (cell body in ganglion) → target organ. Preganglionic fibers are lightly myelinated; postganglionic fibers are unmyelinated.

Compare neurotransmitters for Somatic vs. Autonomic motor systems.

Somatic: Single neuron releases ACh at NMJ (always excitatory). Autonomic: Preganglionic (both sym & parasym) release ACh. Postganglionic Sympathetic releases NE (mostly, except sweat glands use ACh). Postganglionic Parasympathetic releases ACh (with co-transmitters like NO).

Compare the effects of Somatic vs. Autonomic motor systems on target organs.

Somatic: Always stimulatory (excitatory) on skeletal muscle. Autonomic: Stimulatory OR inhibitory depending on the neurotransmitter and receptor type on the effector organ.

Where are preganglionic sympathetic neurons located?

Intermediolateral (IML) cell column of the lateral horn of spinal gray matter, T1-L2. These are the ONLY preganglionic sympathetic neurons, innervating all sympathetic ganglia and adrenal chromaffin cells.

What are the two main groups of sympathetic ganglia?

1) Sympathetic trunk ganglia (paravertebral; sympathetic chain) 2) Prevertebral ganglia (preaortic) - named after major arterial branches (celiac, superior mesenteric, inferior mesenteric).

Which cranial nerves carry parasympathetic outflow and what ganglia do they innervate?

CN III → Ciliary ganglion; CN VII → Pterygopalatine + Submandibular ganglia; CN IX → Otic ganglion; CN X → Vagus (direct innervation to thoracic/abdominal visceral ganglia).

Which cranial nerve does NOT have a parasympathetic role?

CN V (Trigeminal) - purely somatic sensory/motor, no parasympathetic outflow.

What is the sympathetic trunk and how do fibers reach upper/lower body?

A paravertebral chain of ganglia running the entire vertebral column. Postganglionic fibers ascend or descend within the trunk to reach targets above (cervical) or below (lumbar/sacral) the T1-L2 outflow (e.g., fibers from T1-T5 ascend to head/neck).

Compare White vs. Gray Rami Communicantes.

White: Myelinated (white), carries PREganglionic fibers, ONLY exists at T1-L2. Gray: Unmyelinated (gray), carries POSTganglionic fibers, exists at ALL spinal nerve levels.

A patient has a T10 spinal nerve lesion. Why does he lose sweating over the T10 dermatome?

The Gray ramus communicans carries postganglionic sympathetic fibers to sweat glands in the skin. Lesion interrupts this pathway (or the T10 anterior ramus carrying these fibers).

What are splanchnic nerves?

Paired visceral nerves carrying preganglionic sympathetic fibers (GVE) and visceral afferent fibers (GVA) from the sympathetic trunk to preaortic ganglia/plexuses. Exception: Pelvic splanchnic nerves carry PARASYMPATHETIC fibers.

List the thoracic splanchnic nerves and their spinal origins.

Greater splanchnic (T5-T9), Lesser splanchnic (T10-T11), Least splanchnic (T12).

Which prevertebral (preaortic) ganglia supply foregut, midgut, and hindgut?

Foregut = Celiac ganglion (via Greater splanchnic). Midgut = Superior mesenteric ganglion (via Lesser splanchnic). Hindgut = Inferior mesenteric ganglion (via Lumbar splanchnics).

What is the unique exception to the sympathetic "postganglionic" rule?

The adrenal medulla. Chromaffin cells are modified postganglionic sympathetic neurons directly innervated by preganglionic fibers (releasing ACh). They secrete epinephrine (and NE) into the blood as a neurohormone.

Which structures receive ONLY sympathetic innervation vs. ONLY parasympathetic innervation?

Only Sym: Most arterial smooth muscle, arrector pili, sweat glands, chromaffin cells. Only Parasym: Ciliary body (lens), lacrimal gland, salivary glands.

What are the sympathetic effects on the heart and GI tract?

Heart: Accelerates and strengthens heartbeat (positive chronotropy/inotropy). GI: Inhibits digestion (decreases secretion/peristalsis).

What are the parasympathetic effects on the heart and GI tract?

Heart: Slows heart rate (negative chronotropy). GI: Increases secretion and peristalsis (stimulates digestion).

What is the cardiac plexus?

A network of sympathetic cardiac nerves (postganglionic from cervical/thoracic ganglia) + vagal fibers (preganglionic parasympathetic) converging on the heart.

What is the mechanism of referred visceral pain?

Visceral pain (stretch/pressure) via DRG afferents travels to the same spinal cord levels as somatic afferents. The brain interprets the pain as originating from the somatic dermatome supplied by those spinal nerves.

Give two classic examples of referred pain.

1) Cardiac ischemia (MI) → pain perceived in chest/left arm (T1-T4 dermatomes). 2) Appendicitis (inflamed appendix, midgut) → diffuse periumbilical/T10 dermatome pain.

A patient presents with ipsilateral ptosis, miosis, and anhidrosis. What is the likely lesion?

Horner's syndrome - interruption of sympathetic outflow (preganglionic T1-T3 ascending to superior cervical ganglion or postganglionic fibers). Disrupts pupillary dilation, eyelid smooth muscle, and facial sweating.

What constitutes the enteric nervous system and is it autonomous?

Myenteric (Auerbach's) and Submucosal (Meissner's) plexuses. It is semi-independent: it can carry out basic peristalsis and secretion without CNS input (denervated gut still functions), but is normally modulated by sympathetic/parasympathetic input.

A patient is given atropine. What is the mechanism and what clinical signs appear?

Atropine is a muscarinic antagonist blocking ACh at parasympathetic targets. Causes dry mouth, blurred vision (mydriasis), urinary retention, and tachycardia.

Which splanchnic nerves carry parasympathetic fibers?

Only the Pelvic splanchnic nerves (S2-S4). All other splanchnic nerves (thoracic, lumbar, sacral) carry sympathetic fibers.

What is the hypogastric plexus?

The continuation of sympathetic trunks and preaortic plexuses into the pelvis; gives rise to Sacral splanchnic nerves (sympathetic) to pelvic viscera.

What is the extent of the vagus nerve (CN X) parasympathetic innervation?

Innervates heart and GI tract from the esophagus to the middle of the colon (splenic flexure). The transition from vagal supply to pelvic splanchnic supply occurs at the splenic flexure.

What is the "rule of thumb" for tracing visceral sensory innervation?

Sensory fibers to a visceral target travel the same overall path as the efferent fibers to that area (i.e., follow the visceral efferent pathway).

Which fibers convey visceral pain vs. non-pain information?

DRG (dorsal root ganglion) afferents convey visceral PAIN (stretch/pressure). The vagus nerve (vagal ganglia) conveys non-pain sensory info (distension, chemoreception) but does NOT convey pain.

Which spinal levels give rise to the sympathetic innervation of the heart?

T1-T4 via sympathetic cardiac nerves from cervical and thoracic paravertebral ganglia to the cardiac plexus.

What are the sympathetic effects on the urinary bladder and sexual function?

Bladder: relaxes detrusor, constricts internal sphincter (prevents voiding). Sex: Stimulates ejaculation (smooth muscle contraction in vas deferens/prostate).

What are the parasympathetic effects on the urinary bladder and sexual function?

Bladder: contracts detrusor, relaxes internal sphincter (promotes voiding). Sex: Stimulates penile erection (via nitric oxide - vasodilation).

What is the clinical significance of the sympathetic trunk extending the entire vertebral column?

It allows postganglionic fibers to reach peripheral targets above and below the T1-L2 outflow. Lesions at any level can cause localized sympathetic deficits (e.g., Horner's if upper trunk affected).

What are the two main groups of sympathetic ganglia?

1) Sympathetic trunk ganglia (paravertebral; sympathetic chain) 2) Prevertebral ganglia (preaortic) - named after major arterial branches (celiac, superior mesenteric, inferior mesenteric).

Where do preganglionic sympathetic fibers first travel after exiting the spinal cord?

Exit via ventral roots → travel through WHITE rami communicantes (myelinated) to the sympathetic trunk. From there, they may synapse in the trunk or pass through via splanchnic nerves to prevertebral ganglia.

What is the ganglion impar?

The caudal end of the sympathetic trunk, where the two trunks converge in the pelvis (anterior to the sacrococcygeal junction).

What is the difference between Sacral Splanchnic nerves and Pelvic Splanchnic nerves?

Sacral splanchnic nerves (from hypogastric plexus) carry SYMPATHETIC fibers. Pelvic splanchnic nerves (from S2-S4) carry PARASYMPATHETIC fibers.

Which structures are innervated by ONLY parasympathetic (no sympathetic)?

Ciliary body, lacrimal gland, salivary glands. (They receive no sympathetic innervation).

Which structures are innervated by ONLY sympathetic (no parasympathetic)?

Most arterial smooth muscle (vasoconstriction), sweat glands, arrector pili muscles (skin), and chromaffin cells of adrenal medulla.

What is the difference between myenteric and submucosal plexuses?

Myenteric (Auerbach's) is between longitudinal and circular muscle layers (controls peristalsis/motility). Submucosal (Meissner's) is in the submucosa (controls secretion and blood flow).

Which cell type gives rise to the neurons of the ANS ganglia?

Neural crest cells (migrate during development to form peripheral autonomic ganglia).