L13 & 14 & 15 AN2102 - Autonomic nervous system & Abdominal Innervation

What is a cluster of neuron cell bodies outside the CNS called?

A ganglion.

What does the somatic nervous system control?

Voluntary motor function (skeletal muscle) and sensory input from skin, joints, and muscles.

Does the somatic nervous system innervate viscera or glands?

No.

What does the autonomic nervous system (ANS) control?

Involuntary motor function of smooth muscle, cardiac muscle, and glands.

Is the ANS sensory or motor?

Purely motor (visceral sensory fibers travel with it but are not part of the ANS).

What are the three divisions of the ANS?

Sympathetic, parasympathetic, and enteric divisions.

What type of fibers do most peripheral nerves contain?

Mixed motor and sensory fibers.1

How many cranial nerves exist?

12 pairs (CN I–XII).

How many spinal nerves exist?

31 pairs (C8, T12, L5, S5, Co1).

What type of fibers does the ventral root carry?

Pure motor (efferent).

Where are ventral horn neuron cell bodies located

In the ventral horn of spinal gray matter.

What type of fibers does the dorsal root carry?

Pure sensory (afferent).

What is housed in the dorsal root ganglion?

Sensory neuron cell bodies

What are primary rami?

The first branches of a spinal nerve: anterior (APR) and posterior (PPR).

What sensations do GSA fibers carry?

Pain, touch, temperature, pressure, proprioception.

What senses belong to SSA fibers?

Vision, hearing, balance

What is the function of GSE fibers?

Voluntary motor control of skeletal muscle.

What do GVA fibers sense?

Stretch, distension, blood pressure, chemical irritation—no “touch.”

What sensation uses SVA fibers?

Taste.

What do GVE fibers control?

Smooth muscle, cardiac muscle, and glands (ANS motor pathway).

What neuron types make up visceral motor pathways?

Preganglionic neurons (CNS) and postganglionic neurons (ganglia).

What muscles are controlled by SVE fibers?

Muscles from pharyngeal arches: mastication (V), facial expression (VII), pharynx/larynx (IX, X).

What type of neuron are sensory neurons?

Pseudounipolar, with cell bodies in ganglia.

What type of neuron are motor neurons?

Multipolar.

How many neurons does a somatic motor pathway use?

One neuron from CNS → skeletal muscle.

How many neurons does a visceral motor pathway use?

Two (preganglionic + postganglionic).

What does the ANS regulate?

Heart rate, digestion, gland secretion, blood vessel tone, and pupil size.

What fiber type does the ANS use?

General visceral efferent (GVE) fibers.

What is the function of the sympathetic system?

“Fight or flight,” energy expenditure, body-wide effects.

Where are sympathetic preganglionic neurons located?

Lateral horn (IML) of T1–L2 spinal cord.

Why does sympathetic innervation reach the entire body?

Because it controls blood vessel tone, and all body regions contain vessels.

What are paravertebral ganglia?

Ganglia forming the sympathetic chain beside the vertebral column.

What are prevertebral ganglia?

Ganglia located in front of the aorta (e.g., celiac, SMA, aorticorenal).

Through which structure do preganglionic sympathetic fibers enter the chain?

The white ramus communicans.

What are the two main sympathetic pathways?

• Synapse in the sympathetic chain (paravertebral).

• or Pass through chain to synapse in prevertebral ganglia (abdominal/pelvic organs).

What is the function of the parasympathetic system?

“Rest and digest,” energy conservation, targeted effects.

Where do parasympathetic preganglionic neurons originate?

Brainstem (CN III, VII, IX, X) and spinal cord S2–S4.

Where are parasympathetic postganglionic neurons located?

In ganglia near or within the target organ.

Which nerve provides most thoracic and abdominal parasympathetic supply?

The vagus nerve (CN X).

• Heart (slows heart rate)

• Lungs (bronchoconstriction, increased mucus)

• Esophagus

• Stomach

• Liver and gallbladder

• Pancreas

• Small intestine

• Large intestine up to the left colic flexure (splenic flexure)

What provides parasympathetic innervation below the left colic flexure?

Pelvic splanchnic nerves (S2–S4).

supplies all parasympathetic fibers to the organs below the splenic flexure, including:

• Distal descending colon

• Sigmoid colon

• Rectum

• Upper anal canal

• Pelvic organs (bladder, uterus, prostate)

• Erectile tissues (via vasodilation in male and female sexual response)

What is the enteric nervous system?

It independently coordinates peristalsis, secretion, and blood flow within the gut, but is modulated by sympathetic and parasympathetic input.

What two autonomic divisions control abdominal viscera?

he sympathetic (“fight or flight”) and parasympathetic (“rest and digest”) systems

Where do sympathetic preganglionic neurons supplying abdominal organs originate?

In the lateral horn (IML) of spinal cord segments T5–L2.

What are the general effects of sympathetic stimulation on the GI tract?

↓ blood flow, ↓ secretion, ↓ motility; ↑ internal sphincter tone; ↑ glycogen breakdown.

Why does sympathetic innervation reach the entire body while parasympathetic does not?

Only sympathetics control vasoconstriction to body-wall vessels, skin, and limbs.

Parasympathetics go to organs like the heart and gut, but they never go to the body wall or limbs, which is why only sympathetics reach the entire body.

What is the function of white rami communicantes?

Carry preganglionic sympathetic fibers from T1–L2 into the sympathetic chain.

Why do gray rami exist at every spinal level?

Postganglionic sympathetic fibers must reach every dermatome and blood vessel.

What are the four possible pathways of a sympathetic preganglionic fiber once inside the chain?

• Ascend → synapse

• Synapse at same level

• Descend → synapse

  • Pass through without synapsing → become a splanchnic nerve

Which splanchnic nerve targets the foregut?

Greater splanchnic (T5–T9) → celiac ganglion.

Which splanchnic nerve mainly supplies the midgut?

Lesser splanchnic (T10–T11) → SMG and aorticorenal ganglion.

Which nerve targets kidneys and upper ureter?

Least splanchnic (T12).

Which nerves supply hindgut and pelvic organs sympathetically?

Lumbar splanchnic nerves (L1–L2/L3) → inferior mesenteric ganglion & hypogastric plexus.

Why is the adrenal medulla a sympathetic exception?

reganglionic fibers synapse directly on chromaffin cells (modified postganglionic neurons).
No postganglionic neuron exists.

What does the adrenal medulla release?

Epinephrine and norepinephrine into the bloodstream → whole-body sympathetic activation.

Epinephrine and norepinephrine into the bloodstream → whole-body sympathetic activation.

Vagus nerve (CN X).

What parasympathetic nerves supply the GI tract below the left colic flexure?

Pelvic splanchnic nerves (S2–S4).

What are general parasympathetic effects on abdominal viscera?

↑ peristalsis, ↑ secretion, ↑ GI blood flow, ↓ internal sphincter tone.

What has no parasympathetic innervation?

Body wall, limbs, sweat glands, arrector pili muscles.

What plexus controls GI motility in the enteric nervous system?

Myenteric (Auerbach’s) plexus.

What plexus controls secretion and local blood flow in the enteric nervous system?

Submucosal (Meissner’s) plexus.

Most visceral pain from abdominal organs above the mid-sigmoid colon travels with which fibers?

Sympathetic fibers.

Which organs send pain through sympathetic pathways?

• Stomach

• Liver & gallbladder

• Small intestine

• Appendix

• Ascending colon

• Transverse colon

• Most of descending colon

• Upper sigmoid (above midpoint)

Pain from organs below the mid-sigmoid colon travels with which fibers?

Parasympathetic fibers (pelvic splanchnic nerves).

Which organs send pain through parasympathetic (S2–S4) pathways?

• Lower sigmoid colon (below midpoint)

• Rectum

• Upper anal canal (above pectinate line)

• Bladder

• Uterus

• Prostate

• Seminal vesicles

  • Other pelvic organs

What is the key landmark separating pain pathways?

the midpoint of the sigmoid colon.

What fibers travel through white rami communicantes?

• Sympathetic preganglionic motor fibers

  • Visceral pain fibers from organs above the mid-sigmoid
    (These enter the sympathetic chain.)

Do parasympathetic fibers ever travel through white rami?

No. White rami are sympathetic only.

What fibers travel through grey rami communicantes?

Sympathetic postganglionic motor fibers
→ to sweat glands, arrector pili, and blood vessels of the body wall.

What is the functional difference between visceral pain above vs below mid-sigmoid?

• Above mid-sigmoid → sympathetics → referred abdominal pain (T1–L2)

  • Below mid-sigmoid → parasympathetics → pelvic/perineal referred pain (S2–S4)

Why does visceral pain refer to dermatomes?

Visceral and somatic afferents converge on the same spinal segments.

Gallbladder irritation can refer pain to which area? Why?

Right shoulder/neck → via phrenic nerve (C3–C5)

A patient has epigastric pain from a gastric ulcer. Which sympathetic spinal levels carry this pain?

T7–T8.

A tumor compressing the celiac ganglion would affect which organs?

Foregut organs: stomach, liver, gallbladder, pancreas, spleen, proximal duodenum.

The celiac ganglion is the major sympathetic ganglion that supplies the entire foregut, so anything affecting it will alter sympathetic control to these organs.

Which symptoms suggest excessive sympathetic activity in the intestines?

Constipation, decreased bowel sounds, decreased secretions.

Loss of vagus nerve function would most severely impact which phases of digestion?

Gastric motility, pancreatic enzyme secretion, bile release, small intestine motility.

Damage to pelvic splanchnic nerves would impair which GI function?

Defecation (due to loss of internal sphincter relaxation and decreased peristalsis in distal colon).

A patient with severe hypotension loses sympathetic tone. What happens to splanchnic vessels?

Without sympathetics, the splanchnic vessels dilate and trap a huge amount of blood, causing severe pooling that makes hypotension dramatically worse.

Which nerve pathway is responsible for stress-induced reduction in bowel motility during anxiety attacks?

Increased sympathetic output via greater/lesser splanchnic nerves → decreased peristalsis.

Which structure is uniquely innervated directly by preganglionic sympathetic fibers?

Adrenal medulla.

A lesion to the inferior mesenteric ganglion affects which regions?

Hindgut: descending colon, sigmoid colon, upper rectum.

Pain from appendicitis first refers to the umbilical region due to which spinal level?

T10 (midgut visceral afferents).

What spinal levels give rise to the greater splanchnic nerve?

T5–T9.

Where does the greater splanchnic nerve synapse?

Celiac ganglion.

What spinal levels give rise to the lesser splanchnic nerve?

T10–T11.

Where does the lesser splanchnic nerve synapse?

Aorticorenal ganglion and superior mesenteric ganglion.

Which GI organs receive fibers from the lesser splanchnic nerve?

Midgut organs (distal duodenum → proximal 2/3 transverse colon) + kidneys.

What spinal level gives rise to the least splanchnic nerve?

T12 only.

Where does the least splanchnic nerve synapse?

Aorticorenal ganglion (sometimes SMG).

what does the least splanchnic nerve innervate?

Kidneys, upper ureter, part of the hindgut.

Which nerves give sympathetic innervation to the hindgut and pelvic organs?

Lumbar splanchnic nerves (L1–L2/L3).

Where do lumbar splanchnic nerves synapse?

Inferior mesenteric ganglion and the superior hypogastric plexus.

Which organs receive lumbar sympathetic fibers?

Descending colon, sigmoid colon, rectum, bladder, prostate, and erectile tissues (vascular tone).

hy must sympathetic fibers to the hindgut descend before synapsing?

Because no sympathetic outflow exists below L2, so they must descend through the chain before forming the lumbar splanchnic nerves.

How do postganglionic sympathetic fibers reach abdominal organs?

They travel along periarterial plexuses that wrap around the abdominal aorta branches.

Which splanchnic nerve is responsible for the fight-or-flight response that decreases intestinal motility in the jejunum and ileum?

Lesser splanchnic nerve (T10–11).

Which splanchnic nerve has the strongest effect on splenic vasoconstriction during sympathetic activation?

Greater splanchnic nerve (T5–T9) → foregut organs including spleen.

Pain from a gastric ulcer refers to which dermatome? Why?

T7–T8, because foregut visceral afferents return with the greater splanchnic nerve at those spinal levels.

A patient presents with flank pain radiating to the groin. Which splanchnic nerve carries these visceral afferents?

Least splanchnic nerve (T12) → kidney & ureter afferents.

Damage to which nerve could impair emission and cause failure of ejaculation?

Lumbar splanchnic nerves (L1–L2), because emission is sympathetic.

Which splanchnic nerve provides the preganglionic fibers that directly stimulate the adrenal medulla?

Greater splanchnic nerve (T5–T9).