Nerve supply of git

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Here is a comprehensive overview of the nerve supply of the gastrointestinal tract (GIT), drawn from Costanzo Physiology, Ganong's Review, Sleisenger & Fordtran's, and Goldman-Cecil Medicine.

Nerve Supply of the GIT

The GIT has two levels of nervous control: extrinsic (from the CNS via the autonomic nervous system) and intrinsic (the enteric nervous system, or ENS, within the gut wall itself).

1. Extrinsic Innervation

A. Parasympathetic Innervation

Nerve fibers: Long preganglionic fibers that synapse in ganglia located within the gut wall (myenteric and submucosal plexuses).

Region	Nerve	Origin
Esophagus (upper 1/3 - striated muscle), stomach, small intestine, ascending colon, proximal transverse colon	Vagus nerve (CN X)	Dorsal motor nucleus, medulla
Distal transverse colon, descending colon, sigmoid colon, rectum	Pelvic nerve (pelvic splanchnic)	Sacral segments S2-S4

Key facts:

The vagus nerve is 75% afferent (sensory - carries info from mechanoreceptors and chemoreceptors to the CNS) and 25% efferent (motor).
Reflexes with both afferent and efferent limbs in the vagus are called vagovagal reflexes.
Postganglionic parasympathetic neurons are cholinergic (release ACh) or peptidergic (release substance P, VIP).
Overall effect: increases motility, increases gastric/intestinal secretion, relaxes sphincters.

B. Sympathetic Innervation

Nerve fibers: Short preganglionic fibers that synapse in prevertebral ganglia outside the gut wall. Postganglionic fibers are adrenergic (release norepinephrine, NE).

Ganglia	Region supplied
Celiac ganglion	Stomach, duodenum, small intestine
Superior mesenteric ganglion	Small intestine, ascending and transverse colon
Inferior mesenteric ganglion	Descending colon, sigmoid, rectum
Hypogastric (pelvic) ganglion	Rectum, internal anal sphincter

Key facts:

Preganglionic neurons arise from the thoracolumbar intermediolateral column (T5-L2).
Postganglionic fibers synapse on ganglia in the myenteric/submucosal plexuses OR directly on smooth muscle, secretory, and endocrine cells.
Overall effect: inhibits non-sphincteric smooth muscle (relaxation), excites sphincters (contraction), reduces secretion - the classic "fight or flight" response.

Extrinsic (parasympathetic and sympathetic) innervation of the GIT. Parasympathetic fibers release ACh or peptides; sympathetic fibers release NE. Both converge on the myenteric and submucosal plexuses. (Costanzo Physiology, Fig. 8.2)

2. Intrinsic Innervation - The Enteric Nervous System (ENS)

The ENS is called the "mini-brain" because it contains all elements of a nervous system - sensory neurons, interneurons, and motor neurons - and can function autonomously even without extrinsic input.

Two Major Plexuses

Plexus	Location	Main Function
Myenteric plexus (Auerbach's)	Between longitudinal and circular muscle layers	Controls motility - peristalsis and muscle contractions
Submucosal plexus (Meissner's)	Between circular muscle and mucosa	Senses luminal environment, regulates secretion, absorption, and mucosal blood flow

Neuronal Types in the ENS

Sensory (afferent) neurons - respond to mechanical, thermal, osmotic, and chemical stimuli in the mucosa.
Interneurons - integrate sensory information and relay to motor neurons.
Motor neurons - control smooth muscle contraction/relaxation, secretion, and absorption.

Intrinsic (enteric) nervous system of the GIT, showing bidirectional communication between the myenteric plexus, submucosal plexus, mucosa, and muscle layers. Modulated by both parasympathetic and sympathetic input. (Costanzo Physiology, Fig. 8.3)

3. Neurotransmitters and Neuromodulators of the ENS

Substance	Source	Key Actions
Acetylcholine (ACh)	Cholinergic neurons	Contracts wall smooth muscle, relaxes sphincters, stimulates secretions
Norepinephrine (NE)	Adrenergic neurons	Relaxes wall smooth muscle, contracts sphincters
VIP (Vasoactive Intestinal Peptide)	Enteric neurons	Relaxes smooth muscle, increases intestinal/pancreatic secretion
Nitric Oxide (NO)	Enteric neurons	Relaxes smooth muscle (key inhibitory neurotransmitter)
Substance P	Co-secreted with ACh	Contracts smooth muscle, stimulates salivary secretion
Enkephalins (opiates)	Enteric neurons	Contracts smooth muscle, decreases intestinal secretion
Neuropeptide Y	Enteric neurons	Relaxes smooth muscle, decreases intestinal secretion
GRP/Bombesin	Vagal neurons in gastric mucosa	Stimulates gastrin secretion

(Costanzo Physiology, Table 8.1)

4. Summary - Functional Contrast

Feature	Parasympathetic	Sympathetic
Fiber length	Long pre-, short post-	Short pre-, long post-
Ganglia location	In gut wall (within plexuses)	Outside gut wall (prevertebral)
Neurotransmitter	ACh (pre- and post-)	ACh (pre-), NE (post-)
Effect on motility	Increases	Decreases
Effect on secretion	Increases	Decreases
Effect on sphincters	Relaxes	Contracts
Effect on blood vessels	Dilates	Constricts

Sources: Costanzo Physiology 7th Ed., p. 344-346 | Ganong's Review of Medical Physiology 26th Ed., p. 274 | Goldman-Cecil Medicine, p. 1405 | Sleisenger & Fordtran's GI and Liver Disease

Myenteric and aurbeck plexuses in the git means

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Here is a clear, complete explanation of these plexuses.

Myenteric Plexus = Auerbach's Plexus

They are the same structure. "Auerbach's plexus" is the eponymous name (named after Leopold Auerbach, a 19th-century German anatomist), and "myenteric plexus" is the descriptive anatomical name - both refer to the exact same neural network.

The Two Plexuses of the Enteric Nervous System

The gut wall contains two main nerve networks (plexuses), both part of the Enteric Nervous System (ENS):

Feature	Myenteric Plexus (Auerbach's)	Submucosal Plexus (Meissner's)
Location	Between the longitudinal and circular muscle layers (within the muscularis externa)	Between the circular muscle and the mucosa (within the submucosa)
Main function	Controls motility - peristalsis, segmentation, muscular contractions	Controls secretion, absorption, mucosal blood flow, and senses the luminal environment
What it innervates	Longitudinal and circular smooth muscle	Secretory cells, endocrine cells, mucosal epithelium, submucosal blood vessels
Neurons present	Sensory (IPANs), interneurons, excitatory & inhibitory motor neurons	Sensory neurons, secretomotor neurons

Myenteric (Auerbach's) Plexus - In Detail

Located between the two muscle layers of the muscularis externa (inner circular + outer longitudinal).
Has a hierarchical structure: primary plexus (main ganglia network) → secondary plexus (smaller branches) → tertiary plexus (finest branches).
Contains ganglia spaced at regular intervals and connected by interganglionic fascicles.
Excitatory motor neurons release ACh and Substance P → contract smooth muscle → propel contents forward (oral to aboral).
Inhibitory motor neurons release NO, VIP, ATP → relax smooth muscle → allow the segment ahead of a bolus to open (receptive relaxation).
This creates the "law of the intestine" (Bayliss and Starling): contraction ABOVE a bolus + relaxation BELOW = peristalsis.

Submucosal (Meissner's) Plexus - In Detail

Located in the submucosa, closer to the mucosal lining.
Has two layers in humans: one near the mucosa and one near the circular muscle, connected by interganglionic fascicles.
There is also a third, deeper plexus called the Schabadasch plexus (within the circular muscle - but it contains no ganglia).
Functions: regulates glandular secretion, water/electrolyte absorption, and blood flow to the mucosa.

Why the Myenteric Plexus is Clinically Important

Disease	Mechanism
Hirschsprung's disease	Absence of myenteric (and submucosal) ganglia in the distal colon → no peristalsis → functional obstruction in newborns
Achalasia	Loss of inhibitory neurons in the myenteric plexus of the lower esophagus → failure of LES relaxation
Gastroparesis	Damage to myenteric neurons (e.g., in diabetes) → impaired gastric emptying
Chagas disease	Trypanosoma cruzi destroys ENS ganglia → megacolon/megaesophagus

Memory Aid

"Myo" = muscle → Myenteric plexus lies between muscle layers → controls muscle (motility)

"Sub-mucosal" → lies below the mucosa → controls mucosal secretion

Sources: Sleisenger & Fordtran's GI and Liver Disease, p. 1954 | Fischer's Mastery of Surgery, p. 2860 | Mulholland & Greenfield's Surgery | Harrison's Principles of Internal Medicine 22E

Function of it .. relation to sympathetic and parasympathetic

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Excellent sources gathered. Here is a thorough, well-organized answer.

Functions of the Myenteric & Submucosal Plexuses + Their Relationship to Sympathetic/Parasympathetic Systems

Functions of Each Plexus

1. Myenteric Plexus (Auerbach's) - "The Motor Plexus"

Primary role: Controls GIT motility

Function	Mechanism
Peristalsis	Coordinates contraction above a bolus + relaxation below (the "law of the intestine")
Segmentation	Mixing contractions for digestion and absorption
Sphincter control	Relaxes sphincters (LES, pylorus, ileocecal valve, internal anal sphincter)
Tone regulation	Sets baseline smooth muscle tone throughout the gut

Excitatory motor neurons → release ACh + Substance P → contract longitudinal and circular muscle
Inhibitory motor neurons → release NO + VIP + ATP → relax smooth muscle (especially important for sphincters)

2. Submucosal Plexus (Meissner's) - "The Secretory/Sensory Plexus"

Primary role: Controls secretion, absorption, and mucosal blood flow

Function	Mechanism
Secretion	Stimulates glandular epithelium to secrete water, electrolytes, and enzymes
Absorption	Regulates water and electrolyte absorption
Mucosal blood flow	Controls submucosal arterioles (vasodilation during digestion)
Sensory monitoring	Detects chemical and mechanical stimuli from the lumen via mechanoreceptors and chemoreceptors

How Sympathetic & Parasympathetic Connect to the Plexuses

The key concept: Neither the sympathetic nor parasympathetic system talks directly to the smooth muscle as their primary route. Instead, they mostly synapse on the ganglion cells of the myenteric and submucosal plexuses and modulate them. The plexuses are the final common pathway.

Parasympathetic → Plexus Relationship

Vagus nerve (CN X) / Pelvic nerve (S2-S4)
        ↓  [long preganglionic fiber, releases ACh]
Ganglion cells IN the myenteric & submucosal plexuses
        ↓  [short postganglionic fiber, releases ACh or peptides]
Smooth muscle / Secretory / Endocrine cells

Effect on plexus:

Preganglionic fibers synapse directly onto ENS ganglia → the ENS ganglia are the postganglionic parasympathetic neurons
Activates and amplifies ENS activity
Net result: increases motility, increases secretion, relaxes sphincters

"Parasympathetic cholinergic activity generally increases the activity of intestinal smooth muscle." - Ganong's Physiology

Sympathetic → Plexus Relationship

Spinal cord (T5-L3) [short preganglionic, releases ACh]
        ↓
Prevertebral ganglia (celiac, sup/inf mesenteric, hypogastric)
        ↓  [long postganglionic fiber, releases NE]
Synapse on ENS ganglion cells in myenteric/submucosal plexuses
        OR directly onto smooth muscle

Effect on plexus - TWO mechanisms:

Mechanism	Detail
Presynaptic inhibition (main route)	Postganglionic sympathetic fibers end on cholinergic neurons of the ENS. NE activates α2 presynaptic receptors → inhibits ACh release from ENS neurons → suppresses plexus activity
Direct muscle inhibition	Some sympathetic fibers bypass the plexus and act directly on smooth muscle cells → relaxation
Sphincter contraction	NE acts on α1 receptors of sphincter smooth muscle → contraction

Net result: decreases motility, decreases secretion, contracts sphincters

"Sympathetic fibers end on postganglionic cholinergic neurons, where norepinephrine inhibits ACh secretion by activating α2 presynaptic receptors." - Ganong's Review of Medical Physiology, p. 471

The Big Picture - How It All Works Together

                    ┌─────────────────────────────┐
                    │      CENTRAL NERVOUS SYSTEM  │
                    └──────────┬──────────────────┘
                               │
              ┌────────────────┴────────────────┐
              ▼                                  ▼
     PARASYMPATHETIC                        SYMPATHETIC
   (Vagus / Pelvic n.)              (Celiac / Mesenteric ganglia)
   Long preganglionic                  Short preganglionic
   ACh → nicotinic                     ACh → nicotinic
              │                                  │
              ▼                                  ▼
    ┌─────────────────────────────────────────────────┐
    │           ENTERIC NERVOUS SYSTEM (ENS)          │
    │   Myenteric plexus ←──→ Submucosal plexus       │
    │  (Auerbach's)              (Meissner's)          │
    │   ~100 million neurons - can act autonomously    │
    └──────────────────────┬──────────────────────────┘
                           │
           ┌───────────────┼──────────────────┐
           ▼               ▼                  ▼
    Smooth muscle    Secretory cells    Endocrine cells

Summary Table

Feature	Parasympathetic Effect on Plexus	Sympathetic Effect on Plexus
Where synapse occurs	ON ENS ganglia (long pre → ENS as postganglionic)	ON ENS cholinergic neurons (presynaptic inhibition) OR directly on muscle
Neurotransmitter	ACh (at ENS ganglion)	NE (inhibits ENS via α2 receptors)
Effect on motility	Increases (via ENS activation)	Decreases (by suppressing ENS)
Effect on secretion	Increases	Decreases
Effect on sphincters	Relaxes	Contracts (via α1 on sphincter muscle)
Effect on blood vessels	Dilates (via VIP/NO from ENS)	Constricts (NE on vessel α1)
ENS dependency	ENS is the postganglionic neuron	ENS is the target of inhibition

Key Point to Remember

The ENS can function entirely on its own (even after cutting all extrinsic nerves). The sympathetic and parasympathetic systems modulate ENS activity - they don't replace it. The ENS is the master controller; the ANS is its volume dial.

Sources: Ganong's Review of Medical Physiology 26th Ed., p. 471 | Costanzo Physiology 7th Ed., p. 344-346 | Goldman-Cecil Medicine, p. 1405

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