Enteric Nervous System

4 Layers of the Alimentary Canal

Mucosa, Submucosa, Muscularis Externa, and Serosa

What are parts of the mucosa?

Epithelium, laminate propria, muscularis mucosa

What can be found on the submucosa?

o Aeriolar connective tissue & dense connective tissue

o Contains glands

o Contains Submucosal Plexus (or Meissner’s plexus)

Components of the muscularis externa

o Inner circular layer

o Outer longitudinal layer

o Stomach has an additional layer called the inner oblique layer

o In between there is the Myenteric plexus (Auerbach plexus

This is basically the mesothelium

Serosa

If a structure has a serosa it is

intraperitoneal

If it doesn’t have serosa, it is _______. Instead, they are anchored to posterior abdominal wall and called _______.

retroperitoneal, adventitia

What is the function of the circular layer?

Contracts and produces a constriction ring and pushes behind the bolus and pushes it forward

What is the function of the longitudinal layer?

When it contracts it opens the lumen in front of the food.

What does the ascending fibers do to the circular muscle layer?

fibers from the stretch receptors release acetylcholine + substance P to the circular muscle layer  causes it to CONTRACT (by increasing cation influx)

What does the ascending fibers do to the longitudinal muscle layer?

fibers release vasoactive intestinal peptide (VIP) + Nitric Oxide  potassium ions to leave the cell  hyperpolarization  causes the muscle to RELAX

What does the descending fibers do to the circular muscle layer?

fibers release VIP + NO  cause potassium ions to leave the cell  hyperpolarization  RELAX  receptively open

What does the descending fibers do to the longitudinal muscle layer?

fibers release acetylcholine +substance P to the longitudinal layer  CONTRACT  opens up the lumen to accept the bolus

more powerful stimulators for the submucosal plexus are

Chemoreceptors

Function of Chemoreceptors:

release chemicals depending on the food eaten

How are the chemoreceptors stimulated in the context of submucosal plexus?

Chemical substances from food → chemoreceptors stimulated → activate submucosal plexus → stimulate gland to secrete secretions to the lumen to help digest the food

How are the chemoreceptors stimulated in the context of Blood vessels?

Blood vessels dilate - more blood flow - increase absorption of substances to the circulation

Overall results of sympathetic nervous system in GI

- ↓ Motility

- ↓ Blood Flow

- ↓ Secretions

- ↓ Absorption

- constricts sphincters

Overall results of parasympathetic nervous system in GI

 ↑ Motility

 ↑ Secretions

 ↑ Absorption

 Exception: Relaxes Sphincters

are completely peripheral and only involve

the local integration of sensory input with motor output

Short reflexes

Example of short reflexes:

 Afferent: chemo/stretch receptor

 Integration center: plexus

 Effect: dilation, secretion, motility, etc.

have afferent branches that enter the spinal cord or brain and involve the efferent branches

Long reflexes

activated when food stretches the stomach, and so it reduces the tone in the muscular wall of the body of the stomach allowing the wall to bulge progressively outward, accommodating greater and greater quantities of food up to a certain limit.

Vagovagal reflex

inhibitory reflex which is initiated by:

 ↑↑↑ acid in the intestines

 ↑↑↑ distention of stomach

Signals from the colon and small intestine inhibit stomach motility and stomach secretion.

Since the rate of stomach emptying is slower, sufficient time is ensured for adequate digestion in the duodenum and small intestine.

Enterogastric reflex

initiated by feces entering the descending colon and allows defecation

Defecation reflex

Where do the longitudinal muscle layer extend?

Down the intestinal tract

Where do the circular muscle layer extend?

Around the gut

muscle fibers are electrically connected with one another through

Gap junctions

What do you mean by each muscle layer functions as a syncytium

when an action potential is elicited anywhere within the muscle mass, it generally travels in all directions in the muscle

What is the rhythm of contraction of the body of the stomach?

3/min

What is the rhythm of contraction of the duodenum?

12/min

What is the rhythm of contraction of the terminal ileum?

8-9/min

What is the cause of slow waves?

interactions among the smooth muscle cells and interstitial cells of Cajal

These are the electrical pacemakers for smooth muscle cells

interstitial cells of Cajal

What is the function of slow waves?

They do not cause muscle contraction except in the stomach but excite the appearance of intermittent spike potentials

What is the function of spike potentials?

Excite the muscle contraction

When do spike potentials occur?

when the resting membrane potential of the gastrointestinal smooth muscle becomes more positive than about −40 millivolts

What is the normal resting membrane potential in the smooth muscle fibers of the gut?

between −50 and −60 millivolts

In nerve fibers, how are action potentials caused?

by rapid entry of sodium ions through sodium channels to the interior of the fibers.

In gastrointestinal smooth muscle fibers, what causes action potentials?

allow especially large numbers of calcium ions to enter along with smaller numbers of sodium ions and therefore are called calcium-­ sodium channels.

Under normal conditions, what is the average resting membrane potential?

-56 millivolts

What happens when potential becomes less negative (depolarization)

Muscle fibers become more excitable

What happens when potential becomes more negative (hyperpolarization)?

Fibers become less excitable

What are the factors that depolarize the membrane?

Stretching of muscle, stimulation by acetylcholine and stimulation by GI hormones

What are the factors that hyperpolarize the membrane?

Effect of norepinephrine or epinephrine and stimulation of sympathetic nerves

What ions are mainly responsible for smooth muscle contraction?

Calcium ions because they activate the myosin filaments in the fiber, causing attractive forces to develop between the myosin filaments and the actin filaments, thereby causing the muscle to contract.

What are the ions that ennter the smooth muscle fiber due to slow waves?

Sodium ions

Causes of tonic contraction

Continuous repetitive spike potentials, hormones, continuous entry of calcium ions into the cell

Function of the myenteric plexus

Controls mainly the gastrointestinal movements

Function of the submucosal plexus?

Controls GI secretion and local blood flow

What neurotransmitter most often excites GI activity?

Acetylcholine

What neurotransmitter almost always inhibits GI activity?

Norepinephrine and epinephrine

Where does most of the cranial parasympathetic nerve fibers found?

Vagus nerves

Where does the sacral parasympathetics originate?

Second, third, and fourth sacral segments of spinal cord

Where does the postganglionic neurons located?

Myenteric and submucosal plexuses

Where does the sympathetic fibers originate?

between T5 and L1 of the spinal cord

Sensory (towards brain/spinal cord)

Afferent

Motor (away from brain/spinal cord)

Efferent

Secreted by G cells that stimulates gastric acid secretion

Gastrin

Secreted by I cells that stimulates Pancreatic enzyme secretion, bicarbonate secretion, Gallbladder contraction And Inhibits Gastric emptying

Cholecystokinin

Secreted by S cells that stimulates Pepsin secretion, and inhibits Gastrin release and gastric acid secretion

Secretin

Secreted by K sells that stimulates insulin release

Gastric inhibitory peptide or glucose dependent insulinotropic peptide

Secreted by M cells that stimulates gastric and intestinal motility

Motilin