GASTROINTESTINAL TRACT LAYERS

GASTROINTESTINAL TRACT
LAYERS
JAI NARAIN VYAS UNIVERSITY, JODHPUR
ASSISTANT PROFESSOR:- ASHWIN SINGH
CHOUHAN
DEPARTMENT:- PHARMACOLOGY
E-mail:- anshukavya1993@gmail.com

JNVU PHARMACY, JODHPUR
BASIC STRUCTURE
The gastrointestinal tract is a muscular tube lined by a
special layer of cells, called epithelium.
The contents of the tube are considered external to the
body and are in continuity with the outside world at the
mouth and the anus.
Although each section of the tract has specialised
functions, the entire tract has a similar basic structure
with regional variations.

The wall is divided into four layers as follows:
MUCOSA
The innermost layer of the digestive tract has specialised
epithelial cells supported by an underlying connective tissue
layer called the lamina propria.
The lamina propria contains blood vessels, nerves,
lymphoid tissue and glands that support the mucosa.
Depending on its function, the epithelium may be simple (a
single layer) or stratified (multiple layers).
Areas such as the mouth and oesophagus are covered by
a stratified squamous (flat) epithelium so they can survive
the wear and tear of passing food. Simple columnar (tall) or
glandular epithelium lines the stomach and intestines to aid
secretion and absorption.
The inner lining is constantly shed and replaced, making it
one of the most rapidly dividing areas of the body! Beneath
the lamina propria is the muscularis mucosa. This comprises
layers of smooth muscle which can contract to change the
shape of the lumen.

Layers of Tissue Within the Mucosa
Since the mucosa is the innermost layer within the GI
tract, it surrounds an open space known as the lumen.
Food, mucus, and digestive juices pass through the
lumen, and the mucosa comes in direct contact with
digested food (chyme).
The mucosa is made up of three layers:
The epithelium is the innermost layer and it is
responsible for most digestive, absorptive, and secretory
processes.
The lamina propria is a layer of connective tissue that is
unusually cellular compared to most connective tissue.
The muscularis mucosae is a thin layer of smooth
muscle and its function is still under debate.

The mucosae (singular: mucosa) are highly specialized
in each organ of the gastrointestinal tract in order to deal
with different digestive tract conditions. The most
variation is seen in the epithelium tissue layer of the
mucosa.
In the esophagus, the epithelium is stratified,
squamous, and non-keratinizing, for protective purposes.
In the stomach. the epithelium is simple columnar, and
is organized into gastric pits and glands to deal with
secretion.
In the small intestine, the epithelium (particularly the
ileum) is specialized for absorption, with villi and
microvilli increasing surface area.

SUBMUCOSA
The submucosa surrounds the muscularis mucosa and
consists of fat, fibrous connective tissue and larger
vessels and nerves.
At its outer margin there is a specialized nerve plexus
called the submucosal plexus or Meissner plexus.
This supplies the mucosa and submucosa.
The Muscularis
The muscularis in the stomach differs from that of other
GI organs in that it has three layers of muscle instead of
two.
Under these muscle layers is the adventitia—layers of
connective tissue that are continuous with the omenta.

MUSCULARIS EXTERNA
This smooth muscle layer has inner circular and outer
longitudinal layers of muscle fibres separated by the
myenteric plexus or Auerbach plexus.
Neural innervations control the contraction of these
muscles and hence the mechanical breakdown and
peristalsis of the food within the lumen.

The submucosa consists of a dense irregular layer of
connective tissue with large blood vessels, lymphatics,
and nerves that branch into the mucosa and muscularis
externa. It contains Meissner’s plexus, an enteric nervous
plexus, situated on the inner surface of the muscularis
externa.
In the gastrointestinal tract, the submucosa is the layer
of dense irregular connective tissue or loose connective
tissue that supports the mucosa. It also joins the mucosa
to the bulk of underlying smooth muscle (fibers running
circularly within layer of longitudinal muscle).
Blood vessels, lymphatic vessels, and nerves (all
supplying the mucosa) will run through here. Tiny
parasympathetic ganglia are scattered around forming
the submucosal plexus (or Meissner’s plexus) where
preganglionic parasympathetic neurons create synapses
with the postganglionic nerve fibers that supply the
muscularis mucosae. JNVU PHARMACY, JODHPUR

Muscularis
The muscularis is responsible for the segmental
contractions and peristaltic movements in the
gastrointestinal (GI) tract.
The gastrointestinal (GI) tract is composed of four
layers of tissue, known as tunics. Each layer has different
structures and functions. From the inside out they are
called the mucosa, submucosa, muscularis externa, and
serosa.

Structure of the Muscularis Externa
The muscularis externa is responsible for segmental
contractions and peristaltic movement in the GI tract. These
muscles cause food to move and churn together with
digestive enzymes down the GI tract. The muscularis
externa consists of an inner circular layer and a longitudinal
outer muscular layer. It should not be confused with a thin
layer of muscle known as the muscularis mucosa, which lies
within the submucosa, a layer of tissue adjacent to the
muscularis externa.
The muscularis mucosa is made up of smooth muscle, and
is most prominent in the stomach.
Within the muscularis externa, the circular muscle layer
prevents food from traveling backward, while the
longitudinal layer shortens the tract. The layers are not truly
longitudinal or circular, rather the layers of muscle are
helical with different pitches. The inner circular is helical
with a steep pitch and the outer longitudinal is helical with a
much shallower pitch. JNVU PHARMACY, JODHPUR

The coordinated contractions of these layers is called
peristalsis. Between the two muscle layers is the
myenteric or Auerbach’s plexus, which controls
peristalsis.
Peristaltic activity is regulated by these nerve cells, and
the rate of peristalsis can be modulated by the rest of the
autonomic nervous system.
The thickness of muscularis externa varies in each part
of the tract. In the colon, for example, the muscularis
externa is much thicker because the feces are large and
heavy, and require more force to push along.
The outer longitudinal layer of the colon thins out into
three discontinuous longitudinal bands known as tiniae
coli (bands of the colon).
This is one of the three features helping to distinguish
between the large and small intestine.

Occasionally in the large intestine (two to three times a
day), there will be mass contraction of certain segments,
moving a lot of feces along. This is generally when one
gets the urge to defecate.
The pylorus of the stomach has a thickened portion of the
inner circular layer: the pyloric sphincter. Alone among
the GI tract, the stomach has a third layer of muscularis
externa. This is the inner oblique layer, and helps churn
the chyme in the stomach

SEROSA/MESENTERY
The outer layer of the GIT is formed by fat and another
layer of epithelial cells called mesothelium.
The outer lining of organs and body cavities of the
abdomen and chest, including the stomach.
This layer of visceral peritoneum invests the organ to a
varying extent depending on the particular segment of
the organ and its mesenteric attachments.

The Serous Membrane
In anatomy, the serous membrane (or serosa) is a
smooth membrane that consists of a thin connective
tissue layer and a thin layer of cells that secrete serous
fluid. Serous membranes line and enclose several body
cavities, known as serous cavities, where they secrete a
lubricating fluid to reduce friction from muscle
movements.
Serosa is not to be confused with adventitia, a connective
tissue layer that binds together structures rather than
reduces friction between them.

Each serous membrane is composed of a secretory
epithelial layer and a connective tissue layer underneath.
The epithelial layer, known as mesothelium, consists of a
single layer of avascular flat nucleated cells (simple
squamous epithelium) that produce the lubricating serous
fluid. This fluid has a consistency similar to thin mucus.
These cells are bound tightly to the underlying connective
tissue. The connective tissue layer provides the blood
vessels and nerves for the overlying secretory cells, and
also serves as the binding layer that allows the whole
serous membrane to adhere to organs and other
structures.
For the heart, the surrounding serous membranes
include: the outer, inner, parietal pericardium, and
visceral pericardium (epicardium). Other parts of the
body may also have specific names for these structures.
For example, the serosa of the uterus is called the
perimetrium.

The pericardial cavity (surrounding the heart), pleural
cavity (surrounding the lungs) and peritoneal cavity
(surrounding most organs of the abdomen) are the three
serous cavities within the human body. While serous
membranes have a lubricative role to play in all three
cavities, in the pleural cavity it has a greater role to play
in the function of breathing.
The serous cavities are formed from the intraembryonic
coelom and are basically an empty space within the body
surrounded by a serous membrane. Early in embryonic
life, visceral organs develop adjacent to a cavity and
invaginate into the bag-like coelom.
Therefore each organ becomes surrounded by a serous
membrane—they do not lie within the serous cavity. The
layer in contact with the organ is known as the visceral
layer, while the parietal layer is in contact with the body
wall.

GASTROINTESTINAL TRACT LAYERS

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