Gastrointestinal Tract (GIT)

Gastrointestinal Tract

Organs of the digestive system are divided into two main groups: organs within the alimentary canal and accessory digestive organs.

The alimentary canal, which is also called the gastrointestinal (GI) tract or gut, is the entire length of tube that winds through the body from the mouth to the anus. It digests, breaks down and absorbs food through its lining into the blood.

The muscular alimentary canal

Mouth

Pharynx

Esophagus

Stomach

Small intestine

Large intestine

Anus

The accessory digestive organs

Supply secretions contributing to the breakdown of food

Teeth & tongue

Salivary glands

Gallbladder

Liver

Pancreas

The Digestive Process

Ingestion

–Taking in food through the mouth

Propulsion (movement of food)

–Swallowing

–Peristalsis – propulsion by alternate contraction &relaxation

Mechanical digestion

–Chewing

–Churning in stomach

–Mixing by segmentation

Chemical digestion

–Complex food molecules (carbohydrates, proteins and lipids) broken down into chemical building blocks (simple sugars, amino acids, and fatty acids and glycerol)

–Carried out by enzymes secreted by digestive glands into lumen of the alimentary canal

Absorption

– Transport of digested end products into blood and lymph in wall of canal

Defecation

– Elimination of indigestible substances from body as feces.

Layers of GI Wall

Four main layers. From inside (the lumen) to outside the are:

Mucosa,Sub mucosa, Muscularis (external), Serosa ( visceral peritoneum). 

Mucosa is the innermost, moist, epithelial membrane that lines the entire digestive tract.

 (1) It secretes mucus, digestive enzymes, and hormones;

 (2) absorbs digestive end products into the blood and

 (3) protects against infectious disease.

•Consists of a lining epithelium, a lamina propria, and a Muscularis mucosa.

•Epithelium - simple columnar epithelium and goblet cells

•Lamina propria - areola C.T. with capillaries and lymphoid follicles

•Muscularis mucosa - thin layer, produces local movements of the mucosa.

Sub mucosa  is a moderately dense connective tissue layer containing blood and lymphatic vessels, lymphoid follicles, and nerve fibers.

Muscularis (external)  typically consists of smooth muscle and is responsible for peristalsis and segmentation. Contains the my enteric plexus of Auerbach, the other major intrinsic nerve plexus. Located between the two layers of smooth muscle, controls motility of the G.I. tract.

Serosa ( visceral peritoneum) the protective outer layer of the intraperitoneal organs, is the visceral peritoneum.

Electrical Activity Of GIT Smooth Muscle

It is consist of two  types:

Slow waves or basic electrical rhythm (BER) and Spike Potentials.

Slow waves or basic electrical rhythm (BER)

These are slow, undulating changes in resting membrane potential of GIT  smooth muscle.  

Caused By:    

Slow, undulation of the pumping activity of Na-K Pump.

Functions:

The basic electrical rhythm (BER) is a slow spontaneous depolarized wave in the GI.

BER of smooth muscle cells form the basis (action potential) of stomach contractions.

The ability of BER to induce smooth muscle contraction is altered by the activity of  extrinsic nerves and hormones, brought into play by stomach and intestine receptors.

Slow waves are not true action potential, but show undulating changes in the resting membrane potential.

Spike Potentials

These are true Action Potentials that occur when Resting Membrane Potential of GIT smooth muscle rises above-40mV.

CAUSED BY:

Opening of slow  Ca-Na channels.

FUNCTIONS:Ca++ ions that enter GIT smooth muscle fiber during spike potential, cause GIT smooth muscle to contract (i.e. Peristalsis).

Potential Changes Of GIT Smooth  Muscle Membrane

Under normal conditions, the resting membrane potential averages about -56 mill volts, but multiple factors can change this level. When the potential becomes less negative, which is called depolarization of the membrane, the muscle fibers become more excitable. When the potential   becomes more negative,   which is called hyper polarization, the fibers become less excitable.

Factors that depolarize the membrane —that's  make it more  excitable—are :

(1) stretching of the muscle,

(2) stimulation by acetylcholine,

(3) stimulation by parasympathetic nerves that secrete acetylcholine at their endings, and

(4) stimulation by several specific gastrointestinal hormones.

Important factors that make the membrane potential more negative—that is, hyperpolarize the membrane and make the muscle fibers less excitable—are:

(1) the effect of nor epinephrine or epinephrine on the fiber membrane and

(2) stimulation of the sympathetic nerves that secrete mainly nor epinephrine at their endings.

Calcium Ions and Muscle Contraction.

Smooth muscle contraction occurs in response to entry of calcium ions into the muscle fiber. Calcium ions, acting through a calmodulin control mechanism, 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.

The slow waves do not cause calcium ions to enter the smooth muscle fiber (only sodium ions). Therefore, the slow waves by themselves usually cause no muscle contraction. Instead, it is during the spike potentials, generated at the peaks of the slow waves, that significant quantities of calcium ions do enter the fibers and cause most of the contraction.

Tonic Contraction of Some Gastrointestinal Smooth Muscle

Some smooth muscle of the gastrointestinal tract exhibits   tonic contraction as well as or instead of rhythmical contractions.

Tonic contraction is continuous, not associated with the basic electrical rhythm of the slow waves but often lasting several minutes or even hours. The tonic contraction often increases or decreases in intensity.

Tonic contractions (continued)- Caused by:

Continuous repetitive spike potential

Hormonal effects

Continuous entry of Ca.

Neural Control of Gastrointestinal

1. Enteric Nervous System

2. Autonomic Nervous System

3. Sensory Nervous System 

Enteric Nervous System It is Intrinsic system of GIT  extending from esophagus  up to anus. And it is subdivided into two.

Myenteric  or Aurbach’s Plexus. Lies between longitudinal and circular muscles layer. And its function is to control GIT movements.

submucosal plexus or Meissner’s plexus: Lies in submucosa. And its function is to control GIT secretions and blood flow. Sub serves  sensory functions by receiving signals from GIT Epithelium and from stretch receptors of GIT wall.

Autonomic Nervous System

Activity is control by sympathetic and parasympathetic  nervous system.

Parasympathetic Nervous System: Vagus Nerve innervates GIT  from esophagus up to  proximal 2/3 of transverse colon.

Pelvic Parasympathetic Nerve:(S2-S4)Innervates GIT from distal 1/3 of transverse colon.

Functions:

Increase peristalsis and tone.

Relaxes Sphincters.

Increase digestive secretions.

Sympathetic Nervous System:  

Preganglionic sympathetic originate in T5 TO L2 segment of spinal cord, Pass thru sympathetic chain, synapse with

Post ganglionic neuron in celiac and mesenteric and hypo gastric ganglia. Postganglionic fibers innervates in GIT.

 Functions

Decrease peristalsis and tone

Contract sphincters

Decrease digestive secretion

 Sensory Nervous System

Sensory nerve endings originate in the gastrointestinal epithelium or gut wall and send afferent fibers to both plexuses of the enteric system, as well as

(1) To the prevertebral ganglia of the sympathetic nervous system,

(2) To the spinal cord, and

(3) In the Vagus nerves all the way to the brain stem.

Functions

These sensory nerves can elicit local reflexes within the gut wall itself and still other reflexes that are relayed to the gut from either the prevertebral ganglia or the basal regions of the brain.

Movements in the Gastrointestinal Tract

Two types of movements occur in the gastrointestinal tract:

1.Mixing movements: ( Segmentation Contraction)

2 Propulsive movements: (Peristalsis) 

Mixing Movements

First the intestinal wall is in relaxing state when it is distended with chyme

(food which is mixed with secretion)

The distension of intestinal wall causes contraction in intestine and these contraction divides the intestine into segments

Each segment is 1-2cm long As one set of segmentation contraction relaxes, a new set often begins but the contraction this time appears at new sites between the previous contraction.

These contraction help in chopping of chyme  and promotes  mixing of food particles with the secretion of small intestine.

Segmentation contraction occurs  12time/min in duodenum and 8time/min in the ileum.

These contraction last for 5-6sec, they occur through out the digestive period.

These segmentation contraction are controlled by the Myenteric plexuses  of Autonomic Nervous System.

Propulsive peristaltic movements:

Chyme is propelled or pushed through small intestine by peristaltic waves.

These waves are produced and response to stretch .This is called Myenteric Reflex.

They occur in any part of small intestine  and move at a velocity of 0.2-2cm/sec. They usually dies off after travel this distance.

From here it starts a new peristalsis state and move a chyme in forward direction, thus several peristaltic waves occur one after the other and push the small intestine contents at the distal end of the small intestine.

Therefore due to the slow movement of peristalsis waves usually 3-5hours are required for passage of chyme from pyrolysis to the ileocecal valve.

These peristaltic wave don't occur alone but are superimposed upon segmental movement of small intestine. Thus  both segmental and peristalsis movements occurs simultaneously.