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Gastrointestinal Tract

Presented By:

The GI tract (gastrointestinal tract) • 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 WallFour main layers. From inside (the lumen) to outside the are:o Mucosao Sub mucosao Muscularis (external)o 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 externa 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, the protective outer layer of the intraperitoneal organs, is the visceral peritoneum

Electrical Activity Of GIT Smooth Muscle

• Two types:

Slow waves or basic electrical rhythm (BER) Spike Potentials

BER BER

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 System2. Autonomic Nervous System3. Sensory Nervous System

1. Enteric Nervous System• It is Intrinsic system of GIT extending from

esophagus up to anus.• Division Two:a. MYENTERIC OR AURBACH’S PLEXUS:• Lies between longitudinal and circular muscle

layer. Functions: Control GIT movements.

b. SUBMUCOSAL PLEXUS OR MEISSNER’S PLEXUS:• Lies in submucosa. Functions: • Control GIT secretions and blood flow.• Sub serves sensory functions by receiving signals

from GIT Epithelium and from stretch receptors of GIT wall.

2. Autonomic Nervous System• Activity is control by sympathetic and parasympathetic nervous system.a. 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. FUNTIONS: Increase peristalsis and tone. Relaxes Sphincters. Increase digestive secretions.

b. 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

3. 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)

1. 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.

2. 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.

Guyton And Hall Physiology text book:Chapter#62 Page:# 771-777.

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