Introduction to

Gastrointestinal Physiology

What we will learn today …….

• Different parts of gastrointestinal system

• Overall Function

• Physiologic anatomy

• Gastrointestinal smooth muscle

• Electrical activity of GI muscle

• Neural control

Functions of GI tract

1) Digestion and absorption

2) Excretion of waste materials

3) Fluid and electrolyte balance

4) Immunity

5) Intestinal bacterial flora

Each part is adapted to its specific functions:

a. Simple passage of food: Esophagus

b. Temporary storage of food: Stomach

c. Digestion and absorption: Small intestine

Physiologic Anatomy of the Gastrointestinal Wall

Muscle fibers are electrically connected with one another through large numbers of gap junctions allow low-resistance movement of ions from one muscle cell to the next

Electrical Activity of Gastrointestinal Smooth Muscle Smooth muscle of the GIT is excited by almost continual slow, intrinsic electrical activity along the membranes of the muscle fibers.

Two basic types of electrical waves: (1)Slow waves and(2)Spikes,

Voltage of the RMP of the gastrointestinal smooth muscle can be made to change to different levels

"Slow waves" of smooth muscle membrane potential

Not action potentialsSlow, undulating changes in RMP bet -65to 40 mVBER (Basal Electrical Rhythm)- slow wave which oscillate significantly

Frequency ranges: 3/min in the body of the stomach, 12/min in the duodenum, and about 8 or 9/min in the terminal ileum.

Precise cause of the slow waves not completely understood

complex interactions among the smooth muscle cells and specialized cells, called the interstitial cells of cajal, (electrical pacemakers for smooth muscle cells)

Unique ion channels periodically open and produce inward (pacemaker) currents may generate slow wave activity

Do not cause muscle contraction

Spike potentials

True AP

Occur automatically when the RMP >-40 mV

Higher the slow wave potential rises, the greater the freq of the spike potentials: usubet 1 - 10 spikes per sec

Channels responsible for AP :

• Large numbers of Ca ions + smaller numbers of Na ions (calcium-sodium channels)

• Slower to open and close • Long duration of AP (10 - 20 ms)

• Movement of large amounts of Ca ions to the interior of the muscle fiber during AP plays a special role in causing the intestinal muscle fibers to contract

Factors that depolarize the membrane

(1)Stretching of the muscle, (2)Stimulation by Ach released from the endings of parasympathetic

nerves,(3)Stimulation by several specific GIT hormones.

Factors that hyperpolarize the membrane

(1)Effect of NE or E on the fiber membrane and (2)Stimulation of the sympathetic nerves

Neural Control of Gastrointestinal Function

Enteric nervous system

Autonomic nervous system: parasympathetic and sympathetic

Enteric Nervous System

Lies entirely in the wall of the gutesophagus to anusNumber of neurons: about 100 million

Control gastrointestinal movements and secretion

Composed of two plexuses:

Outer plexus/ Myentericplexus / Auerbach's plexus • Bet the long. and circ.

muscle layers

Inner plexus/Submucosalplexus / Meissner's plexus• lies in the submucosa

Controls gastrointestinal movements

Controls gastrointestinal secretion and local blood flow

Submucosal plexus

• Control function within the inner wall of each minute segment of the intestine

Control local intestinal secretion, local absorption,and local contraction of the submucosal muscle that causes various degrees of infolding of the gastrointestinal mucosa

Types of Neurotransmitters Secreted by Enteric Neurons

(1)acetylcholine excites(2) Norepinephrine inhibits(3) adenosine triphosphate,(4) serotonin,(5) dopamine,(6) cholecystokinin,(7) substance P,(8) vasoactive intestinal polypeptide,(9) somatostatin,(10) leu-enkephalin,(11) met-enkephalin, and (12) bombesin.

• Extrinsic sympathetic and parasympathetic fibers connect both the myentericand submucosal plexuses

• Stimulation by these greatly enhance or inhibit gastrointestinal functions

Autonomic nervous system

Sensory nerve endings send afferent fibers to• Both plexuses of the enteric system, • Prevertebral ganglia of the sympathetic

nervous system,• Spinal cord, and • Vagus nerves all the way to the brain stem

Sensory nerves can elicit• Local reflexes within the gut wall itself • Other reflexes that are relayed to the gut

from either the prevertebral ganglia or the basal regions of the brain

What we learned today …….

• Different parts of gastrointestinal system

• Overall Function

• Physiologic anatomy

• Gastrointestinal smooth muscle

• Electrical activity of GI muscle

• Neural control