Introduction

• The primary function of the digestive system is to transfer nutrients, water, and electrolytes from the food we eat into the body’s internal environment.
• Which is an essential energy source/fuel, from which the cells can generate ATP to carry out their activities.

Digestive Tract

• A long muscular tube with many sections and areas.
• Parts of the Digestive Tract 1.Mouth

2.Esophagus 3.Stomach 4.Small Intestine 5.Large Intestine

Accessory Parts

• Organs that are not in the digestive tract but helps in the digestion

1.Teeth 2.Tongue 3.Salivary glands 4.Liver

• Gall bladder
• Pancreas

Digestive System phases/processes

Ingestion – occurs when material enters via the mouth

•             Mechanical Processing –Crushing/Shearing–makes material easier to move (motility) through the tract

•             Digestion—Chemical      breakdown         of            food      into        small      organic compounds for absorption

•             Secretion             –Release             of            water,   acids,     buffers,                enzymes &         salts       by epithelium of GI tract and glandular organs

•             Absorption –Movement of organic substrates, electrolytes, vitamins & water across digestive epithelium •             Excretion –Removal of waste products from body fluids

Physiological Anatomy Of Intestinal Wall

Cross section of the intestinal wall, from the outer surface to inward

1. The serosa,
2. A longitudinal smooth muscle layer
3. A circular smooth muscle layer 4.The submucosa

The mucosa (additional some mucosal muscle)

GI Smooth Muscles—Functional Syncytium

•             Smooth muscle has Parallel arrangement up to 1000 fibers.

•             Connected electrically via gap junctions.

•             Gap junctions allow low-resistance movement of ions from one muscle cell to the other.

•             Each muscle layer functions as a syncytium i.e. When an action potential is elicited anywhere within the muscle, it travels in all directions in the muscle.

•             Distance it travels ,depends on the excitability of the muscle.

Electrical Activity of GIT

Resting membrane potential of GIT

• Averages about (-56mv)
• Can change to different levels because of multiple factors
• Range (-50mv to -60mv)

When the potential becomes less negative i.e. Depolarization (EXITABILITY)

•Factors that depolarize the membrane

•Stretching of muscle

•Stimulation by acetylcholine from

parasympathetic nerves

•Stimulation by specific gastrointestinal

hormones

When the potential becomes more negative

i.e. Hyperpolarization (LESS EXCITABILITY)

•Factors that hyperpolarize the membrane

•Effect of norepinephrine or epinephrine on

the fiber membrane Stimulation of the sympathetic nerves that secrete mainly norepinephrine at their endings.

Two basic types of electrical waves1.Slow waves 2.Spikes

SLOW WAVE POTENTIAL

Slow waves are not true action potentials(wave of partial depolarization).

• Are slow, undulating changes in the resting membrane potential–basic electrical rhythm or BER.
  • Intensity usually varies between 5 and 15 millivolts
  • Frequency              varies              in              different              parts              of gastrointestinal tract from 3 to 12 per minute
    • Body of stomach—3/min
    • Terminal ileum—8-9/min

Duodenum—12/min

SLOW WAVE POTENTIAL

Slow wave potential is usually caused by complex interactions between smooth muscle cells and interstitial cells of cajal

• These specialized cells are placed between the smooth muscle layers
  • Make synaptic-like contacts to smooth muscle cells.
  • Bears unique ion channels that open intermittently and produce inward slow rising currents–generate slow wave activity.
  • Thus interstitial cells of cajal acts as electrical pacemakers for smooth muscle cells.

Spike potential

• Slow waves excite the spike potential.
• Occur   automatically   at   crest   of   slow waves when rmp becomes positive than

−40mv (normal RMP −50&−60mv)

Spike potential

Each GI spike last 10—20msec (10-40 times longer than action potential in nerve).

• In nerve fibers, the action potentials caused by rapid entry of Na+ ions through sodium channels to the interior of the fibers.
  • In GI muscle fibers, action potentials is caused by Ca+2/Na+ channels by which large numbers of Ca+2 to enter along with smaller numbers of Na+ .

These channels are slower to open and close than are the rapid sodium channels of large nerve fibers thus causes longer duration of action potentials.

Entry of Calcium Ions Causes Smooth Muscle Contraction

• Slow waves do not cause calcium ions to enter the smooth muscle fiber, just cause entry of sodium ions.
• Thus   slow   waves   does   not            cause muscle  contraction,            but     just coordinates peristaltic activity.
• It is during spike potential, calcium ions enter and cause contraction.

TONIC CONTRACTION

Some smooth muscles of GI exhibit tonic contractions as well or instead of rhythmical contractions.

• It is continuous, last for several minutes or hours.
  • It varies in intensity but continuous.

Reasons:

1. Repetitive spike potential
2. Hormones that cause partial state of depolarization without causing actual action potential.
3. Entry of calcium ions into cell in such a way that does not cause change

in membrane potential Details still not clear……..

Enteric Nervous System/GUT brain

• The GIT has its nervous system.
• It lies in the wall of gut.
• It begins from the esophagus and extending all the way to anus.
• About 100 million neurons are present in it.
• Contain as many neurons as in spinal cord.
• Has quality of self-regulation.
• Important in controlling movements and secretions.
• Composed of two main plexuses.
• Myenteric plexus (Auerbach’s plexus)
• Sub mucus plexus (Meissner’s plexus)

MYENTERIC PLEXUS OR AUERBACH’S PLEXUS

Outer plexus lying between the longitudinal and circular muscle layers.

• Mostly   of   a   linear   chain   of                many interconnecting                neurons          that extends the entire length of the gastrointestinal tract.
• Concerned mainly with controlling muscle activity along the length of the gut.

Functions

(1)Increased tonic contraction, or “tone,” of the gut wall (2)Increased intensity of the rhythmical contractions (3)Increased rate of the rhythm of contraction

(4)Increased velocity of conduction of excitatory waves causing more rapid movement of the gut peristaltic waves.

MYENTERIC PLEXUS OR AUERBACH’S PLEXUS

This plexus is not entirely excitatory because some of its neurons are inhibitory Because their fiber secrete inhibitory transmitter possibly vasoactive intestinal polypeptide

• Inhibitory signals are especially useful for inhibiting some of the intestinal sphincter muscles that impede movement of food in GIT
  • The pyloric sphincter, which controls emptying of the stomach into the

duodenum, and

• Sphincter of the ileocecal valve, which controls emptying from the small intestine into the cecum.

SUBMUCOSAL PLEXUS OR MEISSNER PLEXUS

Inner plexus that lies in submucosa.

• Function within the inner wall of each minute segment of the intestine
• Many sensory signals originate from the gastrointestinal epithelium and are then integrated in the submucosal plexus.

Functions:

It controls the local intestinal secretion, local absorption and local contraction of submucosa plexus.

NEUROTRANSMITTERS

1. Acetylcholine (excitatory)
2. Norepinephrine (inhibitory) 3.Serotonin

4.Dopamine 5.Cholecystokinin 6.Substance P 7.Somatostatin

Autonomic nervous systemParasympathetic stimulation

Cranial Division

• Mostly vagus except in mouth and pharyngeal region
• Provide innervation to esophagus, stomach, pancreas and up to 1^st half of large intestine.

Sacral division

• Originate from 2nd,3rd and 4^th sacral segments of spinal cord and pass through pelvic nerve.
• Provide innervation to second half of large intestine especially sigmoidal, rectal and anal regions

Post ganglionic fibers OF PARASYMPATHETIC SYSTEM lie in ENS and increase the activity of ENS (stimulate motility & secretions).

Autonomic nervous system

Sympathetic stimulation

It is supplied by the segments T5 and L2.

• Preganglionic after leaving the cord enter the sympathetic chains lie on lateral side of spinal cord.
• many of these fibers then pass on through the chains to outlying ganglia such as to the celiac ganglion and various mesenteric ganglia.
• From here fibers innervate all parts of gut.

Functions—

• It inhibits smooth muscle contractions by secreting Norepinephrine.
• Reduce peristalsis & secretions

Cause Contraction of sphincter muscles

Extrinsic sympathetic and parasympathetic fibers connect both myenteric plexus and submucosal plexus.

Although enteric system can work independently but stimulation by parasympathetic and sympathetic system can greatly enhance or inhibit gastrointestinal functions.

Movements in G.I.T

Two types:

1. Mixing
2. Propulsive

1:Mixing movements:

• Mixing movements are different in different parts of G.I.T
• Local intermittent constrictive contractions occur every few seconds
• Thus shop and shear the content.

2:Propulsive Movements “Peristalsis”—Basic movement

STIMULI:

1. Distention of the gut
2. Irritation of epithelium of gut
3. Parasympathetic stimulation
4. Effective peristalsis require active Myenteric plexus.

Myenteric plexus is “polarized” in the anal direction THUS peristalsis IS ALSO IN in forward direction(Peristaltic waves move toward the anus with downstream receptive relaxation).

• Contractile ring causing the peristalsis normally begins on the orad side and moves toward the distended segment
  • Gut relaxes several centimeters downstream toward the anus, which is called “receptive relaxation,” thus allowing the food to be propelled more easily toward the anus than toward the mouth.

Law of the gut:

• Complex pattern does not occur in the absence of the myenteric plexus—peristaltic reflex.
• The peristaltic reflex plus the anal direction of movement of the

peristalsis is called the “law of the gut.”