Defecation reflex is involuntary

When the pressure in the rectum reaches a certain value, the excitation from the receptors will go along the sensory neuron to the spinal cord (S2-S4). Next, the defecation reflex is formed (the internal rectal sphincter opens). The impulse also spreads through the spinal cord to the cortex (the urge to defecate)

The act of defecation is voluntary. Its center is in the cerebral cortex. When the impulse comes from the cortex to the spinal cord, the motorneurons that control the outer transverse striated sphincter of the rectum are inhibited. Sphincter (external) opens and then defecation occurs.

Neural Control - Enteric Nervous System
In the gastrointestinal tract there is a nervous system called the enteral nervous system.

It lies entirely in the wall of the intestine, beginning with the esophagus and extending to the anus.

The enteral nervous system consists mainly of two plexuses.

1. The outer plexus lying between the muscle layers, called myenteric plexus or Auerbach's plexus, it controls basically the motor activity.

2. The inner plexus, called the submucosal plexus or Meusner's plexus,

       controls the secretion and blood circulation.

Types of Neurotransmitters Secreted by Enteric Neurons
Acetylcholine, norepinephrine, adenosine triphosphate,

serotonin, dopamine, cholecystokinin, substance P, ets.

Autonomic Control of the Gastrointestinal Tract (PSNS&SNS)
The PSNS is represented in two parts:

1) cranial - vagus nerve - X pair;

 Vagus nerve innervates the esophagus, stomach, small intestine and large - up to sigmoid.

2)sacral - sacral nerves, the centers of which are located in 2-4 segments of the spinal cord, innervate sigmoid, rectum.

Stimulation of these parasympathetic nerves causes general increase in activity of the enteric nervous system, the activity of the gastrointestinal tract.

Sympathetic Innervation.

 Sympathetic fibers to the gastrointestinal tract come from the segments of the spinal cord (Th5 - L2).

In general, stimulation of the sympathetic nervous system inhibits the activity of the gastrointestinal tract.

Hormonal Control of Gastrointestinal Motility

Gastrin is secreted by the “G” cells of the antrum of the stomach, activates motility of gastrointestinal tract.

Cholecystokinin is secreted in the duodenum and jejunum, it strongly compresses the gallbladder, and also inhibits gastric contraction.

Secretin is secreted in the duodenum, inhibits motor activity gastrointestinal tract.

Motilin is secreted by the upper duodenum, it increases gastrointestinal motility.

Absorption (Suction)

Suction in the stomach: water, salts, alcohol, glucose, vitamin C.

Suction in the small intestine

Water is absorbed by osmosis (up to 9 liters)

Mineral salts: Na, Ca, K, Fe, Mg, phosphates by active transport.           

Carbohydrates break down to

monosaccharides, proteins to amino acids are absorbed by active transport.

Fat breaks down into glycerol and fatty acids. Which form chylomicrons with bile acids, which enter the enterocytes, and they again synthesize fat, which is absorbed into the lymph.

Suction in the сolon: water, salt, vitamins.