Movement of Digestive Materials

Movement of Digestive Materials

There are two basic types of motor functions in the alimentary canal: mixing movements and propelling movements. When smooth muscles contract rhythmically, mixing occurs. Waves of contractions mix food with digestive juices. In the small intestine, mixing movements are aided by segmentation, involving alter-nating contraction and relaxation of smooth muscle in nonadjacent segments. Materials are not propelled along the tract in one direction because segmentation follows no set pattern. Peristalsis consists of the propel-ling, wave-like movements of the tube (­FIGURE 24-4). Contraction appears in the wall of the tube in a “ring,” whereas the muscular wall immediately ahead of the ring relaxes. The peristaltic wave moves along, pushing contents of the tube toward the anus.

Blood Supply

The arteries that branch from the abdominal aorta, serving the digestive organs and hepatic portal circu-lation, make up the splanchnic circulation. Usually, one-fourth of the cardiac output supplies these arter-ies, which include branches of the celiac trunk serving the liver, spleen, and stomach. They also include the mesenteric arteries serving the small and large intes tines. After a meal, the amount of blood needed from the heart increases. Venous blood that is rich in nutri-ents is collected by the hepatic portal circulation from the digestive viscera. It is then transported to the liver.

Enteric Nervous System

The enteric nervous system is the nerve supply within the alimentary canal. Its semi-autonomous enteric neurons communicate with each other, reg-ulating digestive activities. They make up the bulk of the two primary intrinsic nerve plexuses, which are ganglia that are interconnected by unmyelinated fiber tracts. These submucosal and myenteric nerve plexuses are within the alimentary canal walls. They are inter-connected throughout the GI tract and regulate the activities of digestion all along the tract.

The submucosal nerve plexus is within the submucosa. The myenteric nerve plexus is like a sandwich, between the circular and longitudinal muscle layers that form the muscularis externa. The enteric neurons are the major nerve supply of the GI tract walls. They control motility, which is the motion of substances through the tract. Within the enteric nervous system, there are two types of reflex arcs:

■■ Short reflexes: These are completely under enteric nervous system plexus control. They respond to GI tract stimuli. Segmentation and peristalsis is mostly under automatic control via the pacemaker cells and reflex arcs between the enteric neurons in various organs.

■■ Long reflexes: These are controlled by CNS integration centers and extrinsic autonomic nerves. The enteric nervous system communicates with the CNS over afferent visceral fibers, receiving sympathetic and parasympathetic branches or motor fibers, of the autonomic nervous system that enter intestinal walls and synapse with intrinsic plexus neurons. Long reflexes may be started via stimuli from inside or outside the GI tract. For these reflexes, the enteric nervous system handles autonomic impulses to allow extrinsic controls to affect digestion. Basically, parasympathetic impulses increase digestive activities while sym-pathetic impulses reduce them.

1. Describe the organs of the alimentary canal from beginning to end.

2. Describe the wall of the alimentary canal.

3. Define peristalsis.

4. Describe the functions of the mucosa (mucous membrane) of the alimentary canal.

5. Differentiate between the submucosa, muscularis externa, serosa, and adventitia.

6. Explain how the splanchnic circulation works.