Neural Regulation of Digestion
Neural control of the digestive system is implemented by the autonomic nervous system, specifically its sympathetic and parasympathetic divisions. These influences determine the intensity of glandular secretion and smooth muscle motility.
Secretory activity of the digestive glands is triggered by two main mechanisms:
- Conditioned reflex mechanism — a response to the sight, smell of food, or environment.
- Unconditioned reflex mechanism — a response to direct contact of food with receptors in the oral cavity and stomach.
To support such active work, the digestive system requires a colossal energy resource. This is precisely why the blood supply to the stomach, intestines, liver, pancreas, and spleen accounts for about 30% of the cardiac output.
Gradient of Regulatory Mechanisms
In the gastrointestinal tract, there is a clear pattern: as the food bolus moves along, the leading role in function management changes. This phenomenon is called the regulatory gradient.
- Upper GI tract (oral cavity, pharynx, esophagus, upper stomach). There is a pronounced predominance of central reflex mechanisms (both conditioned and unconditioned).
- Distal GI tract. As chyme moves further down the tract, reflex influences gradually fade. They are replaced by powerful humoral regulation.
- Small and large intestines. In these segments, local mechanisms hold absolute priority. Local mechanical stretching of the walls or chemical irritation of receptors leads to an immediate increase in intestinal activity strictly at the site of direct impact.
Mechanisms of Local Stimuli Action
Chemical and mechanical factors arising during digestion affect the GI system through two main pathways:
- By completing peripheral reflexes at the level of the local neural network.
- Through the stimulation of gastrointestinal hormone release.
The main chemical stimulants of nerve endings in the mucous membrane include:
- Acids;
- Alkalis;
- Products of food hydrolysis (digested proteins, fats, carbohydrates).
The humoral pathway of signal transmission operates as follows: active substances are released into the blood, transported to target glands, and stimulate their activity either directly or indirectly through other structures.
Gastrointestinal Hormones
Humoral regulation is provided by specific gastrointestinal hormones. Their sites of formation are specialized endocrine cells scattered in the mucosa of the stomach, duodenum, jejunum, and in the pancreatic tissue.
Effects within the digestive tract:
- Regulation of motor function (contraction and relaxation of sphincters and walls).
- Control of water, enzyme, and electrolyte secretion.
- Ensuring the absorption of nutrients.
- Modulation of the functional activity of neighboring GI endocrine cells.
Systemic effects (outside the GI tract):
- General influence on body metabolism.
- Effects on the cardiovascular system and other endocrine glands.
- Influence on the central nervous system (it has been proven that some digestive tract peptides are also found in brain structures).
Classification of Regulatory Influences
According to the classification by the prominent physiologist G.F. Korotko, all regulatory influences on the digestive system are divided into two functional groups:
- Trigger mechanisms — initiate the onset of secretion and motility (e.g., conditioned reflexes at the sight of food).
- Correcting mechanisms — come into play later. Their task is the fine adaptation of the volume of secreted juices and their enzymatic composition to the actual quantity and chemical quality of the contents currently present in the stomach and intestines.