Victor Forge
The gastrointestinal tract is a series of tubular organs that process ingested food, absorb nutrients, and eliminate waste. The gastrointestinal (GI) tract uses a carefully coordinated series of muscular contractions to carry out its functions. These contractions propel food along the tract, churning and mixing it with enzymatic secretions to aid digestion. The two fundamental patterns of motility conducted by the digestive tube are propulsion and mixing. The principal type of propulsive motility is peristalsis, which moves food through the gastrointestinal tract. Peristalsis is an automatic, involuntary wave-like movement of the muscles that line the gastrointestinal tract. The GI tract is able to respond to changes in the luminal environment caused by food intake through the control of the autonomic nervous system, hormones, and other intrinsic mechanisms.
| Characteristics | Values |
|---|---|
| Motility patterns | Depend on control of muscles by enteric neurons |
| Muscle type | Smooth muscle |
| Muscle layers | Two layers of circularly or longitudinally oriented muscle bundles |
| Muscle contractions | Coordinated, synchronized, and wave-like |
| Muscle contraction functions | Propulsion, mixing, and grinding |
| Muscle contraction influencers | Neural and hormonal input, food intake, and medications |
| Muscle relaxation | Caused by rectal stretching, mediated by neural pathways |
| Muscle control | Autonomic nervous system, hormones, and other intrinsic mechanisms |
| Muscle cell connections | Gap junctions and electrical signals |
| Muscle cell pacemakers | Interstitial cells of Cajal (ICCs) |
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Peristalsis
Smooth muscle cells form electrical and mechanical junctions between cells, allowing for the coordination of contractions. The patterns of contractile activity in gastrointestinal muscles are determined by inputs from enteric motor neurons that innervate smooth muscle cells and interstitial cells. Slow waves synchronize muscle contractions in the gut by controlling the appearance of a second depolarization event, spike potentials, which cause muscle contractions. Slow-wave frequency differs throughout the gastrointestinal tract, occurring approximately 16 times per minute in the small intestine and 3 times per minute in the stomach and large intestine.
Problems with peristalsis can result in abnormal motility, which may be too fast (hypermotility) or too slow (hypomotility). Hypermotility can lead to diarrhoea and digestion problems, while hypomotility can cause constipation and bacterial overgrowth. Peristalsis issues can be caused by medications, injuries, infections, diseases, hormone fluctuations, and electrolyte imbalances. They can also be related to conditions involving the nervous system, mental/emotional factors, or hormones. Treating peristalsis problems effectively requires understanding their causes.
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Smooth muscle
The contractile activity of smooth muscles is determined by inputs from enteric motor neurons that innervate smooth muscle cells and interstitial cells of Cajal. Excitatory and inhibitory neurons also express neuropeptides that regulate the motor control of gastrointestinal muscles.
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Hormonal control
The gastrointestinal tract is responsible for supplying nutrients to our bodies through ingestion, motility, secretion, digestion, and absorption. This process is coordinated by the endocrine and nervous systems. The GI tract is the largest endocrine organ in the human body, producing mediators that play a crucial role in regulating its functions.
GI hormones can be classified as endocrines, paracrine, or neurocrines based on how they reach their target cells. Endocrine hormones, such as gastrin, cholecystokinin (CCK), secretin, glucose-dependent insulinotropic peptide (GIP), and motilin, are secreted directly into the bloodstream by enteroendocrine cells. Paracrine hormones, like somatostatin and histamine, diffuse through the extracellular space to act locally on target tissues without entering the systemic circulation. Neurocrines, including vasoactive intestinal peptide (VIP), gastrin-releasing peptide (GRP), and enkephalins, are secreted by postganglionic non-cholinergic neurons of the enteric nervous system.
The stomach is the primary site of gastrin production, with some D-cells also found in the duodenum. Somatostatin and histamine are produced in the stomach by enterochromaffin-like (ECL) cells, a subtype of enteroendocrine cells. The small intestines, particularly the duodenum and jejunum, are responsible for the secretion of CCK, secretin, GIP, and motilin.
The duodenal brake, which controls gastric motility and emptying, is primarily induced by secretin and CCK. On the other hand, the ileal brake mechanism for gastric emptying and intestinal transit is controlled by peptide YY (PYY) and likely by GLP-1. Motilin is also significant in triggering phase 3 activities of the gastric antrum and upper small intestine during the interdigestive period, stimulating motility and enhancing gastric emptying.
In summary, gastrointestinal motility is intricately regulated by a complex interplay of GI hormones, endocrine cells, and the nervous system. These hormones, including gastrin, CCK, secretin, GIP, motilin, PYY, and GLP-1, play specific roles in controlling gastric motility, emptying, and intestinal transit.
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Neurotransmitters
The gastrointestinal tract is a complex system controlled by several modulators, including neurotransmitters. Neurotransmitters are chemical messengers that carry signals from one neuron to another target cell, such as a nerve cell, muscle cell, or gland. They are critical for maintaining homeostasis within the gut system in terms of nutrient absorption, blood flow, the gut microbiome, the local immune system, and overall gut motility.
The gastrointestinal tract possesses intrinsic neural plexuses that allow a significant degree of autonomy over GI functions, but the central nervous system (CNS) also provides extrinsic neural inputs that regulate, modulate, and control these functions. The sympathetic nervous system exerts an inhibitory effect on GI muscle and regulates GI blood flow, while the parasympathetic nervous system exerts both excitatory and inhibitory control over gastric and intestinal tone and motility.
Other neurotransmitters involved in gastrointestinal motility include acetylcholine, which is released by most neurons in the autonomic nervous system and regulates gut motility, and γ-aminobutyric acid (GABA), which is the most well-known and essential inhibitory neurotransmitter of the CNS. GABA contributes to GI motility from the stomach to the ileum, as well as the peristaltic reflex in the colon.
In addition, excitatory and inhibitory neurons express neuropeptides that regulate the motor control of gastrointestinal muscles. These include small molecule neurotransmitters such as NO, acetylcholine, and β-NAD/ATR, as well as neuropeptides that are elicited at higher frequencies of enteric nerve firing.
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Gut motility
The muscles in the GI tract are predominantly smooth muscles, except for the muscles in the proximal two-thirds of the oesophagus and in the external anal sphincter, which are skeletal. Smooth muscle fibres are arranged in intertwined, indistinct bundles, aligned in most areas of the tube in circular and longitudinal layers. Individual smooth muscle fibres are connected to neighbouring smooth muscle cells by gap junctions, which allow these cells to be electrically coupled and contract as a functional unit. The contractions serve to propel food through the tract, as well as churning and mixing it with enzymatic secretions to aid digestion.
The two fundamental patterns of motility conducted by the digestive tube are propulsion and mixing. Propulsion occurs when a ring of muscle contraction appears on the oral side of a bolus of food and moves towards the anus, propelling the contents. This is called peristalsis, which is an automatic, wave-like movement of the muscles that line the GI tract. Peristalsis begins in the throat when food is swallowed and continues through the oesophagus, stomach, and intestines. Segmentation contractions are a common type of mixing motility, seen especially in the small intestine, where segmental rings of contraction chop and mix the food.
The control of gut motility is complex and is coordinated by the autonomic nervous system, hormones, and other mechanisms intrinsic to the GI tract. The enteric nervous system, the intrinsic nervous system of the GI tract, coordinates digestion, secretion, and motility for adequate nutrient absorption. It does this through information stimulating the CNS, such as sight and smell, and by local mechanical and chemical receptors found within the GI tract.
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Frequently asked questions
Gastrointestinal motility is the term given to the stretching and contractions of the muscles in the gastrointestinal (GI) tract. The GI tract uses a carefully coordinated series of muscular contractions to aid the digestion and absorption of nutrients.
The two fundamental patterns of motility conducted by the digestive tube are propulsion and mixing. Propulsion occurs when foodstuffs are propelled along the length of the digestive tube to be processed and absorbed. The principal type of propulsive motility is peristalsis, which is an involuntary muscle movement that moves food through the GI tract. Mixing motility, on the other hand, involves segmental contractions that chop and mix the ingesta.
Gastrointestinal motility is controlled by the coordinated activity of smooth muscle cells, interstitial cells of Cajal (ICCs), and the enteric nervous system. Smooth muscle cells in the GI tract contract in a coordinated manner due to electrical signals transmitted through gap junctions. ICCs act as pacemakers, generating a basic electrical rhythm that influences neighbouring smooth muscle cells. The enteric nervous system, an intrinsic nervous system of the GI tract, coordinates digestion, secretion, and motility for adequate nutrient absorption.
