The morphology of the epithelium of the small intestine and colon can be compared. The lamina propria, located beneath the epithelium, consists mainly of loose connective tissue containing collagen and elastin fibrils. This layer also contains various types of glands, lymph vessels and nodules, capillaries, and nerve fibers. The muscularis mucosae, the innermost layer of intestinal smooth muscle, causes the mucosa to have folds and ridges visible through an endoscope. 

Next is the submucosa, which primarily consists of loose connective tissue with collagen and elastin fibrils. In certain regions of the gastrointestinal (GI) tract, glands are found in the submucosa. The submucosa also contains larger nerve trunks, blood vessels, lymph vessels, and one of the plexuses of the enteric nervous system (ENS), known as the submucosal plexus.

The muscularis externa, or muscularis propria, is typically composed of two significant layers of smooth muscle cells: an inner circular layer and an outer longitudinal layer. The circular muscle layer's fibers are oriented circumferentially, while the longitudinal muscle layer's fibers are aligned along the tube's longitudinal axis. Between these layers lies the myenteric plexus, another plexus of the ENS. Contractions of the muscularis externa facilitate mixing and propulsion of the GI tract contents.

The wall of the GI tract contains interconnected neurons. The submucosa houses a dense network of nerve cells called the submucosal plexus, while the myenteric plexus is located between the circular and longitudinal smooth muscle layers. Together, these intramural plexuses form the ENS, which plays a role in integrating the motor and secretory activities of the GI system.

The outermost layer of the GI tract, known as the serosa or adventitia, consists of squamous mesothelial cells. It belongs to the mesentery, which lines the abdominal wall and suspends the abdominal organs. The mesenteric membranes secrete a thin viscous fluid that lubricates the abdominal organs, allowing for movement as the muscle layers contract and relax.

Regulatory mechanisms are essential for GI tract function. Unlike the cardiovascular or respiratory systems, the GI tract experiences periods of relative inactivity (intermeal period) and intense activity after food intake (postprandial period). Therefore, the GI tract must be able to detect and appropriately respond to food intake. Additionally, it needs mechanisms to detect variations in macronutrient content and communicate with associated organs such as the pancreas. Furthermore, since the GI tract is a long tube, there must be mechanisms for signaling events occurring in the proximal portion to the distal parts, and vice versa.

The regulation of GI function involves three principal control mechanisms: endocrine, paracrine, and neurocrine.
•Fig. 27.3 Comparisonofthemorphologyoftheepitheliumofthesmallintestineandcolon.
The lamina propria immediately below the epithelium consists largely of loose connective tissue that contains collagen and elastin fibrils (see
Fig. 27.2 ). The lamina propria is rich in several types of glands and contains lymph vessels and nodules, capillaries, and nerve fibers. The muscularis mucosae is the thin innermost layer of intestinal smooth muscle. When seen through an endoscope, the mucosa has folds and ridges that are caused by contractions of the muscularis mucosae.
The next layer is the submucosa (see
Fig. 27.2 ), which consists largely of loose connective tissue with collagen and elastin fibrils. In some regions of the GI tract, glands (invaginations or folds of the mucosa) are present in the submucosa. The larger nerve trunks, blood vessels, and lymph vessels of the intestinal wall lie in the submucosa, together with one of the plexuses of the enteric nervous system (ENS), the submucosal plexus.
The muscularis externa, or muscularis propria, typically consists of two substantial layers of smooth muscle cells: an inner circular layer and an outer longitudinal layer (see
Fig. 27.2 ). Muscle fibers in the circular muscle layer are oriented circumferentially, whereas muscle fibers in the longitudinal muscle layer are oriented along the longitudinal axis of the tube. In humans and most mammals, the circular muscle layer of the small intestine is subdivided into an inner dense circular layer that consists of smaller, more closely packed cells, and an outer circular layer. Between the circular and longitudinal layers of muscle lies the other plexus of the ENS, the myenteric plexus. Contractions of the muscularis externa mix and circulate the contents of the lumen and propel them along the GI tract.
The wall of the GI tract contains many interconnected neurons. The submucosa contains a dense network of nerve cells called the submucosal plexus (also referred to as Meissner’s plexus). The prominent myenteric plexus (Auerbach’s plexus) is located between the circular and longitudinal smooth muscle layers. These intramural plexuses constitute the ENS. The ENS helps integrate the motor and secretory activities of the GI system. If the sympathetic and parasympathetic nerves to the gut are cut, many motor and secretory activities continue because the ENS directly controls these processes.
The serosa, or adventitia, is the outermost layer of the GI tract and consists of a layer of squamous mesothelial cells (see
Fig. 27.2 ). It is part of the mesentery that lines the surface of the abdominal wall and suspends the organs within the abdominal cavity. The mesenteric membranes secrete a thin viscous fluid that helps lubricate the abdominal organs so movement of the organs can occur as the muscle layers contract and relax.
Regulatory Mechanisms in the Gastrointestinal Tract
Unlike the cardiovascular or respiratory systems, the GI tract undergoes periods of relative quiescence (intermeal period) and periods of intense activity after the intake of food (postprandial period). Consequently the GI tract has to detect and respond appropriately to food intake. In addition the macronutrient content of a meal can vary considerably, and there have to be mechanisms that can detect this and mount appropriate physiological responses. Thus the GI tract has to communicate with associated organs such as the pancreas. Finally, because the GI tract is essentially a long tube, there have to be mechanisms by which events occurring in the proximal portion of the GI tract are signaled to the more distal parts, and vice versa.
There are three principal control mechanisms involved in the regulation of GI function: endocrine, paracrine, and neurocrine (Fig. 27.4