The muscularis externa is a substantial layer of muscle tissue that forms part of the wall of the gastrointestinal (GI) tract. It is one of four distinct layers that make up the tube extending from the esophagus to the anus. Its fundamental purpose is to generate the force necessary to move and mix the contents of the digestive system, a process known collectively as motility. This movement ensures that ingested food is processed mechanically and exposed to digestive enzymes and absorptive surfaces. The coordinated contractions of this muscular sheath are involuntary, operating without conscious thought to propel material through the digestive pathway.
Anatomical Placement in the Digestive Tract
The wall of the gastrointestinal tract is built in a consistent pattern of four layers, arranged concentrically around the lumen. Moving outward, the layers are the mucosa, the submucosa, the muscularis externa, and the serosa or adventitia. The muscularis externa is positioned third in this sequence, lying immediately outside the submucosa and beneath the outermost layer. This placement allows it to exert strong, effective pressure on the contents within the tube.
The muscularis externa is present in nearly every part of the GI tract, from the middle of the esophagus to the anal canal. While its structure is generally consistent, site-specific variations exist. For example, the stomach requires powerful churning for mixing and grinding food, so its muscularis externa includes an additional third layer.
Structural Composition
The muscularis externa is composed of two distinct layers of smooth muscle fibers oriented at right angles to one another. The inner layer is the circular layer; its fibers wrap around the circumference of the tube, and contraction reduces the diameter of the lumen, squeezing the contents.
The outer layer is the longitudinal layer, with muscle fibers running parallel to the length of the GI tract. Contraction of the longitudinal layer causes the segment of the digestive tube to shorten. These two layers work in coordinated opposition to achieve digestive movements. In the upper esophagus, the muscle is skeletal muscle for voluntary control of swallowing, but the remainder is involuntary smooth muscle tissue. The stomach is a notable exception, possessing an innermost oblique layer situated inside the circular layer, which enhances its ability to mix food.
Primary Function: Peristalsis and Segmentation
The circular and longitudinal muscle layers facilitate two primary types of movement: peristalsis and segmentation. Peristalsis is the wave-like movement responsible for propulsion, pushing material forward through the digestive tract. This action is initiated when the inner circular muscles contract behind a mass of food, squeezing the tube and preventing backward movement.
Simultaneously, the outer longitudinal muscles ahead of the food mass contract, shortening that segment of the tract. This coordinated relaxation ahead and contraction behind the bolus creates a net forward movement, propelling the contents distally. Peristalsis occurs throughout the GI tract, driving material from the esophagus down to the colon, and its speed is regulated to allow sufficient time for digestion and absorption to occur in the small intestine.
Segmentation is a localized form of contraction primarily involved in mixing. It occurs when the inner circular muscles contract and relax in alternating, non-consecutive segments of the intestine. This rhythmic squeezing divides the contents into small, temporary partitions, churning the food and ensuring it is thoroughly mixed with digestive juices and enzymes. Segmentation is particularly pronounced in the small intestine. Unlike peristalsis, segmentation does not result in significant net forward movement of the contents.
Nervous System Control
The involuntary motility of the muscularis externa is controlled by a specialized network of nerves known as the enteric nervous system (ENS). This system is sometimes referred to as the “second brain” because it is capable of operating autonomously, independent of the central nervous system. The ENS is further divided into two main nerve plexuses, one of which is the Myenteric Plexus, also called Auerbach’s Plexus.
The Myenteric Plexus is situated between the inner circular and outer longitudinal layers of the muscularis externa. Its primary function is to provide motor innervation to both muscle layers, controlling the strength and frequency of their contractions. The nerve signals originating from this plexus coordinate the synchronized movements required for both peristalsis and segmentation.
While the Myenteric Plexus can generate its own rhythmic activity, it is modulated by the autonomic nervous system. The parasympathetic system generally stimulates the ENS, increasing the intensity and velocity of the muscular contractions. Conversely, the sympathetic nervous system typically inhibits the ENS, slowing down the overall rate of motility. This dual control system allows for precise regulation of digestive movement based on the body’s current state.