The tunica muscularis is the muscular layer that surrounds blood/lymphatic vessels and other hollow organs. In most cases, it is composed of smooth muscle tissue – exceptions are the heart and esophagus, which have striated muscle.
What is the tunica muscularis?
The tunica muscularis is a layer of tissue that surrounds mainly hollow organs. These include organs of the digestive tract such as the esophagus (food pipe), stomach, and intestines, as well as the bile ducts and gallbladder (vesica biliares) and the urinary bladder (vesica urinaria) with its ureter and urethra. Other hollow organs that have a tunica muscularis are the heart and its vessels, and the respiratory tract and lungs with their bronchi. Veins, which carry blood to the heart, have a weaker tunica muscularis than arteries, which carry blood away from the heart. The pumping of the arteries can be felt as a pulse beat, which is why they are also known as the arteries. Women have additional hollow organs with corresponding musculature in the form of the uterus, ovaries and fallopian tubes, and vagina.
Anatomy and structure
Most muscular hollow organs have a tunica muscularis, which consists of smooth muscle tissue. However, the esophagus and the heart are exceptions. The tunica muscularis of the esophagus is composed of striated skeletal muscle-with a tissue of bundled muscle fibers enclosed in a sheath of connective tissue. In contrast, the smooth tunica muscularis consists of single cells that are spindle-shaped and form a cohesive tissue no clear structure with definable areas. This type of muscle is found in the tunica muscularis of most organs. The tunica muscularis is composed of two layers. The outer stratum longitudinale consists of longitudinal fibers. Below this is a thin layer of connective tissue containing neurons and vessels, which is followed by the stratum circulare. This inner layer has fibers that run transverse to the direction of movement of the hollow organ. The tunica muscularis is usually embedded in a wall that has other layers. In blood vessels the tunica muscularis is located in the middle layer (tunica media), in other organs it is found under a tunica submucosa. Nerve endings, vessels and glands are located in this connective tissue layer. Above the tunica submucosa is often another layer of mucosa, namely the tunica mucosa.
Function and tasks
The function of the tunica muscularis is to compress the respective hollow organ. Both the type of movement and its effect depend on the organ involved. Blood and lymph vessels, esophagus, intestine, and other tubular hollow organs have annular tunica muscularis. Not all sections of smooth muscle contract at the same time. In the intestine, for example, the musculature creates an annular constriction that locally displaces the food pulp, forcing it to push further toward the anus. This movement repeats in a wave-like fashion to allow for uniform transport of the intestinal contents. In this way, the colon moves an average of three times per day. In contrast, the tunica muscularis of the esophagus contracts only when the tissue detects liquid or solid substances, and blood vessels respond, among other things, to nerve signals from the circulatory center in the brain. Despite these differences, the different variants of the tunica muscularis have one thing in common: their activity is not subject to voluntary human control. The autonomic nervous system controls smooth muscle with the help of neurons located in the separating layer of connective tissue between the longitudinal and circular stratum. The autonomic nervous system represents a functional unit and has its own nerve fibers. These belong to either the sympathetic or the parasympathetic subsystem and accordingly tend to stimulate activating or regenerative processes. In addition, the gastrointestinal tract has its own nerve supply with the enteric nervous system, which also controls the tunica muscularis of the stomach, intestines and other digestive organs. However, the enteric nervous system is not completely independent of the sympathetic and parasympathetic nervous systems.
Diseases
Depending on the organ affected, the tunica muscularis can cause a variety of symptoms. For example, failures and impairments of the tunica muscularis of the esophagus interfere with the transport of food into the stomach.However, when a person sits upright and swallows, gravity naturally assists the process. Muscle disorders of the esophagus are often due to the failure of the innervating nerves. A low-fiber diet leads to weakening of intestinal movements because the digestive organ lacks the stimulus for muscle activity. The colon extracts water from the digestive pulp. At the same time, if the undulation of its tunica muscularis is absent and it does not push the substrate vigorously enough toward the rectum, constipation may form. Medicine speaks of such constipation when the affected person has not had a bowel movement for three days. Other possible causes include insufficient fluid intake, side effects of medications, lack of exercise, and conditions such as diabetes mellitus, electrolyte imbalances, bowel disease, hypothyroidism, and neurological diseases. In addition, psychological factors may contribute to constipation. Children with encopresis often also suffer from constipation. Encopresis represents a psychological disorder in which children defecate even though they are old enough and physiologically able to regulate bowel movements. In some cases, encopresis is learned, when children feel pain during defecation and therefore want to avoid it – in other cases, the clinical picture is related to other psychological problems such as anxiety, depression or experiences of abuse. Cramps in the uterine muscles can cause abdominal pain. Women often suffer from these contractions during or just before the onset of menstruation, which may be accompanied by other symptoms such as nausea, mood swings, and changes in appetite.