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Visceral muscle is a type of smooth muscle found in the body's visceral structures. Visceral pain, which comes from internal organs and blood vessels, is distinct from somatic pain, which arises in the skin, muscles, and soft tissues. Unlike somatic pain, visceral pain is often vague and challenging to pinpoint, characterised by an aching or squeezing sensation. This is due to the lower density of nerves in the visceral region.
Characteristics and Values of Visceral Muscle
| Characteristics | Values |
|---|---|
| Type | Smooth muscle |
| Location | Internal organs and blood vessels |
| Pain | Aching or squeezing |
| Diagnosis | Harder to pinpoint |
| Fibres | Built up of smaller strands called myofibrils |
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What You'll Learn
Visceral muscle tissue structure
Visceral muscle tissue, also known as smooth muscle, is a type of muscle tissue associated with the internal organs of the body, especially those in the abdominal cavity. Smooth muscles are the most common type of muscle in the human body and form the walls of hollow organs, including the stomach, intestines, and blood vessels.
Unlike skeletal or cardiac muscle, smooth muscle does not have a band-like appearance. Instead, it is composed of bundles of specialised cells, or muscle fibres, that are capable of contraction and relaxation to create movement. These muscle fibres contain nerves, blood vessels, and connective tissue. Each fibre is built from smaller strands called myofibrils, which contain interlaced filaments of muscle proteins. When viewed in cross-section, the structure of smooth muscle resembles a telephone cable, with bundles of smaller cables nested within larger ones.
Smooth muscle cells do not have stripes, but they do contain actin and contractile myosin proteins, which interact to generate tension. These proteins are not arranged in ordered sarcomeres, resulting in the lack of stripes. Instead, they are anchored in dense bodies dispersed throughout the cytoplasm and attached to the muscle membrane.
The contractions of visceral smooth muscles are slow and steady, allowing food and other substances from the digestive tract to pass through the body. To minimise energy requirements, smooth muscles have relatively low energy production, as they need to work uninterrupted for extended periods.
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Visceral muscle pain
Historically, there was a misconception that visceral organs did not receive any sensory innervation and therefore could not perceive any sensation when the viscera were cut, pinched, or pricked. Later, the presence of sensory innervation was recognized, but it was believed that visceral dysfunction could only lead to somatic pain since muscle tenderness and pain were evident during visceral dysfunction.
It took many years to understand that true visceral pain did exist and that the somatic tenderness observed during visceral dysfunction was due to the convergence of visceral and somatic sensory afferents in the spinal cord, a phenomenon commonly known as "referred pain". Recent studies have also revealed that spinal afferents running through the thoracolumbar (LSN) and sacral (PN) pathways are not functionally homogeneous and transmit different qualities of signals to the spinal cord.
In the GI tract, spinal afferents are classified as mucosal, muscle or tension-sensitive, muscle/mucosal (also known as "tension/mucosal"), serosal, and mesenteric afferents, depending on the location of the receptive endings in the gut. Mucosal afferents are believed to monitor the dynamic events of mechanical positioning of the gut and the passage of a bolus through the lumen. The majority of these mucosal mechanosensitive afferents are also chemosensitive and respond to a hyperosmolar solution, acid, bile, 5-HT, ATP, and capsaicin. While there is no direct experimental evidence, it is speculated that these afferents are possibly the first line of the neural pathway that signals pain and discomfort due to changes in the luminal chemical environment, mucosal damage, or inflammation.
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Visceral muscle fibres
Visceral smooth muscle is composed of bundles of specialised cells, or fibres, that are capable of contraction and relaxation to create movement. These fibres are built up from smaller strands called myofibrils, which contain interlaced filaments of muscle proteins. The muscle fibres are joined by gap junctions, allowing the muscle to contract as a single unit. This is known as single-unit smooth muscle, which is the most common type and is found in the walls of all visceral organs except the heart.
Single-unit visceral smooth muscle is myogenic, meaning it can contract regularly without input from a motor neuron. In contrast, multi-unit smooth muscle, which is less common, is neurogenic and receives stimuli from autonomic nerves or hormones rather than stretching. Visceral smooth muscle produces slow, steady contractions that allow substances such as food in the digestive tract to move through the body. The power output of visceral smooth muscle is relatively low, but contractions can continue without using large amounts of energy, allowing for the maintenance of muscle tone in visceral organs with minimal energy expenditure.
The triggers for visceral smooth muscle contraction include hormones, neural stimulation by the autonomic nervous system, and local factors. For example, in the walls of visceral organs, stretching the muscle can trigger its contraction, known as the stretch-relaxation response. This is important for hollow organs such as the stomach or urinary bladder, which expand as they fill. The smooth muscle around these organs helps to maintain muscle tone and prevent "flabbiness" when the organ empties and shrinks.
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Visceral muscle protein composition
Visceral muscles are involuntary muscles that are part of internal organs. Each muscle fibre is made up of smaller strands called myofibrils, and each myofibril contains interlaced filaments of muscle proteins.
Visceral proteins have been used as indirect markers of protein energy malnutrition (PEM) in clinical practice. Serum albumin and prealbumin are well-known visceral proteins that have been traditionally considered useful biochemical laboratory values in a nutrition assessment. However, recent literature disputes this, suggesting that these proteins characterise inflammation rather than describing nutritional status or PEM.
Other studies have also investigated the role of visceral proteins in detecting malnutrition in the elderly. Low values of visceral proteins, except for transferrin, could indicate a poor nutritional status. Serum albumin has been found to be associated with skeletal muscle in elderly men and women.
Visceral proteins have also been studied for their prognostic value in patients with colon cancer. Additionally, the use of retinol-binding protein and prealbumin as indicators of the response to nutrition therapy has been explored.
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Smooth muscle
Single-unit smooth muscle, also known as visceral smooth muscle, is found in the walls of most internal organs (viscera) and lines blood vessels (except large elastic arteries), the urinary tract, and the digestive tract. It is not found in the heart, which has cardiac muscle. In single-unit smooth muscle, a single cell in a bundle is innervated by an autonomic nerve fiber (myogenic). An action potential can be propagated through neighbouring muscle cells due to the presence of many gap junctions between the cells. This allows the whole bundle to form a syncytium that contracts in a coordinated fashion, making the whole muscle contract or relax. For example, during childbirth, the uterine muscles contract in a coordinated fashion.
Multi-unit smooth muscle is found in the trachea, in the iris of the eye, and lining the large elastic arteries. It is neurogenic, meaning that its contraction must be initiated by an autonomic nervous system neuron. However, even in multi-unit smooth muscle, there is some cell-to-cell communication and local production of activators/inhibitors, leading to a somewhat coordinated response.
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Frequently asked questions
Visceral muscles are involuntary muscles that are part of the body's internal organs and visceral structures. They are made up of muscle fibres, nerves, blood vessels, and connective tissue.
Somatic pain arises in the skin, muscles, bones, or soft tissues, and is usually easier to diagnose due to its intensity and localisation. In contrast, visceral pain comes from internal organs or blood vessels and may be more challenging to pinpoint, often feeling like an aching or squeezing sensation.
Yes, visceral pain can be treated, depending on its underlying cause. It is important to consult a healthcare provider for an accurate diagnosis and appropriate treatment options.
