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Smooth muscle cells are always uninucleated, meaning they have one nucleus per cell. Smooth muscle is one of the three main types of muscle in the body, alongside skeletal and cardiac muscle. Smooth muscle is found in the walls of hollow organs such as the intestines, blood vessels, and the bladder. Unlike skeletal and cardiac muscle, smooth muscle is involuntary and non-striated, meaning it does not have the striated appearance seen in skeletal and cardiac muscle under a microscope.
| Characteristics | Values |
|---|---|
| Muscle type | Smooth muscle |
| Muscle cell shape | Spindle-shaped |
| Muscle cell size | Smaller and shorter than skeletal muscle cells |
| Muscle cell nucleus location | Central |
| Muscle cell nucleus number | One |
| Muscle control | Involuntary |
| Muscle contraction speed | Slow and sustained |
| Muscle appearance under a microscope | Non-striated |
| Muscle function | Moving food through the digestive system and regulating blood flow |
| Muscle location | Walls of hollow organs (e.g. intestines, uterus, stomach, bladder) and blood vessels |
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What You'll Learn
Smooth muscle cells are always uninucleated
Smooth muscle tissue contracts slowly and spontaneously compared to striated muscle, and it lacks the striped pattern seen under a microscope in skeletal and cardiac muscle. This is because the contractile proteins actin and myosin are not organised into sarcomeres in smooth muscle cells. Smooth muscle cells have unbranched cells that are controlled by the autonomic nervous system. They function to produce contractions that move body parts, including internal organs, and are essential for activities like moving food through the digestive system and regulating blood flow.
In contrast to smooth muscle, skeletal muscle cells are always multinucleated, meaning they have multiple nuclei per cell. Skeletal muscle is a type of muscle tissue responsible for the voluntary movements of bones. It has a striped appearance under a microscope due to the presence of striations, and it can contract quickly but tires easily. Cardiac muscle cells, found only in the heart, are generally mononucleated but can occasionally have two nuclei. Cardiac muscle is also striated and composed of similar contractile proteins as skeletal muscle, but the striations are less defined due to the presence of relatively large amounts of mitochondria and other organelles.
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Cardiac muscle cells are usually uninucleated
Muscle tissue is composed of specialised cells known as muscle cells or myocytes, which are commonly referred to as muscle fibres. These muscle cells have the ability to contract, resulting in movement. The movement may be related to the movement of the body or body parts, or it can be the movement of blood, food, and other substances within the body.
Cardiac muscle cells, also called cardiomyocytes, are usually uninucleated, meaning they have one nucleus per cell. However, they can occasionally be binucleated. Cardiac muscle cells are shorter and broader compared to skeletal muscle cells and are branched at their ends. They are striated and composed of contractile proteins structurally arranged into sarcomeres, which are the functional unit of cardiomyocyte contraction. The striations in cardiac muscle are not as defined as those in skeletal muscle due to the presence of large amounts of mitochondria and other organelles in the cell. Cardiac muscle is found only in the heart and is responsible for the contractility of the heart and, therefore, the pumping action.
Smooth muscle cells, on the other hand, are always uninucleated. They are smaller and shorter than skeletal muscle cells and are spindle-shaped with long tapered ends. Smooth muscle cells are found in the walls of hollow organs such as the intestines, blood vessels, and the bladder, as well as in the inner lining of organs such as the stomach, oesophagus, bronchi, uterus, urethra, and urinary bladder. They contract slowly and spontaneously compared to striated muscle and are controlled by the autonomic nervous system.
Skeletal muscle cells, unlike cardiac and smooth muscle cells, are always multinucleated, meaning they have multiple nuclei per cell. These nuclei are located at the periphery of the cell. Skeletal muscle cells are cylindrical in shape and are found attached to the bones and tendons to regulate locomotion and maintain body posture. They can be voluntarily controlled and contract quickly but can also tire easily.
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Skeletal muscle cells are always multinucleated
Skeletal muscle is composed of cells collectively referred to as muscle fibers. Each muscle fiber is multinucleated, with its nuclei located along the periphery of the fiber. Each muscle fiber further subdivides into myofibrils, which are the basic units of the muscle fiber. These myofibrils are surrounded by the muscle cell membrane (sarcolemma), which forms deep invaginations called transverse tubules (T-tubules) within the myofibril.
The T-tubules function as a major location for ion exchange. Inward Ca2+ flow causes the release of acetylcholine (ACh) at the neuromuscular junction, which diffuses to the postsynaptic membrane at the muscle fiber. The postsynaptic membrane of the muscle fiber is also known as the motor endplate. ACh binds to the nicotinic receptors located at the motor endplate, initiating the action potentials in the muscle fiber.
Skeletal muscle is found throughout the body and is attached to bones via tendons. It serves many purposes, including producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints. Skeletal muscle comprises approximately 40% of human body weight and contains 50-75% of all body proteins.
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Smooth muscle cells are non-striated
Smooth muscle is one of the three major types of muscle tissue, the others being skeletal and cardiac muscle. Smooth muscle cells are always mononucleated, meaning they contain a single nucleus per cell. They are smaller and shorter than skeletal muscle cells and are found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus. They are also found in the walls of blood vessels, the liver, pancreas, and bile ducts.
Smooth muscle is involuntary muscle, meaning it functions without conscious control. It contracts slowly and can sustain long-term tension. It is also non-striated, meaning it does not have the striped pattern seen in skeletal and cardiac muscle under a microscope. This is because smooth muscle cells do not contain sarcomeres, which are structures composed of contractile proteins actin and myosin that give striated muscles their banded appearance. While smooth muscle cells contain these contractile proteins, they are not organised into sarcomeres, resulting in a non-striated appearance.
The absence of sarcomeres in smooth muscle cells has functional implications. Unlike striated muscles, smooth muscles can stretch and still maintain their contractility, making them essential in organs like the intestines and urinary bladder. Additionally, smooth muscle cells can contract much stronger than striated muscles due to their unique cellular structure. Smooth muscle fibres group in branching bundles, allowing for stronger contractions.
Smooth muscle plays a crucial role in various physiological processes. For example, it is responsible for moving food through the digestive system and regulating blood flow. In the gastrointestinal tract, smooth muscle cells work together with interstitial cells of Cajal and platelet-derived growth factor receptor alpha to form a functional syncytium that facilitates intestinal contractions. Smooth muscle is also found in the tracts of the respiratory, urinary, and reproductive systems, as well as in the eye, where it forms the ciliary muscles, iris dilator muscle, and iris sphincter muscle.
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Skeletal muscle cells are striated
Skeletal muscle is a highly organised tissue that comprises around 40% of human body weight and contains 50-75% of all body proteins. It is made up of bundles of muscle fibres called myofibers, which contain several myofibrils. Each myofiber represents a muscle cell, and the basic cellular unit is the sarcomere.
Skeletal muscle is striated, and this has been known for 300 years, although the molecular understanding of the striations is more recent. The striated appearance is due to the arrangement of actin and myosin, which creates the sarcomeres. When viewed under a microscope, the sarcomeres are arranged longitudinally and include the M line, Z disk, H band, A band, and I band.
The Z line, or Z disk, is the terminal boundary of the sarcomere, where alpha-actinin acts as an anchor for the actin filaments. The M line is the central-most line of the sarcomere, where the myosin filaments are anchored together. The H band contains the M line and is the central region of the sarcomere that contains only myosin filaments. The A band is a larger portion of the sarcomere that contains the entirety of the myosin fibres and includes regions of actin and myosin overlap. The I band is divided into equal parts by the Z disk.
The banding pattern is explained by the thick myosin-containing filaments and the thin actin-containing filaments. The sarcomeres are all the same length, and the individual myofibrils are aligned so that the A and I bands appear continuous across the muscle fibre, giving it its characteristic cross-striated appearance.
The striated appearance of skeletal muscle is in contrast to smooth muscle, which is always uninucleate and non-striated. Smooth muscle is found in the walls of hollow organs such as the intestines, blood vessels, and the bladder.
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Frequently asked questions
Smooth muscle cells are always uninucleated, meaning they contain one nucleus per cell.
Smooth muscles are one of the three main types of muscle in the body. They are found in the walls of hollow organs such as the intestines, blood vessels, and the bladder. Smooth muscles are also found in the uterus and stomach walls.
Uninucleated muscles have a single nucleus, whereas multinucleated muscles have multiple nuclei. Skeletal muscles are always multinucleated.
Smooth muscles are involuntary muscles, meaning they function without conscious control. They contract slowly and can sustain long-term tension. They are essential for activities like moving food through the digestive system and regulating blood flow.
