Dominic Stone
There are three types of muscle cells: skeletal, smooth, and cardiac. Smooth muscle cells are spindle-shaped and contain a single nucleus. They are responsible for involuntary contractions and are found in the walls of blood vessels and hollow organs such as the bladder and uterus. Cardiac muscle cells, on the other hand, are striated and also contain a single nucleus. They form the contractile walls of the heart and contract autonomously and involuntarily. Skeletal muscle cells differ from the other two types in that they are multinucleated, meaning they have more than one nucleus.
| Characteristics | Values |
|---|---|
| Muscle Type | Smooth, Cardiac |
| Shape | Spindle-shaped |
| Nucleus | Single, Central |
| Striation | No visible striations |
| Function | Responsible for involuntary contractions |
| Location | Walls of hollow organs, blood vessels, airways, arteries |
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What You'll Learn
Cardiac muscle cells have a single nucleus
Muscle cells can be categorised into skeletal, smooth, and cardiac muscle cells. Smooth muscle cells are spindle-shaped and contain a single central nucleus. They are the smallest type of muscle cell, ranging from 10 to 600 μm in length. They are elastic and responsible for the expansion of organs such as the kidneys, lungs, and vagina.
Cardiac muscle cells, also called heart muscle or myocardium, constitute the main tissue of the heart wall. The myocardium forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium). Cardiac muscle cells contain many mitochondria that provide the energy needed for the cell in the form of adenosine triphosphate (ATP), making them highly resistant to fatigue. The majority of these cells contain only one nucleus, although some may have two central nuclei.
Skeletal muscle cells, on the other hand, are long, cylindrical, and striated. They are multi-nucleated, meaning they have more than one nucleus. This is because they are formed from the fusion of embryonic myoblasts, and each nucleus regulates the metabolic requirements of the surrounding sarcoplasm. Skeletal muscle cells have high energy requirements and contain many mitochondria to generate sufficient ATP.
The presence of a single nucleus in cardiac muscle cells is a distinctive feature that sets them apart from skeletal muscle cells, which typically have multiple nuclei. This mononuclear characteristic of cardiac muscle cells is shared with smooth muscle cells, contributing to their unique properties and functions within the human body.
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Smooth muscle cells are spindle-shaped with one nucleus
Smooth muscle cells are spindle-shaped and contain a single central nucleus. They are the smallest type of muscle cell, ranging from 10 to 600 μm (micrometers) in length. In a relaxed state, each cell is 30–200 μm in length, thousands of times shorter than a skeletal muscle cell. Smooth muscle cells are elastic and important in the expansion of organs such as the kidneys, lungs, and vagina. They are non-striated, meaning they have no sarcomeres and therefore no striations (bands or stripes). They are found in the walls of hollow organs, including the stomach, intestines, bladder, and uterus, as well as in the walls of blood vessels and lymph vessels.
Smooth muscle differs from skeletal muscle in terms of structure, function, and regulation of contraction. Skeletal muscle cells are long, cylindrical, and striated, with multiple nuclei. Each nucleus regulates the metabolic requirements of the sarcoplasm around it. Skeletal muscle cells have high energy requirements and contain many mitochondria to generate sufficient ATP. They are also larger than smooth muscle cells, typically measuring 2–3 cm in length and 100 μm in diameter.
Smooth muscle can be divided into two subgroups: single-unit and multi-unit smooth muscle. Single-unit smooth muscle can contract regularly without input from a motor neuron, while multi-unit smooth muscle contraction must be initiated by an autonomic nervous system neuron. Smooth muscle is responsible for involuntary contractions and contains gap junctions for the spread of depolarization. It also contains normal cell organelles such as sarcosomes, but in lower numbers.
Smooth muscle tissue demonstrates greater elasticity and functions within a larger length-tension curve than striated muscle. This ability to stretch and maintain contractility is important in organs like the intestines and urinary bladder. In the gastrointestinal tract, smooth muscle works together with interstitial cells of Cajal (ICCs) and platelet-derived growth factor receptor alpha (PDGFRα) to facilitate contractions.
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Skeletal muscle cells are multinucleated
The multinucleated nature of skeletal muscle cells offers several advantages. Firstly, it allows for the generation of sufficient mRNA per cell, ensuring efficient protein translation. Secondly, the presence of multiple nuclei enables protein synthesis close to their functional position, reducing the need for long-distance protein transport. Additionally, the mitochondria, responsible for powering the cell, benefit from the proximity of multiple nuclei as it ensures efficient energy production close to the site of energy usage.
The myonuclei in skeletal muscle cells are arranged quite uniformly along the fiber. Unlike in non-muscle cells where the nucleus is centrally positioned, the myonucleus is elongated and located close to the sarcolemma. Each nucleus has its own myonuclear domain, where it regulates the metabolic requirements of the surrounding sarcoplasm and supports the volume of cytoplasm in that section of the myofiber.
In contrast to skeletal muscle cells, smooth muscle cells and cardiac muscle cells typically have a single central nucleus. Smooth muscle cells are spindle-shaped and responsible for involuntary contractions, such as those in the walls of blood vessels and hollow organs. Cardiac muscle cells, on the other hand, form the cardiac muscle in the walls of the heart chambers and are joined to neighbouring cells by intercalated discs.
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Cardiac muscle cells are striated
There are three types of muscle tissue in the body: skeletal, smooth, and cardiac. Cardiac muscle cells, also known as myocardium, are striated and located in the walls of the heart. They appear striped and are under involuntary control. The individual cardiac muscle cell, or cardiomyocyte, is a tubular structure composed of chains of myofibrils, which are rod-like units within the cell.
In contrast to skeletal muscle cells, which often contain many nuclei, cardiomyocytes typically contain a single, centrally located nucleus. This arrangement ensures that the metabolic requirements of the sarcoplasm are effectively regulated. Additionally, cardiomyocytes contain a high number of mitochondria, which produce large amounts of adenosine triphosphate (ATP) and myoglobin to meet the oxygen demands of muscle contraction.
The outer membrane of the cardiomyocyte is called the sarcolemma, which acts as a barrier between extracellular and intracellular contents. Invaginations of the sarcolemma, known as T-tubules, play a crucial role in the exchange of ions with the extracellular fluid surrounding the cell. These T-tubules run adjacent to enlarged areas of the sarcoplasmic reticulum, forming a structure known as a diad.
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Smooth muscle cells lack striations
Smooth muscle cells are non-striated, meaning they lack the bands or stripes that are typically found in skeletal and cardiac muscles. This is because smooth muscle cells have no sarcomeres, which are the structures that give rise to striations. Smooth muscle is one of the three major types of vertebrate muscle tissue, with the other two being skeletal and cardiac muscle.
Smooth muscle cells are spindle-shaped and contain a single central nucleus. They are the smallest type of muscle cell, ranging from 10 to 600 μm in length. They are elastic and play a crucial role in the expansion of organs such as the kidneys, lungs, and vagina. Smooth muscle is also found in the walls of blood vessels and hollow organs, including the stomach, intestines, bladder, and uterus.
Unlike skeletal muscle cells, which are long, cylindrical, and striated, smooth muscle cells have thick and thin filaments that are not arranged into sarcomeres, resulting in a non-striated pattern. On microscopic examination, smooth muscle appears homogeneous. The smooth muscle cytoplasm contains a large amount of actin and myosin, which are the main proteins involved in muscle contraction.
Smooth muscle differs from skeletal muscle in terms of function as well. Smooth muscle is capable of maintaining tone for extended periods and often contracts involuntarily, without input from a motor neuron. This involuntary contraction is regulated by the autonomic nervous system. In contrast, skeletal muscle cells have high energy requirements and are responsible for voluntary movements.
Smooth muscle plays a vital role in various subsystems throughout the body, including the regulation of blood pressure and tissue oxygenation. Its ability to be contracted and controlled involuntarily allows the nervous system to tightly regulate many of the body's functions without conscious thought.
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Frequently asked questions
Smooth muscle cells and cardiac muscle cells have a single central nucleus.
Smooth muscle cells are spindle-shaped with wide middles and tapering ends. They are elastic and important in the expansion of organs such as the kidneys, lungs, and vagina.
Cardiac muscle cells form the contractile walls of the heart. They are striated and found only in the heart.
