Logan Steele
The human body is made up of about 600 muscles, which is roughly half of a person's body weight. These muscles are made up of thousands of small fibres woven together, and they come in four main shapes: spindle, flat, triangular, and circular. Skeletal muscles, which are the most common type of muscle in the body, are attached to bones and allow for movement. They are also known as voluntary muscles because they are under our conscious control. Skeletal muscle fibres are red and white, and they look striated, or striped. Cardiac muscles are also striated, but smooth muscles are not. The width of skeletal muscle fibres ranges from 3 to 8 micrometres, with a breadth of 18 to 200 micrometres.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | 600-700 |
| Types of muscles | Skeletal, cardiac, and smooth |
| Types of muscle tissue | Visceral, cardiac, and skeletal |
| Skeletal muscle composition | Thousands of elastic fibres bundled tightly together |
| Skeletal muscle fibres | Red and white |
| Skeletal muscle shape | Spindle, flat, triangular, and circular |
| Skeletal muscle mass in men | 42% of body mass |
| Skeletal muscle mass in women | 36% of body mass |
| Skeletal muscle width | 3-8 micrometres |
| Skeletal muscle breadth | 18-200 micrometres |
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What You'll Learn
Skeletal muscle composition
Skeletal muscle is the most common type of muscle in the human body, comprising approximately 30% to 40% of total body mass. It is a highly organised tissue composed of bundles of muscle fibres called myofibers, which contain several myofibrils. Each myofiber represents a muscle cell, with its basic cellular unit being the sarcomere.
The myofibrils are composed of actin (thin filaments) and myosin (thick filaments), repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fibre necessary for muscle contraction. The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance. The striations, or stripes, are due to the arrangement of the sarcomeres, which are the functional units of the muscle fibre. The sarcomeres are composed of an M line, Z disk, H band, A band, and I band. The M line is the central-most line of the sarcomere, where myosin filaments are anchored together. The H band contains the M line and contains only myosin filaments. The A band is a larger portion that contains the entirety of the myosin fibres and includes regions of actin and myosin overlap. The Z line, or Z disk, is the terminal boundary of the sarcomere, where alpha-actinin acts as an anchor for the actin filaments.
Each muscle fibre is surrounded by a layer of connective tissue called the endomysium, which is the innermost layer surrounding individual muscle fibres. Bundles of muscle fibres form a fasciculus, which is surrounded by another layer of connective tissue called the perimysium. The outermost layer is the epimysium, which divides the muscle into compartments. Each compartment contains a bundle of muscle fibres. Skeletal muscles are attached to bones by tendons, which are tough bands of connective tissue.
The skeletal muscle also acts as a storage source for amino acids that different organs of the body can use for synthesising organ-specific proteins. Skeletal muscle plays a central role in maintaining thermostasis and acts as an energy source during starvation.
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Cardiac muscle function
The human body contains three types of muscle tissue: skeletal, smooth, and cardiac. Cardiac muscle tissue is the only type found in the heart. It is made up of cardiac muscle cells, also called cardiomyocytes, which are contractile myocytes. These cells are highly specialised and perform coordinated actions that keep the heart pumping and blood circulating throughout the body.
Cardiac muscle cells are rectangular and are joined at their ends by intercalated discs to form long fibres. Each cell contains myofibrils, which are rod-like units within the cell. Myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organise into thick and thin filaments, called myofilaments. A single myosin filament connects to two actin filaments on either side, forming a single unit of muscle tissue. When a cardiac muscle cell contracts, the myosin filament pulls the actin filaments towards each other, causing the cell to shrink. The sliding of actin and myosin past each other produces the formation of "cross-bridges", which causes contraction of the heart and the generation of force. This process is activated by the release of calcium from the sarcoplasmic reticulum when delivering an action potential to the muscle, in a process called excitation-contraction coupling.
The outside of the cardiomyocyte is surrounded by a plasma membrane called the sarcolemma that acts as a barrier between extracellular and intracellular contents. Invaginations of the sarcolemma into the cardiomyocyte are called T-tubules, which are involved in the rapid transmission of electrical impulses from the cell surface to the cell's core and help to regulate the concentration of calcium within the cell. At the Z-line of the cardiomyocyte, T-tubules run adjacent to enlarged areas of the sarcoplasmic reticulum known as the terminal cisternae, and the combination of a single T-tubule with one terminal cisterna is referred to as a diad.
Cardiac muscle cells contain mitochondria, which convert oxygen and glucose into energy in the form of adenospine triphosphate (ATP). ATP is used to power the contraction of the cardiac muscle cells. Cardiac muscle cells typically contain one nucleus, which houses all of the cell's genetic material, but some have two.
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Smooth muscle appearance
Smooth muscle is a type of muscle tissue found in the walls of hollow organs, such as the intestines, uterus, and stomach, as well as in the walls of passageways, including arteries and veins of the cardiovascular system. It is also present in the tracts of the urinary, respiratory, and reproductive systems, as well as in the eyes, skin, and ducts of exocrine glands.
The shape of smooth muscle cells is described as fusiform, round in the center, and tapering at each end. These cells are narrow and spindle-shaped, with a single, centrally located nucleus. Smooth muscle tissue does not show cross stripes under microscopic magnification, unlike skeletal and cardiac muscles, which have a banded or striped appearance due to the arrangement of protein fibers. The smooth appearance of visceral muscle, another term for smooth muscle, is uniform and stands in contrast to the appearance of other muscle types.
Smooth muscle cells are anchored to the surrounding connective tissue by a basal lamina. The cells form a complex system of branching bundles that contract in waves along the length of the structure. Actin and myosin form continuous chains within the smooth muscle cell, creating a mesh-like network that allows for uniform contraction in a spiral corkscrew fashion. The actin filaments are stretched between dense bodies in the cytoplasm and attachment plaques at the cell membrane, with myosin filaments lying between them. Intermediate filaments such as desmin and vimentin provide additional support to the cell structure.
The cytoplasm of smooth muscle cells is homogeneously eosinophilic, consisting mainly of myofilaments. The cell membrane forms small pouch-like invaginations called caveolae, which are functionally equivalent to the T-tubules found in skeletal muscles. Smooth muscle cells are typically slower to contract than skeletal muscle cells, but they are stronger, more sustained, and require less energy. They are also capable of contracting involuntarily, unlike skeletal muscles, which are under voluntary control.
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Voluntary vs involuntary muscle
Voluntary and involuntary muscles differ in several ways, including their movement, structure, and function. Voluntary muscles are under conscious control, meaning that we can decide when and how to move them. They are attached to our bones and skin and help us move parts of our body, like our arms and legs. These muscles include skeletal muscles, biceps, triceps, quadriceps, diaphragm, pectoral muscles, abdominals, and hamstrings. They exhibit rapid contraction and relaxation compared to involuntary muscles, but they also tire quickly and require regular rest.
Involuntary muscles, on the other hand, function automatically without conscious control. They are associated with internal organs like the heart, stomach, intestines, and blood vessels. These muscles help with essential body functions like breathing, pumping blood, and digesting food. They contract and relax at regular intervals, ensuring that vital processes like circulation and digestion occur smoothly without our intervention. Involuntary muscles include smooth muscles and cardiac muscles.
Voluntary muscles are long, multinucleated cells, with sarcomeres arranged into bundles. They are composed of cylindrical fibres and are attached to the skeleton, allowing for bodily movement. The actions of these muscles are controlled by the somatosensory nervous system.
Involuntary muscles, such as cardiac muscles, are striated and branched. They are controlled by the autonomic nervous system and exhibit slow and regular contractions and relaxations. These muscles are unique to the heart and help maintain proper blood circulation throughout the body. Smooth muscles, another type of involuntary muscle, lack striations when viewed under a microscope. They are found lining the walls of internal organs and blood vessels.
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Muscle shape and size
The human body is made up of over 600 muscles, which can be grouped into three types: skeletal, smooth, and cardiac. Skeletal muscles are the most common type of muscle in the body, comprising 30-40% of total body mass. They are attached to bones and allow us to perform a wide range of movements. These muscles are voluntary, meaning we control how and when they work. Skeletal muscles come in various shapes, including spindle (wide in the middle and tapered at the ends, like the biceps), flat (like a sheet, like the diaphragm), triangular (wider at the bottom, like the deltoid muscles), and circular (ring-shaped, like the muscles around the mouth).
The shape of a muscle is often reflected in its name. For example, the terms "deltoid" and "rhomboid" describe muscles that are triangular and rhombus-shaped, respectively. Other terms like "vastus" and "maximus" indicate huge or large muscles, while "minimus" and "brevis" refer to small and short muscles. The direction of the muscle fibers also influences their names: "rectus" for straight fibers, "transverse" for across, and "oblique" for diagonal.
Cardiac muscle tissue, found only in the heart, is responsible for pumping blood throughout the body. Cardiac muscle is involuntary, contracting and relaxing without conscious input. It is autorhythmic, stimulating itself to contract and pumping blood with each heartbeat. Cardiac muscle cells have a branched structure, forming strong connections with neighboring cells through intercalated discs.
Smooth muscle, the third type, is found within the walls of organs and structures like the esophagus, stomach, intestines, and blood vessels. It is involuntary, contracting without conscious awareness. Smooth muscle plays a vital role in various body systems, including the female and male reproductive systems, urinary system, respiratory system, and digestive system.
The basic composition of these three muscle types is similar, consisting of thousands of elastic fibers bundled together. Each bundle is wrapped in a thin membrane called the perimysium. Individual muscle fibers are made up of protein blocks called myofibrils, which contain myoglobin and other molecules that provide the energy and oxygen necessary for muscle contraction.
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Frequently asked questions
The width of a muscle fibre ranges from 3-8 micrometres.
A muscle is made up of thousands of elastic fibres bundled tightly together. Each bundle is wrapped in a thin membrane called a perimysium.
There are three types of muscles in the human body: skeletal, cardiac and smooth muscle.
Muscles are responsible for movement in the human body. They also support the weight of the body and move substances throughout the body.
Muscles can be spindle-shaped, flat, triangular, or circular.
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