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Muscles are pieces of soft tissue that help the body move, breathe, swallow, and stay alive. They are made of thousands of small fibres woven together, which stretch and press together to move the body and its organs. There are three types of muscle tissue in vertebrates: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles that are anchored by tendons to bones, allowing for a wide range of movements. Smooth muscles are involuntary and are found within the walls of organs and structures such as the oesophagus, stomach, and intestines. The heart is the only organ that is also a muscle, made of cardiac muscle tissue.
| Characteristics | Values |
|---|---|
| Definition | Muscles are pieces of soft tissue throughout the body that help with movement, breathing, swallowing, and keeping the body alive. |
| Types | There are three types of muscle tissue in vertebrates: skeletal, cardiac, and smooth muscle. |
| Composition | Muscles are made of thousands of small fibers woven together. These fibers are composed of contractile proteins called actin and myosin, along with regulatory proteins troponin and tropomyosin. |
| Structure | Each muscle is made up of groups of muscle fibers called fascicles, surrounded by a layer of connective tissue called perimysium. The outermost layer of connective tissue covering each muscle is called the epimysium. |
| Function | Skeletal muscles help with movement, posture, body temperature regulation, nutrient storage, and joint stabilization. Cardiac muscles, found only in the heart, contract involuntarily to pump blood. Smooth muscles line organs and structures like the esophagus, stomach, and intestines, aiding in functions like digestion and waste removal. |
| Embryonic Development | Muscles form during embryonic development through a process called myogenesis. The para-axial mesoderm differentiates and forms somites, which then develop into muscle tissue under the influence of regulatory factors like Wnt, Shh, and BMP4 proteins. |
| Appearance | Skeletal and cardiac muscles are striated, with a striped appearance due to the arrangement of actin and myosin filaments. Smooth muscles are non-striated. |
| Mass | Skeletal muscles comprise approximately 30% to 40% of total body mass, with men having about 36% more skeletal muscle mass than women. |
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Skeletal muscle
Each skeletal muscle is made up of hundreds or thousands of muscle fibres bundled together and wrapped in a connective tissue covering. Each muscle fibre is a single cylindrical muscle cell. The bundles of muscle fibres are known as fasciculi and are surrounded by a connective tissue layer called perimysium. The outermost connective tissue sheath surrounding the entire muscle is known as the epimysium, and the innermost layer surrounding each individual muscle fibre is called the endomysium. Skeletal muscle fibres are red and white and are often called striated or striped muscles.
The two most significant myofilaments that make up the contractile elements of skeletal muscle fibres are actin and myosin, which interact to cause movement. The actin and myosin filaments are arranged to form various bands on the skeletal muscle. Other muscle proteins include the regulatory proteins troponin and tropomyosin.
The development of skeletal muscles in the limbs and trunk depends on the expression of MyoD and Myf5 and their effects on the different myoblasts. These embryonic myoblasts undergo further differentiation to form the primary muscle fibres and eventually secondary myofibers by the union of myoblasts in the fetus.
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Cardiac muscle
The cardiac muscle forms a thick middle layer of the heart wall, between the outer layer (the pericardium) and the inner layer (the endocardium). The endocardium is not cardiac muscle, but simple squamous epithelial cells that form the inner lining of the heart chambers and valves. The pericardium, on the other hand, is a fibrous sac surrounding the heart, consisting of the epicardium, pericardial space, parietal pericardium, and fibrous pericardium. The myocardium, meanwhile, is composed of individual cardiac muscle cells or cardiomyocytes, which are joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix.
The primary function of the cardiac muscle is to pump blood into circulation by generating sufficient force. The mechanism behind each coordinated contraction involves the cardiac muscle and electrical impulses. The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. The cardiac muscle must contract with enough force and blood to meet the metabolic demands of the entire body.
The rhythmic contraction of the cardiac muscle is regulated by the sinoatrial node of the heart, which serves as the heart's pacemaker. The pacemaker cells are only weakly contractile without sarcomeres, and are connected to neighbouring contractile cells via gap junctions. They are distributed throughout the heart and are responsible for generating and sending out electrical impulses.
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Smooth muscle
Single-unit smooth muscle contracts as a syncytium, with the entire bundle or sheet of cells contracting together. In contrast, multi-unit smooth muscle cells are independent and innervated individually, allowing for more precise control. These cells are found in the iris and hair erector muscles.
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Muscle fibres
There are three types of muscle tissue in vertebrates: skeletal, smooth, and cardiac. Skeletal muscles are attached to bones and allow us to perform a wide range of movements and functions. They are voluntary muscles, meaning we can control how and when they work. Smooth muscles, on the other hand, are involuntary and are found within the walls of organs and structures such as the oesophagus, stomach, intestines, and blood vessels. Cardiac muscle, or heart muscle, is also an involuntary muscle and is the only type of muscle that is also an organ.
Skeletal muscles are further classified into two fibre types: type I (slow-twitch) and type II (fast-twitch). Type I muscle fibres are dense with capillaries and rich in mitochondria and myoglobin, giving them a red colour. They can carry more oxygen and sustain aerobic activity. Type II muscle fibres have three subtypes, with type IIa being similar to type I in that it is aerobic and rich in mitochondria and capillaries. Type IIx, or type IId, has a lower density of mitochondria and myoglobin.
The myofibrils of each cell are branched and interlocked with those of adjacent fibres, forming strong junctions. Each myofibril is made up of arrays of parallel thick and thin filaments. The thick filaments have a diameter of about 15 nm and are composed of the protein myosin. The thin filaments, about 5 nm in diameter, are composed mainly of the protein actin, along with smaller amounts of the regulatory proteins troponin and tropomyosin. The arrangement of these filaments gives skeletal muscle its striated, or striped, appearance. The thick and thin filaments come together to form functional units called sarcomeres, which create the distinctive pattern of alternating dark A bands and light I bands.
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Muscle contraction
The process begins with an action potential, a signal generated by the nervous system, which travels through motor neurons to the neuromuscular junction. At this junction, a neurotransmitter called acetylcholine is released, binding to receptors on the muscle fibre membrane. This binding sets off a chain of chemical reactions within the muscle fibre, leading to the release of calcium ions. The calcium ions play a crucial role in facilitating the interaction of proteins actin and myosin, which are essential for muscle contraction.
Actin and myosin are contractile proteins that interact to cause muscle contraction. They are arranged in repeating arrays called sarcomeres, giving muscle fibres their striated appearance. When calcium ions are released, they bind to troponin C, causing a conformational change that allows the myosin heads to attach to the actin filaments, forming cross-bridges. This cross-bridge cycling is fuelled by the binding of ATP to the myosin head, leading to further conformational changes and the eventual contraction of the sarcomere and the muscle fibre.
There are different types of muscle contractions, including concentric and eccentric contractions. Concentric contractions occur when a muscle actively shortens to generate tension and lift something heavy. On the other hand, eccentric contractions happen when a muscle lengthens during normal activities like walking or lowering a heavy object. Another type of contraction is isometric contraction, where a muscle is held at a set length, such as when carrying an object without raising or lowering it.
The various types of muscle contractions work together to enable movement, provide stability to joints, maintain posture, and produce heat to regulate body temperature. These contractions occur in different types of muscle tissues, including skeletal, cardiac, and smooth muscle tissues, each with its unique structure and function.
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Frequently asked questions
Muscles are made of thousands of small fibres woven together, allowing them to stretch and press together to move your body and organs.
The two most significant myofilaments that make up the contractile elements of the muscle fibre are actin and myosin. Actin is a thin filament and myosin is a thick filament.
There are three types of muscle tissue in vertebrates: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles that allow you to perform a wide range of movements and functions. Smooth muscles are involuntary and make up your organs, blood vessels, digestive tract, skin and other areas. Cardiac muscle is also involuntary and is only found in the walls of the heart.
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