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The human body is made up of about 600 muscles, which are pieces of soft tissue that help us do everything from breathing to running marathons. There are three types of muscle tissue: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles that help with movement, while smooth and cardiac muscles contract involuntarily. All muscles are made up of thousands of small fibres woven together, containing contractile proteins called actin and myosin, which interact to cause movement.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | 600+ |
| Types of muscles | Skeletal, cardiac, smooth |
| Skeletal muscle composition | 30-40% of total body mass |
| Skeletal muscle fibres | Actin, myosin, support proteins |
| Skeletal muscle fibre length | 2-3cm |
| Skeletal muscle fibre diameter | 10-100 μm |
| Skeletal muscle fibre colour | Red and white |
| Skeletal muscle fibre type | Type I (slow-twitch), Type II (fast-twitch) |
| Type II muscle subtypes | Type IIa, Type IIx, Type IIb |
| Muscle movement | Voluntary, involuntary |
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Skeletal muscle
The skeletal muscle is derived from the paraxial mesoderm, which divides into segments called somitomeres. These eventually form the skeletal muscles in the head, neck, and trunk regions. The development of skeletal muscles in the limbs and trunk depends on the expression of MyoD and Myf5, which affect the differentiation of embryonic myoblasts into primary muscle fibres and secondary myofibers.
Each skeletal muscle fibre is made up of several hundred to a thousand myofibrils, which are composed of actin (thin filaments) and myosin (thick filaments), along with support proteins. Myofibrils also contain specialised proteins like myoglobin and molecules that provide oxygen and energy for muscle contraction. When stimulated, these filaments fold together, causing the muscle fibre to shorten, resulting in the contraction of the entire muscle.
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Cardiac muscle
The primary function of 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. These contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. Aerobic production is the core utilization process; however, the heart may use anaerobic processes in a limited capacity.
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Smooth muscle
Multi-unit smooth muscle, on the other hand, is neurogenic, meaning its contraction must be initiated by an autonomic nervous system neuron. It is found in the trachea, in the iris of the eye, and lining the large elastic arteries. Smooth muscle cells are spindle-shaped with a wide middle and tapering ends, and a single nucleus. They are shorter than skeletal muscle cells, ranging from 30 to 200 micrometers in length. Smooth muscle cells contain the contractile proteins myosin and actin, which enable them to contract. Myosin, primarily class II in smooth muscle, contains two heavy chains that form a coiled-coil structure.
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Muscle fibres
Skeletal muscle fibres are multinucleated, elongated cells that range from 10 to 100 micrometres in diameter and several centimetres in length. The nuclei of these cells are located along the inside of the cell membrane. Skeletal muscle fibres are composed of myofibrils, which are made up of actin and myosin filaments, also known as myofilaments. These filaments are repeated in units called sarcomeres, which are the fundamental functional, contractile units of muscle fibres. The arrangement of actin and myosin filaments gives skeletal muscle its microscopic striated appearance.
The sarcomere is the basic unit of muscle contraction and is composed of three main regions: the H band, the A band, and the I band. The H band contains the M line, where myosin filaments are anchored together. The A band contains the entirety of the myosin filaments and includes regions of actin and myosin overlap. The I band covers the terminal regions of two adjacent sarcomeres and contains only actin filaments. During muscle contraction, the H and I bands shorten, while the A band remains constant in length.
The composition of muscle fibre types is flexible and can vary depending on environmental factors. For example, in fish, different fibre types are expressed at different water temperatures. Cold temperatures require a more efficient muscle metabolism and fatigue resistance, while in tropical environments, fast and powerful movements may be more advantageous. Similarly, the diet, exercise habits, and lifestyle of humans can influence the proportions of fibre types in their muscles.
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Muscle movement
There are three types of muscle tissue in vertebrates: skeletal, cardiac, and smooth muscle. Skeletal muscles are under voluntary control, meaning an individual can decide to move them. They make up a significant proportion of total body mass, ranging from 30% to 40%. Skeletal muscles are attached to bones by tendons and work together with bones, tendons, and ligaments to support body weight and enable movement. These muscles consist of flexible muscle fibres that contract, allowing for a wide range of movements. Each skeletal muscle contains multiple fascicles, or bundles of muscle fibres, surrounded by connective tissue layers called the endomysium, perimysium, and epimysium. The endomysium contains capillaries and nerve tissue, supplying individual muscle fibres. The perimysium surrounds bundles of fascicles, and the epimysium forms a fibrous envelope around the entire muscle.
Cardiac and smooth muscles are involuntary muscles controlled by the autonomic nervous system. Cardiac muscle is found in the heart walls and is responsible for involuntary movements like the heart beating. Smooth muscle is present in the walls of organs and structures such as the oesophagus, stomach, intestines, and blood vessels, facilitating movements like the expansion and contraction of the bronchi during breathing.
The muscle fibres within skeletal muscles are composed of myofibrils, which are made up of actin and myosin filaments. These filaments form sarcomeres, the basic functional units of muscle contraction. The arrangement of actin and myosin gives skeletal muscle its striated appearance. The interaction between actin and myosin proteins causes muscle movement. Additionally, muscle fibres have multiple mitochondria to meet energy demands during movement.
The type of muscle fibre present in skeletal muscles can vary depending on environmental factors and the species of organism. There are two main types of muscle fibres: type I (slow-twitch) and type II (fast-twitch). Type I fibres are dense with capillaries and mitochondria, giving them a red colour. Type II fibres have three subtypes, with varying contractile speeds, mitochondrial densities, and anaerobic capabilities. The composition of muscle fibres can be influenced by factors like diet, exercise, and lifestyle in humans, while temperature and movement requirements play a role in other species.
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Frequently asked questions
Muscles are composed of thousands of small fibres woven together. These fibres are made up of proteins called actin and myosin.
There are three types of muscle tissue: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles, which means you have conscious control over them. Smooth and cardiac muscles contract involuntarily, without conscious intervention.
Muscles move your body and support your organs. They also store and release energy that your body uses as part of your metabolism.
