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The human body is made up of around 600 muscles that help us perform a variety of functions, from pumping blood and supporting movement to lifting heavy weights or giving birth. Muscles are made up of thousands of elastic fibres bundled tightly together, with each bundle wrapped in a thin, transparent membrane called a perimysium. Each muscle fibre is made up of blocks of proteins called myofibrils, which contain a specialised protein called myoglobin, as well as molecules that provide the oxygen and energy required for muscle contraction. The brain, nerves and skeletal muscles work together to cause movement, and this is known as the neuromuscular system.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | 600 |
| Function | Pumping blood, supporting movement, lifting heavy weights, giving birth, etc. |
| Types of functions | Voluntary, involuntary |
| Types of muscles | Skeletal, smooth, cardiac |
| Skeletal muscle shapes | Spindle, flat, triangular, circular |
| Source of energy | Glucose from carbohydrates |
| Other nutrients required | Calcium, magnesium, potassium, sodium |
| Muscle composition | Actin, myosin, troponin, tropomyosin, titin, nebulin |
| Muscle cell composition | Myofibrils, myofilaments |
| Myofibril composition | Thick and thin filaments |
| Thick filament diameter | 15 nm |
| Thin filament diameter | 5 nm |
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Skeletal muscle
Each skeletal muscle is made up of hundreds or thousands of muscle fibers bundled together and wrapped in a connective tissue covering. Each muscle is surrounded by a connective tissue sheath called the epimysium. The connective tissue covering provides support and protection for the delicate cells and allows them to withstand the forces of contraction. The connective tissue is present in all muscles as deep fascia, which separates the groups of muscles into compartments.
The muscle fibers are organized into bundles called fascicles, surrounded by a middle layer of connective tissue called the perimysium. Each muscle fiber is encased in a thin connective tissue layer of collagen and reticular fibers called the endomysium. The endomysium surrounds the extracellular matrix of the cells and plays a role in transferring force produced by the muscle fibers to the tendons.
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Smooth muscle
Multi-unit smooth muscle, on the other hand, is neurogenic, meaning that 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. However, the terms single- and multi-unit smooth muscle represent an oversimplification, as smooth muscle is controlled and influenced by a combination of different neural elements, with some cell-to-cell communication and locally produced activators/inhibitors leading to a coordinated response even in multi-unit smooth muscle.
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Cardiac muscle
The individual cardiac muscle cell is a tubular structure composed of chains of myofibrils, which are rod-like units within the cell. The 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. The myofilaments slide past each other as the muscle contracts and relaxes, forming "cross-bridges" that cause the contraction of the heart and generate force. The concentration of calcium in the myocyte determines how much force is generated with each contraction.
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Muscle contraction
At these junctions, the motor neuron releases a neurotransmitter called acetylcholine, which binds to receptors on the muscle fibre, initiating a chain reaction. This leads to the activation of proteins involved in the contraction process, primarily a protein called myosin, which interacts with another protein, actin, to generate force.
The interaction between myosin and actin is key to muscle contraction. Myosin molecules have a head-like structure that can bind to specific sites on the actin filaments. When an action potential reaches the muscle fibre, it triggers a release of calcium ions from a specialized structure called the sarcoplasmic reticulum. These calcium ions then bind to troponin, a regulatory protein complex associated with the actin filaments, exposing the binding sites on the actin for myosin.
ATP, the energy currency of the cell, plays a crucial role in this process. The binding of myosin to actin, along with the energy from ATP hydrolysis, causes a conformational change in the myosin head, resulting in a power stroke that pulls the actin filament past the myosin, leading to contraction. This cycle repeats, with myosin detaching from the actin, then reattaching at a different site, resulting in sustained contraction as long as the calcium signal is present and ATP is available.
The overall effect of these individual contractions is the generation of force on a larger scale, which leads to muscle shortening and, ultimately, movement. The process is highly regulated, allowing for precise control of muscle function, whether it's the subtle movement of a finger or the powerful contraction of a leg muscle during a sprint.
Additionally, it's worth noting that there are different types of muscle fibres, generally categorized as slow-twitch and fast-twitch fibres, which contract at different speeds and are suited to different types of activities. Slow-twitch fibres are used for endurance activities, while fast-twitch fibres are used for more powerful but shorter-duration movements. The ratio of these fibre types in a muscle determines its characteristics and function.
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Muscle fibres
The human body is made up of around 600 muscles, which help us perform a variety of functions, from involuntary actions like breathing to voluntary actions like walking. Muscles work by contracting and relaxing, either voluntarily or involuntarily, to cause movement.
Skeletal muscle is the specialised tissue that is attached to bones and allows movement. Each skeletal muscle is made up of individual bundles of muscle fibres, known as fasciculi. These bundles are wrapped in a thin, transparent membrane called the perimysium. Each muscle fibre is made up of myofibrils, which contain myofilaments. When bundled together, the myofibrils form a unique striated pattern, creating sarcomeres, which are the fundamental contractile unit of a skeletal muscle. The two most significant myofilaments are actin and myosin, which are arranged to form bands on the skeletal muscle.
The myofibrils of each cell are branched and interlock with those of adjacent fibres, forming strong junctions. The thick filaments have a diameter of about 15 nm and are composed of the protein myosin, while the thin filaments have a diameter of about 5 nm and are composed of the protein actin, along with smaller amounts of troponin and tropomyosin. The sarcomere is bordered by the Z-line, which serves as the site of origin of the thin actin myofilaments that project towards each other as they partially overlap the myosin filaments. The regulatory proteins troponin, tropomyosin, titin, and nebulin play a key role in the myofilaments sliding mechanism leading to contraction.
The outermost connective tissue sheath surrounding the entire muscle is called the epimysium, while the innermost sheath surrounding individual muscle fibres is called the endomysium. The primary artery supplying blood to a skeletal muscle runs parallel to the longitudinal axis of the muscle fibre and gives off tributaries known as feed arteries. These feed arteries run perpendicularly to the primary artery and proceed towards the perimysium.
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Frequently asked questions
Muscles are made up of thousands of elastic fibres bundled tightly together. There are about 600 muscles in the human body, and they have a range of functions from pumping blood and supporting movement to lifting heavy weights or giving birth.
Muscles work by contracting and relaxing to cause movement. This movement may be voluntary (made consciously) or involuntary (done without our conscious awareness). The brain, nerves and skeletal muscles work together to cause movement, and this is collectively known as the neuromuscular system.
There are three different types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are attached to bones and allow movement. Smooth muscles are located in various internal structures including the digestive tract, uterus, and blood vessels. Cardiac muscles make up the walls of the heart, powering its contraction and relaxation to enable circulation.
