Victor Forge
Muscles are tissues in the body that can contract and relax to produce movement. There are three major types of muscles in the human body: skeletal, cardiac, and smooth muscle. Skeletal muscles are attached to bones and are responsible for movement and posture. Cardiac muscle, also known as heart muscle, is responsible for circulating blood throughout the body. Smooth muscle is present in the gastrointestinal, reproductive, urinary, vascular, and respiratory systems, and it helps with functions such as moving food along the oesophagus and controlling blood flow and pressure. The human body is susceptible to various muscle-related conditions, such as tendonitis, sarcopenia, strain, myopathies, and muscular dystrophy, which can affect muscle function and overall health.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Types of muscle tissue | Skeletal, cardiac, smooth |
| Muscle composition | Thousands of small fibres woven together |
| Skeletal muscle composition | Red and white fibres |
| Skeletal muscle mass in men | 42% of total body mass |
| Skeletal muscle mass in women | 36% of total body mass |
| Skeletal muscle functions | Chewing, swallowing, breathing, moving, maintaining posture |
| Cardiac muscle function | Pumps blood throughout the body |
| Smooth muscle function | Moves waste through intestines, helps lungs expand during breathing |
| Smooth muscle location | Walls of organs and structures like oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels |
| Skeletal muscle types | Type I (slow-twitch), Type II (fast-twitch) |
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What You'll Learn
Skeletal muscle
Muscle fibres are composed of myofibrils, which are made up of actin and myosin filaments called myofilaments. These are repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fibre necessary for muscle contraction. Skeletal muscle fibres are multinucleated with the nuclei often referred to as myonuclei.
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Cardiac muscle
The cardiac muscle forms a thick middle layer between the outer layer of the heart wall (the pericardium or epicardium) and the inner layer (the endocardium). The heart wall is a three-layered structure, with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium. The inner endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels that connect to the heart. The outer epicardium forms part of the pericardial sac that surrounds, protects, and lubricates the heart.
Cardiomyocytes are 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 organize into thick and thin filaments, called myofilaments. Thin myofilaments contain the protein actin, and thick myofilaments contain the protein myosin. The myofilaments slide past each other as the muscle contracts and relaxes, producing the formation of “cross-bridges”, which causes contraction of the heart and generation of force.
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Smooth muscle
At a cellular level, smooth muscle has a unique structure. It consists of narrow, spindle-shaped cells with a single, centrally located nucleus. These cells do not exhibit cross stripes under microscopic magnification, resulting in a non-striated appearance. Smooth muscle contains thick and thin filaments of actin and myosin, which are proteins that enable muscle contraction. The shape of each smooth muscle cell is fusiform, with a round centre and tapered ends. This structure contributes to its elastic properties, allowing it to tense and relax effectively.
The development of smooth muscle is influenced by neural crest cells, which play a vital role in regulating blood vessels. Vascular smooth muscle cells, also known as mural cells, arise from multiple origins and contribute to the site-specific localisation of vascular diseases. Smooth muscle cell embryonic lineage may also be a factor in determining the location and presentation of certain diseases.
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Muscle fibres
There are three types of muscle fibre: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, but in varying proportions. The speed of contraction depends on how quickly myosin's ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibres hydrolyze ATP about twice as rapidly as slow fibres, resulting in much quicker cross-bridge cycling.
Skeletal muscle fibres are further classified into two types: type 1 and type 2. Type 1 fibres utilize oxygen to generate energy for movement and have a higher density of energy-generating organelles called mitochondria, which makes them dark. Type 2 fibres can be further divided into subtypes 2A and 2B. Type 2A fibres can use oxygen to generate energy for movement, but they contain fewer mitochondria, making them light. Type 2B fibres, on the other hand, do not use oxygen to generate energy. Instead, they store energy for short bursts of movement and contain even fewer mitochondria than type 2A fibres, giving them a white appearance.
Cardiac muscle fibres have their own rhythm and are branched and interconnected. They are stimulated by special cells called pacemaker cells, which generate impulses that cause the cardiac muscle to contract. Smooth muscle fibres, on the other hand, are found within the walls of organs and structures such as the oesophagus, stomach, intestines, and blood vessels. They are much shorter than skeletal muscle fibres and have a more uniform, oblong shape.
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Muscle tissue
Muscle is a soft tissue and one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle.
Skeletal muscle is the only voluntary muscle tissue in the human body, controlled by the somatic nervous system. Skeletal muscles are attached to the bones by tendons and are used to effect skeletal movement such as locomotion and to maintain posture. They consist of flexible muscle fibres that range from less than half an inch to just over 3 inches in diameter. Skeletal muscle tissue is striated, consisting of elongated, multinucleate muscle cells called muscle fibres. Skeletal muscle fibres are red and white and are further classified into two types: type I (slow-twitch) and type II (fast-twitch).
Cardiac muscle tissue is found only in the walls of the heart as myocardium. It is an involuntary muscle controlled by the autonomic nervous system. Cardiac muscle tissue is striated like skeletal muscle, containing sarcomeres in highly regular arrangements of bundles. Cardiac muscle tissue cannot be controlled consciously and is considered autorhythmic or intrinsically controlled.
Smooth muscle is an involuntary muscle that lines the inside of some organs. It is found within the walls of organs and structures such as the oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin. Smooth muscle is non-striated and is often used to describe visceral muscle because of its very smooth, uniform appearance when viewed under a microscope.
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Frequently asked questions
Muscles are made of tissue that can tighten and relax to produce movement. There are three major types of muscles found in the human body: skeletal, cardiac, and smooth muscle. Each type has unique cellular components, physiology, specific functions, and pathology.
Skeletal muscles are attached to bones and have the main function of contracting to facilitate movement of our skeletons. Cardiac muscle encompasses the heart, which keeps the human body alive. Smooth muscle is present throughout the gastrointestinal, reproductive, urinary, vascular, and respiratory systems.
Some common muscle injuries include tendonitis, which is inflammation of the tendons that connect muscles with bones; strain, which is caused by overstretching or overusing a muscle; and myopathies, which are medical conditions that weaken the integrity of the muscles and cause life-threatening diseases.
Muscles are responsible for producing movement in the body. They can also sense temperature and pain. Additionally, certain types of muscles have specialized functions, such as the cardiac muscle's role in circulating blood throughout the body.
