Victor Forge
The human body is made up of three types of muscle tissue: skeletal, smooth, and cardiac. Skeletal muscle, also known as muscle, is the most common type of muscle in the body, comprising 30-40% of total body mass. These muscles are attached to bones by tendons and allow us to perform a wide range of movements and functions. Skeletal muscles are also unique in that they are voluntary muscles, meaning we control how and when they work. Each skeletal muscle contains multiple fascicles or bundles of muscle fibers, and each fiber can contain thousands of nuclei. The arrangement of protein fibers inside skeletal muscle cells gives them a striped appearance, indicating that they are very strong.
| Characteristics | Values |
|---|---|
| Definition | Skeletal muscle is the most common type of muscle in the body. |
| Types of Muscle Tissue | There are three types: skeletal, cardiac, and smooth muscle. |
| Muscle Composition | Skeletal muscle is composed of bundles of muscle fibres called myofibers, which contain several myofibrils. |
| Muscle Control | Skeletal muscles are voluntary muscles, controlled consciously. Cardiac and smooth muscles are involuntary. |
| Muscle Function | Skeletal muscles enable movement, maintain posture, control body temperature, and stabilise joints. |
| Muscle Appearance | Skeletal muscle tissue is striated, with a striped appearance due to the arrangement of sarcomeres. |
| Muscle Structure | Each muscle is composed of fascicles, surrounded by connective tissue layers: endomysium, perimysium, and epimysium. |
| Muscle Connectors | Tendons attach skeletal muscles to bones, allowing for skeletal movement. |
| Muscle Size | Skeletal muscles vary in size, ranging from less than 0.5 inches to over 3 inches in diameter. |
| Muscle Weight | Skeletal muscles comprise approximately 30-40% of total body mass in humans. |
| Muscle Disorders | Skeletal muscle disorders include muscular dystrophies, congenital myopathies, and inflammatory disorders. |
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Skeletal muscle
Sarcomeres are the basic contractile units of the muscle fibre, necessary for muscle contraction. The sarcomere consists of a central M-line, with thick myosin filaments anchored together. The M-line is bordered by the Z-line, where thin actin filaments originate and project towards each other, partially overlapping the myosin filaments. The H-band contains the M-line and includes only myosin filaments, while the A-band contains the entirety of the myosin filaments and overlaps with actin. The I-band covers the terminal regions of 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 muscle fibres are bundled together to form fascicles, which are surrounded by another layer of connective tissue called the perimysium. The perimysium may enclose anywhere from 10 to 100 fascicles. These fascicles are then grouped to form a muscle, encased by the epimysium, a fibrous tissue envelope. Each muscle can contain thousands of muscle fibres, and they contract in response to a stimulus, allowing for movement, maintaining posture, controlling body temperature, and stabilising joints.
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Cardiac muscle
The human body contains three kinds of muscle tissue: skeletal, smooth, and cardiac. Cardiac muscle, also called the myocardium, is one of three major categories of muscles found within the vertebrate body. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart.
The cardiac muscle forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium). The endocardium is not cardiac muscle and is made up of simple squamous epithelial cells, forming 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 cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. The heart may use anaerobic processes in a limited capacity. The cardiac action potential lasts approximately 200 ms and is divided into 5 phases: resting, upstroke, early repolarization, plateau, and final repolarization.
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Smooth muscle
Additionally, smooth muscle is responsible for goosebumps, with cells in the skin causing hairs to stand erect in response to cold temperatures or fear. In the eyes, smooth muscles, such as the ciliary muscles and iris dilator and sphincter muscles, control the shape of the lens and the dilation and constriction of the pupils, respectively. Smooth muscle is also involved in contractions during labour in the uterus.
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Muscle composition
The composition of skeletal muscle includes muscle fibres, intramuscular connective tissue, and intramuscular fat. Muscle fibres, which make up about 90% of skeletal muscle, are individual cylindrical muscle cells surrounded by a layer of connective tissue called the endomysium. Bundles of these muscle fibres, known as fasciculi, are encased in another layer of connective tissue, the perimysium. The entire muscle is then enveloped by the epimysium, another layer of connective tissue. This connective tissue provides support and protection for the muscle fibres, enabling them to withstand the forces of contraction.
The connective tissue surrounding muscle fibres and bundles is a loose type of connective tissue composed of cells and an extracellular matrix (ECM). The ECM consists primarily of collagen fibres forming a composite network with proteoglycans. Collagen, a fibrous protein, forms a helical structure through the coiling of three polypeptide chains. These structures are stabilised by various types of bonds, resulting in the formation of fibrils. The ECM plays a crucial role in muscle development, growth, and repair, as well as effective muscle contraction and force transmission.
The composition of muscle fibres can vary depending on several factors. For instance, it can depend on the species, breed, gender, age, physical activity levels, environmental temperature, and feeding practices. For instance, the pig Longissimus muscle has a different composition of type I, IIA, IIX, and IIB fibres compared to the bovine Longissimus muscle. Additionally, muscle composition can be influenced by in vivo and postmortem factors, such as nutritional and environmental factors, slaughtering conditions, and postmortem processing, which in turn affect meat quality.
To maintain healthy muscles, regular exercise and a nutritious, balanced diet are essential. Muscle-strengthening exercises, such as lifting weights or using resistance bands, help to improve muscle strength and mass. Adequate protein intake, along with good quality carbohydrates and healthy fats, is necessary for building and maintaining muscle mass. However, it is important to note that muscle abnormalities, weakness, and pain can occur due to various conditions or injuries, requiring medical attention and appropriate treatment.
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Muscle function
The human body has more than 600 muscles that help with a wide range of functions, from involuntary actions like breathing, circulation, and digestion to voluntary actions like walking or speaking. The body has three types of muscles: cardiac, skeletal, and smooth.
Cardiac muscle makes up the walls of the heart, powering contraction and relaxation to enable circulation. Cardiac muscle is an involuntary muscle that is controlled by the autonomic nervous system.
Skeletal muscles are the most common type of muscle in the body. They connect to bones, tendons, and ligaments to support movement and make up around 30% to 40% of total body mass. Skeletal muscles are voluntary, meaning individuals control how and when they work. Skeletal muscles serve many purposes, including producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints. Skeletal muscle fibres are striated, multinucleated cells ranging from 10 to 100 micrometers in diameter and several centimetres long.
Smooth muscle makes up the inside of organs such as the bladder, stomach, and intestines and plays a role in involuntary functions like the digestive and urinary systems.
The principal functionality of muscles is rooted in their ability to contract and relax. The foundation for muscle contraction is the sarcomere, found in all muscle cells. Sarcomeres contain a motor protein called myosin, which powers the muscle to contract by "grabbing" onto another protein called actin and "flexing". When the myosin releases the actin, the muscle relaxes.
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