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The human body is composed of over 600 muscles that help us move, breathe, and perform a wide range of functions. There are three types of muscles: skeletal, cardiac, and smooth muscle. Skeletal muscles are the most common type and make up 30-40% of our total body mass. They are composed of muscle fibres, also known as myocytes, which are long and cylindrical cells that can range from less than half an inch to over 3 inches in diameter. Each skeletal muscle may contain hundreds or thousands of these fibres, bundled together and wrapped in connective tissue. These fibres contract, allowing us to move our bones and perform various movements. The structure of these fibres, from their arrangement to the proteins within them, is what determines the shape and function of our muscles.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Types of muscles | Skeletal, cardiac and smooth muscle |
| Muscle composition | Thousands of small fibers woven together |
| Muscle movement | Contraction and extension |
| Muscle functions | Movement, posture, balance, breathing, digestion, maintaining body temperature, storing nutrients, stabilising joints |
| Skeletal muscle composition | Connective tissue sheaths called epimysium, perimysium, and endomysium |
| Skeletal muscle mass | 30% to 40% of total body mass |
| Skeletal muscle control | Voluntary |
| Cardiac and smooth muscle control | Involuntary |
| Muscle fibers | Actin (thin filaments), myosin (thick filaments), and support proteins |
| Muscle fiber size | 0.5 to 3 inches in diameter |
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What You'll Learn
Skeletal muscle composition
Skeletal muscle is the most common type of muscle in the human body, comprising approximately 30% to 40% of total body mass. These muscles are attached to bones by tendons, allowing for a wide range of movements and functions. Skeletal muscles are under voluntary control, meaning individuals can consciously decide how and when they work.
Skeletal muscles are composed of bundles of muscle fibres called myofibers, which contain several myofibrils. Each myofiber represents a muscle cell, with its basic cellular unit being the sarcomere. These sarcomeres are the functional units of muscle fibres, composed of actin (thin filaments) and myosin (thick filaments), along with support proteins. The arrangement of actin and myosin gives skeletal muscle its striated appearance. Myosin is the protein responsible for muscle contraction, while actin forms a helical structure that constitutes the bulk of the thin filament mass.
The sarcolemma, or the cell membrane of muscle fibres, acts as a conductor for electrochemical signals stimulating muscle cells. Transverse tubules (T-tubules) are connected to the sarcolemma, aiding in carrying these signals into the centre of the muscle fibre. The sarcoplasmic reticulum serves as a storage site for calcium ions, which are essential for muscle contraction. Additionally, skeletal muscles contain mitochondria, which break down sugars to provide energy in the form of ATP for active muscles.
The development of skeletal muscles begins during embryogenesis, with the paraxial mesoderm undergoing stepwise differentiation to generate muscle tissue. The dorsomedial aspect of the myotome differentiates into the epaxial myotome, giving rise to back muscles. Conversely, the ventrolateral aspect forms the hypaxial myotome, resulting in the muscles of the body wall. After birth, satellite cells function as stem cells, contributing to the growth and development of skeletal muscles.
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Muscle fibres and movement
The human body has more than 600 muscles, which help us do everything from breathing to moving our bodies. Skeletal muscles, which are voluntary muscles that we can control consciously, make up 30% to 40% of our total body mass. These muscles are connected to our bones and allow us to perform a wide range of movements and functions.
Skeletal muscles are made up of thousands of small muscle fibres woven together. These fibres can be as small as less than half an inch in diameter and as large as just over 3 inches in diameter. Each muscle can contain thousands of fibres, and these fibres are composed of myofibrils. Myofibrils are the contractile structures of the cell and are made up of many protein fibres arranged into repeating subunits called sarcomeres. The sarcomere is the functional unit of muscle fibres and is composed of actin and myosin filaments called myofilaments.
Actin forms a helical structure that makes up the bulk of the thin filament mass. Myosin, on the other hand, is the protein that causes muscles to contract. These actin and myosin filaments work together to produce muscle movement. When a muscle contracts, the actin and myosin filaments slide past each other, causing the muscle to shorten and generate force. This process is known as the sliding filament mechanism.
There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). SO fibres contract slowly and use aerobic respiration to produce ATP. They have a low rate of fatigue and are suited for endurance activities like marathon running. FO fibres, on the other hand, have fast contractions and can use both aerobic and anaerobic respiration. They produce ATP relatively quickly and can generate moderate amounts of tension. FG fibres have fast contractions and primarily use anaerobic glycolysis to produce ATP. These fibres fatigue quickly and are suited for short, powerful movements like sprinting and weight-lifting.
The type of muscle fibres and their organisation determine the strength and precision of our movements. Muscles that perform fine movements, like those of the eyes or fingers, have fewer muscle fibres in each motor unit, allowing for precise control. In contrast, muscles that require more strength, like leg or arm muscles, have many muscle cells in each motor unit. The body can control the strength of a muscle by determining how many motor units to activate for a given function. This is why the same muscles can be used for both picking up a pencil and lifting a bowling ball.
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Types of muscle tissue
There are three major types of muscle tissue in the human body: skeletal, cardiac, and smooth muscle. Each muscle type has unique cellular components, physiology, specific functions, and pathology. Skeletal 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 facilitate a wide range of movements and functions. They are also known as fast-twitch muscles due to their ability to contract quickly and use short bursts of energy. Skeletal muscles are voluntary, meaning individuals have control over their movement.
Cardiac muscle, or myocardium, forms the middle layers of the heart. It is an involuntary, striated muscle that encloses the chambers of the heart. Cardiac muscle is unique in that it is the only type of muscle tissue found in the heart. Cardiac muscle is also involuntary, meaning individuals do not have control over its movement.
Smooth muscle is found throughout the gastrointestinal, reproductive, urinary, vascular, and respiratory systems. It lines some organs and is involuntary, allowing the body to function without conscious thought. Smooth muscle helps the body rid itself of waste and toxins.
Additionally, skeletal muscle can be classified into two types: Type I (slow oxidative) and Type II (fast-twitch). The diversity in the makeup of skeletal muscle results in variations in contraction speed and length among different muscle groups.
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Muscle shape and function
The human body has over 600 muscles that help us perform a variety of functions, from breathing and swallowing to running marathons. These muscles are made up of thousands of small fibres woven together, which stretch and press together to move our organs and body.
Muscles are classified based on their shape, size, direction, and function. For example, the deltoids have a triangular shape, the serratus muscles have a serrated or saw-like shape, and the rhomboid major is diamond-shaped. The size of a muscle can be used to distinguish between two muscles in the same region, such as the gluteal region's maximus, medius, and minimus muscles. The direction in which the muscle fibres run can also be used to identify a muscle. In the abdominal region, the rectus abdominis runs straight up and down, the transverse abdominis runs transversely, and the obliques run at an angle.
The shape and function of a muscle are closely related. The fascicular architecture and fibre length of a muscle determine its shape and function. For example, the gluteal muscles have numerous thick, short fascicles, while the sartorius muscle has longer and more slender fascicles. These differences in shape and fibre arrangement allow skeletal muscles to perform a wide range of tasks effectively.
There are three types of muscles in the human body: skeletal, cardiac, and smooth muscle. Skeletal muscles are voluntary muscles that we control consciously. They make up 30-40% of our total body mass and are responsible for a wide range of movements. Skeletal muscles rarely work alone and often work in groups to produce precise movements. Cardiac muscle, or myocardium, makes up the middle layers of the heart and is responsible for pumping blood through the cardiovascular system. Smooth muscles are involuntary muscles that line the inside of some organs, such as the walls of tubular structures and hollow organs. They modify the diameter or size of these structures to expel or contain contents.
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Muscle growth and exercise
The human body has more than 600 muscles, comprising 30% to 40% of total body mass. These muscles are made up of thousands of small fibres woven together. Skeletal muscles are the most common type of muscle in the body, and they are attached to the bones by tendons. These muscles are voluntary, meaning we control how and when they work.
Muscle growth, or hypertrophy, occurs when the muscle fibres sustain damage or injury. The body repairs these damaged fibres by fusing them, increasing the mass and size of the muscles. This process is influenced by hormones, including testosterone, human growth hormone, and insulin growth factor. Age, sex, and genetics also play a role in the rate of muscle growth. For instance, men tend to lose muscle mass as they age, and testosterone levels gradually decline, making it harder to build and maintain muscle.
Exercise is an important factor in muscle growth. Strength training is particularly effective for building muscle, and free weights are often better for this than machines. However, it is important to allow for rest and recovery between workouts, as insufficient rest can slow fitness progression and increase the risk of injury. Cardiovascular exercise, or cardio, is also beneficial for muscle growth and overall health, though it may be less efficient for protein deposition than a moderately restricted diet.
To build muscle, it is recommended to consistently challenge the muscles with higher levels of resistance or weight. This can be achieved through compound exercises such as squats, deadlifts, and lunges, which work multiple muscle groups simultaneously. Additionally, adequate nutrition is crucial, as a nutritionally balanced and calorically adequate diet is required for optimal muscle growth.
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Frequently asked questions
Muscles are pieces of soft tissue that help the body perform various functions, from holding the body still to complex physical activities.
There are three types of muscles in the human body: skeletal, cardiac and smooth muscle. Skeletal muscles are voluntary muscles that connect to bones and allow you to perform a wide range of movements. Cardiac and smooth muscles are involuntary muscles that are controlled by the autonomic nervous system.
Skeletal muscles are made of thousands of small fibres woven together. Each muscle is surrounded by a connective tissue sheath called the epimysium. Each muscle fibre is a single cylindrical muscle cell.
Muscles contract when stimulated by signals from their motor neurons. Motor neurons release neurotransmitter chemicals at the neuromuscular junction (NMJ) that bond to a special part of the sarcolemma known as the motor end plate. The sarcolemma acts as a conductor for electrochemical signals that stimulate muscle cells.
