Victor Forge
Muscle is one of the four primary tissue types of the human body. There are about 600 muscles in the human body, and they make up about 40% of our body weight. The three main types of muscle tissue are skeletal, smooth, and cardiac. Skeletal muscles are attached to bones and facilitate movement. Smooth muscles are located in various internal structures, including the digestive tract, uterus, and blood vessels. Cardiac muscles are found in the heart and facilitate heartbeats. All muscles share several properties, including contractility, excitability, extensibility, and elasticity.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | 600 |
| Muscle composition | Tissue composed of specialized cells/fibres |
| Muscle fibres | Actin and myosin filaments |
| Muscle tissue classification | Striated or non-striated/smooth |
| Most common type of muscle tissue | Skeletal muscle |
| Skeletal muscle composition | Connective tissue layer of fascia |
| Skeletal muscle fibres | Red and white |
| Skeletal muscle mass | 30% to 40% of total body mass |
| Types of muscle movements | Voluntary and involuntary |
| Muscle functions | Movement, metabolism, posture, body temperature, joint stability, etc. |
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What You'll Learn
- Skeletal muscle: Voluntary movement, making up 30-40% of body mass
- Cardiac muscle: Involuntary movement, e.g. heart beating
- Smooth muscle: Involuntary movement, e.g. digestive tract
- Muscle fibres: Made of myofibrils, actin and myosin filaments
- Muscle function: Movement, posture, temperature, joint stability
Skeletal muscle: Voluntary movement, making up 30-40% of body mass
Skeletal muscles are the most common type of muscle in the human body, comprising 30-40% of total body mass. They are attached to bones by tendons, which are tough connective tissues. These muscles are under voluntary control, meaning that you can consciously decide how and when they move. This is in contrast to the other two types of muscles in the body, cardiac and smooth muscle, which are involuntary.
Skeletal muscles are made up of thousands of muscle fibres, also known as myocytes. These fibres are woven together and slide past each other to produce contractions, which move the body. Each muscle can contain thousands of fibres, which are surrounded by different types of sheaths. The fibres can contract quickly, using short bursts of energy (fast-twitch muscles), or more slowly (slow-twitch muscles).
Skeletal muscles allow for a wide range of movements and functions, such as raising your arm to reach for a book. They also help to maintain posture and support body weight. These muscles can be found all over the body, including the shoulders, hamstrings, and abdomen.
Skeletal muscle disorders can manifest as muscle weakness, and include muscular dystrophies, congenital myopathies, inflammatory disorders, and diseases affecting the neuromuscular junction.
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Cardiac muscle: Involuntary movement, e.g. heart beating
The human heart consists of cardiac muscles, which are involuntary muscles that contract and relax to pump blood around the body. These involuntary movements happen automatically without conscious thought. This is distinct from voluntary movements, which are actions that an individual consciously controls.
Cardiac muscle has a unique property known as automaticity, which is the ability to contract without direct neural stimulation. This intrinsic rhythmicity is driven by pacemaker cells located in the sinoatrial (SA) node, the heart's natural pacemaker. These specialised cells generate electrical impulses that initiate and coordinate contractions, ensuring a consistent and reliable heartbeat. This automaticity is critical for continuous blood circulation.
Another important property of cardiac muscle is excitability, which refers to its ability to respond to electrical stimuli. When the SA node generates an impulse, it triggers a cascade of electrical activity that spreads across the heart through intercalated discs. These discs contain gap junctions that enable the direct passage of ions and electrical impulses between cardiac muscle cells, facilitating rapid signal transmission.
The contractility of cardiac muscle refers to its ability to contract forcefully in response to stimulation. This property determines the strength of each heartbeat and the subsequent amount of blood ejected from the heart with each contraction. The contractile force is influenced by factors such as the availability of calcium ions and the condition of the myocardial tissue.
Additionally, cardiac muscle exhibits a high level of metabolic efficiency. It primarily relies on aerobic metabolism and has a rich supply of mitochondria to meet its energy demands. This ensures a continuous supply of adenosine triphosphate (ATP), which is necessary for sustained contractions. Cardiac muscle can also adapt to varying demands, such as during exercise or stress, by undergoing physiological hypertrophy, where muscle cells enlarge to increase the heart's pumping capacity.
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Smooth muscle: Involuntary movement, e.g. digestive tract
Smooth muscle is one of the three types of muscle tissues in the human body, the other two being skeletal and cardiac muscle. It is present throughout the body, including in the stomach, intestines, urinary system, arteries, and veins. Smooth muscle is responsible for various involuntary movements and functions that occur without conscious thought.
Smooth muscle plays a crucial role in the digestive process. In the gastrointestinal tract, smooth muscle enables the movement of food through the system via a process called peristalsis. Peristalsis involves the rhythmic, wave-like contraction of both longitudinal and circular smooth muscle fibres within the digestive tract. This contraction forces food through the tract, beginning in the throat when swallowing and continuing through the oesophagus, stomach, and intestines during digestion. Segmentation is another type of involuntary muscle movement that occurs mainly in the intestines. It activates circular muscles that contract to move food back and forth, allowing it to mix with gastric juices and facilitating digestion.
Smooth muscle also regulates the diameter of blood vessels, controlling blood flow and pressure. This regulation is vital for maintaining tissue oxygenation and meeting the body's oxygen demands during exercise. Smooth muscle is further involved in the disease process and is targeted by various medications. For example, bronchodilators relax airway smooth muscle in asthma treatment, and nitrates are used in combination with ace inhibitors to treat ischemic heart disease.
Smooth muscle differs from skeletal muscle in several ways, most notably in its ability to contract and be controlled involuntarily. Skeletal muscles are voluntary muscles that individuals can control consciously. They are responsible for a wide range of movements and functions and make up 30% to 40% of total body mass. Smooth muscle, on the other hand, is controlled by the autonomic nervous system, allowing the body to regulate various subsystems without conscious effort.
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Muscle fibres: Made of myofibrils, actin and myosin filaments
Muscle fibres are responsible for the movement of the human body. Each muscle fibre contains many myofibrils, which are bundles of actin and myosin filaments. Myofibrils are long contractile fibres, which run parallel to each other on the long axis of the myocytes. The myocytes, in turn, run parallel to each other on the long axis of the cell.
Actin and myosin filaments are proteins that slide past one another, producing contractions that move body parts, including internal organs. These proteins are not exclusive to muscle cells. However, myocytes are characterised by an abundance of these proteins within their cytoplasm. In the case of myocytes, actin and myosin filaments are generally oriented along a single axis, thereby eliciting movement in a linear fashion.
The thick filaments are composed of strands of the protein myosin, and the thin filaments are strands of the protein actin, along with two other muscle regulatory proteins, tropomyosin and troponin. Under the influence of adenosine triphosphate (ATP), actin and myosin form a contractile compound, actomyosin, which is required for muscle contraction. The binding of ATP dissociates myosin from actin. ATP hydrolysis then induces a conformational change that displaces the myosin head group. This is followed by the binding of the myosin head to a new position on the actin filament.
The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance and creates functional units called sarcomeres. Sarcomeres are the repeating units that form myofibrils. The shortening of the individual sarcomeres leads to the contraction of the individual muscle fibres, leading to muscle contractions.
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Muscle function: Movement, posture, temperature, joint stability
The human body has over 600 muscles that help us perform a wide range of tasks, from moving our bodies to breathing and maintaining our posture. These muscles are made up of thousands of small fibres that work together to move our organs and body parts.
Movement
There are two types of movements: voluntary and involuntary. Voluntary movements are actions we consciously control, such as scrolling on a phone or sprinting. Involuntary movements, on the other hand, happen automatically without conscious thought, like the beating of our heart or breathing. Muscles surrounding synovial joints are responsible for moving the body in space, with movements often occurring in pairs, such as flexion and extension or abduction and adduction. For example, flexion refers to decreasing the angle between two bones, like bending, while extension involves increasing the angle, like straightening a bend.
Posture
Posture refers to how our body holds itself during muscular inactivity or activity. It is an individual and dynamic aspect, constantly adapting to different situations and environments. Our posture is influenced by our muscles, joints, perception, and emotions. Even when we are seemingly still, our body makes tiny adjustments and movements to maintain stability. For example, certain muscles in our back help with posture by moving slowly to keep us upright. Impairments, such as piriformis syndrome or tight hip adductors, can affect our posture by causing unlevelness in the base of the sacrum or a lateral pelvic tilt.
Temperature
Temperature is an important factor in muscle function, influencing the efficiency of muscle contractions during physical activities like cycling. Research has shown that elevating muscle temperature can increase mechanical efficiency in younger individuals, while decreasing efficiency in older individuals. This is due to differences in recruitment patterns and changes in muscle fibre type with age.
Joint Stability
Joint stability refers to a joint's ability to maintain or return to proper alignment through the equalization of forces. This requires the coordination of bones, joint capsules, ligaments, muscles, tendons, and sensory receptors. Dynamic joint stability is specifically influenced by the neuromuscular control of muscles, which involves the unconscious activation of restraints to maintain and restore stability. The ligaments in most joints are equipped with mechanoreceptors that, when triggered, activate the muscles associated with that joint, enhancing stability. Additionally, muscles exert compressive forces on the spine, contributing to stability.
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Frequently asked questions
Muscle tissue shares several properties across all types: contractility, excitability, extensibility, and elasticity.
Contractility is the ability of muscle cells to forcefully shorten. This allows muscle tissue to pull on its attachment points and shorten with force.
Extensibility is the ability of a muscle to be stretched or extended.
Elasticity is the ability of a muscle to return to its original length when relaxed.
There are three types of muscle tissue: skeletal muscle, smooth muscle, and cardiac muscle.
