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The human body is made up of more than 600 muscles, which help us do everything from breathing to running marathons. These muscles are made of thousands of small fibres woven together, and they come in three types: skeletal, cardiac and smooth muscle. Skeletal muscles are the most common type and they help us perform a wide range of movements and functions. They are attached to our bones and are under our voluntary control. Cardiac muscles, on the other hand, are located in the walls of the heart and are under involuntary control. Smooth muscles are found in the walls of visceral organs like the liver and intestines and are also involuntary. Together, these muscles form the muscular system, allowing us to move, maintain our posture, and even keep us alive.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Muscle composition | Thousands of small fibres woven together |
| Types of muscle tissue | Cardiac, smooth, and skeletal |
| Most common type of muscle | Skeletal muscle |
| Percentage of total body mass that skeletal muscles make up | 30% to 40% |
| Control over skeletal muscles | Voluntary |
| Control over cardiac and smooth muscles | Involuntary |
| Function of muscles | Movement, maintenance of posture and body position |
| Cell membrane of muscle fibres | Sarcolemma |
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Skeletal muscle
Each skeletal muscle consists of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue. Skeletal muscles vary in size, shape, and arrangement of fibres. They can range from tiny strands such as the stapedium muscle in the middle ear to large masses like the thigh muscles. The fibres within each muscle can be parallel to the long axis, converge to a narrow attachment, or be oblique.
The skeletal muscle fibres are made up of thousands of small fibres that contract to enable movement. These fibres are either red or white and are surrounded by a layer of connective tissue called the endomysium. The endomysium provides support and protection to the delicate muscle fibres, allowing them to withstand the forces of contraction. It also serves as a pathway for blood vessels and nerves.
The skeletal muscle receives signals from motor neurons, which release neurotransmitter chemicals at the neuromuscular junction (NMJ). These chemicals bond to a part of the cell membrane called the motor end plate, stimulating the muscle to contract. The sarcoplasmic reticulum within the muscle fibre stores calcium ions (Ca2+), which are essential for muscle contraction.
The skeletal muscle is supplied with blood by a primary artery that runs parallel to the longitudinal axis of the muscle fibre. This artery gives rise to smaller branches called feed arteries, which eventually form transverse arterioles and terminal arterioles. The terminal arterioles perfuse capillaries within the endomysium, supplying blood to the skeletal muscle.
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Cardiac muscle
The individual cardiomyocytes are joined together at their ends by intercalated discs, creating a syncytium of cardiac cells. These intercalated discs are complex adhering structures that enable the rapid transmission of electrical impulses through the network, facilitating synchronized contraction of the myocardium. They consist of three types of cell junctions: fascia adherens, desmosomes, and gap junctions. The fascia adherens and desmosomes provide structural support, while the gap junctions allow ions to move directly between cardiomyocytes, facilitating the spread of depolarization and rapid transmission of action potentials.
The sarcolemma, or cell membrane of muscle fibers, acts as a conductor for electrochemical signals that stimulate muscle cells. Connected to the sarcolemma are transverse tubules (T-tubules) that help carry these signals into the middle of the muscle fiber. The sarcoplasmic reticulum serves as a calcium ion storage facility, vital for muscle contraction. The release of calcium from the sarcoplasmic reticulum during an action potential triggers the contraction of the heart.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force through coordinated contractions. These contractions are controlled by pacemaker cells, which set the rhythm of the heart. The pacemaker cells are specialized cardiomyocytes that generate and send out electrical impulses, initiating the beating of the heart.
Diseases affecting cardiac muscle, such as ischemic heart disease and various cardiomyopathies, can have a significant impact on health and lead to morbidity and mortality worldwide.
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Smooth muscle
The smooth muscle fibres group in branching bundles, allowing for stronger contractions than those of skeletal muscle. These bundles do not run strictly parallel but consist of a complex system. The actin filaments are stretched between dense bodies in the cytoplasm and attachment plaques at the cell membrane, with myosin filaments lying between them. The smooth muscle cell is typically 3-10 µm thick and 20-200 µm long.
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Muscle fibres
The human body has over 600 muscles that help us move, breathe and survive. These muscles are made up of thousands of small muscle fibres, which stretch and press together to move our organs and body.
Skeletal muscle fibres are classified into two types: type 1 and type 2. Type 1 fibres utilise 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. Type 2A fibres can also use oxygen to generate energy, but they contain fewer mitochondria, making them lighter. Type 2B fibres, on the other hand, do not use oxygen to generate energy. Instead, they store energy for short bursts of movement and appear white.
The speed of contraction in muscle fibres depends on how quickly myosin's ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibres produce cross-bridge cycling twice as fast as slow fibres. People who excel at endurance sports tend to have a higher number of slow-twitch fibres, while sprinters tend to have more fast-twitch fibres. However, it is important to note that the number and type of muscle fibres cannot be increased through exercise. Muscle fibres grow larger through muscle cell growth and the addition of new protein filaments.
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Muscle contractions
Types of Muscle Contractions
Isometric contractions, on the other hand, happen when a muscle is held at a set length, maintaining a specific position without lengthening or shortening. Isotonic contractions, meanwhile, involve a constant muscle tension despite changes in muscle length. This occurs when the muscle's force of contraction matches the total load on the muscle.
Mechanisms of Muscle Contraction
The process of muscle contraction, known as excitation-contraction coupling, begins with an action potential causing depolarization in the myocyte membrane. This depolarization spreads through transverse (T) tubules, leading to conformational changes in receptors and the release of calcium from the sarcoplasmic reticulum. Calcium binds to troponin C, shifting tropomyosin and allowing myosin heads to attach to actin filaments, forming cross-bridges. ATP then binds to the myosin head, initiating cross-bridge cycling.
The contraction cycle continues with the dissociation of myosin from actin, breaking the cross-bridge. ATP hydrolysis results in conformational changes in the myosin head, causing it to move towards the positive end of the actin filament. Subsequently, phosphate and ADP are released, and the ADP-bound myosin binds to a new site on the actin filament. The release of ADP returns the myosin to its original position, pulling on the actin filament and causing the sarcomere and muscle fibre to contract.
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Frequently asked questions
The three types of muscle tissue are skeletal, cardiac, and smooth muscle.
Skeletal muscles are the most common type of muscle in the human body. They are attached to the bones and allow you to perform a wide range of movements and functions. They are also known as striated muscles and are under voluntary control.
Cardiac muscles are located in the walls of the heart and are under involuntary control. They help in pumping blood throughout a person's lifetime.
Smooth muscles are located in the walls of hollow visceral organs such as the liver, pancreas, and intestines. They are spindle-shaped and under involuntary control.
Some examples of highlighted muscles in the human body include the fibularis longus, flexor digitorum superficialis, extensor pollicis longus, and the biceps brachii.
