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The human body has over 600 muscles that help us perform a wide range of functions, from involuntary actions like breathing, pumping blood, and digestion to voluntary actions like walking, running, and speaking. The brain, nerves, and skeletal muscles work together to cause movement, collectively known as the neuromuscular system. Skeletal muscles are the only muscles that can be consciously controlled and account for approximately 40% of a person's weight. Cardiac muscles, on the other hand, are involuntary and encompass the heart, keeping us alive. Smooth muscles are also involuntary and are present throughout the body, aiding in digestion, reproduction, and more.
| Characteristics | Values |
|---|---|
| Number of muscles in the body | More than 600 |
| Types of muscles | Skeletal, smooth, cardiac |
| Muscle composition | Thousands of elastic fibres bundled tightly together |
| Muscle movement | Contraction, relaxation |
| Types of movement | Voluntary, involuntary |
| Neuromuscular system | Brain, nerves, skeletal muscles |
| Muscle function | Create force and movement, maintain posture, control body temperature, store nutrients, protect bones and organs, pump blood, support movement, lift heavy weights, give birth, circulate blood, control heartbeat, breathing, digestion, vision, speech, chewing, running, jumping |
| Muscle disorders | Myopathy, weakness, pain, loss of movement, paralysis, injury or overuse, sprains, strains, cramps, tendonitis, bruising, metabolic, endocrine or toxic disorders, cancers |
| Muscle fibre types | Type I (slow oxidative), slow-twitching, low glycogen content, low rate of fatigue, slow contractile speed, low myosin ATPase activity, Type II (fast-twitch) |
| Muscle composition | Actin (thin filaments), myosin (thick filaments), support proteins (titin, desmin, myomesin, C protein, nebulin, plectin) |
| Muscle contraction | Signalling from the nervous system, release of acetylcholine, binding of myosin and actin, release of calcium, activation of ryanodine receptors, binding of calcium and troponin, removal of tropomyosin, detachment of myosin and actin using adenosine triphosphate (ATP) |
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What You'll Learn
Muscles and movement
The human body has more than 600 muscles that help us perform a wide range of functions, from involuntary actions like breathing, circulation and digestion to voluntary actions like walking, running, jumping, and speaking. The muscular system's main function is to enable movement, but muscles also help with posture, stability, and temperature regulation.
Muscles are pieces of soft tissue made up of thousands of elastic muscle fibres, or myofibrils, bundled tightly together. Each bundle is wrapped in a thin membrane called a perimysium. Skeletal muscle fibres are multinucleated cells ranging from 10 to 100 micrometers in diameter and many centimetres long. The nuclei are located in the cell's periphery, adjacent to the sarcolemma, a tubular sheath that encases and defines each muscle fibre.
The brain, nerves, and skeletal muscles work together to cause movement, collectively known as the neuromuscular system. The brain sends a message to the motor neurons, triggering the release of acetylcholine from the presynaptic terminals. The muscle responds by contracting. This contraction is made possible by the myosin in the muscle fibres, which "grabs on" to actin, another protein, and "flexes". When the myosin releases the actin, the muscle relaxes.
There are two types of movement: voluntary and involuntary. Voluntary movements are actions we choose to perform, like sprinting or scrolling on a phone, and are controlled by the nervous system. Involuntary movements happen automatically without conscious thought, like the beating of the heart.
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Involuntary vs voluntary movements
The human body is home to over 600 muscles, which help us do everything from moving our bodies to breathing and pumping blood. These muscles work by contracting or relaxing, and they can be controlled either voluntarily or involuntarily.
Voluntary movements are self-generated, willed actions performed as a result of cognitive processes. They are controlled by the nervous system and include actions such as scrolling through an article on a phone or sprinting around a track. Voluntary movements are mediated by several descending motor pathways, which are modified and regulated by two main loops: the basal ganglia loop and the cerebellar loop. The capacity for voluntary control is seen as essential to human movement, giving us a sense of agency and control.
Involuntary movements, on the other hand, happen automatically without conscious thought. They are produced by a non-intentional, pathological activation anywhere within the final common pathways or the two main loops. Involuntary movements can be divided into four major groups: tremor, myoclonus, ballism/chorea, and dystonia/athetosis. Tremors are characterized by sudden, brief, shock-like movements, while myoclonus involves irregular, phasic movements that are often mimicked by normal subjects. Ballism/chorea is marked by sudden, unexpected movements, and dystonia/athetosis involves sustained, long-duration muscle contractions.
While the mechanisms underlying voluntary movement are not yet fully understood, recent neuroscientific tools have identified networks of different brain areas, including the prefrontal cortex, supplementary motor area, and parietal cortex, that are involved in voluntary action. Dysfunction in these areas can result in semi-voluntary movements, which lie in the borderland between voluntary and involuntary movement.
In summary, voluntary movements are intentional and consciously controlled, while involuntary movements occur automatically without conscious thought. Both types of movements are essential for human functioning and are mediated by complex interactions between the brain, nerves, and muscles.
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Types of muscle
There are over 600 muscles in the human body, which can be categorised in various ways. One way is to group them by location, such as chest, leg, or back muscles. They can also be classified by the type of movement they perform, for example, abductors, flexors, or extensors.
Another way to classify muscles is by their shape. For example, deltoid muscles are triangular in shape, serratus muscles have a serrated or saw-like shape, and the rhomboid major muscle is shaped like a rhombus or diamond. The size of muscles can also be used to distinguish between them, as in the case of the gluteal muscles, which are differentiated by size into gluteus maximus (large), gluteus medius (medium), and gluteus minimus (smallest).
Muscles can also be classified by their function. For example, muscles that produce movement are known as agonists or prime movers, and they always work in conjunction with antagonist muscles that produce the opposite effect. For instance, the biceps brachii muscle flexes the arm at the elbow, while its antagonist, the triceps brachii muscle, extends the arm at the elbow. Synergist muscles support the movements of agonists by stabilising and reducing extraneous movements. Fixator muscles assist in movement by holding the origin stable.
Beyond these classifications, there are three types of muscle tissue in vertebrates: skeletal, cardiac, and smooth muscle. Skeletal muscle is the most common type of muscle in the body and is under voluntary control. It is attached to the skeleton and allows for body movement. Cardiac muscle is located in the walls of the heart and is under involuntary control. It is self-contracting and autonomically regulated, contracting in a rhythmic fashion for the entirety of an organism's life. Smooth muscle is located in the walls of hollow visceral organs such as the liver, pancreas, and intestines, as well as in hollow organs like the urinary bladder and uterus. It is also under involuntary control and functions to modify the diameter or size of these structures to expel or contain their contents.
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Muscle disorders
Additionally, cancers such as soft tissue sarcoma can affect muscles. Neuromuscular disorders are a specific type of muscle disorder that affects the nerves controlling voluntary muscles and those communicating sensory information to the brain. Unhealthy or dying nerve cells disrupt communication between the nervous system and muscles, leading to muscle atrophy and weakness. Some neuromuscular disorders may be inherited or caused by spontaneous gene mutations or immune system disorders. Muscular dystrophy, for instance, causes muscle weakness and the wasting away of muscle tissue, and is inherited from parents.
Skeletal muscle disorders, such as congenital myopathies and inflammatory disorders, can also manifest as muscle weakness. Skeletal muscle fibres are composed of actin, myosin, and support proteins, which give them their striated appearance. These fibres are bundled tightly together, forming the basic structure of skeletal, smooth, and cardiac muscles. Skeletal muscles, in particular, are serviced by motor neurons, allowing for voluntary movement.
If you experience chronic muscle pain, soreness, or weakness, it is important to seek medical advice. While occasional muscle aches are normal, long-term symptoms could indicate a more serious underlying issue.
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Muscle maintenance
The three main types of muscle are skeletal, smooth, and cardiac. Skeletal muscle is found throughout the body and is responsible for producing movement, sustaining body posture, maintaining body temperature, storing nutrients, and stabilizing joints. Skeletal muscle fibres are striated, multinucleated cells ranging from 10 to 100 micrometers in diameter and many centimetres long. They are composed of myofibrils, which contain actin and myosin filaments, giving skeletal muscle its striated appearance. These filaments fold together to contract the muscle, causing movement.
To maintain muscle health, strength training is recommended. This can include weight training or endurance exercises. The benefits of strength training increase as you get older, as it can slow down or prevent age-related muscle loss. It is important to start slowly and build up intensity over time, focusing on form and smooth, steady movements. Additionally, diet plays a role in muscle maintenance, with protein being essential for building muscle mass. Consuming a meal with an appropriate carbohydrate-to-protein ratio after a workout can maximize muscle growth and improve recovery.
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