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The human body is an intricate machine, housing over 600 muscles that work in tandem to facilitate movement and essential functions. These muscles are classified into three types: skeletal, smooth, and cardiac. Skeletal muscles, attached to bones, are the most common and enable voluntary movements like speaking, walking, or writing. Smooth and cardiac muscles are involuntary, working without conscious thought to support vital processes such as pumping blood and digestion. Each muscle is a complex network of fibres, sheaths, and cells, with unique shapes and functions, working together to keep the body in motion.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | 600-700 |
| Muscle composition | Thousands of elastic fibres bundled together |
| Types of muscle tissue | Visceral, cardiac, skeletal |
| Types of muscle movement | Voluntary, involuntary |
| Types of muscle fibres | Actin, myosin |
| Types of muscle shapes | Spindle, flat, triangular, circular |
| Types of muscle sheaths | Epimysium, perimysium, endomysium |
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What You'll Learn
Skeletal muscle
The connective tissue covering is called the endomysium, the perimysium, and the epimysium, with the epimysium being the outermost layer surrounding the entire muscle. The perimysium surrounds bundles of muscle fibres, and the endomysium surrounds individual muscle fibres. Skeletal muscles have an abundant supply of blood vessels and nerves, and before a skeletal muscle fibre can contract, it must receive an impulse from a nerve cell. Nerve cells called motor neurons control the skeletal muscles, and when a motor neuron receives a signal from the brain, it stimulates all the muscle cells in its motor unit simultaneously.
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Cardiac muscle
Unlike skeletal muscle, cardiac muscle is under involuntary control. Cardiac muscle fibres have their own pacemaker cells like the sinoatrial (SA) node that spontaneously depolarizes. These depolarizations occur at a consistent pace, but the pacemaker cells can also receive input from the autonomic nervous system to decrease or increase the heart rate depending on the requirements of the body. The myocardial action potential occurs in five steps, beginning with rapid depolarization during Phase 0, followed by initial partial repolarization during Phase 1, a plateau period of Phase 2, then a rapid repolarization during Phase 3, leading to stabilization at the resting potential during Phase 4.
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Smooth muscle
At a cellular level, smooth muscle contains thick and thin filaments of actin and myosin, which are the main proteins involved in muscle contraction. These filaments attach to dense bodies spread throughout the cell, which can be observed as dark areas under an electron microscope. Another important structure in smooth muscle is the calcium-containing sarcoplasmic reticulum, which aids in sustaining contraction.
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Voluntary vs involuntary movement
The human body has over 600 muscles, which can be categorised into three types: skeletal, cardiac and smooth muscle. Skeletal muscles are the only voluntary muscles in the human body, allowing us to perform a wide range of movements and functions. These muscles are attached to bones and are controlled by the somatic nervous system. When a motor neuron receives a signal from the brain, it stimulates the muscle cells, causing them to contract and relax, resulting in movement.
Voluntary movements are actions that we consciously control and perform. Examples include flicking a thumb to scroll on a phone, sprinting, speaking, and writing. These movements are mediated by several descending motor pathways and require precision and stability.
In contrast, involuntary movements occur automatically without conscious thought. They are produced by non-intentional, pathological activation within the final common pathways or modulation loops. These movements can be further divided into four major groups: tremor, myoclonus, ballism/chorea, and dystonia/athetosis. Examples of involuntary movements include the beating of the heart, breathing, and reflexes such as the startle response.
While involuntary movements are essential for the body's functioning, they can sometimes cause challenges in daily tasks if they lead to tremors or unsteadiness. For example, activities like navigating a mouse cursor or writing with a pencil require stable and controlled movements. In such cases, adjustments like anti-tremor filtering or decreasing cursor speed can help improve the user experience.
Understanding the distinction between voluntary and involuntary movements is crucial for both maintaining overall health and addressing movement-related challenges.
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Muscle fibres
Sarcomeres are defined as the region of a myofibril contained between two cytoskeletal structures called Z-discs (or Z-lines or Z-bands). The striated appearance of skeletal muscle fibres is due to the arrangement of thick and thin myofilaments within each sarcomere. The thick filaments are composed of myosin, and the thin filaments of actin. The thick filaments are anchored at the middle of the sarcomere (the M-line) by a protein called myomesin, and the thin filaments are anchored at the Z-discs by a protein called α-actinin.
Each skeletal muscle fibre is surrounded by a plasma membrane called the sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells. The nuclei of muscle fibres are called myonuclei, and a single muscle fibre can contain from hundreds to thousands of nuclei. These nuclei are quite uniformly arranged along the fibre, with each nucleus having its own myonuclear domain where it is responsible for supporting the volume of cytoplasm in that particular section of the myofibre.
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Frequently asked questions
There are about 600 muscles in the human body. The three main types of muscles are skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles that you can control consciously. They are attached to bones and help with movement. Smooth muscles are involuntary muscles that help with functions like digestion. Cardiac muscles are also involuntary and are located in the walls of the heart.
Skeletal muscles are the only voluntary muscles in the human body. They are attached to bones and are controlled by the somatic nervous system. Skeletal muscles are also the most common type of muscle in the body, comprising 30-40% of total body mass. They are made up of thousands of muscle fibers that contract to allow for movement.
Smooth and cardiac muscles are both involuntary muscles, meaning they function without conscious control. Smooth muscles are located in the walls of visceral organs like the liver, pancreas, and intestines. Cardiac muscles are located in the heart and help with functions like pumping blood.
