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The human body is made up of over 600 muscles that help us perform a variety of functions, from pumping blood and supporting movement to lifting heavy weights and giving birth. Each muscle is made up of thousands of small fibres woven together like a quilt that covers our body. These fibres are made up of proteins and molecules that provide the energy and oxygen required for muscle contraction. The three main types of muscles in the body are skeletal, smooth, and cardiac.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | 600 |
| Types of muscles | Skeletal, Smooth, Cardiac |
| Muscle composition | Thousands of small fibres woven together |
| Muscle function | Contraction and relaxation |
| Muscle movement | Voluntary, Involuntary |
| Muscle fibres | Actin, Myosin |
| Muscle tissue | Blood vessels, lymphatics, contractile muscle fibres, connective tissue sheaths |
| Muscle development | Differentiation of muscle fibres, controlled by distinct transcriptional mechanisms and specific gene regulatory activity |
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What You'll Learn
Muscles are made of thousands of fibres
The human body is an intricate structure, comprising about 600 muscles that work in tandem to perform a myriad of functions. These muscles are made up of thousands of small fibres woven together like a quilt, stretching and pressing to facilitate movement. This intricate network of fibres forms the basis of our ability to move, breathe, swallow, and perform various voluntary and involuntary actions.
At the microscopic level, each muscle fibre is a bundle of proteins called myofibrils, which contain specialised proteins and molecules that provide the energy and oxygen necessary for muscle contraction. Myofibrils are composed of filaments that fold together upon receiving a signal to contract, resulting in a shortening of the muscle fibre. This contraction occurs through the action of motor proteins called myosin, which "grab" onto another protein called actin and "flex," with the process being regulated by troponin.
The formation of these muscle fibres begins during embryogenesis, with the para-axial mesoderm differentiating to generate muscle tissue. This process is influenced by specific regulatory factors, including Wnt, Shh, and BMP4 proteins, which stimulate the differentiation of somites into a dermomyotome and sclerotome. The dermomyotome then undergoes a transition to form a unique myotome, which, upon stimulation, differentiates to create skeletal muscles.
These muscle fibres are grouped into three main types: skeletal, smooth, and cardiac. Skeletal muscles, also known as voluntary muscles, are attached to bones, tendons, and ligaments, enabling conscious movement. Smooth muscles, on the other hand, line our organs and facilitate involuntary functions like digestion and urinary processes. Lastly, cardiac muscles are unique to the heart, powering its contraction and relaxation for circulation.
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Embryonic development of muscles
The development of the muscular system begins in the fourth week of gestation. At this time, the embryo is a flat, two-layered structure, with the epiblast on the dorsal side and the hypoblast on the ventral side. A line called the primitive streak appears on the epiblast, and cells migrate along it during gastrulation, resulting in a three-layered embryo. The three layers are the endoderm, ectoderm, and mesoderm.
The mesoderm layer is the embryonic layer from which muscle tissues develop. It differentiates into segmented series of tissue blocks called somites, which give rise to sclerotomes and dermomyotomes. The sclerotome forms the vertebra and ribs, while the dermomyotome will differentiate into dermatome cells that form the dermis of the back and neck, and myotome cells that form the skeletal muscles.
Myotome cells first differentiate into myoblasts (embryonic muscle cells) through elongation of their nuclei and cell bodies. Myoblasts are a type of embryonic progenitor cell that proliferates and fuses to form long, multinucleated, cylindrical muscle fibres. These muscle fibres are attached by collagenous connective tissues, and the entire muscle is enclosed in a fibrous capsule.
The formation of muscle fibres from the fusion of myoblasts is called myogenesis. In early embryonic development, myoblasts proliferate in the presence of fibroblast growth factor (FGF). When FGF runs out, they cease division and secrete fibronectin onto their extracellular matrix. The second stage involves the alignment of the myoblasts into myotubes, which then become muscle fibres.
The development of limb muscles follows a slightly different pattern of regulation to the development of trunk muscles, but the essential stages are the same. The first muscle fibres to appear are called primary fibres, and they appear around embryonic day 11-14 in mice. Secondary fibres form around the primary fibres when innervation begins to be established (around embryonic day 14-16).
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Muscle contraction and relaxation
The process of muscle contraction involves the excitation of muscle cells, which generate electrical signals by controlling the movement of charged ions across their membranes. This electrical gradient creates a membrane potential, which is the basis for neural signalling and muscle contraction. When an action potential is generated, it spreads across the muscle fibre, leading to the release of calcium ions (Ca++) from the sarcoplasmic reticulum. These calcium ions bind to troponin, exposing active sites on the actin strands, which are one of the two essential myofilaments that make up the contractile elements of the muscle fibre.
Cross-bridges then form between the myosin heads and the active sites on actin, initiating the contraction. This process is powered by the hydrolysis of ATP, with repeated cycles of cross-bridge binding, pivoting, and detachment resulting in filament sliding and the shortening of the muscle fibre. The contraction continues as long as calcium ions are available to bind to troponin and ATP is present.
Muscle relaxation occurs when the motor neuron stops releasing its chemical signal, acetylcholine (ACh), into the synapse. This leads to the repolarization of the muscle fibre, closing the calcium channels and stopping the release of calcium ions. ATP-driven pumps move calcium ions out of the sarcoplasm back into the sarcoplasmic reticulum, causing the tropomyosin to re-cover the binding sites on actin and preventing further cross-bridge formation. Without the ability to form cross-bridges, the muscle fibre loses its tension and relaxes, returning passively to its resting length.
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Muscle disorders
Some examples of muscle disorders include rippling muscle disease, STAC3 disorder, tubular aggregate myopathy, rhabdomyolysis, critical illness myopathy, and neuromyopathy. Muscular dystrophy is another disorder that causes weakness and the wasting away of muscle tissue. It is inherited and leads to the eventual loss of strength. While there is currently no cure for neuromuscular disorders, treatments such as medications, physical therapy, occupational therapy, and surgery can help manage symptoms and enhance patients' quality of life.
The differentiation of muscle fibres is controlled by distinct transcriptional mechanisms and specific gene regulatory activity. During embryogenesis, the para-axial mesoderm undergoes stepwise differentiation to generate muscle tissue. The myotome then differentiates to form the skeletal muscles in the body after receiving stimulation from the Sonic Hedgehog (Shh) signaling molecule. The development of skeletal muscles in the limb and trunk depends on the expression of specific factors and their effects on embryonic myoblasts, which eventually form primary and secondary muscle fibres.
Healthcare providers organise muscles into three types of tissue: skeletal, smooth, and cardiac (myocardium). Skeletal muscles are part of the musculoskeletal system and work with bones, tendons, and ligaments to support the body and enable movement. Smooth muscle tissue lines some organs, but most organs are made of other types of tissue as well. Cardiac muscle, found only in the heart, is a unique type of muscle tissue that beats to keep us alive.
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Types of muscles
The human body has more than 600 muscles, and they are all made of the same material: a type of elastic tissue. Each muscle is made up of thousands of small fibres woven together. There are three types of muscles in the body: skeletal, cardiac, and smooth.
Skeletal Muscles
Skeletal muscles are attached to bones and give the body structure and strength. They are voluntary muscles, which means they are under our conscious control. They come in many different sizes and shapes and allow us to perform specific movements. Skeletal muscles are also responsible for maintaining the body's posture, storing amino acids, and maintaining core body temperature via shivering. They are derived from the paraxial mesoderm. Some of the biggest and most powerful skeletal muscles are the calf and thigh muscles.
Cardiac Muscles
Cardiac muscles make up the walls of the heart, allowing blood to be pumped throughout the body. The heart is the only organ that is also a muscle. Cardiac muscles are involuntary muscles, meaning they are not under conscious control. There are two types of cardiac muscle cells: autorhythmic and contractile. Autorhythmic cardiac cells do not contract but set the pace of contraction for other cardiac muscle cells. Contractile cardiac cells, or cardiomyocytes, constitute the majority of the heart muscle and can contract. Cardiac muscles are derived from the lateral splanchnic mesoderm.
Smooth Muscles
Smooth muscles are usually found in sheets or layers, with one layer of muscle behind the other. They are involuntary muscles, meaning we cannot control them. Smooth muscles are found throughout the body, including in the blood vessels, gastrointestinal tract, bronchioles, uterus, and bladder. They are responsible for various functions, such as helping with digestion and vomiting. Smooth muscles are derived from the splanchnic mesoderm.
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Frequently asked questions
Muscles are pieces of soft tissue throughout the body that help us move, breathe, swallow and stay alive. There are over 600 muscles in the human body.
Muscles are made of thousands of small fibres woven together. Each muscle fibre is made up of blocks of proteins called myofibrils, which contain a specialised protein (myoglobin) and molecules to provide the oxygen and energy required for muscle contraction.
There are three types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are attached to bones, tendons, and ligaments to support movement. Smooth muscles line some of our organs, such as the bladder, stomach, and intestines, and play a role in involuntary functions. Cardiac muscle makes up the heart, powering its contraction and relaxation to enable circulation.
Muscles work by contracting and relaxing, causing movement. This movement can be voluntary, such as walking, or involuntary, such as breathing. The foundation for muscle contraction is the sarcomere, found in all muscle cells. Sarcomeres contain a motor protein called myosin, which powers the muscle to contract by "grabbing" onto another protein called actin. When the myosin releases the actin, the muscle relaxes.
