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The human body has more than 600 muscles, which help us do everything from breathing and moving to keeping us alive. Muscles are pieces of soft tissue, and there are three types: skeletal, cardiac, and smooth muscle. Skeletal muscles are voluntary muscles, which means we control how and when they move. They make up between 30% and 40% of our total body mass and are responsible for a wide range of movements and functions. Cardiac and smooth muscles are involuntary, meaning they work without our conscious control. Cardiac muscle is found in the heart and is responsible for pumping blood throughout the body. Smooth muscle is found in the walls of organs and structures such as the esophagus, stomach, and intestines, and it helps move substances through the body.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Muscle composition | Thousands of small fibers woven together |
| Types of muscle tissue | Skeletal, cardiac, and smooth |
| Skeletal muscle composition | Bundles of muscle fibers (myofibers) containing myofibrils |
| Skeletal muscle mass in humans | 30-40% of total body mass |
| Skeletal muscle fiber types | Type I (slow-twitch) and Type II (fast-twitch) |
| Muscle function | Movement, breathing, and supporting internal organs |
| Muscle movement | Voluntary and involuntary |
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Skeletal Muscle
Each skeletal muscle fibre is a muscle cell composed of several hundred to several thousand myofibrils. Myofibrils are made up of actin (thin filaments), myosin (thick filaments), and support proteins. The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance and creates functional units called sarcomeres. These sarcomeres are the basic contractile units of muscle fibres, ranging from 3 to 8 micrometres in width and 18 to 200 micrometres in breadth.
Bundles of myofibers form fascicles, and bundles of fascicles form muscle tissue. The outermost layer of skeletal muscle is the epimysium, while the endomysium is the innermost layer surrounding individual muscle fibres. The endomysium is a layer of connective tissue that surrounds each muscle fibre. The microvascular unit, including the terminal arteriole and its supplied capillaries, is the smallest unit in the skeletal muscle where blood flow can be regulated.
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Cardiac Muscle
There are three types of muscle tissue in the human body: skeletal, cardiac, and smooth muscle. Cardiac muscle, also known as myocardium or heart muscle, is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart. The heart has three layers—the pericardium, myocardium, and endocardium. The endocardium is not cardiac muscle and is comprised of simple squamous epithelial cells that form the inner lining of the heart chambers and valves.
The myocardium forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium). The myocardium is surrounded by a thin outer layer called the epicardium (or visceral pericardium) and an inner endocardium. The cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action. The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force.
Specialized modified cardiomyocytes known as pacemaker cells set the rhythm of the heart contractions. They are located in the sinoatrial node (the primary pacemaker) on the wall of the right atrium. Other pacemaker cells are found in the atrioventricular node (secondary pacemaker). They are distributed throughout the heart and are responsible for generating and sending out electrical impulses.
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Smooth Muscle
Additionally, smooth muscle has specific functions in certain parts of the body. For example, in the skin, it causes hairs to stand on end, resulting in goosebumps. In the eyes, smooth muscles control how the eyes focus and how the pupils dilate or constrict. In the uterus, smooth muscles are responsible for contractions during labour. Smooth muscle is also important in the disease process, with treatments for conditions such as asthma and ischemic heart disease targeting the signalling pathways that affect smooth muscle.
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Muscle Tissue
Skeletal muscle is the most common type of muscle in the body. Skeletal muscles are attached to bones and allow you to perform a wide range of movements and functions. They make up between 30% and 40% of your total body mass. Skeletal muscles are voluntary, meaning you control how and when they move. Skeletal muscles are broadly classified into two fibre types: type I (slow-twitch) and type II (fast-twitch).
Smooth muscle is non-striated and involuntary. It is found within the walls of organs and structures such as the oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin.
Cardiac muscle, or myocardium, is found only in the walls of the heart. It is an involuntary muscle that beats on its own without your input. Cardiac muscle has branching fibres, one nucleus per cell, striations, and intercalated discs.
Muscles are made of thousands of small fibres woven together. These fibres stretching and pressing together is what moves your organs and body.
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Muscle Composition
Each skeletal muscle fibre is a single cylindrical muscle cell, and an individual skeletal muscle may be made up of hundreds or thousands of muscle fibres bundled together and wrapped in a connective tissue covering. The connective tissue consists of cells and an extracellular matrix (ECM) that is made up of collagen fibres and proteoglycans. The ECM plays a role in muscle development, growth, and repair, as well as the transmission of contractile force. The connective tissue sheath that surrounds each muscle is called the epimysium, and the connective tissue outside the epimysium is called the fascia. The epimysium projects inward to divide the muscle into compartments, each containing a bundle of muscle fibres called a fasciculus, which is surrounded by the perimysium. Each individual muscle cell within the fasciculus is surrounded by the endomysium.
The composition of muscle fibres can vary depending on the species, breed, gender, age, physical activity, environmental temperature, and feeding practices. For example, the pig Longissimus muscle has a different composition of type I, IIA, IIX, and IIB fibres compared to the bovine Longissimus muscle.
Smooth muscle is located in various internal structures such as the digestive tract, uterus, and blood vessels, and its motion occurs without our conscious awareness. Cardiac muscle is specific to the heart and also contracts and relaxes without our conscious awareness.
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