Dominic Stone
The human body is home to over 600 muscles, which are pieces of soft tissue that help us move, breathe, swallow, and stay alive. They are attached to bones or internal organs and blood vessels and are responsible for movement. The three types of muscles are skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles, meaning we control how and when they move. Smooth and cardiac muscles are involuntary, meaning they work without our conscious input. Skeletal muscles make up 30-40% of our body mass and are essential for a range of movements and functions. Smooth muscles line the inside of some organs and play an important role in many body systems, including the reproductive, urinary, respiratory, and digestive systems. Cardiac muscles are found only in the heart and are responsible for pumping blood throughout the body.
| Characteristics | Values |
|---|---|
| Number of muscles in the body | More than 600 |
| Muscle types | Skeletal, cardiac, smooth |
| Skeletal muscle composition | Bundles of muscle fibres called myofibres, which contain several myofibrils |
| Skeletal muscle mass | 30% to 40% of total body mass |
| Skeletal muscle control | Voluntary |
| Cardiac muscle control | Involuntary |
| Smooth muscle control | Involuntary |
| Smooth muscle location | Inside organs like the intestines, stomach, bladder, and blood vessels |
| Smooth muscle function | Regulating the menstrual cycle, removing waste, breathing, maintaining blood circulation |
| Cardiac muscle location | Inside the heart |
| Cardiac muscle function | Pumping blood throughout the body |
| Muscle functions | Movement, breathing, swallowing, maintaining body temperature, stabilising joints, protecting bones and organs, absorbing shock, reducing friction in joints |
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Skeletal muscles
The brain, nerves, and skeletal muscles work together to cause movement, collectively known as the neuromuscular system. Skeletal muscles are serviced by between 50 and 200 (or more) branches of specialised nerve cells called motor neurones. These neurones plug directly into the skeletal muscle at a point of contact called the neuromuscular junction. The brain then sends a message to the motor neurones, triggering the release of the chemical acetylcholine. The muscle responds to this chemical by contracting.
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Smooth muscles
Smooth muscle is one of the three major types of muscle tissue in the body, the other two being skeletal and cardiac muscle. Smooth muscle is an involuntary muscle, meaning it works without conscious input. It is found throughout the body, including the walls of hollow organs such as the stomach, intestines, bladder, and uterus. It is also found in the walls of blood vessels, the urinary tract, and the digestive tract. Smooth muscle plays a crucial role in various bodily functions, such as:
Blood Pressure and Circulation
Smooth muscle helps regulate blood pressure and circulation by contracting and relaxing to control the width of blood vessels. This ensures adequate tissue oxygenation and helps maintain the body's subsystems.
Digestion
Smooth muscle is present in the digestive tract, where it helps with digestion and nutrient absorption. It moves waste through the intestines and aids in the removal of toxins from the body.
Vision
Smooth muscle in the eyes, specifically in the ciliary body and iris, controls how the eyes focus by adjusting the shape of the lens and dilating or constricting the pupils.
Respiration
Smooth muscle is found in the tracts of the respiratory system, where it helps with lung expansion during breathing.
Reproduction
Smooth muscle contractions in the uterus during childbirth are essential for labour.
Skin
Smooth muscle in the skin causes hairs to stand on end, resulting in goosebumps or a response to cold temperature and fear.
Smooth muscle is controlled by the autonomic nervous system and exhibits greater elasticity and function within a larger length-tension curve compared to skeletal muscle. It is composed of spindle-shaped cells with a wide middle and tapering ends, containing the proteins actin and myosin, which enable contraction.
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Cardiac muscles
There are over 600 muscles in the human body, and they all have a job to do. Muscles are pieces of soft tissue that help us move, breathe, swallow, and stay alive.
One type of muscle in the body is the cardiac muscle, also called the myocardium. This is the muscle specific to the heart. The heart contracts and relaxes without our conscious awareness. Cardiac muscle is one of three major categories of muscles, along with smooth muscle and skeletal muscle. The heart is made up of three layers—the pericardium, myocardium, and endocardium. The myocardium is the thick middle layer of the heart, sandwiched between the inner endocardium and the outer epicardium (also known as the visceral pericardium).
The primary function of the cardiac muscle is to pump blood into circulation by generating sufficient force. The cardiac muscle must contract with enough force and blood to supply the metabolic demands of the entire body. The heart pumps blood through the contraction and relaxation of cardiac muscle cells or cardiomyocytes. The cells are joined by intercalated discs to form long fibers. The cardiomyocytes contain myofibrils, which are made up of repeating sections of sarcomeres. These are the fundamental contractile units of the muscle cells. The sarcomeres are composed of long proteins that organize into thick and thin filaments, called myofilaments. When the heart contracts, the myofilaments slide past each other, producing the formation of "cross-bridges", which causes contraction and the generation of force.
Cardiac muscle cells also contain branched fibers connected via intercalated discs that contain gap junctions and desmosomes. These interconnections allow the cardiomyocytes to contract together synchronously to enable the heart to work as a pump. The coordinated contraction of the heart is vital for its pumping action.
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Muscle disorders
There are three types of muscles in the body: skeletal, cardiac, and smooth muscle. Skeletal muscles are voluntary muscles that you control consciously. They make up 30% to 40% of your total body mass and are responsible for a wide range of movements and functions. Skeletal muscle mass varies from person to person and tends to be higher in males. It decreases with age.
Cardiac and smooth muscles are involuntary. They work without conscious awareness and are controlled by the autonomic nervous system. Smooth muscles line the inside of some organs and play an important role in many body systems, including the reproductive, urinary, respiratory, and digestive systems.
Some common muscle disorders include congenital myopathy, extra-abdominal desmoid tumors, familial periodic paralyses, and mitochondrial disorders. Neuromuscular disorders are another type of muscle disorder that affects the nerves controlling voluntary muscles and the communication of sensory information to the brain. These disorders can result in muscle weakness, atrophy, and fatigue that progresses over time. Symptoms of neuromuscular disorders can begin in infancy, childhood, or adulthood, depending on the type and affected areas.
Muscle strains, or pulled muscles, are the most common type of muscle injury. They occur when a muscle is overused, causing the muscle fibers to stretch beyond their limit and tear.
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Muscle contractions
There are three types of muscles in the human body: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and give the body structure and strength. They are the only type of muscle that can be controlled consciously. Cardiac muscle tissue is found in the heart and is responsible for pumping blood throughout the body. Cardiac muscle is controlled by the body's autonomic nervous system and cannot be controlled consciously. Smooth muscle is found in the blood vessels, gastrointestinal tract, bronchioles, uterus, and bladder. It is also under involuntary control and do not contain the troponin complex required for skeletal muscle contraction.
The process of muscle contraction is called excitation-contraction coupling. It begins when an action potential causes depolarization in the myocyte membrane. This causes a conformational change that releases calcium, which binds to troponin C. This shift allows the myosin heads to attach to the actin filaments, creating a cross-bridge. When ATP binds to the myosin head, cross-bridge cycling begins, and the muscle contracts.
There are several types of muscle contractions, including isotonic and isometric contractions. Isotonic contractions can be concentric or eccentric. Concentric contractions occur when a muscle shortens while generating force, such as when lifting a heavy weight. Eccentric contractions result in the elongation of a muscle while it is still generating force, such as when slowly lowering a weight. Isometric contractions occur when a muscle is under tension but neither shortens nor lengthens.
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Frequently asked questions
Muscles are pieces of soft tissue throughout your body that help you move, breathe, swallow, pump blood, and stay alive. There are over 600 muscles in the human body, making up about half of a person's body weight.
There are three types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles that you control consciously, whereas smooth and cardiac muscles are involuntary, working without your conscious input. Skeletal muscles are attached to bones and allow for a wide range of movements. Smooth muscles line the inside of some organs, helping with essential functions like digestion and breathing. Cardiac muscles are found in the heart and are responsible for pumping blood throughout the body.
Muscles work by contracting and relaxing, resulting in movement. Skeletal muscles, for example, contract in response to neural inputs, allowing for conscious control of movements. The brain, nerves, and skeletal muscles work together as part of the neuromuscular system to cause movement. Skeletal muscles also contribute to maintaining body posture, temperature regulation, and joint stability.
