Logan Steele
The human body has over 600 muscles that work with the skeletal system to enable movement, posture, and balance. Muscles are made of thousands of small fibres woven together, and they are categorised into three types: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles that help with movement, smooth muscles are involuntary muscles that line the inside of some organs, and cardiac muscles are also involuntary and make up the heart. The muscular system is responsible for several functions, including mobility, stability, posture, circulation, breathing, digestion, temperature regulation, and vision.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Types of muscles | Skeletal, cardiac, smooth, striated, visceral |
| Muscle control | Voluntary, involuntary, reflexive |
| Muscle composition | Thousands of small fibers, tendons, nerves, blood vessels |
| Muscle functions | Movement, posture, balance, chewing, swallowing, breathing, heartbeat, digestion, temperature regulation, vision |
| Muscle systems | Musculoskeletal, cardiovascular, reproductive, urinary, respiratory, digestive |
| Muscle heat | 85% of body heat is produced by muscle contraction |
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What You'll Learn
Skeletal muscles
The muscles are made up of myofibrils, which are composed of actin and myosin filaments (myofilaments) that are repeated in units called sarcomeres. These sarcomeres are the basic functional, contractile units of the muscle fibre necessary for muscle contraction. Muscles are predominantly powered by the oxidation of fats and carbohydrates, but anaerobic chemical reactions are also used, particularly by fast-twitch fibres. These chemical reactions produce adenosine triphosphate (ATP) molecules that are used to power the movement of the myosin heads.
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Smooth muscles
Smooth muscle is one of the three major types of muscle tissue, the other two being skeletal and cardiac muscle. Smooth muscle is an involuntary muscle, meaning it contracts and relaxes without conscious input. It gets its name from its smooth appearance, which is different from most other types of muscle. Smooth muscle is found throughout the body and is involved in many important functions, including vision, digestion, and blood pressure regulation.
Smooth muscle is made up of spindle-shaped cells with a wide middle and tapering ends. These cells contain large amounts of the proteins actin and myosin, which are responsible for muscle contraction. Actin filaments attach to dense bodies within the cell, and the cell also contains a calcium-containing structure called the sarcoplasmic reticulum, which helps to sustain contraction. Smooth muscle differs from skeletal and cardiac muscle in terms of structure, function, and regulation of contraction, demonstrating greater elasticity and functioning within a larger length-tension curve.
Smooth muscle is controlled by the autonomic nervous system and can be divided into two subgroups: single-unit and multi-unit smooth muscle. Single-unit smooth muscle, also known as visceral smooth muscle, makes up the majority of smooth muscle in the body. It is found in the walls of most internal organs, blood vessels (excluding large elastic arteries), the urinary tract, and the digestive tract. In single-unit smooth muscle, a bundle of cells contracts as a unit, and this type of muscle can contract regularly without input from a motor neuron. Multi-unit smooth muscle, on the other hand, requires stimulation from the autonomic nervous system to contract. It is found in the trachea, the iris of the eye, and lining the large elastic arteries.
Smooth muscle plays a critical role in maintaining homeostasis and regulating various body systems. For example, in the skin, smooth muscle causes hairs to stand on end, resulting in goosebumps. In the eyes, smooth muscle controls how the pupils dilate or constrict and helps the eyes focus. In the uterus, smooth muscle contractions occur during labour. Smooth muscle is also involved in the respiratory, urinary, digestive, and reproductive systems, highlighting its diverse and essential roles in the body's functioning.
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Cardiac muscles
The human body has three types of muscles: visceral, cardiac, and skeletal. Cardiac muscle, also called myocardium, is an involuntary, striated muscle that constitutes the main tissue of the heart wall. It forms a thick middle layer of the heart, between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium). The endocardium is not cardiac muscle, but simple squamous epithelial cells that form the inner lining of the heart chambers and valves. The pericardium, on the other hand, is a fibrous sac surrounding the heart, consisting of the epicardium, pericardial space, parietal pericardium, and fibrous pericardium.
Cardiac muscle is composed of individual cardiac muscle cells or cardiomyocytes, which are joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix. Cardiomyocytes are the contractile myocytes of the cardiac muscle, and their primary function is to contract, generating the pressure needed to pump blood through the circulatory system. The coronary arteries supply blood to the cardiac muscle, and cardiac veins drain this blood.
Cardiac muscle cells are roughly rectangular in shape and contain myofibrils, specialised protein contractile fibres of actin and myosin that slide past each other during contraction and relaxation. This process is activated by the release of calcium from the sarcoplasmic reticulum, which is triggered by electrical stimulation in the form of a cardiac action potential. The sliding of actin and myosin past each other produces the formation of “cross-bridges”, which causes contraction of the heart and the generation of force.
The coordinated contraction of cardiac muscle cells is vital for pumping blood throughout the cardiovascular system. Neighbouring cardiomyocytes are joined together at their ends by intercalated discs to create a syncytium of cardiac cells. Gap junctions between adjacent cardiomyocytes allow for the rapid transmission of action potentials from one cell to the next, resulting in synchronized contraction. This electrical coupling ensures that each cardiomyocyte contracts in coordination with its neighbouring cells, allowing the heart to pump efficiently.
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Voluntary and involuntary movements
The muscular system is responsible for the movement of the body, maintaining its posture, and the circulation of blood cells. It includes more than 700 muscles, making up about 40% of the body weight. These muscles are attached to bones, blood vessels, and other internal organs.
Voluntary muscles help us perform purposeful actions like writing, jumping, or smiling, all of which are initiated by our conscious decision to move. These muscles are controlled by the somatic nervous system, which allows for conscious movement and coordination. When we decide to move, our brain sends signals to these muscles, making them contract. For example, the muscles in our arms that we use to lift objects are under our voluntary control. Voluntary muscles are more prone to fatigue after intense use and need time to recover and repair.
In contrast, involuntary muscles manage functions that happen automatically, such as controlling heartbeats, breathing, and digestion. These processes are essential for survival and occur without our conscious involvement. Involuntary muscles are highly resistant to fatigue as they contract slowly and rhythmically, allowing them to maintain long-term functions without exhausting the body. For example, the heart contracts rhythmically throughout our lifetime to pump blood. These muscles are controlled by the autonomic nervous system in the body.
Reflexes are another type of involuntary movement, which are automatic muscle responses to particular stimuli. For example, the rapid withdrawal of your hand after touching something hot or the "knee jerk" response. These reflexes help maintain balance and safety, such as when you jump off a chair and your muscles contract to help you "stick the landing".
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Muscle tissue
Cardiac muscle tissue is found only in the walls of the heart as myocardium, and it is an involuntary muscle. It squeezes and relaxes to pump blood through the cardiovascular system. Cardiac muscle cells appear striped (striated) and are under involuntary control.
Smooth muscle is involuntary and non-striated. It lines some organs and is divided into two subgroups: the single-unit (unitary) and multi-unit smooth muscle. Within single-unit cells, the whole bundle or sheet contracts as a syncytium. Multi-unit smooth muscle tissues innervate individual cells, allowing for fine control and gradual responses. Smooth muscle fibres are located in the walls of hollow visceral organs such as the liver, pancreas, and intestines.
In addition to these three types, there are also Type I and Type II muscle fibres, which are distinguished by their relative density of mitochondria and myoglobin. Type I muscle fibres are dense with capillaries and rich in mitochondria and myoglobin, giving the muscle tissue a red colour. Type II muscle fibres have three major kinds, ordered by increasing contractile speed: Type IIa, Type IIx (also known as Type IId), and Type IIb. Type IIx is the fastest muscle type in humans, capable of exerting a large amount of force very quickly, but it can only sustain short, anaerobic bursts of activity.
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Frequently asked questions
The muscular system's main function is to allow movement. Muscles also contribute to stability, posture, circulation, temperature regulation, and vision.
There are three types of muscles in the human body: skeletal (striated), smooth, and cardiac. Skeletal muscles are the most common type, making up between 30% and 40% of total body mass. They are attached to bones and are under voluntary control. Smooth muscles are involuntary and found in the walls of blood vessels and organs like the urinary bladder, intestines, and stomach. Cardiac muscle is also involuntary and makes up the heart, stimulating its own contractions to form our heartbeat.
Skeletal muscles work with bones and joints to form lever systems. The muscle acts as the effort force, the joint acts as the fulcrum, the bone that the muscle moves acts as the lever, and the object being moved is the load.
