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The human body is made up of more than 600 muscles, which help us do everything from breathing to moving our bodies and keeping us alive. There are three types of muscle tissue: visceral, cardiac, and skeletal. Visceral muscle is found inside organs like the stomach and intestines, while skeletal muscles are attached to bones and allow us to perform a wide range of movements. Cardiac muscle, on the other hand, is only found in the heart and is responsible for pumping blood throughout the body. Skeletal muscles are the most common type of muscle in the body, comprising 30% to 40% of our total body mass. These muscles are voluntary, meaning we control how and when they work.
| Characteristics | Values |
|---|---|
| Number of muscles in the body | More than 600 |
| Muscle tissue types | 3 (skeletal, cardiac, visceral/smooth) |
| Muscle composition | Thousands of small fibres woven together |
| Muscle function | Movement, breathing, keeping internal organs functioning |
| Muscle weight | 30-40% of total body mass |
| Muscle control | Voluntary (skeletal) and involuntary (cardiac, visceral) |
| Muscle groups | Flexors, supinators, adductors, agonists, antagonists |
| Muscle layers | Superficial (extrinsic), intermediate, intrinsic (deep) |
| Muscle protection | Hypodermis (bottom layer of skin) |
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What You'll Learn
Skeletal muscles
The human body has more than 600 muscles, which help us do everything from breathing and staying alive to performing complex physical movements. There are three types of muscle tissue: visceral, cardiac, and skeletal.
The skeletal muscles are made up of thousands of small fibres woven together, which contract and stretch to move the body. These fibres can contract quickly, using short bursts of energy (fast-twitch muscles), or contract slowly, like back muscles that help with posture. Skeletal muscles can be named based on their function, their connection to bones, or a combination of both. For example, the biceps brachii muscle flexes the arm at the elbow, while the sternocleidomastoid connects the sternum and clavicle to the mastoid process of the skull.
It is important to keep skeletal muscles strong and healthy. They support our weight and allow us to move, so injuries and pain are common. Skeletal muscles can be strengthened through exercise and training, and it is recommended to visit a healthcare provider if you experience muscle pain for more than a week.
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Cardiac muscles
There are three types of muscle in the human body: smooth muscle, skeletal muscle, and cardiac muscle. Cardiac muscle, also known as myocardium or heart muscle, is an involuntary, striated muscle that constitutes the main tissue of the heart wall. It is the thick middle layer of the heart, sandwiched between the outer layer of the heart wall (the pericardium or epicardium) and the inner layer (the endocardium).
Cardiac muscle is composed of individual cardiac muscle cells, or cardiomyocytes, joined by intercalated discs, and encased by collagen fibres and other substances that form the extracellular matrix. Cardiomyocytes are tubular structures composed of chains of myofibrils, which are rod-like units within the cell. The myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organize into thick and thin filaments, called myofilaments. Thin myofilaments contain the protein actin, and thick myofilaments contain the protein myosin. The myofilaments slide past each other as the muscle contracts and relaxes, producing the formation of “cross-bridges”, which causes contraction of the heart and generation of force.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. The heart may use anaerobic processes in a limited capacity, but aerobic production is the core utilization process. The contractility of the heart changes with metabolic demand. The coordinated contraction of cardiomyocytes is facilitated by gap junctions, which allow for the propagation of coordinated action potentials from one cell to the next, and desmosomes, which anchor cardiac muscle fibres together and are vital in maintaining the structural integrity of the heart.
Cardiac muscle is under involuntary control, with hormones and signals from the brain adjusting the rate of contraction. The natural pacemaker of the heart is made of cardiac muscle tissue that stimulates other cardiac muscle cells to contract. Because of its self-stimulation, cardiac muscle is considered to be autorhythmic or intrinsically controlled.
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Smooth muscles
Smooth muscle is one of the three main types of muscle, the others being skeletal and cardiac muscle. It is a type of muscle tissue that works automatically and is found throughout the body. Smooth muscle is controlled by the autonomic nervous system and is an involuntary muscle, meaning it works without conscious thought. This is in contrast to skeletal muscle, which is a voluntary muscle that responds to a person's control. Smooth muscle is also structurally different from skeletal muscle, with a smooth, uniform appearance when viewed under a microscope, compared to the banded appearance of skeletal muscle.
Smooth muscle serves a variety of functions in the body, including blood pressure regulation, circulation, vision, and digestion. It is found in the walls of many internal organs, including the stomach, intestines, bladder, and uterus, as well as in the urinary, digestive, and reproductive tracts. In the skin, smooth muscle causes hairs to stand on end, resulting in goosebumps. In the eyes, it controls how the pupils dilate or constrict and helps with focusing. Smooth muscle is also involved in the contraction of the uterus during labour.
Smooth muscle can be divided into two subgroups: single-unit and multi-unit smooth muscle. Most smooth muscle is of the single-unit type, which contracts as a whole unit and can be found in the walls of most internal organs, blood vessels (excluding large elastic arteries), the urinary tract, and the digestive tract. Multi-unit smooth muscle, on the other hand, is found in the trachea, iris of the eye, and lining the large elastic arteries.
Smooth muscle differs from skeletal muscle in several ways, including structure, function, and regulation of contraction. Smooth muscle cells have a different shape, with narrow ends, a wider middle, and a shorter length compared to skeletal muscle cells. Smooth muscle also demonstrates greater elasticity and the ability to stretch while maintaining contractility, which is important in organ systems like the intestines and urinary bladder.
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Voluntary and involuntary movements
Voluntary movements are intentional and are caused by skeletal muscles. These muscles are attached to the bones of the skeletal system and allow for a wide range of movements and functions. For example, when reaching for a book on a shelf, the skeletal muscles in the neck, arm, and shoulder are activated. These muscles are controlled by the brain and an individual can decide how and when they work.
On the other hand, involuntary movements are non-intentional and are caused by smooth muscle. These movements occur without conscious control and are crucial for everyday life. Reflexes, one of the simplest types of involuntary movements, are automatic muscle responses to particular stimuli. For instance, when touching something hot, the hand rapidly withdraws. This reflex occurs without the involvement of the brain or conscious attention but instead depends on circuits of neurons near the spinal cord. Other examples of involuntary movements include the knee-jerk response and the flexion crossed extension reflex, which helps maintain balance.
Involuntary movements are not limited to reflexes but also include pathological non-intentional movements, which require clinical treatment. Examples of pathological non-intentional movements are myoclonus, which are sudden, brief, shock-like involuntary movements, and dystonia/athetosis, which is a sustained, long-duration muscle contraction.
Additionally, certain muscles in the body, such as visceral muscle and cardiac muscle, are also considered involuntary muscles as they cannot be directly controlled by the conscious mind. Visceral muscle is found inside organs like the stomach and intestines, and it contracts to move substances through the organ. Cardiac muscle, found only in the heart, stimulates itself to contract and pump blood throughout the body. While hormones and signals from the brain can adjust the rate of contraction, this process ultimately occurs without conscious control.
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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 for a wide range of movements and functions. They are voluntary muscles, meaning that you can control how and when they move. Skeletal muscle tissue is striated, consisting of elongated, multinucleate muscle cells called muscle fibres. Skeletal muscle tissue is also highly oxygen-consuming. The density of mammalian skeletal muscle tissue is about 1.06 kg/litre, making it approximately 15% denser than fat tissue.
Cardiac muscle tissue is found only in the walls of the heart and is responsible for pumping blood throughout the body. It is involuntary, meaning that it cannot be controlled consciously. Cardiac muscle tissue is also striated, with branching fibres, one nucleus per cell, and intercalated discs.
Smooth muscle is involuntary and non-striated. It is located in the walls of hollow visceral organs such as the liver, pancreas, and intestines. Smooth muscle is divided into two subgroups: single-unit and multiunit.
In addition to these three main types, there are also two further types of muscle tissue based on their myofibre composition: Type I, slow-twitch muscle, which is dense with capillaries and rich in mitochondria and myoglobin; and Type II, fast-twitch muscle, which has three major kinds, ordered by increasing contractile speed.
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Frequently asked questions
Muscles are pieces of soft tissue throughout your body that help you do everything from holding your body still to running a marathon. There are more than 600 muscles in the human body, and they make up about half of a person's body weight.
The skin is beneath the muscles. It is the body's largest organ and is made of water, protein, fats and minerals.
The three types of muscle tissue are skeletal, smooth (or visceral) and cardiac.
Skeletal muscles are the most common type of muscle in the body. They are attached to the 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.
Smooth and cardiac muscles are involuntary muscles, meaning they work without you having to think about it. Smooth muscle is found in the heart, while visceral muscle is found in organs like the stomach, intestines and blood vessels. Cardiac muscle is responsible for pumping blood throughout the body.
