Logan Steele
The human body is made up of more than 600 muscles, which are responsible for movement and keeping the body healthy. There are three types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are attached to the bones and are under voluntary control, meaning you can control how and when they move. Smooth muscles, on the other hand, are usually found in layers and are involuntary, meaning they contract and relax without conscious thought. Cardiac muscles make up the heart and are also involuntary, contracting and relaxing automatically to pump blood throughout the body. While all muscles are made of the same material, a type of elastic tissue, they can be distinguished by their appearance, location, and function.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Muscle tissue types | Visceral, cardiac, and skeletal |
| Skeletal muscle voluntary/involuntary | Voluntary |
| Cardiac muscle voluntary/involuntary | Involuntary |
| Smooth muscle voluntary/involuntary | Involuntary |
| Skeletal muscle appearance | Striated, or striped |
| Cardiac muscle appearance | Striated, or striped |
| Smooth muscle appearance | Smooth, or uniform |
| Skeletal muscle location | Between bones |
| Cardiac muscle location | Heart |
| Smooth muscle location | Walls of hollow visceral organs (except the heart) |
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What You'll Learn
Skeletal muscles
These muscles consist of flexible muscle fibres that range from less than half an inch to just over 3 inches in diameter. Each muscle can contain thousands of fibres. The fibres contract (tighten), allowing the muscles to move bones so you can perform various movements. Skeletal muscle fibres are red and white and have a striped appearance, so they are often called striated muscles.
The outermost layer of tissue surrounding the entire muscle is called the epimysium. The middle layer surrounding bundles of muscle fibres is the perimysium. The innermost layer surrounding individual muscle fibres is the endomysium. Together, these layers are called mysia.
Deep fascia, a type of connective tissue, is present in all muscles. It encloses each muscle fibre as endomysium, each muscle fascicle as perimysium, and each individual muscle as epimysium. Deep fascia also separates the groups of muscles into muscle compartments.
The functions of skeletal muscles include producing movement, maintaining body posture, controlling body temperature, and stabilizing joints. They also act as a storage source for amino acids, which different organs of the body can use for synthesizing organ-specific proteins.
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Cardiac muscles
The human body has more than 600 muscles, and these can be categorised into three types: skeletal, smooth, and cardiac. Cardiac muscles, also known as myocardium, are found only in the heart. They are involuntary muscles, meaning they contract without conscious control.
Cardiac muscle cells, or cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are striated, branched, and contain many mitochondria. The striations are caused by lighter I bands composed mainly of actin, and darker A bands composed mainly of myosin. These bands give cardiac muscles a striped or striated appearance when viewed under a microscope. The cells are surrounded by an extracellular matrix produced by supporting fibroblast cells.
Cardiac muscle cells are the contracting cells that allow the heart to pump. Each cardiomyocyte needs to contract in coordination with its neighbouring cells to efficiently pump blood from the heart. The cells are joined at their ends by intercalated discs to form long fibres. The intercalated discs contain gap junctions and desmosomes, which are vital in maintaining the structural integrity of the heart.
The heart is made up of three layers—the pericardium, myocardium, and endocardium. The endocardium is not cardiac muscle and forms the inner lining of the heart chambers and valves. The pericardium is a fibrous sac surrounding the heart. The cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action.
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Smooth muscles
The term "smooth muscle" is often used interchangeably with visceral muscle, referring to the weakest type of muscle tissue found inside organs like the stomach, intestines, and blood vessels. These visceral muscles are controlled by the unconscious part of the brain, making them involuntary. They play a crucial role in organ function by contracting to move substances through the organ, such as during digestion.
The structure of smooth muscles includes thick and thin filaments of actin and myosin, the primary muscle contraction proteins. These filaments are not organized into sarcomeres, resulting in a non-striated appearance. The calcium-containing sarcoplasmic reticulum is another important structure that enables contraction in smooth muscles. Smooth muscle cells are typically 3-10 µm thick and 20-200 µm long, with a centrally located cigar-shaped nucleus during contraction.
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Voluntary vs involuntary muscles
The human body has over 600 muscles, which can be grouped into three types: skeletal, cardiac, and smooth muscle. Skeletal muscles are voluntary, while cardiac and smooth muscles are involuntary.
Voluntary muscles are controlled by the somatic nervous system and are involved in the movement of body parts and locomotion of the body. They exhibit rapid contraction and relaxation compared to involuntary muscles. However, they also tire quickly and require regular intervals of rest. Examples of voluntary muscles include skeletal muscles, such as the biceps, triceps, quadriceps, diaphragm, pectoral muscles, abdominals, and hamstrings. These muscles are composed of long, multinucleated cells with sarcomeres arranged into bundles and cylindrical fibers, which are usually attached to bones and the skin.
Involuntary muscles, on the other hand, are controlled by the autonomic nervous system and are associated with organs that exhibit slow and regular contractions and relaxation. These muscles cannot be controlled by will or conscious thought. They are often found lining the walls of internal organs like the stomach, intestine, urinary bladder, and blood capillaries. Involuntary muscles include cardiac and smooth muscles. Cardiac muscles are found only in the heart and are responsible for pumping blood throughout the body. They are composed of individual heart muscle cells called cardiomyocytes, which are joined by intercalated discs and encased by collagen fibers. Smooth muscles, also known as visceral muscles, are found in the internal walls of organs like the intestines, stomach, uterus, and blood vessels. They aid in the passage of fluids and food in the digestive system and help maintain proper blood circulation by controlling the internal diameter of the blood vessels.
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Muscle injuries
There are three main types of muscle tissue: visceral, cardiac, and skeletal. Visceral muscle is found inside organs like the stomach and intestines and is controlled by the unconscious part of the brain, making it an involuntary muscle. Cardiac muscle is found in the heart and is responsible for pumping blood throughout the body. Cardiac muscle is also involuntary, as it cannot be consciously controlled. Skeletal muscles are the most common muscles in the body and are attached to the bones, allowing for a wide range of movements. They are voluntary muscles, meaning you control how and when they move.
Muscle strains, or pulled muscles, are the most common type of muscle injury. They occur when the muscle fibres tear due to overuse or excessive force. Strains can happen suddenly or gradually and are often associated with eccentric muscle action, such as sprinting, twisting, or jumping. They can also be caused by repetitive movements, overtraining, or undertraining. Acute injuries are typically the result of a single traumatic event and cause noticeable symptoms. On the other hand, overuse or chronic injuries occur over time due to repetitive micro-traumas and may be more challenging to diagnose.
Contusions are another type of muscle injury that occurs due to compressive forces, usually in contact sports like football or rugby. Bruises, a type of haematoma, can also occur in muscles due to blunt trauma. While small bruises are usually not dangerous, they can sometimes lead to more severe internal bleeding or fractures. Muscle cramps are sudden, involuntary contractions that can be painful.
It is important to seek medical advice for muscle injuries, especially if the pain persists for more than a week or causes trouble with movement. Treatment options for muscle injuries may include the RICE method (rest, ice, compression, elevation), pain relievers, and physical therapy. Severe tears may require surgery to stitch the muscle back together.
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Frequently asked questions
There are three types of muscles in the human body: skeletal, smooth, and cardiac.
Skeletal muscles are voluntary muscles, meaning you can control how and when they move. They are attached to the bones and help with a wide range of movements and functions.
Smooth muscles are involuntary muscles, meaning they work without conscious thought. They are found in the stomach, digestive system, bladder, uterus, and eyes.
Cardiac muscles are involuntary muscles that make up the heart. They contract and relax to pump blood throughout the body.
A pulled muscle or a muscle strain happens when a muscle or tendon is stretched beyond its limit and tears. It can cause pain, swelling, and bruising. If you experience these symptoms, it is best to visit a healthcare provider.
