Dominic Stone
The human body is made up of over 600 muscles, which are responsible for movement, posture, and balance. These muscles are made of thousands of small fibres woven together, allowing us to do everything from moving our bodies to breathing and staying alive. There are three types of muscle tissue: skeletal, smooth, and cardiac. Skeletal muscles are the most common type, attached to bones by tendons, and are the only category of muscles that can be controlled consciously. Smooth muscles are found in various organ systems, including the reproductive, urinary, respiratory, and digestive systems, and work involuntarily. Cardiac muscles make up the heart and are responsible for its rhythmic contractions.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Muscle types | Skeletal, smooth, cardiac/visceral |
| Muscle composition | Thousands of small elastic fibres |
| Muscle shape | Delta or triangular, rhombus or diamond, etc. |
| Muscle functions | Pumping blood, supporting movement, lifting heavy weights, giving birth, breathing, swallowing, etc. |
| Muscle movement | Contraction, relaxation |
| Muscle fuel | Glucose, calcium, magnesium, potassium, sodium |
| Muscle injuries | Strains, sprains, cramps, tendonitis, bruising, pulled tendon, etc. |
| Muscle disorders | Myopathy, metabolic disorders, endocrine disorders, toxic disorders, etc. |
| Muscle training | Warm-up, stretching |
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Skeletal muscles
The brain, nerves and skeletal muscles work together to cause movement, known as the neuromuscular system. Skeletal muscle cells are much longer than other muscle tissue cells and are also known as muscle fibres. These fibres are made up of myofibrils, which contain actin and myosin filaments called myofilaments. These filaments fold together to contract the muscle fibres, causing the muscle to shorten. Skeletal muscles are powered by the oxidation of fats and carbohydrates, and anaerobic chemical reactions, which produce adenosine triphosphate (ATP) molecules to power movement.
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Smooth muscles
At a cellular level, smooth muscles consist of thick and thin filaments that do not form sarcomeres, resulting in a non-striated pattern. These filaments are composed of actin and myosin, the main proteins involved in muscle contraction. The actin filaments attach to dense bodies within the cell, which can be observed as dark areas under an electron microscope. Another critical structure is the calcium-containing sarcoplasmic reticulum, which aids in sustaining muscle contraction.
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Cardiac muscles
There are three types of muscle tissue in the human body: skeletal, smooth, and cardiac. Cardiac muscle, also called myocardium, is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart. The heart is made up of three layers: the pericardium, myocardium, and endocardium. The myocardium forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium).
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 contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. The mechanism behind each coordinated contraction involves the cardiac muscle and electrical impulses. Coronary arteries supply blood to the cardiac muscle, and cardiac veins drain this blood.
Cardiac muscle cells (cardiomyocytes) are the contractile myocytes of the cardiac muscle. They are surrounded by an extracellular matrix produced by supporting fibroblast cells. Cardiomyocytes are the individual cells that make up the cardiac muscle. Each cardiomyocyte needs to contract in coordination with its neighboring cells, working to efficiently pump blood from the heart. The cells are joined by intercalated discs to form long fibers. The intercalated discs contain gap junctions and desmosomes, which are intercellular structures that anchor cardiac muscle fibers together and are vital in maintaining the structural integrity of the heart.
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). These cells carry the impulses responsible for the beating of the heart.
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Muscle disorders
Neuromuscular disorders affect the nerves that control voluntary muscles and the nerves that communicate sensory information back to the brain. When nerve cells (neurons) become unhealthy or die, communication between the nervous system and muscles breaks down, resulting in muscle weakness and atrophy. Neuromuscular disorders can be inherited or caused by a spontaneous gene mutation, or immune system disorders. There is currently no cure for neuromuscular disorders, but treatments aim to manage symptoms, delay disease progression, and enhance patients' quality of life.
Muscular dystrophy is a neuromuscular disease that causes weakness and the wasting away of muscle tissue. It is inherited from parents and involves the eventual loss of strength.
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Muscle fibres
There are three types of muscle fibre: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, although in varying proportions. Skeletal muscle fibres are further classified into two types: type 1 and type 2. Type 2 is further broken down into subtypes 2A and 2B. Type 1 fibres use oxygen to generate energy for movement and have a higher density of energy-generating organelles called mitochondria, which makes them dark. Type 2A fibres also use oxygen to generate energy, but they contain less mitochondria, making them light. Type 2B fibres don't use oxygen to generate energy; instead, they store energy for short bursts of movement. They contain even less mitochondria than type 2A fibres and appear white.
The speed of contraction of a muscle fibre depends on how quickly myosin's ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibres hydrolyze ATP approximately twice as rapidly as slow fibres, resulting in much quicker cross-bridge cycling. The extraocular muscles that position the eyes have a high proportion of fast-twitch fibres, while the soleus muscle in the leg has a high proportion of slow-twitch fibres. The number of slow and fast-twitch fibres in the body varies between individuals and is determined by genetics. However, the types of fibres can be influenced by training. For example, sprint training can improve the power generated by slow-twitch fibres, while endurance training can increase the endurance level of fast-twitch fibres.
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Frequently asked questions
There are more than 600 muscles in the human body.
There are three main types of muscles: skeletal, smooth and cardiac. Skeletal muscles are attached to bones and allow movement. Smooth muscles are involuntary muscles that line the inside of some organs. Cardiac muscles make up the walls of the heart.
Skeletal muscles include biceps, triceps, hamstrings, and quadriceps femoris.
