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Mitochondria are tiny structures found within cells, commonly referred to as the cell's power plants. They produce energy through cellular respiration, which is then converted into a molecule called ATP. This energy is essential for muscle function, as it is required for both contraction and relaxation. As muscle cells have a high demand for energy, they contain a large number of mitochondria to meet their energy needs. The presence of mitochondria in muscle cells is particularly evident during strenuous exercise, where the rate of energy use in skeletal muscles can increase almost instantly. Research has also shown that mitochondria within muscle cells form a grid-like network, allowing for the quick distribution of energy.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Muscle composition | Thousands of elastic fibres bundled tightly together |
| Muscle movement | Contraction or relaxation |
| Types of movements | Voluntary, involuntary |
| Types of muscles | Skeletal, smooth, cardiac |
| Muscle disorders | Myopathy, muscular dystrophies, myasthenia gravis |
| Muscle nutrients | Glucose, calcium, magnesium, potassium, sodium |
| Muscle shapes | Triangular, circular, serrated, diamond |
| Muscle fibres | Type I, Type II A, Type II B |
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Skeletal muscle
To keep your skeletal muscles strong and healthy, it is important to do regular strength conditioning and resistance exercises, eat a nutritious and balanced diet, maintain a healthy body weight, and stretch and warm up your muscles before physical activity.
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Smooth muscle
Single-unit smooth muscle, also known as visceral smooth muscle, is found in the walls of most internal organs, blood vessels (excluding large elastic arteries), the urinary tract, and the digestive tract. It is not found in the heart, which has cardiac muscle. In single-unit smooth muscle, a single cell in a bundle is innervated by an autonomic nerve fiber (myogenic). This means that it can contract regularly without input from a motor neuron. An action potential can be propagated through neighbouring muscle cells due to the presence of many gap junctions between the cells.
Multi-unit smooth muscle, on the other hand, is neurogenic, meaning its contraction must be initiated by an autonomic nervous system neuron. It is found in the trachea, in the iris of the eye, and lining the large elastic arteries. However, it is important to note that the terms single- and multi-unit smooth muscle are oversimplifications, as smooth muscle differs in structure, function, regulation of contraction, and excitation-contraction coupling from skeletal and cardiac muscle.
While conditions that specifically affect smooth muscle are not common, they can be severe when they occur. Some conditions that can affect smooth muscle include anti-smooth muscle autoantibodies, where the immune system attacks and damages smooth muscle, and visceral myopathies, which are a group of smooth muscle diseases that interfere with their function and can be deadly when affecting certain organs. Smooth muscle is also vulnerable to more general muscle conditions, such as spasms, injuries, and paralysis.
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Cardiac muscle
The primary function of the cardiac muscle is to pump blood into circulation by generating sufficient force through contraction and relaxation. This process requires a constant supply of oxygen and nutrients to meet the energy demands of the muscle. Coronary arteries supply blood to the myocardium, and cardiac veins drain this blood. The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
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Muscle disorders
There are several types of muscle disorders, including:
- Neuromuscular disorders: These affect the nerves that control voluntary muscles and the nerves that communicate sensory information back to the brain. When the nerve cells (neurons) become unhealthy or die, communication between the nervous system and muscles breaks down, leading to muscle weakness and atrophy. Examples include Myasthenia gravis (MG), the most common chronic autoimmune neuromuscular disorder, and muscular dystrophy, which causes weakness and the wasting away of muscle tissue.
- Toxic myopathies: This occurs when a toxin or medication interferes with muscle structure or function.
- Rippling muscle disease: There is limited information available about this disease.
- STAC3 disorder: There is limited information available about this disorder.
- Tubular aggregate myopathy: There is limited information available about this type of myopathy.
While many muscle disorders don't have a cure, medication and other therapies can help manage symptoms and slow down their progression. Treatment options may include physical and occupational therapy, speech therapy, nutrition therapy, and assistive devices to help with mobility.
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Muscle shape
The shape of a muscle is determined by its architecture, or the physical arrangement of muscle fibres at the macroscopic level. There are several different types of muscle architecture, including parallel, pennate (or pinnate), and hydrostats. The direction of the muscle fibres relative to the force-generating axis determines the type of architecture. For example, the parallel muscle group includes spindle-shaped muscles where the muscle belly is wider than the origin and insertion, such as the biceps brachii and psoas major.
The pennate subgroup has three types: unipennate, bipennate, and multipennate. Multipennate muscles, such as the deltoid muscle in the human shoulder, have fibres oriented at multiple angles along the force-generating axis. Muscular hydrostats, the third subcategory of muscle architecture, function independently of a hardened skeletal system and are supported by a membrane of connective tissue.
Fusiform muscles are wider and cylindrically shaped in the centre, tapering off at the ends, resembling a spindle. An example of this type of muscle architecture is the biceps brachii in humans. Convergent, or triangular muscles, are wider at the bottom and tapered at the top, like the deltoid muscles of the shoulder. Circular muscles are ring-shaped, like the muscles that surround the mouth, pupils, and anus, also known as sphincters.
The shape of a muscle can also be influenced by the length and number of sarcomeres, or muscle fibres, within it. The length of a muscle can be measured at the peak of the length-tension relationship (L0) to ensure all sarcomeres are the same length. Longer fibres will shorten at greater absolute speeds than similar muscles with shorter fibres, and muscles with short fibres will have higher force production per unit muscle mass.
Building muscle through exercise and diet can also influence muscle shape. Strength training is the best type of exercise for building muscle, but cardiovascular activity can also be beneficial. Current guidelines recommend that adults engage in muscle-strengthening exercises at least twice a week and participate in at least 150 minutes of moderate-intensity or 75 minutes of vigorous-intensity physical activity each week. To build muscle, it is recommended to train heavy and safely, performing moves like squats, deadlifts, pull-ups, and bench presses, which stimulate multiple muscle groups simultaneously. Consuming sufficient protein is also important for muscle growth, as it increases protein synthesis.
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Frequently asked questions
Yes, muscles have mitochondria.
Mitochondria are tiny structures found in cells that produce energy in the form of ATP. They are often referred to as the "powerhouses" of the cell.
Muscles need a lot of energy to contract and relax. Mitochondria provide this energy through the process of cellular respiration, so muscles require a high concentration of mitochondria to meet their energy demands.
The clearance of unhealthy mitochondria is vital for maintaining skeletal muscle health. Dysfunctional mitochondria can be removed through a process called mitophagy, which helps to maintain a healthy population of organelles within the muscle.
Endurance exercise training has been shown to promote mitochondrial biogenesis and improve mitochondrial function. Chronic contractile activity can increase anti-apoptotic levels, reduce apoptotic protein release from mitochondria, and decrease DNA fragmentation, potentially preserving muscle mass.
