Logan Steele
Muscle cells are specialised to contract, creating a pulling force to stabilise or move parts of the body. There are three types of muscle cell: skeletal, visceral, and cardiac. All muscle cells contain organelles, which are membrane-bound structures that perform specific functions. One such organelle is the mitochondrion, which generates most of the energy needed to power the cell's biochemical reactions. Mitochondria play a central role in muscle cell metabolism, energy supply, and calcium homeostasis. Another organelle found in muscle cells is the sarcoplasmic reticulum, which regulates calcium homeostasis in the context of muscle contraction.
| Characteristics | Values |
|---|---|
| Organelle | Mitochondria |
| Description | Membrane-bound cell organelles that generate most of the chemical energy needed to power the cell's biochemical reactions |
| Muscle Type | Skeletal muscle |
| Muscle Cell Metabolism | Mitochondria play a central role in muscle cell metabolism |
| Energy Supply | Mitochondria play a central role in energy supply |
| Regulation of Energy-Sensitive Signaling Pathways | Mitochondria play a central role in the regulation of energy-sensitive signaling pathways |
| Reactive Oxygen Species Production/Signaling | Mitochondria play a central role in reactive oxygen species production/signaling |
| Calcium Homeostasis | Mitochondria play a central role in calcium homeostasis |
| Regulation of Apoptosis | Mitochondria play a central role in the regulation of apoptosis |
| Muscle Contraction | Mitochondria play a role in muscle contraction |
| Other Organelles | Other organelles include the sarcoplasmic reticulum, sarcomeres, myofibrils, and sarcosomes |
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What You'll Learn
Mitochondria and muscle health
Mitochondria are membrane-bound organelles that generate most of the energy needed to power a cell's biochemical reactions. Colloquially, they are referred to as "the powerhouses of the cell". They are critical for muscle cell metabolism, energy supply, and regulating energy-sensitive signalling pathways. They also play a role in calcium homeostasis and the regulation of apoptosis.
Mitochondria are essential for skeletal muscle health. Skeletal muscle comprises around 40% of total body mass in young, healthy individuals and is important for locomotion and whole-body metabolism. Skeletal muscle health is dependent on the optimal function of its mitochondria. Mitochondria are largely localized in distinct regions of the muscle, below the sarcolemma, termed subsarcolemmal (SS) mitochondria, and between the myofibrils, called intermyofibrillar (IMF) mitochondria. These two types of mitochondria have different biochemical and morphological characteristics and respond differently to exercise and disease.
The remarkable plasticity of mitochondria allows them to adjust their volume, structure, and capacity under conditions such as exercise, which can improve metabolic health in individuals with various diseases and/or advancing age. Exercise induces robust changes in mitochondrial content and quality, increasing metabolic health and muscle force. It is the most potent behavioural therapeutic approach for improving mitochondrial health, not only in muscle but also potentially in other tissues.
With advancing age, decrements in numerous mitochondrial variables are evident in muscle. This decline is due in part to reduced physical activity and in part to age-related alterations in mitochondrial synthesis and degradation. The age-related loss of muscle mass was termed sarcopenia in 1988. Mitochondrial dysfunction has been implicated in a large number of adverse events and conditions affecting skeletal muscle health, including sarcopenia, sepsis-induced muscle wasting, cachexia, and pathology-specific muscle atrophy and dysfunction occurring in chronic obstructive pulmonary disease or amyotrophic lateral sclerosis.
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Mitochondria and muscle aging
Mitochondria are membrane-bound organelles that generate most of the energy needed to power a cell's biochemical reactions. They are essential for muscle cell metabolism, energy supply, and regulating energy-sensitive signalling pathways. Mitochondria are also involved in reactive oxygen species production and calcium homeostasis.
Mitochondrial dysfunction has been linked to various adverse events and conditions affecting skeletal muscle health. Age-related muscle deterioration, muscle atrophy, sepsis-induced muscle wasting, and pathology-specific muscle atrophy are all associated with mitochondrial dysfunction. As mitochondria play a central role in energy production and muscle function, their decline with age contributes to reduced muscle health and physical performance. Studies have shown that the abundance of mtDNA and mRNA, which are essential for mitochondrial function, declines with age, leading to reduced mitochondrial ATP production and muscle dysfunction.
Research has also found that physical activity levels play a crucial role in the decline of mitochondrial function with age. Older adults tend to become less active over time, which further accelerates the decline in mitochondrial capacity and muscle health. However, regular exercise training can help mitigate the effects of aging by stimulating mitochondrial biogenesis and improving mitochondrial function.
Further studies are needed to fully understand the complex relationship between mitochondrial function, muscle physiology, and the aging process. By examining the mechanisms regulating mitochondrial dynamics, morphology, and quality control processes, we can enhance our knowledge of the role mitochondria play in skeletal muscle health and pathophysiology.
In conclusion, mitochondria are vital organelles for muscle health, and their dysfunction contributes to age-related muscle deterioration. While aging is a primary factor in mitochondrial decline, maintaining adequate physical activity levels through regular exercise can help counteract the negative effects of aging on muscle function.
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Mitochondria and muscle diseases
Mitochondria are membrane-bound organelles that generate most of the chemical energy needed to power a cell's biochemical reactions. They are critical for muscle cell metabolism, energy supply, and calcium homeostasis. Mitochondrial dysfunction can lead to various muscle diseases, including ageing-related deterioration of muscle function, disuse-induced skeletal muscle atrophy, sepsis-induced muscle wasting, cachexia, and pathology-specific muscle atrophy.
Mitochondrial diseases are a group of conditions that affect the mitochondria's function in cells. Mitochondrial myopathy, a type of mitochondrial disease, specifically causes muscular problems, while mitochondrial encephalomyopathy causes both muscular and neurological issues. These problems include muscle weakness, exercise intolerance, hearing loss, trouble with balance and coordination, seizures, and learning deficits. The diagnosis of mitochondrial diseases involves a comprehensive evaluation, including medical history, physical and neurological examinations, metabolic tests, DNA testing, and advanced imaging or biochemical analyses.
The prognosis and treatment options for mitochondrial-related muscle diseases vary. While there is no cure for mitochondrial diseases, treatments can help prevent life-threatening complications. For example, exercise can improve mitochondrial function and slow down deterioration in skeletal muscle disorders. Vitamin therapy, such as riboflavin and coenzyme Q, may also improve fatigue and energy levels in some patients.
Understanding the mechanisms regulating mitochondrial biology, including dynamics, morphology, and quality control processes, is crucial for advancing our knowledge of mitochondrial function in skeletal muscle physiology and pathophysiology. By studying these aspects, we can enhance our comprehension of the role mitochondria play in both healthy and diseased muscle cells.
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Sarcoplasmic reticulum and muscle contraction
The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells. It is a network of tubules that extend throughout muscle cells, wrapping around the contractile units of the cell (myofibrils). SR is a form of smooth endoplasmic reticulum, and its main function is to store and release calcium ions (Ca2+), which is necessary for muscle contraction and relaxation.
During muscle contraction, the sarcomere shortens, and the Z disks move closer together. This shortening is caused by the release of calcium ions from the SR, which then bind to troponin C on the actin (thin) filaments. This changes the conformation of tropomyosin, allowing the myosin (thick) filament cross-bridges to attach and detach, pulling along actin and shortening the sarcomere. The released calcium ions also increase the rate of contraction.
Calcium ion release from the SR occurs through ryanodine receptors (RyR) in the junctional SR/terminal cisternae, which are enlarged regions of the SR that store calcium. There are three types of ryanodine receptors: RyR1 (in skeletal muscle), RyR2 (in cardiac muscle), and RyR3 (in the brain). In cardiac and smooth muscle, an electrical impulse triggers calcium ions to enter the cell through an L-type calcium channel, which then binds to and activates the RyR. In skeletal muscle, the L-type calcium channel is bound to the RyR, so activation of the L-type calcium channel directly activates the RyR, causing calcium release.
The SR is also involved in calcium ion absorption. The longitudinal SR is thinner and runs between the terminal cisternae, containing ion channels necessary for calcium ion absorption. SERCA pumps remove calcium ions from the cytosol and pump them back into the SR, allowing the muscle to relax. Phospholamban (PLB), a protein found in cardiac muscle, can prevent SERCA from working by decreasing its attraction to calcium, leading to decreased muscle contraction.
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Myofibrils and muscle contraction
Muscle cells are composed of long, tubular muscle fibres, which contain chains of myofibrils. Myofibrils are cylindrical microfilamentous organelles, with a diameter of approximately 1 micrometre, and they typically extend throughout the entire muscle fibre. They are composed of long proteins, including actin, myosin, and titin, as well as other proteins that hold them together. These proteins are organised into thick, thin, and elastic myofilaments, which repeat along the length of the myofibril in sections or units of contraction called sarcomeres.
Sarcomeres are the smallest fundamental contractile unit of the myofibril. Each sarcomere is bordered by structural support proteins, predominantly consisting of α-actinin, that collectively comprise the Z-line. The Z-line is a prominent dense line that can be observed in the centre of the I band, a light band of low density. The area between two Z-lines, a sarcomere, can be considered the primary structural and functional unit directly responsible for muscle contraction.
During muscle contraction, the sarcomere shortens, and the Z-disks are pulled closer together. The H-zone becomes smaller and smaller due to the increasing overlap of actin and myosin filaments, and the muscle shortens. The thick and thin filaments are responsible for the movement and force developed during contraction, and this process is known as the cross-bridge cycle. The thick and thin filaments are organised in a parallel fashion along the entire length of the myofibril, and this repeating pattern gives skeletal muscle its microscopically striated appearance.
The release of calcium into the sarcoplasm is essential for muscle fibre contraction. Calcium is stored in the terminal cisternae of the sarcoplasmic reticulum, which is bound to the acidic protein calsequestrin. The sarcoplasmic reticulum cisternae are in contact with invaginations of the sarcolemma, forming structures known as triads. These triads are located between the A and I bands of muscle fibres, with two terminal cisternae flanking each T-tubule at the I-A junctions.
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Frequently asked questions
Muscles need mitochondria, a membrane-bound organelle that generates most of the chemical energy needed to power a cell's biochemical reactions.
Mitochondria regulate many critical cellular processes for skeletal muscle physiology. They play a central role in muscle cell metabolism, energy supply, calcium homeostasis, and the regulation of apoptosis.
Other organelles found in muscle cells include the sarcoplasmic reticulum, myofibrils, and sarcosomes. The sarcoplasmic reticulum is a highly ordered structure that regulates calcium homeostasis in the context of muscle contraction. Myofibrils are organelles that contain sarcomeres, which are the smallest functional unit of a skeletal muscle fiber. Sarcosomes provide the required energy for contraction in smooth and cardiac muscle cells.
