Dominic Stone
The human body is made up of more than 600 muscles that help us do everything from moving our bodies to breathing and staying alive. Muscles are pieces of soft tissue that move and support our organs. There are three major muscle types: skeletal, cardiac, and smooth muscle. Each muscle type has unique cellular components, physiology, specific functions, and pathology. Comparative physiology is important in understanding the mechanisms, diversity, and adaptation in skeletal muscle physiology and mechanics. It helps us examine variations in contraction rates and force–length and force–velocity relationships across diverse muscles.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Muscle composition | Thousands of small fibers woven together |
| Types of movements | Voluntary, involuntary |
| Muscle types | Skeletal, cardiac, smooth |
| Skeletal muscle composition | Actin and myosin fibers covered by a cell membrane (sarcolemma) |
| Skeletal muscle classifications | Type I (slow oxidative), Type II (fast-twitch) |
| Cardiac muscle composition | Individual cardiomyocytes with cytoskeletal and contractile elements |
| Smooth muscle composition | Actin and myosin fibers arranged in sheets |
| Smooth muscle function | Contractile force for shortening and propelling contents across organ systems |
| Comparative physiology deviations | Force–length and force–velocity relationships, high muscle forces during lengthening |
| Comparative physiology mechanisms | Changes to actin–myosin lattice spacing, cooperative crossbridge binding, changes to titin upon muscle activation |
| Skeletal muscle atrophy causes | Overuse, compartment syndrome, prescription/OTC/illicit drugs, disuse, denervation, systemic illness, chronic glucocorticoid use, malnourishment |
| Types of muscle atrophy | Physiologic, pathologic, neurogenic |
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What You'll Learn
Skeletal muscle physiology
Skeletal muscles are found throughout the body and are responsible for producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints. They are also associated with the diaphragmatic, esophageal, and eye muscles. Skeletal muscle contraction is primarily in response to a voluntary stimulus, with the muscle serving as an organ that consists of integrated tissues such as blood vessels, nerve fibres, and connective tissue.
Each skeletal muscle has three layers of connective tissue (mysia) that enclose it, providing structure and allowing the muscle to move independently. The outermost layer, or epimysium, is a sheath of dense, irregular connective tissue that separates the muscle from other tissues and organs. The middle layer, or perimysium, is a layer of connective tissue that surrounds the bundles of muscle fibres, or fascicles. The innermost layer, or endomysium, is a thin layer of connective tissue that surrounds each individual muscle fibre.
The skeletal muscle fibres themselves are striated, multinucleated cells ranging from 10 to 100 micrometers in diameter and several centimetres long. The nuclei are located in the cell's periphery, adjacent to the sarcolemma, which is a tubular sheath that encases and defines each muscle fibre. The sarcolemma is composed of a plasma membrane and a polysaccharide coating that fuses with tendon fibres. Invaginations within the sarcolemma, called transverse tubules (T tubules), serve as a major location for ion exchange.
Cross-bridge cycling is the mechanism by which skeletal muscle contracts. Myosin is initially bound tightly to actin in a step called rigor. When ATP binds to the myosin head, it causes a conformational change that releases the actin-myosin cross-link. After ATP is hydrolyzed to adenosine diphosphate (ADP) and inorganic phosphate, the myosin head moves toward the positive end of actin. With adequate calcium to maintain an uncovered actin-binding site, the myosin head forms a cross-bridge with actin, and the release of ADP and inorganic phosphate causes the power stroke, where the myosin head moves toward the negative end of actin.
Skeletal muscle disorders can manifest as muscle weakness, and multiple types of disorders have been identified, including muscular dystrophies, congenital myopathies, inflammatory disorders, and diseases affecting the neuromuscular junction. Skeletal muscle injuries can result from overuse, compartment syndrome, or drug use, while atrophy can be caused by disuse, denervation, systemic illness, chronic glucocorticoid use, or malnourishment. Malignant hyperthermia is a life-threatening condition primarily affecting individuals with a mutation in the ryanodine receptor of the sarcoplasmic reticulum, leading to massive intracellular calcium release and severe hyperthermia.
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Muscle contraction mechanisms
Excitation-contraction coupling (ECC) is a fundamental mechanism in muscle contraction. It involves the conversion of electrical stimuli, known as action potentials, into mechanical responses, resulting in muscle fibre contraction. During ECC, voltage-gated calcium channels open, allowing an influx of calcium ions (Ca2+). This influx triggers the release of acetylcholine (ACh) at the neuromuscular junction, which then diffuses to the postsynaptic membrane, also known as the motor endplate. ACh binds to nicotinic receptors, initiating a new action potential in the muscle fibre.
The cross-bridge cycle is another critical mechanism in muscle contraction. This cycle occurs during ECC and involves the sliding of thick and thin filaments past each other to generate contraction. Myosin and actin, two key proteins in muscle fibres, play central roles in the cross-bridge cycle. Initially, myosin is tightly bound to actin in a state called rigor. With the presence of ATP, the myosin head undergoes a conformational change, releasing the actin-myosin cross-link. ATP is then hydrolysed to adenosine diphosphate (ADP) and inorganic phosphate, re-cocking the myosin head toward the positive end of actin. The presence of calcium ions (Ca2+) facilitates the formation of a cross-bridge between myosin and actin. The subsequent release of ADP and inorganic phosphate triggers the power stroke, where the myosin head moves toward the negative end of actin, resulting in muscle contraction.
It is important to note that muscle contraction can be classified into two types: concentric contraction and eccentric contraction. Concentric contraction occurs when the force of contraction exceeds the force of resistance, leading to muscle shortening. On the other hand, eccentric contraction takes place when the force of resistance is greater than the force of contraction, resulting in muscle lengthening.
Comparative physiology provides insights into the deviations from classic ECC theory observed in certain muscles. For instance, sustained force has been noted in the forearm muscles of ranid frogs and the jaw muscles of southern alligator lizards during prolonged mate-holding behaviours. These muscles exhibit incomplete relaxation before subsequent contractions, leading to high sustained forces. Additionally, variations in contraction kinetics, force-length relationships, and force-velocity relationships have been observed across different muscles and species.
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Muscle force and velocity
The force-velocity relationship is a fundamental principle of skeletal muscle physiology, with the velocity of muscle shortening and the magnitude of isometric force being inversely related. This means that as the velocity of muscle shortening increases, the force generated decreases, and vice versa. This relationship was first described by A.V. Hill in 1938 through groundbreaking studies in isolated frog muscles, and it has been further elaborated on by Huxley in 1957, who explained the mechanisms underlying the constant velocity of isotonic shortening.
The force-velocity relationship has important implications for muscle efficiency and fatigue and plays a role in understanding the pathophysiology of various myopathies and the mechanisms of muscle contraction. The shape of the force-velocity relationship has been described as linear, hyperbolic, or double-hyperbolic, with the most common description being a rectangular hyperbola. However, recent evidence suggests that the relationship in skeletal muscles follows a double-hyperbolic pattern, deviating from the hyperbolic function at both ends of the relationship.
The force-velocity curve can be shifted to the right through strength and power training, resulting in an improved rate of force development (RFD) and increased explosiveness in athletes. Training on only one part of the force-velocity curve may lead to improved performance in that specific section but could result in a reduction in muscle contractile velocity. Different exercises and intensities have been categorised based on the force-velocity curve, with some exercises, such as counter-movement jumps, producing high movement velocity but low levels of force, while others produce high levels of force but low movement velocity.
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Muscle fibre types
Skeletal muscles are made up of thousands of small fibres woven together, allowing for movement. Skeletal muscle fibres can be classified based on two criteria: the speed of contraction and the method of regenerating ATP. Using these criteria, there are three main types of skeletal muscle fibres: slow oxidative (Type I), fast oxidative (Type IIa), and fast glycolytic (Type IIx).
Slow oxidative fibres, or slow-twitch fibres, contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They produce low-power contractions over long periods and are slow to fatigue. These fibres can be found in the legs and thighs of turkeys, for example, giving the meat its dark colour.
Fast oxidative fibres, or fast-twitch fibres, contract relatively quickly and also use aerobic respiration to generate ATP. They produce higher tension contractions than slow oxidative fibres but are not as powerful as fast glycolytic fibres.
Fast glycolytic fibres use anaerobic glycolysis as their primary energy source, which allows them to produce ATP quickly. They have a large diameter and a high volume of glycogen. Due to their reliance on anaerobic metabolism, these fibres fatigue quickly and are only used for short periods. However, they can produce rapid and forceful contractions, resulting in quick and powerful movements.
The speed of contraction for each fibre type depends on how quickly myosin's ATPase hydrolyzes ATP to produce cross-bridge action. Resistance exercises, such as strength training, can cause the formation of more actin and myosin, increasing the structure of muscle fibres. Endurance training can also lead to fibre type-specific increases in certain proteins and enzymes, making slow fibres more efficient.
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Muscle atrophy
Malnutrition and nutritional deficiencies can also contribute to muscle atrophy. Diets low in lean protein, fruits, and vegetables can impair muscle growth and maintenance. Malnutrition-related muscle atrophy may develop due to medical conditions that impair the body's ability to absorb nutrients. Cachexia, a wasting syndrome caused by underlying diseases such as cancer, HIV, or multiple sclerosis (MS), leads to extreme weight loss and muscle atrophy. Sarcopenia, an age-related muscle atrophy, can be slowed by exercise as it is caused by a reduction in proteins that promote muscle growth.
The treatment for muscle atrophy depends on the underlying cause and may include physical therapy, ultrasound therapy, and in some cases, surgery. Exercise and adequate nutrition are often recommended, and functional electrical stimulation (FES) is another effective treatment. Given the limited treatment options, minimizing immobility during injury or illness is critical to preventing muscle atrophy.
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Frequently asked questions
Muscles are pieces of soft tissue throughout the body. Humans have more than 600 muscles that help us do everything from moving our bodies to breathing and staying alive.
There are three major muscle types: skeletal, cardiac, and smooth muscle. Each type has unique cellular components, physiology, specific functions, and pathology.
Comparative physiology examines the mechanisms, diversity, and adaptation in skeletal muscle physiology and mechanics. It reviews how realistic muscle physiology has supported and refuted theories of muscle activation and the generation of force, work, and power.
Comparative physiology has demonstrated significant deviations from the classic ECC theory in muscles that produce sustained force or contract asynchronously. Additionally, the shortening side of the isotonic force-velocity relationship is well-characterized in comparative muscle physiology, showing a more than 40-fold variation in Vmax across muscles and species.
