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There are three types of muscles in the body: skeletal, cardiac, and smooth muscle. Skeletal muscles are attached to the bones and allow us to perform a wide range of movements and functions. They are also the most common type of muscle in the body, comprising 30% to 40% of our total body mass. Skeletal muscle fibres are red and white and have a striped appearance. They are classified into two types: Type 1 and Type 2, with Type 2 further divided into subtypes 2A and 2B. These subtypes differ in the way they generate energy for movement and vary in colour from dark to light to white.
| Characteristics | Values |
|---|---|
| Appearance | Striated or non-striated with a striped or banded appearance |
| Cell size | Large, 20-100 μm in diameter and several centimetres long, with the longest fibres being about 12 cm |
| Composition | Sarcolemma (cell membrane), fibrils, sarcosomes (mitochondria), sarcoplasm, and sarcoplasmic reticulum |
| Contraction | Caused by a change in electric charge (depolarization) initiated by nerve impulses or pacemaker cells |
| Types | Slow oxidative (SO), fast oxidative (FO), fast glycolytic (FG), slow-twitch (ST), fast-twitch (FT), type 1, type 2 (2A and 2B), type I, type II |
| Function | Control movement of the body and internal organs |
| Plasticity | Can adapt to changing demands by changing size or fibre type composition |
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What You'll Learn
Skeletal muscle fibres are red and white
Skeletal muscles are attached to the skeleton by tendons and control the voluntary movements of the body. They are the most common type of muscle in the human body, comprising 30% to 40% of total body mass. They are also referred to as striated muscles because they have a striped appearance due to the repeating thick and thin filaments that make up the muscle fibres.
Skeletal muscle fibres are classified into two types: type 1 and type 2. Type 1 muscle fibres utilise oxygen to generate energy for movement and have a higher density of mitochondria, giving them a dark appearance. Type 2 fibres are further divided into subtypes 2A and 2B. Type 2A fibres can also use oxygen to generate energy but contain fewer mitochondria, making them lighter in colour. Type 2B fibres, on the other hand, do not use oxygen and instead rely on stored energy for short bursts of movement. They contain even fewer mitochondria, giving them a white appearance.
The colour of skeletal muscle fibres, whether red or white, is influenced by their protein content and energy demands. Red muscle fibres have a greater abundance of slow isoforms of contractile proteins and higher oxidative enzyme content, resulting in a larger oxidative capacity. This is due to their higher mitochondrial content compared to white muscle fibres. White muscle fibres, on the other hand, are characterised by a predominance of glycolytic enzymes and fast isoforms of contractile proteins. They are used for rapid, forceful contractions and quick, powerful movements but fatigue quickly.
The different types of skeletal muscle fibres allow for a wide variety of capabilities and functions in the human body. Training and exercise can also influence the characteristics of skeletal muscle fibres, affecting their endurance, force production, and resistance to fatigue.
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Type 1 fibres have a high density of mitochondria
Muscle fibres are classified into two types: type 1 and type 2. Type 1 muscle fibres, also known as slow-twitch fibres, have a high density of mitochondria. They utilise oxygen to generate energy for movement, which is why they are considered slow-twitch fibres. Type 1 fibres are found in high abundance in elite endurance athletes, such as long-distance runners and cyclists. They are also relatively fatigue-resistant.
Type 1 fibres have a high density of energy-generating organelles called mitochondria. Mitochondria are responsible for producing ATP (adenosine triphosphate), which provides energy for muscle contraction. The more mitochondria a muscle fibre has, the more ATP it can produce, and the longer it can contract before fatiguing. This is why type 1 fibres are considered slow-twitch fibres and are useful for endurance activities.
In contrast, type 2 muscle fibres have fewer mitochondria and are further classified into subtypes: type 2A and type 2B. Type 2A fibres, also known as intermediate fibres, can also use oxygen to generate energy but contain fewer mitochondria than type 1 fibres. Type 2B fibres do not use oxygen to generate energy and instead rely on anaerobic metabolism, which produces less ATP and causes faster fatigue.
The ratio of type 1 to type 2 muscle fibres can vary among individuals and can be predictive of sports performance. Individuals with a higher proportion of type 1 fibres may excel in slower, longer-distance events, while those with more type 2 fibres may be better suited to higher velocity, shorter-duration activities.
Endurance training can increase the oxidative capacity of muscle fibres by increasing the number of mitochondria and improving aerobic metabolism. This allows the muscles to produce more ATP and contract for longer periods before fatiguing, enhancing endurance performance.
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Type 2 fibres are further divided into subtypes
Skeletal muscle fibres are classified into two types: Type 1 and Type 2. Type 2 muscle fibres are further divided into subtypes: Type 2A and Type 2B.
Type 1 muscle fibres utilise oxygen to generate energy for movement and have a higher density of energy-generating organelles called mitochondria, which makes them dark. Type 2A muscle fibres can also use oxygen to generate energy for movement but contain fewer mitochondria, making them light. Type 2B muscle fibres, on the other hand, do not use oxygen to generate energy. Instead, they rely on anaerobic glycolysis as their primary energy source, producing ATP quickly to facilitate rapid and forceful contractions. This energy source allows for quick, powerful movements but also causes Type 2B fibres to fatigue quickly, limiting their use to short periods.
Type 2A fibres are sometimes referred to as intermediate fibres because they exhibit characteristics of both fast and slow fibres. They produce ATP faster than Type 1 fibres, resulting in higher tension levels. Type 2A fibres are oxidative, possessing higher amounts of mitochondria, and they do not fatigue as quickly as Type 2B fibres. However, they lack significant amounts of myoglobin, giving them a lighter colour compared to Type 1 fibres.
Type 2B fibres, also known as fast glycolytic (FG) fibres, have a large diameter and contain high levels of glycogen. They are distinguished by their white colour due to the limited presence of mitochondria and myoglobin. FG fibres are responsible for generating rapid and forceful contractions, making them suitable for short bursts of powerful movements. However, their quick fatigue limits their sustained use.
The distinction between Type 2A and Type 2B fibres is significant in understanding muscle performance and training adaptations. Type 2A fibres exhibit characteristics of both endurance-oriented Type 1 fibres and power-oriented Type 2B fibres, showcasing their versatility in different athletic contexts. Type 2B fibres, with their rapid energy production and powerful contractions, are particularly advantageous for high-velocity, short-duration activities.
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Smooth muscles are involuntary
Smooth muscles differ from skeletal muscles, which are attached to the skeleton and are under voluntary control. Skeletal muscle fibers are striated, with a striped appearance due to repeating thick and thin filaments. In contrast, smooth muscles are non-striated, with a more uniform appearance and an oblong or fusiform shape. They are also thousands of times shorter than skeletal muscle fibers.
The primary function of smooth muscle is contraction, and it consists of two types: single-unit and multi-unit. Single-unit smooth muscle consists of multiple cells that can be stimulated synchronously from a single synaptic input, causing uniform contraction in a spiral corkscrew fashion. Smooth muscle cells are essential for vascular development and stability, wrapping around larger vessels and regulating blood flow, endothelial network growth, and vessel stability.
Smooth muscle plays a crucial role in the disease process throughout the body. For example, bronchodilators are used to relax airway smooth muscle in asthmatic patients, while medications like metoclopramide stimulate smooth muscle signaling to promote gastric emptying. Smooth muscle contraction and relaxation are important in organ systems like the urinary bladder, where the preservation of contractile tone is necessary.
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Striated and smooth muscles have different mechanisms
Muscle fibres are a type of muscle cell that help to control the physical forces within the body. There are three types of muscle tissue in the body: skeletal, cardiac, and smooth muscle. Skeletal muscles are attached to the skeleton by tendons and control the voluntary movements of the body. Cardiac muscles are only found in the heart and have their own rhythm. Smooth muscles are involuntary, meaning that you cannot control them.
Skeletal muscles (also known as striated muscles) are the most common type of muscle in the body, making up between 30% and 40% of total body mass. They are called striated muscles because they have a striped appearance due to the repeating thick and thin filaments that make up the muscle fibres. Skeletal muscle fibres are classified into two types: type 1 and type 2. Type 1 fibres utilise oxygen to generate energy for movement and have a higher density of mitochondria, giving them a dark appearance. Type 2 fibres are further divided into subtypes: Type 2A, which can also use oxygen to generate energy but contain fewer mitochondria, and Type 2B, which do not use oxygen and instead store energy for short bursts of movement.
Smooth muscles, on the other hand, do not have a striated appearance. They have a more uniform, homogenous appearance and an oblong or fusiform (round in the centre and tapering at the ends) shape. Smooth muscles are thousands of times shorter than skeletal muscle fibres. Unlike skeletal muscles, smooth muscles are involuntary and can be contracted and controlled without conscious thought. The nervous system uses hormones, neurotransmitters, and other receptors to control smooth muscle function. Smooth muscle cytoplasm contains large amounts of actin and myosin, which are the main proteins involved in muscle contraction.
While the basic process of muscle contraction is similar for both striated and smooth muscles, the exact mechanism differs. In striated muscles, the process begins with depolarization, a change in electric charge that can be initiated by a nerve impulse. This leads to a complex chain reaction within the muscle fibres, resulting in a release of energy and muscle contraction. Smooth muscles, on the other hand, have a unique mechanism involving membrane potential, which can be stimulated by factors such as local humoral factors, circulating hormones, or mechanical stimulation. Smooth muscles are also able to maintain tone for extended periods, which is important for regulating organ systems such as the urinary bladder.
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Frequently asked questions
Muscle fibres are red and white and have a striped appearance. They are also known as striated muscles.
There are three types of muscle fibres in the body: skeletal, cardiac and smooth muscle. Skeletal muscle fibres are further classified into type 1 and type 2, with type 2 having subtypes 2A and 2B.
Type 1 muscle fibres have a higher density of mitochondria, which makes them dark. They also utilise oxygen to generate energy for movement. Type 2 muscle fibres, on the other hand, have less mitochondria, with type 2B having the least, making them appear light or white. Type 2B fibres do not use oxygen to generate energy and instead rely on anaerobic glycolysis to produce rapid and powerful movements.
Muscle fibres work with muscles to cause movement in the body. Skeletal muscles, which are the most common type, are attached to the skeleton by tendons and control voluntary movements such as walking, bending and picking up objects. Smooth muscles are involuntary and are found in internal organs and eyes, helping with functions like digestion and changing pupil size. Cardiac muscles are also involuntary and facilitate heartbeats.
