Dominic Stone
Skeletal muscles, the most common type of muscle in the human body, are made up of thousands of small fibres woven together. These fibres are organised into bundles called fascicles, surrounded by a layer of connective tissue called the perimysium. Each individual muscle fibre is encased in a thin layer of connective tissue called the endomysium, which contains capillaries and nerve tissue. The endomysium is surrounded by the perimysium, which may encase anywhere from 10 to 100 fascicles. The perimysium is then surrounded by a fibrous tissue envelope called the epimysium.
| Characteristics | Values |
|---|---|
| Connective tissue layer | Fascia |
| Connective tissue sheath | Epimysium |
| Connective tissue layer surrounding each muscle fiber | Endomysium |
| Middle layer surrounding bundles of muscle fibers | Perimysium |
| Connective tissue outside the epimysium | Fascia |
| Muscle fibers | Contain actin (thin filaments), myosin (thick filaments), and support proteins |
| Muscle fiber type | Type I (slow) and Type II (fast) |
| Muscle fiber subtypes | Type IIA (oxidative) and Type IIX (glycolytic) |
| Muscle fiber structure | Cylindrical muscle cells |
| Muscle fiber nuclei | Myonuclei |
| Muscle fiber innervation | Motor neurons |
| Muscle fiber blood supply | Artery, vein, and capillaries |
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What You'll Learn
Connective tissue
Each individual muscle is surrounded by a connective tissue sheath called the epimysium. The epimysium is a fibrous tissue envelope that encases a muscle fascicle. Fascicles are bundles of muscle fibres. Portions of the epimysium project inward to divide the muscle into compartments. Each compartment contains a bundle of muscle fibres.
Each bundle of muscle fibres is called a fasciculus and is surrounded by a layer of connective tissue called the perimysium. The perimysium is the middle layer that surrounds bundles of muscle fibres.
Within each fasciculus, each individual muscle cell or muscle fibre is surrounded by a thin connective tissue layer of collagen and reticular fibres called the endomysium. The endomysium surrounds the extracellular matrix of the cells and transfers the force produced by the muscle fibres to the tendons.
The connective tissue covering provides support and protection for the muscle cells and allows them to withstand the forces of contraction.
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Muscle contraction
The physiological concept of muscle contraction is based on two variables: length and tension. Muscle shortening and muscle contraction are not synonymous. Tension within the muscle can be produced without any change in length, for example, when holding a dumbbell in the same position. When muscle contraction ends, muscle relaxation occurs, with the muscle fibres returning to a low-tension state.
There are three types of muscles in mammals: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and give the body structure and strength. They are made up of multiple fascicles, or bundles of muscle fibres, surrounded by a layer of connective tissue called the perimysium. Each muscle fibre is a single cylindrical muscle cell surrounded by a thin layer of connective tissue called the endomysium.
During skeletal muscle contraction, the protein filaments within each muscle fibre slide past each other to produce a contraction. This is known as the sliding filament theory. The contraction can be described as a twitch, summation, or tetanus, depending on the frequency of action potentials. Skeletal muscle contraction is neurogenic, requiring synaptic input from motor neurons.
There are several types of muscle contractions, including concentric, eccentric, and isometric. A concentric contraction occurs when a muscle is actively shortened, such as when lifting a heavy box. An eccentric contraction happens when a muscle is actively lengthened, such as when walking or lowering a heavy object. An isometric contraction occurs when a muscle is held at a set length, such as when carrying an object in front of you.
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Muscle types
There are three types of muscle tissue: skeletal, smooth, and cardiac. Skeletal muscles are attached to the skeleton and are under voluntary control. They are responsible for movements that are consciously controlled, such as flicking your thumb or sprinting. Skeletal muscles have an abundant supply of blood vessels and nerves, which is related to their primary function of contraction. Each skeletal muscle fibre is a single cylindrical muscle cell, and a single muscle can contain hundreds of thousands of muscle fibres bundled together and wrapped in a connective tissue covering.
Smooth muscle tissue lines some of the body's organs and is under involuntary control. It contracts slowly and rhythmically. Smooth muscle fibres are located in the walls of hollow visceral organs such as the liver, pancreas, and intestines. They are spindle-shaped and have one central nucleus.
Cardiac muscle, found in the walls of the heart, is also under involuntary control. It contracts involuntarily, strongly, and rhythmically to pump blood through the cardiovascular system. Cardiac muscle cells have one central nucleus and are rectangular in shape.
Within skeletal muscles, there are two types of muscle fibres: Type I, which is slow, and Type II, which is fast. Type II is further divided into type IIA (oxidative) and type IIX (glycolytic). These fibres have distinct metabolic, contractile, and motor unit properties. Slow oxidative (Type I) fibres contract slowly and use aerobic respiration to produce ATP. Fast oxidative (Type IIA) fibres contract quickly, primarily use aerobic respiration, and can fatigue more quickly as they may switch to anaerobic respiration.
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Muscle composition
Skeletal muscles are the most common type of muscle in the human body, comprising 30% to 40% of total body mass. They are attached to bones and allow humans to perform a wide range of movements and functions. These muscles are made up of thousands of small fibres woven together, which stretch and press together to move the body and its organs.
Each skeletal muscle fibre is a single cylindrical muscle cell. Each muscle is surrounded by a connective tissue sheath called the epimysium, and connective tissue outside the epimysium, called fascia, separates groups of muscles into compartments. Each compartment contains a bundle of muscle fibres, known as a fasciculus, which is surrounded by a layer of connective tissue called the perimysium. Inside each fascicle, each muscle fibre is encased in a thin connective tissue layer of collagen and reticular fibres called the endomysium.
Each muscle fibre is composed of several hundred to several thousand myofibrils, which are composed of actin (thin filaments), myosin (thick filaments), and support proteins. The arrangement of actin and myosin gives skeletal muscle its microscopic striated appearance and creates functional units called sarcomeres. The sarcomere is the smallest functional unit of a skeletal muscle fibre and is a highly organised arrangement of contractile, regulatory, and structural proteins. The shortening of these individual sarcomeres leads to the contraction of individual skeletal muscle fibres and ultimately the whole muscle.
There are two types of muscle fibre: Type I, which is slow, and Type II, which is fast. Type II has two divisions: type IIA (oxidative) and type IIX (glycolytic). These fibres have distinct metabolic, contractile, and motor unit properties. Slow oxidative (type I) fibres contract slowly and use aerobic respiration to produce ATP. Fast oxidative (type IIA) fibres contract quickly and primarily use aerobic respiration but can switch to anaerobic respiration, leading to quicker fatigue.
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Muscle innervation
The number of nerve fibres that innervate a muscle is smaller than the number of muscle fibres. Within a muscle, nerve fibres branch out and innervate several muscle fibres. The nerve and the group of muscle fibres that are simultaneously innervated by it are called a motor unit. Motor units vary in size, with muscles that move the eyeball containing an average of about seven muscle fibres per motor unit, while a motor unit in the leg may contain more than 1,000 muscle fibres. The excitatory impulses of the motor units can be recorded as an electromyogram (EMG), which is a recording of the extracellular potential of the muscle.
Voluntary movement of limbs that is regulated by the contraction of skeletal muscles is made possible by the presence of neuromuscular junctions (NMJ), which form the interface between the nervous and musculoskeletal systems in the body. Trauma to the muscle can result in muscle fibres becoming denervated, which critically affects muscle development and regeneration. In the absence of innervation, primary myotubes can still form but cannot mature; hence with limited regeneration, muscle atrophy occurs and functional properties are lost.
The autonomic nervous system (ANS) innervates the smooth muscle organs and tissues, while the skeletal motor system innervates striated muscles.
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Frequently asked questions
Skeletal muscle is the most common type of muscle in the human body. It is found throughout the body and functions to contract in response to a stimulus. Skeletal muscle serves many purposes, including producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints.
Each muscle fiber is surrounded by a thin layer of connective tissue called the endomysium. The endomysium surrounds the extracellular matrix of the cells and plays a role in transferring force produced by the muscles to the tendons.
Within a muscle fiber, proteins are organized into organelles called myofibrils that run the length of the cell and contain sarcomeres connected in series. The sarcomere is the smallest functional unit of a skeletal muscle fiber and is a highly organized arrangement of contractile, regulatory, and structural proteins.
There are two main types of muscle fiber: Type I, which is slow, and Type II, which is fast. Type II has two divisions: type IIA (oxidative) and type IIX (glycolytic). These fiber types have distinct metabolic, contractile, and motor unit properties.
Muscle fibers are woven together and stretch and press against each other to move the body and its organs. The nervous system triggers specific movements of muscles by activating a subset of muscle fibers within a fascicle of the muscle.
