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Skeletal muscle is a highly organized tissue made up of bundles of muscle fibres called myofibers, which contain several myofibrils. Each skeletal muscle is an organ that consists of various integrated tissues, including skeletal muscle fibres, blood vessels, nerve fibres, and connective tissue. Each muscle is enveloped in a fibrous tissue envelope called the epimysium, which is a sheath of dense, irregular connective tissue. The epimysium is one of the three layers of connective tissue (mysia) that enclose skeletal muscle, providing structure and allowing the muscle to contract and move powerfully while maintaining its structural integrity.
| Characteristics | Values |
|---|---|
| Definition | Skeletal muscle is a highly organized tissue composed of bundles of muscle fibers called myofibers. |
| Composition | Skeletal muscle is composed of myofibers, myofibrils, sarcomeres, and connective tissue. |
| Function | Skeletal muscle serves to produce movement, sustain body posture and position, maintain body temperature, store nutrients, and stabilize joints. |
| Control | Skeletal muscle contraction is mostly under voluntary control, receiving neural inputs that allow conscious control of muscles. |
| Blood Supply | Skeletal muscles have an abundant supply of blood vessels for nourishment, oxygen delivery, and waste removal. |
| Nerve Supply | Each skeletal muscle fiber is supplied by the axon branch of a somatic motor neuron, which signals the fiber to contract. |
| Connective Tissue | Skeletal muscle is surrounded and subdivided by connective tissue sheaths called the epimysium, perimysium, and endomysium. |
| Fascia | Fascia is a sheath of connective tissue that surrounds and supports muscles, tendons, ligaments, tissues, organs, nerves, joints, and bones. |
| Location | Fascia is found throughout the body, with different types such as thoracolumbar fascia, fascia lata, and plantar fascia. |
| Layers | There are four layers of fascia: superficial, deep, visceral, and parietal. |
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What You'll Learn
The role of fascia
Fascia is a sheath of stringy, white connective tissue that surrounds and supports every organ, blood vessel, bone, nerve fibre, tendon, ligament, and muscle in the body. It is made up of multiple layers with a liquid called hyaluronan in between, which helps the layers work smoothly together. Fascia is mostly made of collagen, a type of protein that provides strength and flexibility.
Healthy fascia is smooth, slippery, and flexible, allowing it to stretch with the body as it moves. It is also rich in mechanoreceptors and nerve endings, making it one of the richest sensory organs in the body. It plays a major role in the perception of posture and movement, affecting proprioception and coordination. When the body changes its posture or moves, the fascial tissues' mechanoreceptors deform and activate, sending afferent information to the spinal cord and brain.
Fascia provides structure and support throughout the body. It holds muscles together, allowing them to contract and stretch, and provides a smooth surface for muscles, joints, and organs to slide against each other without creating friction or tears. It also separates muscles, eases muscle tension, and stabilizes body structures, giving the body strength.
When fascia is unhealthy, it can become thicker, stickier, drier, and tighter. This can restrict movement and cause painful health conditions, such as compartment syndrome, plantar fasciopathy, and myofascial pain syndrome. Treatment for fascia pain often involves a combination of therapies, such as heat therapy, anti-inflammatory diets, yoga therapy, and guided imagery. Maintaining healthy fascia involves staying physically active, stretching frequently, and maintaining good posture.
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Epimysium, perimysium, and endomysium
Each skeletal muscle has three layers of connective tissue, or "mysia", that enclose it and provide structure: the epimysium, perimysium, and endomysium.
The epimysium is a sheath of dense, irregular connective tissue that surrounds the entire muscle. It allows a muscle to contract and move powerfully while maintaining its structural integrity. It also separates the muscle from other tissues and organs in the area, allowing the muscle to move independently. The epimysium is continuous with the joining of various muscle groups or the tendon attaching muscle to bone.
The perimysium is a layer of connective tissue that surrounds bundles of muscle fibres, known as fascicles. The perimysium may surround anywhere from 10 to 100 fascicles. It is contiguous with the epimysium and separates smaller groups of myofibers, or fasciculi, from one another.
The endomysium is a delicate network of connective tissue fibres, blood vessels, lymphatic vessels, and nerves that surrounds individual muscle fibres. It contains the extracellular fluid and nutrients to support the muscle fibre. These nutrients are supplied via blood capillaries located in the endomysium.
These three layers of connective tissue make up a network of collagen and elastin fibres embedded in a matrix of proteoglycans. Perimysium represents about 90% of total connective tissues in muscles, while the endomysium is a network of fine and slightly wavy fibrils.
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How muscles contract
Skeletal muscle is a specialised contractile tissue that is a distinguishing characteristic of animals. Skeletal muscle contraction is a complex process that involves the nervous system, motor neurons, muscle cells, and various proteins. The process can be summarised in three steps:
- A message is sent from the nervous system to the muscular system, triggering chemical reactions. This message, or impulse, is called an action potential and travels through a type of nerve cell called a motor neuron. When the nervous system signal reaches the neuromuscular junction (where the motor neuron reaches a muscle cell), a chemical message is released by the motor neuron. This chemical message, a neurotransmitter called acetylcholine, binds to receptors on the outside of the muscle fibre, starting a chemical reaction within the muscle.
- The chemical reactions lead to the muscle fibres reorganising themselves in a way that shortens the muscle. The proteins inside muscle fibres are organised into long chains that can interact with each other, reorganising to shorten and relax. This reorganisation is understood to be driven by the interaction of myosin and actin proteins, which form sarcomeres. The H and I bands of the sarcomere shorten with muscle contraction, while the A band remains a constant length.
- When the nervous system signal is no longer present, the chemical process reverses, and the muscle fibres rearrange again, causing the muscle to relax.
There are different types of muscle contractions, including isometric, isotonic, concentric, and eccentric contractions. Isometric contraction generates tension without changing length, for example, when the muscles of the hand and forearm grip an object. In isotonic contraction, the tension in the muscle remains constant despite a change in muscle length, and this occurs when a muscle's force of contraction matches the total load on the muscle. In concentric contraction, muscle tension is sufficient to overcome the load, and the muscle shortens as it contracts. Eccentric contraction occurs when the tension generated while isometric is insufficient to overcome the external load on the muscle, and the muscle fibres lengthen as they contract.
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Skeletal muscle composition
Skeletal muscles are the most common type of muscles in the body, comprising 30% to 40% of total body mass. They are attached to bones and allow for a wide range of movements and functions. Skeletal muscles are voluntary muscles, meaning that we have conscious control over how and when they move.
Each skeletal muscle fibre is a single cylindrical muscle cell. Each muscle is surrounded by a connective tissue sheath called the epimysium. Fascia, a stringy, white substance made mostly of collagen, is a connective tissue outside the epimysium that surrounds and separates the muscles. Portions of the epimysium project inward to divide the muscle into compartments, with each compartment containing 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. Within the fasciculus, each individual muscle cell, or muscle fibre, is surrounded by connective tissue called the endomysium. The endomysium is the innermost layer surrounding individual muscle fibres.
Each muscle fibre is composed of several hundred to several thousand myofibrils. Myofibrils 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 basic cellular unit of the muscle cell. The sarcomere includes the M line, Z disk, H band, A band, and I band. The M line is the central-most line of the sarcomere, where myosin filaments are anchored together. The H band contains the M line and contains only myosin filaments. The A band is a larger portion of the sarcomere that contains the entirety of the myosin fibres and includes regions of actin and myosin overlap. The I band covers the terminal regions of two adjacent sarcomeres and contains only actin filaments. The H and I bands shorten with muscle contraction, while the A band remains a constant length.
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Types of fascia
Fascia is a thin layer of connective tissue that surrounds and supports all the organs, muscles, bones, nerves, and blood vessels in the body. It is composed of multiple layers with a liquid called hyaluronan (hyaluronic acid) between them. Fascia is designed to stretch as you move, but when it dries up and tightens around muscles, it can limit mobility and cause painful knots.
There are four main types of fascia:
- Superficial fascia: This is the lowermost layer of the skin and is found directly under the skin. It varies in thickness depending on the body part. For instance, it is thicker in the chest and abdomen, while it is thinner in the arms and legs. It also surrounds organs, glands, and neurovascular bundles, and fills empty spaces in the body.
- Deep fascia: This type of fascia wraps around bones, muscles, nerves, and blood vessels. It can be divided into two types: aponeurotic fascia, which is thicker and easily detachable from muscles, and epimysial fascia, which is thinner and more strongly connected to muscle fibers.
- Visceral fascia: This type of fascia covers organs within the body cavity, such as the heart, lungs, and abdominal organs.
- Parietal fascia: While this type of fascia is mentioned in sources, there are no further details on its specific functions or location in the body.
Maintaining healthy fascia is important as it can impact both your appearance and quality of life. When fascia is healthy, it is smooth, slippery, and flexible. However, factors such as limited physical activity or repetitive movements can cause fascia to become "gummy" and adhered, leading to painful knots and restricted movement.
Various treatments can be used to address fascia issues, including massage therapy, acupuncture, heat therapy, and yoga.
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Frequently asked questions
An enveloping muscle is a term used to describe the connective tissue covering that surrounds and protects muscles. This covering is called the epimysium.
The epimysium is a sheath of dense, irregular connective tissue that envelops entire muscles. It allows muscles to contract and move powerfully while maintaining structural integrity.
The epimysium is made of connective tissue, primarily collagen. Collagen is a type of protein that provides strength and flexibility.
