MUSCULAR TISSUE
Skeletal Muscle Tissue:
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Mesodermic Origin
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Function: allows the movement of the skeleton and organs
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Contraction: fast, voluntary, vigorous and discontinuous
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Structure: the Skeletal Muscle tissue is formed by very long, cylindrical and multinucleated cell bundles, that contain many filaments called myofibrils that constitute skeletal muscular fibers.
Muscle tissue is composed of contractible cells. Actin microfilaments and associated proteins allow the muscle contraction by using energy from ATP molecules. Muscle cells are originated by mesodermic cells and they differentiate themselves by a gradual process of cell lengthening synthesizing myofibrilar proteins: actin and myosin. Contraction is caused by the sliding interaction of thick myosin filaments along thin actin filaments.
There are three types of muscle tissue and each have different roles in the human body: Skeletal muscle, Cardiac Muscle and Smooth muscle.
It is important to highlight that some components of muscle cells are given special names:
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Cell Membrane - Sarcolemma
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Cytosol - Sarcoplasm
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Smooth endoplasmic reticulum - Sarcoplasmic Reticulum
Skeletal muscle
Skeletal Muscle:
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Skeletal muscle is one of three major muscle types, the others being Cardiac muscle and Smooth muscle.
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It is a form of striated muscle tissue which is under the voluntary control of the somatic nervous system.
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Most skeletal muscles are attached to bones by bundles of collagen fibers known as tendons.
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Each organ or muscle consists of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue.
Skeletal muscle fibers: originated in the embryo through the fusion of elongated cells called myoblasts. The cells' nuclei is located on the outskirts of the fibers, near the sarcolemma, which helps distinguish skeletal muscle from cardiac muscle (central nuclei), along with another unique feature of the cardicac muscle tissue - intercalated discs.
From: Wikipédia
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Organization - Skeletal Muscle
In a muscle, its fibers are organized in fiber bundles that are wrapped by three different layers of connective tissue:
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Epimysium: connective tissue layer that wraps the set of bundles (wraps the entire muscle).
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Perimysium: thin connective tissue septa departing from the epimysium and heading to the interior of the muscle, separating the bundles.
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Endomysium: wraps each muscle fiber individually and is formed by the basal lamina of the muscle fiber associated to reticular fibers. This connective tissue layer presents sparse cell population which is composed by some connective tissue cells, mainly fibroblasts.
These layers keep the muscle fibers together, allowing the contraction force generated by each fiber to act on the entire muscle.
a) Schematic image of muscle tissue showing the three different surrounding layers of connective tissue.
b) Histological image of muscle tissue showing the three different surrounding layers of connective tissue.
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Organization – Skeletal Muscle Fibers
When observed through the optical microscope, skeletal muscle fibers show transverse striations, due to alternate dark and light bands. The dark bands are called A bands due to being anisotropic (identical properties in equal directions). On the other hand, the light bands are isotropic (identical properties in all directions) and, as such, called I bands. At the center of each I band, there’s a dark transversal line – the Z line.
The myofibril striation is caused by the repetition of equal basic unities, the sarcomeres. Each sarcomere is formed by the part of the myofibril that lies between two successive Z lines and contains a A band separating two half I bands.
Main proteins found on myofibrils:
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Innervation
The contraction of skeletal muscle fibers is stimulated by the terminals of motor nerve fibers. Near the surface of the muscle cell, each nerve terminal loses the myelin sheath and expands, forming the neuromuscular junction.
Innervation of skeletal muscle
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Vascularization
Skeletal muscle tissue has blood vessels that enter the muscle through septa located in the connective tissue. These vessels form an extensive network of capillaries that are distributed through the muscle fibers.
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Regeneration
Skeletal muscle has a weak ability of reconstitution. It is assumed that satellite cells are responsible for such regeneration. After an injury or other stimuli, satellite cells become activated, proliferate by mitotic division and fuse together to form new skeletal muscle fibers.
Schematic image of satellite cells on muscle tissue
All images were taken from Google Images, unless said otherwise.
Image showing the myofibril striation and the alternate dark and light bands