Skeletal muscle

* Multinucleated
* Multiple mitochondria
* Striated
* Voluntary contractions
* No cell-cell junctions

Skeletal muscles as a whole are an organ that consists of many integrated tissues - skeletal muscle fibres, blood vessels, nerve fibres, and three layers of connective tissue (called "mysia"); epimysium, perimysium, endomysium. Inside skeletal muscle, the muscle fibres, separated by **endomysium**, are organised into bundles called **fascicles**, separated by the **perimysium**. The **epimysium** is a sheath of dense, irregular connective tissue which wraps around each muscle and allows it to maintain its structural integrity during contraction, as well as allowing the muscle to move independently as it separates it from other tissues and organs.

Skeletal muscle is striated due to organisation of repeating sarcomeres - bands of actin and myosin - along the myofibril. However, unlike cardiac muscle which is also striated, the skeletal myocytes are longer and contain multiple nuclei. These myocytes are arranged in long cylindrical organisation creating a muscle fibre.

#### Formation

Skeletal muscle is formed in a process known as myogenesis during embryonic development. Skeletal myoblasts (muscle fibres) are formed from the fusion of hundred of embryonic myoblasts - single mononucleated cells - fusing together and forming a multinucleated myotube (developing muscle fibre). Each nucleus of each contributing myoblast remains intact in the mature skeletal muscle cell. The number of skeletal muscle cells is set during development, however satellite cells help repair skeletal muscle tissue.

Acetylcholine receptors are initially present along the membrane of the myoblasts, but formation of neuromuscular junctions is stimulated by the release of growth factors due to spinal nerve innervation. The distribution of slow and fast muscle fibres in skeletal muscle is influenced by the electrical signals of the activating neurons.

#### Regeneration

Satellite cells, found underneath the basal laminar of skeletal muscle, are mononucleated quiescent cells which are stimulated to divide when the muscle is damaged, fusing with existing fibres to repair any damage. Satellite cells mainly repair damage to living cells and can only regenerate muscle fibers to a limited extent by facilitating the protein synthesis required for repair and growth. Unlike smooth muscle, skeletal myocytes are unable to divide themselves, however they are able to hypertrophy by laying down new protein.

If a cell is damaged to the extent where it cannot be repaired by satellite cells, it is replaced by scar tissue (fibrosis) which decreases the strength of the muscle as it cannot contract.

#### Cell junctions

Skeletal muscle has no intercellular junctions between its myocytes. Skeletal myocytes are one long cylindrical fibre that span the length of the muscle. However there are neuromuscular junctions present between skeletal muscle and neurons, allowing for signalling for contraction of the skeletal muscle.

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References and extra reading:

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1. Baxter, R. 2022. *Types of muscle cells.* Available at: https://www.kenhub.com/en/library/anatomy/types-of-muscle-cells#section15 [Accessed: 19-03-22]
2. Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., DeSaix, P. 2013. Skeletal Muscle. *Anatomy and Physiology.* Available at: https://opentextbc.ca/anatomyandphysiologyopenstax/chapter/skeletal-muscle/ [Accessed: 19-03-22]
3. Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., DeSaix, P. 2013. Development and Regeneration of Muscle Tissue. *Anatomy and Physiology.* Available at:https://opentextbc.ca/anatomyandphysiologyopenstax/chapter/development-and-regeneration-of-muscle-tissue/ [Accessed: 19-03-22]
4. Faculty of Biological Sciences at Leeds. 2003. *Muscle: Skeletal Muscle.* Available at:https://www.histology.leeds.ac.uk/tissue\_types/muscle/muscle\_skeletal.php [Accessed: 19-03-22]
5. Faculty of Biological Sciences at Leeds. 2003. *Muscle:* *Muscle regeneration.* Available at:https://www.histology.leeds.ac.uk/tissue\_types/muscle/muscle\_regeneration.php [Accessed: 19-03-22]
6. Faculty of Biological Sciences at Leeds. 2003. *Muscle:* *Cell Junctions.* Available at: https://www.histology.leeds.ac.uk/tissue\_types/muscle/Muscle\_cell\_junctions.php [Accessed: 19-03-22]