Cardiac muscle (myocardium)

* Singular nucleus
* Multiple mitochondria
* Branched
* Striated
* Involuntary contractions
* Communicate via intercalated discs (only myocyte to have intercalated discs) - gap junctions and desmosomes

Cardiac muscle tissue is ONLY found in the heart, where the cardiomyocytes coordinate contractions causing the cardiac muscle to contract in a rhythmical pattern and pumping blood through the chambres of the heart and into the circulatory system. Cardiac muscle is striated due to organisation of repeating sarcomeres - bands of actin and myosin - along the myofibril. However, unlike skeletal muscle which is also striated, the cardiomyocytes are shorter and contain only ONE central nucleus. These myocytes are arranged in a branched organisation, to allow for faster signal propagation and contraction of the heart and connected to each by intercalated discs instead of being fused together.

#### Formation

In cardiac muscle, the myoblasts that form cardiomyocytes do not fuse, therefore cardiomyocytes are NOT multinucleate. The gap junctions develop in the cardiac muscle in the early stages of development.

#### Regeneration

Cardiomyocytes are unable to divide effectively, therefore if they are destroyed, they cannot be replaced and are replaced by scar tissue which cannot contract. This puts more of a strain on the other cardiomyocytes, as they have to work harder to achieve the same cardiac output, causing the cardiomyocytes to grow larger (hypertrophy). In cardiac muscle there are NO cells similar to that of the satellite cells found in skeletal muscle.

However, due to stem cells found in the blood some minor regeneration may occur in cardiac muscle if they enter the tissue.

#### Cell junctions

Out of the three different muscle types, myocytes of cardiac muscle are the ONLY cell type to be connected to each other via **intercalated discs**, which run at right angles to the striations of the muscle. The intercalated disc is a specialized portion of the sarcolemma and contain three distinct types of intercellular junctions.

##### **1. Gap** j**unctions**

* Responsible for **electrical coupling** - provide channel between adjacent cardiac myocytes to facilitate electrical communication, allowing for waves of depolarisation produced by cations to pass from cell-cell and spread rapidly over the entire heart.
* Responsible for the transfer of small molecules between cells.

##### **2. "spot" desmosomes**

* Desmosomes also known as adherens junctions.
* Sites of strong intercellular adhesion - help to keep the cardiomyocytes connected when they contract.
* Anchor the intermediate-filament cytoskeleton of the cell

##### **3. "sheet" desmosomes (or fascia adherens junctions)**

* Desmosomes also known as adherens junctions.
* Sites of strong intercellular adhesion - help to keep the cardiomyocytes connected when they contract.
* Actin filaments attach thin filaments in the muscle sarcomeres to the cell membrane.
* Anchor the contractile apparatus.

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Reference list 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. Cardiac Muscle. *Anatomy and Physiology.* Available at:https://opentextbc.ca/anatomyandphysiologyopenstax/chapter/cardiac-muscle-tissue/ [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. Cornell, B. 2016. *Cardiac Muscle.* Available at: https://ib.bioninja.com.au/options/option-d-human-physiology/d4-the-heart/cardiac-muscle.html [Accessed: 19-03-22]
5. 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]
6. 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]
7. Severs, N. J. 1985. Intercellular junctions and the cardiac intercalated disk. *Adv Myocardiol* 5. doi: 10.1007/978-1-4757-1287-2\_18