Overview of Cardiac Muscle Cells
Cardiac muscle cells, also known as cardiomyocytes or alternatively mycocardiocytes, are cells
that form the heart muscle walls called the myocardium. They are responsible for the formation
of all the chambers of the heart. These cells have many functions, but the most prominent
function in simple is enabling blood to be pumped throughout the body by cardiac muscles. This
occurs due to the relaxation-contraction cycle of cardiomyocytes. Cardiomyocytes are deemed to
be essential as the human heart begins to beat 22-23 days after fertilization3. The cardiac
myocyte takes a specific physical form to be able to accomplish its important tasks. For example,
the plasticity of these cells is the reason they do not become physically strained13. As we know,
the anatomy of cells has a direct correlation to its physiological functionality.
The Anatomy of Cardiac Muscle Cells
Cardiac muscle cells need to be able to uniformly contract over and over to carry out their role
which is the responsibility of ensuring the contraction of the heart. This function can be carried
out repeatedly and efficiently due to the physical form of these cells. Unlike many other cells,
cardiomyocytes have been conditioned to be resistant to fatigue, this is because they need to be
continuously contracting for the heart to function properly1.The reason they are highly resistive
to fatigue is due to the presence of several mitochondria4. Cardiomyocytes can be related to
neurons such that they both contain excitable membranes, organized myofibrils, and aligned
sarcomeres which results in a striated appearance of these specialized cells2. The repetition of
these aligned and striated sarcomeres is the make up of the myofibrils. Additionally, these
myofibrils contain fiber-like structures called myofilaments. Myofilaments make up
approximately 45-60 percent of the cardiomyocyte3. Some cardiac muscle cells contain two or
more nuclei, but the average nucleus only contains one nucleus at the centre of the cell, this is
because these individual cells don’t fuse with one another3. Cardiomyocytes come to contact
with one another for joined functionality at a site called an intercalated disk5. At these sites,
cardiomyocytes are connected to more identical cells as shown in Figure 1. These cells are in a
branch structure which is essential in respect to 3-dimensional processes3. Figure 2 shows some
of the main physical characteristics discussed above.
Figure 1 – View of intercalated disc5
Figure 2 - The 5 Main Characteristics of Cardiac Myocytes3
Cardiomyocytes are miniscule measuring at only 10-20µm in diameter and 50-100µm in length
as they contain the ability to shorten and lengthen depending on need2. The individual cardiac
muscle cell is wrapped in a strong, elastic, closely enveloping layer of connective tissue6.
Additionally, every individual myocyte is surrounded by a cell membrane, this membrane is
called the sarcolemma. The sarcolemma contains channels of charged calcium ions and highly
branched tubes called the transverse tubules and they are the part of the cell that is most directly
linked to the physical contraction of mycocardiocytes. These transverse tubules position the
calcium channels in a way that they can interact with sensory neurons to carry out the excitationcontraction process7. On a larger scale, these processes occur when cardiomyocytes are
connected to one another adjacently and end to end. These connected cells can directly
communicate because of the presence of the gap junction. They are held together as stated due to
the presence of desmosomes8.
Figure 3 – Branched and Interconnected Cardiac Myocytes3
Approximately 2-3 billion cardiomyocytes can be found in the heart. Cardiomyocytes only
account for less than one third of the total cells in the heart organ. In the human heart, there is a
notion know as z-lines. The distance between z-lines on a myofilament stretch from 1.6-2.2 µm.
This distance is considered the amount of basic contractile units found in a cardiomyocyte3. The
cardiomyocytes’ structure is so advanced in performing its task and is so mechanically reliable
that research using carbon-14 showed that an average 50-year-old heart still contains more than
half the cardiac muscle cells that the heart was developed at birth9. Not all cardiomyocytes serve
the same purpose. There are specialized cardiomyocytes known as cardiac pacemaker cells
which differ from general cardiac muscle cells due to an alternation of a single gene. This
alteration is what makes them specialized for example for their functionality in the sinoatrial
node16.
The structural anatomy of cardiomyocytes is intricate and contain many regions of importance
and examples of them are what was discussed in this section as these cardiomyocytes are
physical formed in such a way to carry out various tasks with maximum efficiency and reliability
for the heart.
The Physiology of Cardiac Muscle Cells
The cardiac muscle tissue is composed of cardiomyocytes. Cardiomyocytes are these specialized
cells that expand and contract in response to signals from the nervous system. Cardiac muscle
cells work in unison to involuntarily produce a rhythmic and smooth contraction that pumps
blood from the heart to the rest of the body, after contraction it releases until it needs to pump
again. This contraction and release cycle is what makes the human heartbeat10. This intricate
process may be the most important function of any organelle, so what mechanisms are used and
how is this exactly done?
As mentioned previously, cardiomyocytes begin to development before any other cells in the
human body, and this is because for a fetus to begin development it must have access to a source
which can send energy and oxygen to the different parts of these developing organs for them to
furthermore strengthen and construct the anatomy accordingly1. This is the task of the heart. This
task would not be possible without the existence of cardiomyocytes which creates the structure
and walls of the heart and its different chambers. Excitation-contraction coupling is required to
be carried out by cardiomyocytes which converts action potential in neurons to muscle
contraction3.
Figure 4 – Schematic of excitation-contraction coupling3
Figure 5 – Diagram of Components of Contraction15
The excitation-contraction process can be broken down and observed as steps. The process starts
off by an action potential being induced by connected pacemaker cells. It travels to depolarize
the cell membrane along the sarcolemma and down the transverse tubules. The action potential
now moves to activate the calcium channels which are in the t-tubules. This allows for calcium
to enter the cell which results in a signal being sent to the sarcoplasmic reticulum which indicates
a usage of calcium cells. Consequently, the sarcoplasmic reticulum releases stored calcium to
maintain cellular homeostasis. After this step, free calcium binds troponin-C to the thin
filaments. The troponin-C comes from the regulatory complex. The binding process frees up
troponin from the actin binding site and pushes actin it to bind myosin itself. As a result,
contraction occurs as the actin and myosin filaments now slide past one another and shortening
the sarcomere length. At the end of the contraction, calcium pumps suck up the calcium ions and
send them back to the sarcoplasmic reticulum for storage again. The drop in calcium availability
therefore releases myosin-actin binding and the sarcomere is returned to its initial length3. This
process is the reason for example for the right ventricle to contract and the push of the
contraction is what pumps oxygen-depleted blood to the lungs for oxygenation11.
The Cardiac Muscle Cell: Function vs Form
Earlier, the concept that form follows function was briefly touched upon. This is the idea that for
a function to be carried out, the organelle or body part carrying out that function must be
physically equipped to be able to perform the task. This is no different when it comes to cardiac
muscle cells. Cardiac muscle cells have adapted functionally and physically to be able to carry
out their vital function. Clearly, there is a relationship between the physiology and anatomy of
cardiac muscle cells. If the relationship between the anatomy and physiology of these cells were
not parallel with one another, diseases or disorders may develop.
It was also mentioned earlier that cardiomyocytes are unique in their functions as they are highly
resistive to fatigue. The reason for this is because they are overloaded with mitochondria more so
than most cells. Mitochondria is the powerhouse of the cell and creates ATP for the cell. The
presence of a high number of mitochondria means that power is always being supplied to the
cells and they wouldn’t need a rest. To put into perspective the importance of these organelles,
approximately 30 percent of the volume of the cardiac muscle is from mitochondria12. Another
physical and structural characteristic that can be observed is the branched structure of these cells
as mentioned in relation to anatomy. The function that this provides is the ability for allowing
faster signal propagation and contraction of mycocardiocytes in a 3-dimensional space. The
ability for the different chambers to pump separately without interfering with one another makes
the heart more efficiently working. This is possible due to the separation between the
interconnected network of cells3. It has been determined that cardiac muscle cells are
consistently working with no breaks. This may give an idea that these cardiomyocytes may begin
to wear down. This concern is mitigated by the plasticity layer over the cardiomyocyte which
protects from physical strain and tension and allows for the cell to regenerate at a much faster
rate than it could be catabolized13. Furthermore, sarcomeres are important to the structure of the
cell as they help maintain mechanical integrity of the cell1. Specialized cardiac muscle groups
called pacemaker cells were mentioned earlier. The reason these cells are considered specialized
is due to an alteration in a single gene that gives these cells a specific function. That function is
to consistently generate and receive electrical impulse and develop a consistent systemic pace of
beat for the chambers it is associated with. Hence the name, pacemaker cells.
To summarize this deep dive into cardiac muscle cells, it can be stated that cardiomyocytes are
essential to the human from day one of development till their rest of our living lives. This is
evident as these cells are the first cells to be developed because of the importance of their
functionality and their reliability in keeping the heart contracting to pump blood to the rest of the
body. This would not be possible without the intricate physical structure of these cardiomyocytes
as they are formed to perform tasks and are build for the job at hand. From the plasticity layer
which protects from physical strain to the overly packed mitochondrial content and many more
physical attributes in correlation to their function, Alas we can conclude that form determines
function, and function determines form.
REFERENCES:
1. ” Cardiomyocytes, ** Structure, Function and Histology,” MicroscopeMaster, Articles on
Microbiology and Cell Biology. Web. 24 September 2022.
https://www.microscopemaster.com/cardiomyocytes.html
2. Martini, F.H., Nath, J.L., Bartholomew, E.F., Fundamentals of Anatomy & Physiology
Eleventh Edition, Pearson Education Inc.
3. Rachael, “Cell Organelles and Biology, Cardiomyocytes” Rs’ Science Blog posts. Web.
24 September 2022.
https://rsscience.com/cardiomyocytes/
4. Emma, “Why does the heart muscle have so many mitochondria?” Superprof resources.
Web. 24 September 2022.
https://www.superprof.co.uk/resources/questions/biology/why-do-cells-of-heart-musclecontain-so-manymitochondria.html#:~:text=Mitochondria%20are%20the%20organelles%20in%20the%2
0cell%20that,is%20constantly%20contracting%20and%20relaxing%2C%20and%20neve
r%20tires.
5. S. Sheffield, “Cardiac Muscle Tissue” Get Body Smart Interactive Anatomy and
Physiology. Web. 24 September 2022.
https://www.getbodysmart.com/circulatory-system/cardiac-muscletissue/#:~:text=The%20cardiac%20muscle%20cell%20or%20fiber.%20They%20are,cont
ains%20a%20single%20nucleus%2C%20which%20is%20centrally%20positioned.
6. “Structure and Function of Muscle and Nervous Tissue” Yale Team-Based Learning.
Web. 24 September 2022.
http://medcell.org/tbl/structure_and_function_of_muscle_and_nerves/reading.php#:~:text
=Similar%20to%20skeletal%20muscle%20cells%2C%20cardiomyocytes%20have%20ac
tin,muscle%20consists%20of%20smaller%2C%20interconnected%20cells%20called%2
0cardiomyocytes.
7. R. Ripa, T. George, Y. Sattar, “Physiology, Cardiac Muscle” National Library of
Medicine, National Center for Biotechnology Information. Web. 24 September 2022.
https://www.ncbi.nlm.nih.gov/books/NBK572070/
8. Elsevier B.V., “TRANSPORT SYSTEM IN THE HEART, BLOOD VESSELS AND
BODILY FLUIDS: HAMEOPAITIC SYSTEM” ScienceDirect Journals and Books.
Web. 24 September 2022.
https://www.sciencedirect.com/topics/engineering/cardiac-cell
9. N. Van Prooyen “Heart Cells Grow Throughout Life span” NIH…Turning Discoveries
into Health, National Institutes of Health. Web. 24 September 2022.
https://www.nih.gov/news-events/nih-research-matters/heart-cells-grow-throughout-lifespan
10. J.Eske, “What to know about cardiac muscle tissue” MedicalNewsToday. Web. 25
September 2022.
https://www.medicalnewstoday.com/articles/325530
11. “Heart Health, Right Ventricle” Healthline, The Healthline Editorial line. Web. 25
September 2022.
https://www.healthline.com/human-body-maps/right-ventricle#1
12. “Mitochondria and Cardiac Health: Exploring the Connection” MHS, MCISAAC
HEALTH SYSTEMS INC. Web. 25 September 2022.
https://www.mcisaachealthsystems.com/mitochondria-and-cardiac-muscle/
13. R.Gong, Z. Jiang, N. Zagidullin, T. Liu, B.Cai, “Regulation of cardiomyocyte fate
plasticity” Signal Transduction and Targeted Therapy, Springer Nature. Nature.Web. 25
September 2022.
https://www.nature.com/articles/s41392-020-004132#:~:text=Cardiomyocyte%20plasticity%20plays%20a%20critical%20role%20in%20cardiac,after
%20injury%2C%20bringing%20hope%20to%20patients%20with%20MI.
14. V. Rajagopal, “Components of a contractile cardiac myocyte” ResearchGate, ResearchGate
GmbH. Web. 25 September 2022.
https://www.researchgate.net/figure/A-diagram-showing-the-components-of-a-contractilecardiac--adapted-from-2-We_fig5_49627803
15. A. R. Mandla, C. Jung, V. Vedantham, “Transcriptional and Epigenetic Landscape of Cardiac
Pacemaker Cells” Frontiers, Frontiers Media S.A. Web. 26 September 2022.
https://www.frontiersin.org/articles/10.3389/fphys.2021.712666/full