Skeletal muscle cells
Masters of Movement and Force
Credit: Art by Sam Esquillon. Set in motion by Dr. Emanuele Petretto. Words by Dr. Agnieszka Szmitkowska. Project coordination: Dr. Masia Maksymowicz. Series Director: @Radhika Patnala Sci-illustrate Endosymbiont
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The columns that build our muscles
Human skeletal muscle cells, also known as muscle fibres or myocytes, are specialised cells that are pivotal in the movement and posture of the human body. They build the skeletal muscles, which comprise 30%-40% of human body mass (1). Cylindrical in shape, just like columns of the buildings, they keep the body stable and robust. But those columns have a particular function — they can contract, which allows us to move however we decide.
Muscle cells like no others
Skeletal myocytes differ from other muscle cell types. They are involved in our body’s voluntary movements and are under our conscious control, unlike smooth muscles found in organs or cardiac muscle cells in the heart (2). They are called skeletal as they are attached by tendons to our skeleton, letting us breathe, move and stand. Their structure is unique, too. Skeletal muscle cells are long, cylindrical cells that can be several centimetres long. They are multinucleated, meaning each cell has more than one nucleus (blue dots on the image). This feature results from the fusion of multiple precursor cells, or myoblasts, during development, forming a syncytium — a large, multinucleated cell (3).
Internally, skeletal myocytes are highly organised. They contain long myofibrils, which are thread-like filaments made of proteins (an example of which is visible on top of the cell in the illustration). The key proteins building them are actin and myosin. Myofibrils are further organised into sarcomeres — repeating basic functional units of muscle contraction. The arrangement of these sarcomeres gives skeletal muscle its characteristic striated appearance under a microscope. It is due to the precise configuration and interaction of two main types of protein filaments: thin ones made of actin and thick ones made of myosin. These filaments are organised in a repeating pattern that creates alternating dark and light bands (4).
The sarcolemma and sarcoplasm are other crucial structural components of skeletal muscle cells. Their cell membrane, sarcolemma, not only encloses the cell’s contents, but also plays a vital role in conducting electrical signals necessary for initiating muscle contractions and interacting with the extracellular matrix. Muscle cells’ cytoplasm, called sarcoplasm, is rich in mitochondria and myoglobin, reflecting the high energy demands of muscle activity. It also serves as storage for energy reserves, like glycogen and lipids, and houses the sarcoplasmic reticulum which is crucial for regulating calcium ion concentrations essential for muscle contraction (5).
How does muscle contraction work?
Muscle contraction involves the interaction between actin and myosin filaments within the sarcomeres — a process known as the sliding filament theory. The heads of myosin filaments attach to and ‘walk’ along the actin filaments, pulling them closer together and causing the muscle to contract. This action requires energy supplied as adenosine triphosphate (ATP) (6). Skeletal muscle cells need a substantial energy supply, especially during intense physical activity. But, because the amount of ATP stored in muscles is limited, they must continuously regenerate it. This regeneration is accomplished through various pathways, including aerobic respiration, anaerobic glycolysis, and using creatine phosphate, a high-energy compound stored in muscles (7).
Not all skeletal muscle cells are the same. They can be broadly classified into slow-twitch (type I) and fast-twitch (type II) fibres. Slow-twitch fibres are more efficient at using oxygen to fuel continuous, extended muscle contractions over a long time. They are best suited for endurance activities like long-distance running or cycling. On the other hand, fast-twitch fibres are able to generate short bursts of strength or speed but fatigue more quickly. They are ideal for sprinting or weightlifting (8).
Adaptation and Regeneration
One of the unique features of skeletal muscle cells is their ability to adapt and regenerate. When subjected to regular exercise, especially resistance training, these cells grow in size, a process known as hypertrophy. This growth results from an increase in the size of the myofibrils and a corresponding increase in the number of actin and myosin filaments (9).
While skeletal muscle cells do not divide, they can regenerate to a certain extent with the help of satellite cells. They are a type of stem cells located on the outer layer of the muscle fibre. When a muscle is injured, satellite cells multiply and fuse with the damaged muscle fibres, aiding in repair and regeneration (10).
Role in Health and Disease
Skeletal muscle cells are vital for more than just movement; they are crucial in overall health and metabolic processes. As major contributors to the basal metabolic rate (BMR), they significantly impact the body’s energy expenditure, even at rest. Skeletal muscles continuously use energy, thereby influencing overall metabolism and weight management. Additionally, they are instrumental in glucose regulation, aiding in the uptake and utilisation of glucose from the bloodstream, which is relevant for maintaining healthy blood sugar levels. This regulatory function is particularly significant in metabolic disorders like diabetes (11).
Skeletal muscles are also substantial sites for fat oxidation, playing a role in lipid metabolism and energy balance. However, conditions like sarcopenia, characterised by age-related loss of muscle mass and strength, and muscular dystrophy, a group of diseases causing progressive muscle weakness and loss, highlight the importance of maintaining healthy skeletal muscle cells (12, 13). These conditions impact a person’s ability to move and live independently, highlighting how closely muscle health is linked to overall body function and well-being. They also have wider effects on our body’s metabolism, showing just how vital our muscles are for more than just movement.
Conclusion
In summary, skeletal muscle cells are essential for human movement, posture, and overall health. Their unique structure enables controlled voluntary movements, and their ability to adapt and regenerate is crucial for maintaining physical health. These cells play a significant role in metabolic processes, affecting energy expenditure, glucose regulation, and fat oxidation. However, their vulnerability is highlighted by conditions like sarcopenia and muscular dystrophy, emphasising the importance of maintaining muscle health for overall well-being.
Recognising labs working on the subject:
- Neuromuscular Bioengineering Laboratory, Departments of Orthopaedic Surgery & Bioengineering UC San Diego, California, USA https://ortho.ucsd.edu/research/neuromuscular-bioengineering.html
- Department of Physical Medicine and Rehabilitation, Vanderbilt University School of Medicine, Suite 1318, 2201 Children’s Way, Nashville, USA https://www.vumc.org/pmr/
- Faculty of Sports and Nutrition, Amsterdam University of Applied Sciences, Dr. Meurerlaan 8, 1067 SM Amsterdam, the Netherlands https://www.amsterdamuas.com/faculty/fsn/faculty-of-sports-and-nutrition.html
- Research Institute for Sport and Exercise Science, Liverpool John Moores University, Liverpool, United Kingdom https://www.ljmu.ac.uk/research/centres-and-institutes/research-institute-for-sport-and-exercise-sciences
- Department of Biomedical and Biotechnological Sciences, Human Anatomy and Histology Section, School of Medicine, University of Catania, Italy https://www.unict.it/en/university/biomedical-and-biotechnological-sciences
- Laboratory of Muscle Stem Cells and Gene Regulation, NIAMS, NIH, Bethesda, MD, USA https://www.niams.nih.gov/labs/sartorelli-lab
- Brack Laboratory for Skeletal Muscle Regeneration and Aging, Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research, Department of Orthopaedic Surgery, University of California, San Francisco, USA, https://orthosurgery.ucsf.edu/research/laboratories/Brack-Lab
- Muscle Research Laboratory, Department of Neurology, Boston University School of Medicine, Boston, USA, https://www.bumc.bu.edu/neurology/research-2/muscle-research-laboratory/
- The Gilbert Lab, Institute of Biomedical Engineering, Donnelly Centre for Cellular and Biomolecular Research, Toronto, Canada, https://www.gilbert-lab.com/ @GilbertLab
References:
1. Frontera WR, Ochala J. Skeletal muscle: a brief review of structure and function. Calcified tissue international. 2015;96(3):183–95.
2. Tieland M, Trouwborst I, Clark BC. Skeletal muscle performance and ageing. Journal of cachexia, sarcopenia and muscle. 2018;9(1):3–19.
3. Stephenson RS, Agger P, Lunkenheimer PP, Zhao J, Smerup M, Niederer P, et al. The functional architecture of skeletal compared to cardiac musculature: Myocyte orientation, lamellar unit morphology, and the helical ventricular myocardial band. Clinical Anatomy. 2016;29(3):316–32.
4. Rall JA. What makes skeletal muscle striated? Discoveries in the endosarcomeric and exosarcomeric cytoskeleton. Advances in Physiology Education. 2018;42(4):672–84.
5. Trovato FM, Imbesi R, Conway N, Castrogiovanni P. Morphological and functional aspects of human skeletal muscle. Journal of functional morphology and kinesiology. 2016;1(3):289–302.
6. Squire JM, Paul DM, Morris EP. Myosin and actin filaments in muscle: structures and interactions. Fibrous proteins: Structures and mechanisms. 2017:319–71.
7. Roberts TJ, Eng CM, Sleboda DA, Holt NC, Brainerd EL, Stover KK, et al. The multi-scale, three-dimensional nature of skeletal muscle contraction. Physiology. 2019;34(6):402–8.
8. Gong HM, Ma W, Regnier M, Irving TC. Thick filament activation is different in fast‐and slow‐twitch skeletal muscle. The Journal of Physiology. 2022;600(24):5247–66.
9. Krzysztofik M, Wilk M, Wojdała G, Gołaś A. Maximizing Muscle Hypertrophy: A Systematic Review of Advanced Resistance Training Techniques and Methods. Int J Environ Res Public Health. 2019;16(24).
10. Domingues-Faria C, Vasson M-P, Goncalves-Mendes N, Boirie Y, Walrand S. Skeletal muscle regeneration and impact of aging and nutrition. Ageing research reviews. 2016;26:22–36.
11. Frankenberg NT, Mason SA, Wadley GD, Murphy RM. Skeletal muscle cell-specific differences in type 2 diabetes. Cellular and Molecular Life Sciences. 2022;79(5):256.
12. Mukund K, Subramaniam S. Skeletal muscle: A review of molecular structure and function, in health and disease. Wiley Interdisciplinary Reviews Systems Biology and Medicine. 2019;12.
13. van Dronkelaar C, van Velzen A, Abdelrazek M, van der Steen A, Weijs PJ, Tieland M. Minerals and sarcopenia; the role of calcium, iron, magnesium, phosphorus, potassium, selenium, sodium, and zinc on muscle mass, muscle strength, and physical performance in older adults: a systematic review. Journal of the American Medical Directors Association. 2018;19(1):6–11. e3.
About the author:
DR. AGA SZMITKOWSKA
Content Editor The League of Extraordinary Celltypes, Sci-Illustrate Stories
Aga did her Ph.D. in Biochemistry at the CEITEC/Masaryk University in Brno, Czech Republic, where she was a part of the Laboratory of Genomics and Proteomics of Plant Systems. She is a passionate public speaker and science communicator. After graduation, she became a freelance content coordinator and strategist in a start-up environment focused on lifestyle and longevity.
About the artist:
SAM ESQUILLON
Contributing Artist The League of Extraordinary Celltypes, Sci-Illustrate Stories
Sam worked for a couple of educational children’s shows as an illustrator, puppeteer, art director, and production designer. He still works as a production designer for international films and tv/ online streaming shows.
About the animator:
DR. EMANUELE PETRETTO
Animator The League of Extraordinary Celltypes, Sci-Illustrate Stories
Dr. Petretto received his Ph.D. in Biochemistry at the University of Fribourg, Switzerland, focusing on the behaviour of matter at nanoscopic scales and the stability of colloidal systems. Using molecular dynamics simulations, he explored the delicate interaction among particles, interfaces, and solvents.
Currently, he is fully pursuing another delicate interaction: the intricate interplay between art and science. Through data visualisation, motion design, and games, he wants to show the wonders of the complexity surrounding us.
About the series: The League of Extraordinary Celltypes
The team at Sci-Illustrate and Endosymbiont bring to you an exciting series where we dive deep into the wondrous cell types that make our bodies tick ❤.
