Edited by Dr. Bennion
The muscular system is a very active organ system during exercise. As with any heavily scientific subject there are common misconceptions on how the system works. Understanding the structure and the basic components of the system will give the reader, hopefully, a glimpse into how it all functions together. This series will work from micro to macro to get into the nitty gritty of muscle growth.
Skeletal muscle is bundles of muscle tissue wrapped in connective tissue sheaths where the bundles are layered, growing in girth.
Connective tissue sheaths conglomerate together to create tendons at ends of muscle
Skeletal muscles also contain secondary structures, necessary for cellular health, under the category of sarcoplasm
Vocabulary Terms and Definitions:
Fascicles - A small bundle of fibers wrapped in a connective tissue sheath within a muscle
Perimysium - The connective tissue sheath surrounding each muscle fasciculus
Fascia - Fascia is a thin casing of connective tissue that surrounds and holds every organ, blood vessel, bone, nerve fiber and muscle in place.
Muscle Fibers - An individual muscle cell
Endomysium - A sheath of connective tissue that covers each muscle fiber
Fibril - A small bundle of myofibrils wrapped in a connective tissue sheath within a muscle
Myofibril - A contractile element of skeletal muscle
Sarcomeres - A basic functional unit of a myofibril
Tendons - a flexible but inelastic cord of strong fibrous collagen tissue attaching a muscle to a bone
Sarcoplasm - All other necessary organelles and structures besides the contractile proteins and connective tissue
There is a law in the world of kinesiology called Wolff’s Law. It states that the body will adapt to the load placed upon it (Teichtahl et al. 2015). This law is sometimes called the “Use it or Lose it” law. When any tissue in the body is exercised it will usually stimulate growth. This applies for skeletal muscle but also for other tissue types, like the brain and bone. Some of the ways tissue can grow is in: density, thickness, or cross sectional area. So, I would like to put a disclaimer out that the types of growth that will be discussed in future parts of this series are not limited to skeletal muscle, but in this series of blog posts they will focus specifically on growth of skeletal muscle.
Before I can define the functional processes a more refined description of skeletal muscle is in order. In the ATP and Sliding Filament Theory blog, I taught that skeletal muscles are made up of concentric layers; let's dig a bit deeper on that! Concentric layers means the layers are often circles within bigger circles, they can also be squares of triangles but for skeletal muscle it is circular. These layers are bundles of muscle tissue wrapped in a sheath, like the sheath of a sword. The outermost layer of muscle tissue is the muscle belly. Muscle bellies have an outer connective tissue layer called fascia.
The muscle bellies are groupings of fascicles. Fascicles are individually wrapped in a connective tissue called the perimysium (Haun, et al., 2019). Fascicles are groupings of muscle fibers. Muscle fibers are individually wrapped in connective tissue called endomysium (Haun, et al., 2019). This process keeps getting smaller from muscle fiber to myofibril (Haun, et al., 2019). This microscopic look finishes with sarcomeres being within individual myofibrils (Haun, et al., 2019).
To help you visualize the structure, I recommend you perform a fun object lesson: Take a dry spaghetti noodle and wrap it in plastic wrap. You now have the basic idea of a muscle fiber and its accompanying connective tissue layer. Take four or five more spaghetti noodles and individually wrap those in plastic wrap. Bundle the noodles together and wrap the bundle in plastic wrap. You now have an object that represents one fascicle. Repeat the process of bundling the individually wrapped noodles together a few times. Now place the bundles, that represent fascicles, together and wrap that larger bundle in plastic wrap. This large bundle of bundles represents a muscle belly. This is the basic organization of skeletal muscle. Each muscle in your body is like this.
Skeletal muscle is composed of more than just contractile proteins, like the sarcomeres. All these sheaths of connective tissue extend past the muscle belly and wrap together to form tendons. Therefore, tendons are categorized under the muscular system in the human body. As well, about 20-30 percent of skeletal muscle is all the secondary structures of the myofibril, including the sarcolemma, mitochondria, t-tubules, etc., called sarcoplasm (Haun, et al., 2019). The secondary sarcoplasmic structures are for adenosine triphosphate (ATP) production, impulse signaling, cell reproduction, electrolyte disbursement and other essential productions.
The skeletal muscle structure is made up of concentric layers of muscle tissue interspersed with connective tissue and sarcoplasmic structures. Each of these three parts of the muscle: myofibrils, connective tissue, and sarcoplasmic can experience growth due to exercise, which will be the second part of this series.
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Haun, C. T., Vann, C. G., Osburn, S. C., Mumford, P. W., Roberson, P. A., Romero, M. A., . . . Roberts, M. D. (2019). Muscle fiber hypertrophy in response to 6 weeks of high-volume resistance training in trained young men is largely attributed to sarcoplasmic hypertrophy. Plos One, 6.
Haun, C. T., Vann, C. G., Roberts, B. M., Vigotsky, A. D., Schoenfeld, B. J., & Roberts, M. D. (2019). A critical evaluation of the biological construct skeletal muscle hypertrophy: Size matters but so does the measurement. Frontiers in Physiology, 247.
Teichtahl, A. J., Wluka, A. E., Wijethilake, P., Wang, Y., Ghasem-Zadeh, A., & Cicuttini, F. M. (2015). Wolff’s law in action: a mechanism for early knee osteoarthritis. Arthritis Research & Therapy, 17(1), 207.