Muscle Cells (Myocyte)

Original Editor - Lucinda hampton

Top Contributors -  

Introduction[edit | edit source]

Note: This article is about skeletal myocytes.

Sarcolema SkeletalMuscle.png

Myocytes, sometimes called muscle fibers, form the bulk of muscle tissue. They are bound together by perimysium, a sheath of connective tissue, into bundles called fascicles, which are in turn bundled together to form muscle tissue. Myocytes contain numerous specialized cellular structures which facilitate their contraction and therefore that of the muscle as a whole.

  • Muscles are composed of long bundles of Myocytes (Muscle fibers).
  • Myocytes contain thousands of Myofibrils.
  • Each Myofibril is composed of numerous Sarcomeres, the functional contracile region of a striated muscle.
  • Sarcomeres are composed of myofilaments of myosin and actin, which interact using the sliding filament model and cross-bridge cycle to contract.[1]
  • Have lots of mitochondria for energy generation[2]

Key Terms

  • Sarcoplasm: The specialized cytoplasm of a muscle cell that contains the usual subcellular elements along with the Golgi apparatus, abundant myofibrils, a modified endoplasmic reticulum known as the sarcoplasmic reticulum (SR), myoglobin and mitochondria.
  • Sarcoplasmic reticulum: The equivalent of the smooth endoplasmic reticulum in a myocyte.
  • Sarcolemma: The cell membrane of a myocyte.
  • Sarcomere: The functional contractile unit of the myofibril of a striated muscle[1].

Structure[edit | edit source]

Fascia surrounding muscle fibre.png

Myocytes can be incredibly large

  • Diameters of up to 100 micrometers
  • In the anterior thigh, a muscle fiber may be a meter long. In contrast, muscle fibers making up the stapedius, a small muscle of the inner ear, are only a few millimeters in length[3].

The Sarcoplasm

  • Rich with glycogen (a form of stored energy) and myoglobin (a molecule that can store some O2[2]), both required for energy generation
  • Almost completely filled with myofibrils, the long fibers composed of myofilaments that facilitate muscle contraction.

The Sarcolemma of Myocytes

  • Contains numerous invaginations (pits) called transverse tubules which are usually perpendicular to the length of the myocyte.
  • Transverse tubules play an important role in supplying the myocyte with Ca+ ions, which are key for muscle contraction.
Myogenesis Schematic of satellite cell myogenesis and markers typical of each stage.jpg

Each Myocyte

  • Contains multiple nuclei due to their derivation from multiple myoblasts, progenitor cells that give rise to myocytes.[4].
  • Nuclei of skeletal muscle tissue are oval-shaped and located at the periphery of the cell. They are accompanied by satellite cells between the external lamina and sarcolemma.
  • Satellite cells are precursors to skeletal muscle cells and are responsible for the ability of muscle tissue to regenerate. ie These embryonic cells remain in the adult and can replace damaged muscle fibers to some degree.[3]

Myofibril (thousands contained in a myocyte)[edit | edit source]

Sarcomere.gif

Most of the muscle cell is filled with myofibrils.[2]

Myofibrils are contractile units (within the muscle cell) that consist of an ordered arrangement of longitudinal myofilaments (thin actin filaments and thick myosin filaments).

The characteristic 'striations' of skeletal muscle readily observable by light microscopy are the thin filaments (light) the thick filaments (dark).

The Z-line defines the lateral boundary of each sarcomere.[3]

Shortening or contraction of skeletal muscle fibers is a result of sarcomere shortening.

  • Thick filaments are composed of myosin, which is a protein polypeptide. Each myosin molecule has two globular heads which are involved in the contraction through binding thin filaments.
  • Thin filaments include actin (contains a binding site for myosin heads), tropomyosin and troponin (has three subunits: troponin T, troponin I and troponin C).

These sarcomere structures give skeletal muscle its striated appearance and are readily visible on electron microscopy[5].

Image R:  The Actin-Tropomyosin-Troponin complex. In the resting state, tropomyosin, held in place by troponin, blocks the site on actin to which myosin binds. However, in the presence of Ca++, it binds to troponin, and when troponin binds Ca++ it changes conformation, moving tropomyosin away from the myosin binding site on actin.

Sliding Filament Model of Contraction[edit | edit source]

Actin-tropomyosin-troponin.png

The sliding filament model describes the process used by muscles to contract. It is a cycle of repetitive events that causes actin and myosin myofilaments to slide over each other, contracting the sarcomere and generating tension in the muscle.

Key points

  • The sarcomere is the region in which sliding filament contraction occurs.
  • During contraction, myosin myofilaments ratchet over actin myofilaments contracting the sarcomere.
  • Within the sarcomere, key regions known as the I and H band compress and expand to facilitate this movement.
  • The myofilaments themselves do not expand or contract

Cross-bridge cycle

  • The molecular mechanism whereby myosin and acting myofilaments slide over each other is termed the cross-bridge cycle. During muscle contraction, the heads of myosin myofilaments quickly bind and release in a ratcheting fashion, pulling themselves along the actin myofilament.
  • At the level of the sliding filament model, expansion and contraction only occurs within the I and H-bands. The myofilaments themselves do not contract or expand and so the A-band remains constant.
  • The amount of force and movement generated generated by an individual sarcomere is small. However, when multiplied by the number of sarcomeres in a myofibril, myofibrils in a myocyte and myocytes in a muscle, the amount of force and movement generated is significant[4].

The Muscle Contraction Process[edit | edit source]

The Muscle Contraction Process.png

Muscles will contract or relax when they receive signals from the nervous system. The neuromuscular junction is the site of the signal exchange. The steps of this process in vertebrates occur as follows:

Image R: Labels: A: Motor Neuron Axon B: Axon Terminal C. Synaptic Cleft D. Muscle Cell

(1) The action potential reaches the axon terminal. (2) Voltage-dependent calcium gates open, allowing calcium to enter the axon terminal. (3) Neurotransmitter vesicles fuse with the presynaptic membrane and acetylcholine (ACh) is released into the synaptic cleft via exocytosis. (4) ACh binds to postsynaptic receptors on the sarcolemma. (5) This binding causes ion channels to open and allows sodium ions to flow across the membrane into the muscle cell. (6) The flow of sodium ions across the membrane into the muscle cell generates an action potential which travels to the myofibril and results in muscle contraction[6].

Muscle Cell Growth[edit | edit source]

Muscle-up.gif

Although muscle cells can change in size, new cells are not formed when muscles grow. Instead, structural proteins are added to muscle fibers in a process called hypertrophy, so cell diameter increases. Conversely when structural proteins are lost and muscle mass decreases atrophy is said to occur.

Cellular components of muscles can also undergo changes in response to changes in muscle use. The changes in muscle differ depending on the type of exercise undertaken.

  • Endurance exercise causes an increase in cellular mitochondria, myoglobin, and capillary networks in slow twitch fibers. Endurance athletes have a high level of slow twitch fibers relative to the other fiber types.
  • Resistance exercise causes hypertrophy. Power-producing muscles have a higher number of fast twitch fibers than of slow fibers.[7][2].
  • The secretory profile of the skeletal muscle cells is changed by strength and/or endurance exercising eg in addition to its role in inflammation, immune responses and hematopoiesis, interleukin-6 (IL-6) released by muscles during contraction, influences lipid and glucose metabolism[8]

Secretome of skeletal muscle cells[edit | edit source]

Cytokine release.jpg

The secretome is the rich, complex set of molecules secreted from living cells.

  • Skeletal muscle accounts for approximately 40% of the total body weight and contains between 50 and 75% of all body proteins.

Skeletal muscle is an active endocrine organ containing cells that may communicate in an auto-, para- or endocrine manner thanks to the secretion of mediators like myokines.

Myokines are defined as: cytokines and other peptides that are produced, expressed, and released by muscle fibers and exert either paracrine or endocrine effects”

  • Myokines mediate metabolic regulation, inflammatory processes, angiogenesis and myogenesis.
  • Myogenesis is the process during which the muscle stem cells, or satellite cells, proliferate and differentiate into mature muscle fibers, or myotubes. This process is crucial for maintenance and repair of muscle tissue.
  • Myokines are likely to play important roles in the pathophysiology of diseases like sarcopenia, insulin resistance or type-2 diabetes[8].

References[edit | edit source]

  1. 1.0 1.1 Skeletal Muscle Fibers. (2020, August 14). Retrieved March 7, 2021, Available from: https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/9%3A_Muscular_System/9.2%3A_Smooth_Muscle/9.2A%3A_Skeletal_Muscle_Fibers (last accessed 7.3.2021)
  2. 2.0 2.1 2.2 2.3 Austincc ed Muscle Available from: https://www.austincc.edu/sziser/Biol%202404/2404LecNotes/2404LNExII/Muscle%20Physiology.pdf (accessed 8.3.2021)
  3. 3.0 3.1 3.2 Ken Hub Skeletal Muscle Available from: https://www.kenhub.com/en/library/anatomy/histology-of-skeletal-muscle (last accessed 8.3.2021)
  4. 4.0 4.1 Sliding Filament Model of Contraction. (2020, August 15). Retrieved March 7, 2021, from: https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.4%3A_Muscle_Contraction_and_Locomotion/38.4C%3A_Sliding_Filament_Model_of_Contraction (last accessed 7.3.2021)
  5. Braithwaite JP, Al Khalili Y. Physiology, Muscle Myocyte. 2020 Available from: https://www.ncbi.nlm.nih.gov/books/NBK544225/ (accessed 7.3.2021)
  6. https://commons.wikimedia.org/wiki/File:The_Muscle_Contraction_Process.png
  7. BC Campus Exercise and muscle performance Available from: https://opentextbc.ca/anatomyandphysiology/chapter/10-6-exercise-and-muscle-performance/ (last accessed 8.3.2021))
  8. 8.0 8.1 Florin A, Lambert C, Sanchez C, Zappia J, Durieux N, Tieppo AM, Mobasheri A, Henrotin Y. The secretome of skeletal muscle cells: A systematic review. Osteoarthritis and Cartilage Open. 2020 Mar 1;2(1):100019.Available from: https://www.sciencedirect.com/science/article/pii/S2665913119300226 (accessed 8.3.2021)