Skeletal Muscle Organization
SKELETAL MUSCLE HISTOLOGY
skeletal muscle hierarchy
- Skeletal muscles divide into fascicles.
- Fascicles are units of muscle cells (aka skeletal myocytes, skeletal muscle fibers).
- Skeletal muscle cells comprise myofibrils and other organelles (notably, mitochondria).
- Myofibrils comprise proteins, notably thick and thin myofilaments.
- Myofilaments arrange into functional contractile units, called sarcomeres.
connective tissue hierarchy
- Skeletal muscle is covered in epimysium.
- Fascicles are covered in perimysium.
- Muscle cells are covered in endomysium.
Muscle Organization
Epimysium
- Envelopes the muscle (epi = upon, my = muscle).
- Dense irregular connective tissue.
Perimysium
- Divides the muscle into multiple wedges (peri = around); it covers each fascicle.
- Each fascicle comprises skeletal muscle cells (aka skeletal myocytes or muscle fibers).
Endomysium
- Surrounds the muscle cells.
- Loose areolar connective tissue that maintains the extracellular environment for proper muscle cell functioning.
Muscle Fascicle
Muscle cells
- They are multinucleated (meaning, each cell contains many nuclei).
- This reality reflects the actual process of muscle cell formation. Mature muscle cells form from fused myoblast cells (embryonic cells) and each myoblast contributes its nucleus to the adult muscle cell.
- Each skeletal myoctye comprises numerous myofibrils.Also show that each cell comprises numerous myofibrils.
Myofibrils contain Myofilaments
- Thick, myosin filaments
- Numerous thin, actin filaments.
- Thin filaments form a hexagonal shape around the thick filaments.
Sarcolemma
- The plasma membrane of the muscle cell.
Sarcoplasm
- The muscle cell cytoplasm.
- Muscle cell nuclei lie within the periphery of the cell.
- During development, the nuclei transition from a central location to a peripheral one.
- Muscle cells comprise numerous mitochrondria.
Muscle fibers divide into 3 types based on their: myoglobin content and contraction speed, and, in related fashion, their number of mitochondria.
- Type 1 (slow, red)
- Type 2a (fast, intermediate)
- Type 2b (fast, white)
External lamina (sometimes referred to as the basal lamina)
- Lies external to the muscle cell.
- Within it, lie satellite cells, which are skeletal muscle stem cells: inactive myoblasts, lying in-wait: think: Army Reserves.
- Upon muscle injury, they enter mitosis, fuse with other satellite cells to form differentiated muscle fibers. Centrally located nuclei are a hallmark of regenerating muscle cells; whereas mature muscle cells contain peripherally located nuclei.
myofibril histology: Internal
Thick filaments.
- Form from myosin
- The A band refers to the length of the thick filaments, "think "A" for d-a-rk – they are aniosotropic (or birefringent) in polarized light.
- H Zone is a zone of only thick filaments.
- M line bisects the A band.
Thin filaments
- Form from actin
- The I band is the region along the thin filaments (between the thick filaments).
- Think "I" for L-i-ght – they are "isotropic" (do not alter polarized light).
Z disks
- Transverse bands at the ends of the thin filaments.
Electron micrograph of a muscle fiber
we see that the repeating light and dark bands of the sarcomere gives muscle fibers a striated appearance.
- Indicate the A Band, remember "A" for "dark", "anisotropic".
- Indicate the I Band, remember "I" for "light", "isotropic".
- Z disk bisects the I band.
- M line bisects A Bands; they contain myomesin and creatine kinase.
- Sarcomere comprises the area between the Z-disks.
- Triad at the A-I junction (which comprises T tubules and terminal cisternae of the sarcoplasmic reticulum).
Clinical Correlation: Muscle Atrophy (aka Muscle Wasting)