Muscle Flashcards
Nuclei of skeletal muscle
Multinucleated, peripherally placed under sarcolemma
Skeletal muscle cell structure
Long, cylindrical, multibucleated
Length varies up to few mm to 1m
Satellite cells
Small population of reserve progenitor cells remain adjacent to many mature skeletal muscle cells
Endomysium
Delicate layer of reticular fibers that immediately surrounds individual muscle fibers
Perimysium
Thicker layer of CT that surrounds fascicles
Fascicle
Bundle of muscle fibers
Epimysium
Thick external sheath of dense CT that surrounds the entire collection of fascicles that make up the muscle
- Carries/supports larger nerves, blood vessels and lymphatics
Myofibrils
Bundle of myofilaments (actin & myosin)
Basic functional unit of muscle cell
Sarcomere
Sarcomere
- basic functional repetitive contractile subunit of striated muscle
- segment of myofibril between two Z lines
Z line
Bisects light (I) band, anchors adjacent thin filaments (actin)
H band
Bisects dark (A) band
Only myosin, adjecent to actin
M line
Bisects H band
Adjacent thick filaments (myosin) link
I bands during contraction
- Actin filament slides past myosin filaments
- Shortens during contraction
A bands during contraction
Individual myosin filaments do not move, width does not change
H band during contraction
Narrows as actin overlaps myosin
What are the contractile units of skeletal muscle
Myofilaments (actin & myosin)
What are the striations shown in longitudinal sections of skeletal muscle?
Alternating dark & light bands due to organized parallel arrangement of myofibrils
Dark bands
A bands - mark extent of myosin filaments, actin overlaps
Light bands
I bands - contain only actin
Primary proteins of thin myofilaments
F-actin, tropomyosin, Troponin
F-actin
Forms double stranded helix
Tropomyosin
Double helix of 2 polypeptides, forms filaments that run in the groove between F-actin molecules in the thin filament
- in resting muscle works with Troponin to mask myosin binding sites
Troponin
Complex of 3 subunits:
- Troponin-C (TnC) - smallest subunit, binds Ca in initiating contraction
- Troponin-T (TnT) - binds to tropomyosin, anchoring Troponin complex
- Troponin-I (TnI) - Binds to actin inhibiting actin-myosin interaction
Thick myofilament structure
- 2 heavy chains twisted together as myosin tails, and 4 light chains
- globular projections containing he 4 light chains form head at 1 end of heavy chain
Sarcoplasmic reticulum
- Smooth ER of muscle cell
- forms highly organized tubular networks around myofibrils, extending between A-I junctions
Terminal cisternae
At the A-I junction, SR forms ring-like channel
Function of terminal cisternae
Reservoir for Ca
Transverse (T) tubule system
Numerous tubular invaginations of the sarcolemma reach to the A-I junction
The T-tubule and 2 terminal cisternae form ______
a triad
Function of triad complex
Triggers Ca release when sarcolemma is depolarized to initiate contraction of Sarcomeres
Repair & healing of skeletal muscle
- mature skeletal muscle cells are permanent (non-dividing)
- limited ability to repair via satellite cells
- more typically, fibroblasts repair injury site with scar tissue
Epicardium
External CT covering around heart muscle (myocardium), lined by mesothelium
Intercalated discs
Junctions between cardiac muscle cells forming strong intercellular adhesion and gap junctions
Gap junction
Allow for communication allowing cardiac muscle to act as a syncytium allowing informational macromolecules to pass rapidly from cell to cell
Cardiac vs skeletal muscle fibers
- Sarcomeres organized and function similarly to skeletal muscle
- myofibrils sparser than skeletal muscle (moth eaten appearance)
- Autonomic nerves regulate contraction
Repair and healing of cardiac muscle
- permanent cells (non-dividing)
- lack satellite cells - very little potential to regenerate after injury (most likely from nearby stem cells)
- myocardial infarction - healing by fibroblasts laying down scar tissue
Motor end plates / neuromuscular junctions
Contact made by terminal branches of motor axons with the skeletal muscle fiber
Smooth muscle characteristics
- non-striated, random placement of actin & myosin (no Sarcomeres)
- elongated cells with tapered ends, one central nucleus
- cells linked by gap junctions
- No T-tubule system (disorganized SR)
Smooth muscle contractile proteins
myosin light-chain kinase (MLCK) and calmodulin
Calmodulin
Ca binding protein
Dense bodies
Attachment sites in cell membrane anchoring Actin myofilaments
Location of smooth muscle
Major muscle component of blood vessels, digestive, respiratory, urinary, and reproductive tracts
Repair and healing in smooth muscle
Capable of more active regenerative response - stabile (quiescent) cells
What structure is located in the I-band and attaches to the thin filaments?
Z-line
