Histology of Skeletal Muscle Flashcards
Skeletal muscle
Elongated, multi-uncleared cells grouped into bundles surrounded by CT sheaths and extend from their proximal site of origin to their distal attachment
Epimysium
AKA deep fascia, dense CT surrounding the entire muscle
Perimysium
CT that surrounds a group of fibers, forms fascicles, coveys large blood vessels and nerves into the muscle
Endomysium
Delicate CT surrounding individual muscle fibers
Type I muscle fibers
AKA slow twitch/red, more efficient over long periods of time, used for postural maintenance or endurance exercise, ATP generated by ox phos, high levels of myoglobin, speed of contraction low and large motor units
Type II muscle fibers
AKA fast twitch/white, better for short bursts of speed, fatigue quickly, ATP from anaerobic glycolysis, high levels of glycogen, high speed of contraction and small motor units
Skeletal muscle in embryo
Formed by the fusion of myoblasts
Result of the fusion of myoblasts
Production of a post mitotic, multinucleated myotube
Myotube
Matures into long muscle cell
Sarcolemma
Plasma membrane of muscle cells, contains numerous ion transport proteins and receptors, surrounded by a basal lamina
Transverse tubule system
AKA T-tubules, sarcolemma finger-like invaginations that make contact with sarcoplasmic reticulum, transmits depolarization signal deep into muscle cell
Sarcoplasmic reticulum
Internal membrane network, no connection to extracellular space, makes junction complexes with T-tubules, forms triads, site of intracellular calcium storage and release
Myofibrils
Occupies most of the cytoplasm, composed of thin filaments (actin) and thick filaments (myosin), surrounded by mitochondria, SR, and T-tubules
Desmin
Anchored to inside of sarcolemma, extend from one myofibril to another, helps organize the cytoplasm to facilitate the coordinated contraction of myofibrils
Sarcomeres
Repeating contractile elements of myofibrils, contain Z-lines, I-band, A-band, H-band, M-line
Z-lines
Define boundaries of each sarcomere, contains the actin binding proteins, alpha actin
I-band
Composed exclusively of actin thin filaments, width decreases during contraction
A-band
Composed of overlapping thick and thin filaments, width remains constant during contraction
H-band
Central region of the A-band which contains only thick filaments, width decreases during contraction
M-line
Middle of the H-band, overlapping tails of thick filaments
Thin filament composition
G-actin monomers that bind to make F-actin filament, tropomyosin, the troponin complex (I, T, and C), nebulin
G-actin monomers
Bind each other to form double-stranded and twisted F-actin, displays binding site for myosin
F-actin
Many regularly spaced myosin binding sites, complexes with tropomyosin and troponin complex
Tropomyosin
Two polypeptides twisted around each other running in the groove formed by the F-actin filaments, binds troponin-T of troponin complex
Troponin complex components
Troponin-I, troponin-T, troponin-C
Troponin-T
Bound by tropomyosin
Troponin-I
Inhibits the binding of myosin to actin
Troponin-C
Binds calcium ions
Nebulin
Extends from the Z-line along the length of the thin filament to stabilize
Thick filament composition
Myosin with two identical heavy chains and two pairs of light chains
Myosin heavy chain
Consists of globular head, binding site for actin and an ATPase domain, long alpha-helical coiled tail, enables several myosin molecules to self-assemble into a filament, hinge region separates head and tail
Myosin light chains
Play role in regulating activity of myosin head
Titin
Large protein extending from Z-line to center of sarcomere, functions to control assembly of the thick filament and provide elasticity fo the sarcomere
Sliding filament model of muscle contraction
Sliding of thin filaments toward the center of the sarcomere during contraction, length of filaments does not change, rather thick and thin filaments slide past one another’s causing the Z-lines to move closer together, decreasing the width of the H- and I-bands
Cross-bridge cycle
ATP binds myosin head, causes dissociation from actin thin filament, ATP hydrolyzed by myosin head ATPase, myosin conformational change, movement at the hinge to increase angle between head and tail, head lined up with new myosin binding site, attaches to the thin filament, causes release of Pi and conformational change in myosin, movement in hinge so angle between head and tail decreases, causes sliding of the thin filament toward center or sarcomere, ADP released
Calcium and muscle contraction
Depolarization spreads from sarcolemma to T-tubules, transporting the signal to interior of muscle cell, sensed by L-type calcium channels in T-tubule membrane in SR, calcium stored in SR released into cytoplasm of muscle fiber, calcium binds troponin-C, conformational change to troponin complex, altering conformation of tropomyosin, uncovering of myosin binding sites on actin, calcium depended they ATPase mediates the return of calcium to the SR when relaxed
Skeletal muscle fibers
Inner gated by an axon of a motor neuron
Motor end plate
AKA neuromuscular junction, where the axon interacts with the muscle fiber
Release of acetylcholine
From synaptic vesicles in motor neurons, begins contraction
Acetylcholine binding
Binds receptor on muscle sarcolemma, depolarization of cell membrane, action potential spreads to T-tubules, which transmit the signal to interior of muscle cell to cause release of calcium ions from SR
Acetylcholine activity eliminated
Through the action of acetylcholinersterase (present in basal lamina of synaptic cleft)
Motor unit
Motor axon and all of the muscle fibers it innervates, can be small or large
Small motor units
Have fewer muscle cells innervated per axon, found in muscles requiring fine motor control
Large motor unit
Can have several hundred muscle fibers per axon, found in large muscles for gross/powerful movements
Muscle fiber change
If the inner sting motor neuron changes from one type to another following nerve injury or changes in signal it produces
Following denervation
Myofibers undergo atrophy, flattened and angulated
Reinnervation
Restoration of fiber size and strength, but dennervated myofiber may be made a part of a different motor unit and switches fiber type
Fiber type grouping
Ongoing atonal or neuronal damage or drop out that leads to residual motor axons to innervate increasingly large numbers of myofibers, leads to enlargement of motor units, compromised of single type of muscle fiber, susceptible to group atrophy
Proprioreceptors
Sensory organs in skeletal muscle, tendons and joints
Muscle spindles
Found in all human skeletal muscle, consist of intramural muscle fibers and affront nerve endings
Muscle stretching
Stretches the enclosed spindle, stimulates nerve ending to stimulate the appropriate motor neurons to control stretched muscles, signals for the antagonistic muscle to relax at the same time
Golgi tendon organ
Bare sensory nerve ending in tendons to inhibit muscle contraction when it senses excess tension places on the tendon by a muscle, causes contraction of the antagonistic muscle group at the same time
