Muscle 1 Flashcards
Semester 1 year 1
What is skeletal muscle responsible for?
-voluntary movement of bones
-control of inspiration
-skeletal muscle pump
Describe the appearance of skeletal muscle
Striated
Describe the structure of skeletal muscle
-muscle cell/fibre surrounded by endomysium
-muscle cells come together to make fascicles surrounded by perimysium
-muscle units made up of many fascicles surrounded by the epimysium
-myofibrils run through muscle cells + are made up of sarcomeres
What are sarcomeres made up of?
-thick filaments (myosin)
-thin filaments (actin)
How do the I and A bands appear and why?
-I band appears light - predominantly actin
-A band appears dark - overlap of actin and myosin
Where is the Z line/disc in the sarcomere?
At the ends
What are the 2 other proteins in the sarcomere and what do they do?
-nebulin protein is linked around actin
-titin protein anchors myosin to z disc
When the muscle contracts, what changes in the sarcomere?
Actin is pulled inward, so the I band becomes smaller and the A band stays the same
How is muscle contraction initiated?
-ACh released at neuromuscular junction causing action potential in plasma membrane of muscle fibre
-depolarisation passes along sarcolemma + through T-tubule network to cell interior
-T tubule runs near 2 areas of sarcoplasmic reticulum, forming a triad (skeletal muscle)
-depolarisation causes increases in intracellular calcium
How many actin filaments are around myosin?
6
How are cross-bridges formed?
-myosin head is tightly bound with actin
-ATP binds to myosin head, weakening its binding to actin, so it dissociates
-ATP is hydrolysed, causing myosin head to return to resting configuration
-myosin head binds loosely to new actin molecule, forming a cross-bridge
How does the sarcomere contract?
-after cross-bridge is formed, phosphate is released, causing a strong association between actin + myosin
-myosin head changes from open to closed position, pulling actin towards middle of sarcomere
-ADP released, returning head to original position
How can the myosin head attach to a new actin molecule further down the filament?
When the myosin head returns to resting configuration, it’s in an open position, lengthening it
What happens to calcium ions after contraction?
Taken out of cytoplasm, so muscle goes into relaxation period
What happens to skeletal muscle at an increased frequency (10 Hz)?
-muscle doesn’t fully relax before next contraction
-stronger contraction
-temporal summation
What happens to skeletal muscle at 25 Hz?
-unfused tetanus
-contraction force plateaus
What happens to skeletal muscle at 50 Hz?
-fused tetanus
-not time for calcium to be taken out of cytoplasm
-constant generation of force
What are the 3 classes of muscle fibres?
-type I - slow oxidative
-type IIa - fast oxidative
-type IIb - fast glycolytic
Are the classes of muscle fibres fast or slow?
-type I - slow
-type II - fast
Are slow fibres larger or smaller, and do they take longer to contract?
-slow fibres are half the diameter of fast fibres
-take longer to contract
Describe the speed and force of the 3 classes of muscle fibre
-type I - react slowly + generate a low level of force
-type IIb - react quickly + high level of force
-type IIa - in between type I and IIb
Describe the unfused tetanus force + fatigability of the 3 muscle fibre classes
-type I - muscle doesn’t tire, continually generates the same force
-type IIa - generates a large force, sustains for a while but drops off quickly
-type IIb - generates a large force, drops off quickly
What is the difference between isometric and isotonic muscle contraction?
-isometric - muscle remains at fixed length
-isotonic - muscle stimulation causes a change in length
Describe how transmission occurs at the neuromuscular junction
-action potential enters the neuron
-sodium channels open + sodium moves in, causing depolarisation
-activates calcium channels + calcium moves in
-causes fusion of vesicles containing ACh with membrane
-ACh released unto synaptic cleft + stimulates AChR receptors
-depolarisation in muscle, so muscle sodium channels open + wave of depolarisation can continue
What does acetylcholinesterase do?
Breaks down ACh to prevent overstimulation of muscle
Why do potassium ion channels open in the neuron at the neuromuscular junction?
Potassium ions move out, repolarising neuron, so new wave of depolarisation can occur
What are the V snares and T snares and where are they?
-V snare in vesicle - synaptobrevin
-T snares in membrane - SNAP-25 + syntaxin
What is the function of T snares and v snares?
Complex forms between them, causing the vesicle containing ACh to fuse with the membrane and release ACh into cleft
How do botulinum toxins act as inhibitors?
Botulinum toxins cleave V and T snares, so vesicle doesn’t fuse with the membrane, so ACh not released
Which types of botulinum toxin cleave which snares?
-A,C,E - SNAP-25
-B,D,F,G - synaptobrevin
-C - syntaxin
