Muscles Flashcards
What subtype of muscle is the skeletal muscle?
Striated
How are skeletal muscles organized?
- Epimysium (dense irregular connective) surrounds the perimysium, blood vessels and nerves
- perimysium surrounds each fascicle (dense elastic connective tissue)
- each fascicle has 20-60 muscle fibres
How are myofibers organized?
Endomysium surrounds myofibrils
What are myosatellite cells?
Cells that help repair damaged myofibres
How do myofibres produce multiple nuclei?
Many myoblasts fuse together
What is the descending order of muscle organization?
Epimysium>perimysium>endomysium>
Muscle>fascicle>myofibre>myofibril
What are the names for the muscles’ plasma membrane, endoplasmic reticulum, and cytoplasm
Sarcolemma, sarcoplasmic reticulum, sarcoplasm
What two structures do the T-tubules connect?
Sarcolemma and sarcoplasmic reticulum
Where do motor neurons contact myofibres?
NMJ (motor end plate + synaptic cleft + axon terminal)
What is the transmembrane potential of a myofibre at rest?
Slightly more negative on the inside of the sarcolemma than outside (-85mV)
What are the concentrations of sodium, potassium and calcium, large anions and chloride inside and outside the cell?
Na: ICF=12 ECF= 145 (more + outside the cell)
K: ICF= 139 ECF= 4 (more + inside the cell)
Cl-: ICF=4 ECF=120 (more - outside the cell)
A-: ICF=140 ECF= 0 (more negative inside the cell)
Ca2+: ICF=0.0002 ECF= 1.8 (more positive outside the cell)
At rest, which ion can move through ion channels?
Potassium
What do motor neurons release?
Neurotransmitter (ACh) that allows the sodium ions into the myofibre
What kind of receptors are ACh receptors?
ligand-gated ion channels—only lets in ions if ach is bound to outer surface
What are the steps of muscle cell excitation?
- Electrical activity in the motor neuron triggers release of vescicles containing NT into synaptic cleft
- NT binds to the receptor proteins, which open and allow Na+ into the cell
- Enzymes in the extracellular space (AChE) get rid of the neurotransmitter
What is the effect of Curare (interferes with ACh binding to the ACh receptors)?
What its the effect of novichok (blocks enzyme that breaks down ACh in the synaptic cleft)?
Curare: flaccid paralysis because muscle cannot depolarize
Novichok: rigid paralysis: causes spasms/seizures
How does depolarization spread through the NMJ?
By electro tonic current
What does the spread of depolarization of the motor neuron trigger? What does this do?
Voltage gated sodium channels, enhances depolarization
What structure brings the plasma membrane action potential into the vicinity of the sarcoplasmic reticulum? What does this do?
T-tubule; triggers release of calcium ions in the sarcoplasmic reticulum
What proteins are involved in transducing the electrical signal into the release of stored Ca2+ from the sarcoplasmic reticulum?
DHPR: dihydropyridine receptor changes its shape
RyR: ryanodine receptor: reacts to the movement of the DHPR by opening an ion channel to let calcium flow
How can targeted electrical stimulation make your muscles contract? Why does this increase muscle mass?
- the electrical signal is hitting the nerve
- work causes microtears and doing more work creates signals for muscle fibrils to grow width wise and get more myosatellite cells
What are thin filaments connected by and what is it made out of?
The Z line; actinin
What is the thin filament made out of?
F-actin: twisted strand of G-actin
Nebulin: a protein that extends along the F-actin strand in the cleft— holds together
Tropomyosin: double-stranded molecule that is bound to troponin
Troponin: three globular subunits
What is the function of tropomyosin?
Covers the active sites of G-actin
What are the functions of the three troponin subunits?
- One subunit binds to tropomyosin
- One subunit binds to actin
- One subunit binds to 2 calcium ions
What is the structure of the thick filaments?
Myosin molecules, each made of myosin subunits twisted around each other
What is the sliding filament theory?
1) H bands and I bands get smaller
2) the zones of overlap and get larger
3) Z-lines move closer together
4) width of A bands remain constant
Where are the H, I and A bands?
H: the middle section of the thick filaments
A: entire thick filament, including zone of overlap
I: thin filament region
Where does a sarcomere begin and end?
From one Z line to the next Z line
During contraction, what gets shorter?
The sarcomere
The thin filament is made out of ________ and the thick filament is made out of ___________
Actin, myosin
What structure attaches the thick filament to the Z-line?
Titin, a large elastic protein
What does calcium bind to that allows the myosin to bind to actin?
Calcium binds to troponin and reveals the binding site on the G-actin
What are the steps of the power stroke?
- Myosin binds to binding site on actin, neck changes conformation
- ATP breaks the bond by binding to myosin neck
- After myosin detaches, ATP is hydrolyzed and the neck position is reset
What would happen if a muscle fibre ran out of ATP?
- calcium would build up in the sarcoplasm because ATP is needed to operate Ion pumps that remove Ca2+
- myosin heads would not be able to reset
What determines the amount of tension produced by a muscle?
- The number of myosin heads that are able to bind to actin in a contraction cycle
- the number of fascicles in the muscle
What are the three phases of a single muscle “twitch” contraction
1) latent period: time it takes for excitation of MN to release ACh, trigger AP and spread to T tubules
2) contraction phase: increasing tension during contraction is due to Ca build up
3) relaxation: decrease in tension during the relaxation phase is due to Ca being removed from the sarcoplasm
What is a motor unit?
The motor neuron and ALL the muscle fibres it contacts
How many NMJ’s does each muscle fibre have?
One
What is asynchronous recruitment?
Skeletal muscle contraction must be maintained for an extended period of time, motor units rotate so that each unit is only active part of the time
