Muscle Flashcards
What is the method of excitation for skeletal muscle?
alpha motor-neurons
What is the method of excitation for cardiac muscle?
self-depolarization
The regular arrangement of ____ within the myofibrils gives them a striated appearance.
Proteins
Invaginations of the sarcolemma, called the____, extend into the cell itself.
Transverse-tubules (t-tubules)
The tunnels of the t-tubules provide a quick delivery system for the spread of ____ to the skeletal muscle cell.
Action potentials
Excitation of skeletal muscle requires the release of _____ from the sarcoplasmic reticulum.
intracellular calcium
The triad allows for a close coupling of what?
action potentials with intracellular calcium release
What is the contractile unit of a skeletal muscle cell?
sarcomere
Actin, thin filament, is composed of a globular protein called G-actin arranged into a double stranded helical structure called…
F-actin
What myosin light chain serves as a structural role?
alkali light chain
What myosin light chain is involved in regulating the ATPase capability of the myosin head?
regulatory myosin light chain
Filamentous protein that lies in the groove of the double-stranded actin.
Tropomyosin
What has a binding site for the myosin head, which is covered up by tropomyosin?
Each G-actin
Each tropomyosin extends over …
7 G-actin molecules
When calcium binds to the TnC subunit, this leads to …
a conformational change in troponin
The conformational change in troponin from calcium binding to TnC, allows ….
tropomyosin to move out of the actin groove
When tropomyosin moves out of the actin groove it …
exposes the actin active sites for myosin to bind to
What binds tropomyosin?
TnT
What is the function of TnI?
binds to actin, and inhibits actin and myosin from interacting
The central point of the sarcomere to which the thick filaments are anchored by associated proteins
M line
The anchor points for the thin filaments and the boundaries for the sarcomeres
Z-discs
A region that contains the entire length of the thick filaments
A band
A region that contains the portion of the thick filaments that does not overlap with the thin filaments
H zone
A region that contains the portion of the thin filaments that does not overlap with the thick filaments
I band
Structural proteins maintain a _____ of fibers within a sarcomere, with the thin filaments arranged around the thick filaments.
hexagonal cross-section
During a muscle contraction in which the sarcomere shortens, what will happen to the length from Z-disc to Z-disc?
decrease
During a muscle contraction in which the sarcomere shortens, what will happen to the length of the H zone?
decrease
During a muscle contraction in which the sarcomere shortens, what will happen to the length of the A band?
stays the same
During a muscle contraction in which the sarcomere shortens, what will happen to the length of the I band?
decrease
Converts the electrical signal of an action potential into the chemical signal of neurotransmitter.
Alpha motor neuron
The steps of excitation-contraction coupling occur where?
at the neuromuscular junction
In the first step of the ECC, the action potential spreads down the axon of the alpha motor neuron, leading to a …
depolarization of the end-bulb (fast-gated sodium channels)
In the ECC, the depolarization of the end-bulb leads to the opening of …
voltage-gated calcium channels, and calcium flows down its electrochemical gradient into the cell
In the ECC, the influx of calcium into the cell leads to the fusion of membrane-bound vesicles, which contain …
the neurotransmitter acetylcholine
The membrane bound vesicles containing acetylcholine, fuses with what?
The active zone of the plasma membrane
In ECC when the vesicles fuse with the plasma membrane, acetylcholine undergoes …
exocytosis and diffuses through the synaptic cleft
In the ECC, the neurotransmitter is released at an area of the muscle fiber called the …
motor end-plate
The binding of acetylcholine to its receptors on the motor end-plate leads to the opening of …
ligand-gated cation channels
What type of receptors does the acetylcholine bind to?
ligand-gated cation channels; nicotine cholinergic receptors
Acetylcholine is enzymatically digested in the …
synaptic cleft
The enzyme that digests acetylcholine is called
acetylcholinesterase
What portion of the digested ACh is typically taken up by the alpha motor neuron and can then be recycled to create new molecules of acetylcholine?
Choline portion
The binding of acetylcholine to ligand-gated receptors at the motor end-plate opens what?
non-specific cation channels
Both sodium and potassium pass through the non-specific cation channels, but because sodium influx is much greater than potassium efflux, …
the cell (motor end-plate) depolarizes
Why is it impossible to generate an action potential in the motor end-plate region?
There are no fast voltage-gated sodium channels
Instead of an action potential the motor end-plate generates an …
end-plate potential (EPP)
What is an end-plate potential (EPP)?
A localized electrical signal; an example of the excitatory post-synaptic potentials (EPSPs)
What sends electrical signals bidirectionally away from the motor end-plate to adjacent regions of the sarcolemma?
EPP
The electrically stimulated adjacent regions of the sarcolemma do contain … that can open once threshold is reached. This allows an action potential to spread through out the sarcolemma and t-tubules.
voltage-gated sodium channels
What occurs due to quantal release of ACh from the terminal button?
miniature end-plate potentials (MEPP)
The motor end-plate lacks …
a stable RMP
It is hypothesized that quantal release of ACh causing MEPPs occurs to maintain …
the integrity of the neuromuscular junction
How does the production of MEPPs maintain the integrity of the neuromuscular junction?
the quantal release of ACh may keep the ACh receptors localized to the motor end-plate
The depolarization from a MEPP is not of sufficient magnitude to …
depolarize adjacent regions of the sarcolemma to threshold
What are the 3 “safety” factors to ensure neuromuscular transmission?
- More ACh is released than needed to depolarize the motor end-plate.
- The MEP contains 4-12 times more ACh receptors than needed to ensure neuromuscular transmission
- The EPP magnitude is 3-4 times greater than required to initiate depolarization of the sarcolemma
Physostigmine is an anti-acetylcholinesterase, what is the direct effect on neuromuscular transmission?
Prolonged post-synaptic response to ACh
w-Conotoxin blocks voltage-gated calcium channels in nerves, what is the direct effect on neuromuscular transmission?
Prevents exocytosis of ACh
Dendrotoxin blocks voltage-gated K+ channels, what is the direct effect on neuromuscular transmission?
Prevents repolarization of presynaptic membrane and increases release of ACh
Tetrodotoxin blocks voltage-gated Na+ channels, what is the direct effect on neuromuscular transmission?
Prevents nerve and muscle action potentials
Tetanus blocks synaptic vesicle exocytosis in inhibitory interneurons, what is the direct effect on neuromuscular transmission?
Produces skeletal muscle spasms
Curare blocks the acetylcholine receptor, what is the direct effect on neuromuscular transmission?
Produces flaccid paralysis
In order for the action potential to initiate muscular contraction , …levels must rise.
intracellular calcium
The main source for calcium influx is via an organelle called the …
Sarcoplasmic Reticulum (SR)
The SR contains a calcium binding protein called …
calsequestrin
Calsequestrin helps to maintain a …
lower concentration gradient in the SR for free calcium
What serves as reservoirs of calcium within the cell?
Junctional SR regions/terminal cisterns
As the action potential spreads down the t-tubules, … in the sarcolemma are activated.
“L-type” calcium channels - dihydropyridine receptors (DHPR)
DHPRs serve as both … in skeletal muscle and voltage-gated calcium channels inn cardiac muscle.
voltage sensors
In skeletal muscle, each DHPR is linked to a calcium release channel in the membrane of the SR, called a …
ryanodine receptor (RYR)
As the wave of depolarization spreads along the sarcolemma and down the T-tubules, ____ are stimulated.
L-type calcium channels (DHPR)
DHPR stimulates the … in the SR to open.
ryanodine calcium release channels
Once _____ opens, calcium flows down its concentration gradient from the SR and into the cytosol (sarcoplasm)
Ryanodine Receptor (RYR)
Because there is no influx of calcium from the ECF, the form of calcium release is called…
Calcium channel induced calcium release (CCICR)
What form of calcium release is not essential in skeletal muscle, but used from cardiac muscle excitation?
Calcium induced calcium release (CICR)
What type of calcium release does require the influx of calcium rom the ECF?
CICR
In CICR, depolarization of the t-tubule stimulates the ____ to open, and calcium moves down its concentration gradient into the cell.
L-type voltage-gated calcium channels to open
The calcium in the cell directly stimulates the RYR to release calcium from the …
SR stores
T-tubules are located at the junction of the …
A and I bands
In ECC, calcium flows down its electrochemical gradient, from the SR into the sarcoplasm, where it binds to the regulatory protein _____.
troponin
The binding of calcium to TnC leads to a conformational change that moves _____ from the actin active sites.
tropomyosin
When calcium binds to TnC & tropomyosin is removed from the actin active sites, the myosin head is free to bind to the thin actin filaments to …
initiate contraction
In what state of the cross-bridge cycle does the myosin head use its ATPase capability to hydrolyze a bound ATP into ADP and phosphate?
Cocked state
In the cross-bridge cycle the hydrolysis of ATP allows the myosin head to “cock” and move into an appropriate binding position ….
further away from the M-line
With the ADP bound, the myosin head has a high affinity for ____
the actin filament
Cross-bridge formation can only occur if there is a …
high intracellular concentration of calcium
If there is a high intracellular concentration of calcium and tropomyosin has moved away from the actin groove, the myosin head …
binds to the actin filament, forming a bridge between the two filaments (cross-bridge state)
In what state of the cross-bridge cycle does the myosin head release its bound ADP and inorganic phosphate?
Power-stroke state
What powers the movement of the myosin head from a 90 degree angle to a 45 degree angle?
Energy from the ATP hydrolysis
In what state of the cross-bridge cycle does the myosin head remain attached to the actin filament?
Attached state
The binding of an ATP molecule to the myosin head allows the myosin to …
release its attachment to actin
With ATP bound, the myosin has a ____ for the actin filament.
low affinity
In what state of the cross-bridge cycle does the binding of ATP to the myosin head allow it to release from actin?
Released state
If ATP levels become severely depleted, cross-bridges will remain in the attached state leading to …
sustained skeletal muscle contractions (rigor mortis)
As long as ___ is present in the sarcoplasm, and ___ are plentiful, the contraction process will continue.
Calcium; ATP stores
In order to terminate muscle contraction, the cell must resequester calcium into the SR, this occurs largely through use of a …
Sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) pump
The SERCA pump moves calcium from the ____ into the ____
sarcoplasm into the SR
What form of transport does the SERCA pump use?
moves calcium against its concentration gradient using primary active transport
SERCA channels are also associated with a regulatory protein called …
phospholamban
When ____, phospholamban produces an increase in the activity of the SERCA pump.
phosphorylated
In the relaxation of skeletal muscle the regulatory protein phospholamban produces an increase in the activity of the SERCA pump, thus increasing the ….
Resequestration of calcium in the SR
In the relaxation of skeletal muscle when calcium is pumped into the SR, it is subsequently bound to …
Calsequestrin
Calsequestrin maintains a ____ concentration of calcium in the SR.
low free
When calsequestrin maintains a low free concentration of calcium in the SR it provides a ____ for the SERCA channel to operate against.
lower concentration gradient
Another function of calsequestrin, besides maintaining a low free concentration of calcium, is localizing calcium in the _____
Junctional SR
By calsequestrin localizing calcium in the junctional SR, it makes it available for easy release during …
CICR or CCICR
As SR calcium levels drop, calcium …., and tropomyosin ….; preventing further interaction between the myosin and actin filaments, until another excitation occurs.
calcium dissociates from TnC, & tropomyosin re-covers the actin active site
