Physiology Of Skeletal Muscle Flashcards
Why must resting muscle membrane potential exist for an action potential to occur?
AP is the change in voltage, if the action potential does not exist, the voltage cannot be changed/depolarized
Action potential and release of neurotransmitter chemical is controlled by
Nervous system
What do ligand gated ion channels bind to in postsynaptic membrane?
Neurotransmitter chemical, Ach
In the post synaptic membrane, what ion does the voltage gated ion channels allow to cross?
Na+
What are the two stages of action potential
Depolarization and repolarization
What event makes the membrane less polar (<-85mV)
Depolarization
What mark does depolarization need to reach in order to propagate an action potential
Threshold
Define repolarization
Return to resting membrane potential
Briefly describe how Ach and Na+ work to depolarize a myofiber
Ach is released from presynaptic terminal in response to an AP.
Ach binds to Ach receptor site on ligand gated ion channels.
This allows Na+ to enter the cell (polarity is changed slightly)
Once the polarity is changed enough by the Na+, voltage gated Na+ channels open up and allow more Na+ to enter the cell
This initiates an AP that is propogated along the membrane of the T-tubules.
What ion enters the presynaptic terminal to trigger the release of Ach by synaptic vesicles and how does it enter the cell?
Ca++ enters the cell when an AP reaches the presynaptic terminal and opens voltage gated Ca++ channels
What is the definition of excitation-contraction coupling
Mechanism where an action potential causes muscle fiber contraction
Describe a triad
A triad consists of a t-tubule, and it’s two adjacent terminal cisternae
What organelle has specialized Ca ion release mechanisms
Sarcoplasmic reticulum, terminal cistern are
Describe how the action potential of the t tubule encourages muscle cntx
The terminal cisternae are so close to the t tubule that the AP propogated along the membrane of the t tubule opens up voltage gated Ca++ channels in the terminal cisternae.
The Ca++ then binds to the troponin which reveals the active sites and allows myosin/actin cross bridges to form.
How does the Sarcoplasmic reticulum have so much Ca++
An active transport pump works continuously to stockpile Ca++ inside the terminal cisternae
Which chemical does Ca++ bind to on the actin myofilament?
Troponin
How does Ca++ binding to the troponin allow muscle cntx?
When Ca++ binds to the troponin, it slides the troponin-tropomysin complex into the groove of the g-actin chains and the active sites are revealed where cross bridges can form
Name the enzyme connected to the myosin heads that breaks ATP apart to cock the head
ATPase
each actin myofilament is composed of
2 strands of fibrous actin (f actin)
a series of tropomyosin molecules
a series of troponin molecules
how are the strands of f actin arranged?
double helix
each f actin strand is a polymer of
globular actin (g actin)
each g actin monomer has a(n) __________ to which myosin molecules form crossbridges
active site
what molecule covers seven G actin active sites
tropomyosin
troponin is composed of three subunits, what do they bind to?
one binds to actin
one binds to tropomyosin
one is a binding site for Ca++
myosin molecules are shaped like
golf clubs
each myosin molecule is composed of
two myosin heavy chains wound together to form a rod portion
two myosin heads that extend laterally
4 myosin light chains attached to the head of each myosin head
the heads of the myosin myofilaments are _____ enzymes that break down _______
ATPase
ATP
what changes the permeability of SMF
opening and closing of voltage-gated ion channels
repolarization results from the closing of _____________ and the opening of ____________ so that ____ stops moving into the cell and more ____ moves out of the cell
v-gated Na+ channels
v-gated K+ channels
Na+
K+
describe the all-on-none principle in regards to APs
a stimulus strong enough (or even much stronger) to reach threshold, all the permeability changes happen and an AP is produced; if the stimulus is too weak to reach threshold, none of the permeability changes happen and the membrane returns to the resting membrane potential without producing an AP
action potentials propogate by
triggering neighboring APs, not by actually traveling along the plasma membrane
action potential frequency
the number of APs produced per unit of time
AP frequency increases as the strength of the stimulus
increases, once its reached threshold
what ion crosses the membrane via of the presynaptic terminal to initiate the release of synaptic vesicles
Ca++
what causes Ca++ to move into the presynaptic terminal
an AP that was propagated down the axon opens voltage gated Ca++ channels so the Ca++ can diffuse into the cell
what keeps Ach from accumulating in the synaptic cleft?
acytelcholinesterase breaks Ach down into choline and acetic acid
how does acetylcholinesterase ensure that one presynaptic AP yields only one postsynaptic AP
the rapid break down of acetylcholine
what molecules are actively reabsorbed by the presynaptic terminal?
choline
reabsorbed choline is combined with acetic acid in the presynaptic cleft as a recycling mechanism to create more
acetylcholine
excitation-contraction coupling involves
sarcolemma
triad
Ca++
troponin
describe the process of cross-bridge cycling
the myosin molecules goes through the cycle of cross-bridge formation, movement, release and return to the original position many times
how many ATP molecules is required for each mysoin head per each cycle of cross-bridge cycling?
1
how does the myosin head utilize ATP
ATP binds to the myosin head which causes the cross-bridge to break. it breaks it down into P and ADP using the ATPase, returns to cocked position, and it stores the energy from the breakdown to create the power stroke to slide the actin over the myosin if the active sites are still uncovered
movement of the myosin head while the cross bridge is attached is called the
power stroke–will slide the actin molecule
when Ach is no longer released at the NMJ, the muscle cntx stops. why?
Na+ movement across the membrane ceases so the APs cease. No AP means no release of Ca++ from the terminal cisternae. No Ca++ available to bind to troponin means G actin active sides are covered up again by tropomyosin and cross bridges can no longer be formed.
ATP is required to actively transport which ions in order to return to resting membrane potential?
Na+ and K+
How is ATP used to create muscle contraction?
sodium-potassium pump requires ATP to pump Na+ and K+ out of the cell to return to resting membrane potential
required to detach myosin heads from active sites and return to resting position
active transport of Ca++ into sarcoplasmic reticulum
why does a muscle fiber take at least twice as long to relax as it does to cntx?
reuptake of Ca++ is slower than its release
why is the placement of triad in SMF significant
it allows Ca++ to be released from terminal cisternae immediately adjacent to the actin/myosin overlap
describe muscle twitch
response to a stimulus that causes AP in one or more muscle fibers
what are the three phases of a muscle twitch
lag (or latent phase)
contraction phase
relaxation phase
a motor unit is
a single neuron and all the muscle fibers it innervates
motor neurons that innervate only a few muscle fibers probably perform
delicate movements (ex. those that move the eye)
fluid movements of skeletal muscle happens because
motor units vary in how sensitive they are to stimuli for cntx
each motor unit responds in a(n) _________ fashion
all-or-none
what causes a whole muscle to increase the amount of tension created?
a larger number of motor units is stimulated
multiple-wave summation results when
multiple APs are produced in a SMF
when there is not enough time to relax in between each additional AP causes release of Ca++ from the SR, ion levels in sarcoplasm remain elevated and this produces a
tetanic cntx
is the tension produced by a multiple-wave summation greater than or less than the tension produced by a single muscle twitch?
greater
why is there increased tension produced by multiple-wave summation?
greater sarcoplasmic concentration of Ca++
treppe is the tension produced in response to
a maximal stimulus at a low frequency
describe treppe
a muscle is stimulated at a low frequency and (assuming all stimuli are maximal stimulus) the tension produced increases
incomplete tetanus occurs when the AP frequency is low enough to allow
partial relaxation in between
complete tetanus occurs when the AP frequency is high enough that
no relaxation of the SMF occurs
isometric cntx
occur when muscle remains the same length but tension is still produced (as postural muscles that maintain a tension but do not change their length)
isotonic cntx
a muscle produces constant tension and shortens during cntx (as in finger and hand movements)
describe the muscle cntx in a biceps brachii curl
a concentric cntx where muscle produces tension as it shortens
in eccentric cntx, a muscle produces _____ as it ________ lengthening
tension as it resists as in slowly descending a flight of stairs
what is encompassed in the lag phase of the muscle twitch
NTC release by presynaptic terminal depolarization of the sarcolemma terminal cisternae releases Ca++ Ca++ binds to troponin/tropomyosin complex binding sites are exosed cross-bridges form
cross-bridge cycling happens in which phase of muscle twitch
cntx phase
Ca++ _______ to SR, cocked myosin heads ______ to bind on active site and sarcomere returns to _________ are all part of the relaxation phase of muscle twitch
returned
unable
original, resting length
submaximal stimuli
stimuli that is above threshold but not yet at maximal stimuli
maximal stimulus
the stimulus strength that gets the greatest tension out of the muscle
supramaximal stimulus
stimulus strength that is above that of the maximal stimulus, but still doesn’t create more tension in the muscle
subthreshold stimulus
not enough stimulus to cause a local AP that is big enough to cause an AP
active tension
the force applied to an objet to be lifted when a muscle contracts
passive tension
tension applied to the load when a muscle stretches but is not stimulated–tension exists because the muscle and its connective tissue have some elasticity
when does a muscle create the highest tension
when the actin and myosin are at optimal overlap
why is stretching good for muscle tension
it creates proper alignment between actin and myosin and allows most optimal tension
why is less tension created in stretched muscle?
not enough cross-bridging allowed to happen
why does crumpled muscle create less tension?
myofilaments are already contracted together and can’t contract further
describe muscle fatigue
decreased capacity to work and reduced efficiency of performance
what are the three types of muscle fatigue
psychological
muscular
synaptic
which type of muscle fatigue results from ATP depletion
muscular
psychological muscle fatigue depends on what
the emotional state of the individual
if a presynaptic terminal can’t recycle Ach, this would cause what type of muscle fatigue
synaptic
physiological contracture occurs due to
lack of ATP and neither cntx or relaxation can occur
development of rigid muscles several hours after death
rigor mortis
