lecture 6 Flashcards
muscle contraction and relaxation
T/F, skeletal muscle is a voluntary muscle?
true
what part of the nervous system controls the skeletal muscle?
central nervous system
what is skeletal muscle composed of?
bundles of muscle fibers called fasciculus
what are the muscle striations the result of?
thick and thin filaments
Z line to Z ine represents what?
one sarcomere
the sarcomere is a _______ unit (fill in the blank)
contractile
this band contains thin filaments?
I band
what are the thin filaments called?
actin
what is the area between the two I bands?
the A band
what type of filaments comprise the A band and what are these called?
thick filaments; myosin
what does the dark area of the A band represent?
myosin and actin overlap
this area contains thick filaments (myosin) only?
H band
where do the actin fibers extend?
from the Z line to the edge of the H band
what do actin filaments over lap with?
they overlap with myosin (thick filament) in the A band
what happens to the A bands upon muscle contraction? the I bands? H zone? Z lines?
the A bands do not change their length; the I bands and H zone shorten which results in the Z lines coming closer
cardiac muscles have _____ synapses?
T/F, these synapses are used to initiate cardiac function?
chemical; F, modulate NOT initiate
what triggers cardiac muscle?
electrical signals from the neighbor cells
where are the cardiac electrical signals made?
in the SA node
the SA node is the _____ pacemaker of the heart?
internal
what carries out the electrical communications between cells?
the gap junctions found in the sarcolemma of cardiac muscle to maintain the action potential with neighboring cells
what controls the skeletal muscles?
somatic nervous system controlled by the CNS with efferent signals
how many muscle fibers can one nerve innervate?
multiple muscle fibers
how many innervations can one muscle fiber have?
one innervation
what is the importance of the way the skeletal muscles are innervated?
they prevent confusion
describe the tranverse tubules of the skeletal muscles?
they are the extensions of the plasma membrane that penetrate the muscle cells at two points in each sarcomere: the junctions of the A and I bands
what is the effect on the muscle cells as the skeleton grows?
muscle cells lengthen by forming more sarcomeres at the ends of muscle cells
what happens to the limb with the muscle in a shortened position? the muscle cell?
its immobilized; the length of a cell decreased with sarcomeres at the ends eliminated
what is the effect of the changes in muscle length?
it affects velocity and extent of the shortening but not the amount of force
what is muscle hypertrophy?
slow increase in strength and diameter of a muscle like working out in the gym
what is muscle hyperplasia?
skeletal muscles have limited ability to form new muscle fibers, like being immobilized in a cast
what is the effect of being immobilized in a cast do to the body?
loss of muscle mass (atrophy) and weakness
what are the two ways synaptic input to smooth muscle differs from skeletal muscle?
1) the neurons are part of the autonomic nervous system rather than the somatic nervous system.
2) the neuron makes multiple contacts with a smooth muscle cell.
also;
1) Smooth muscle cell may get input from more than one neuron (this will never happen in a skeletal muscle cell)
2) No end plate in smooth muscle
how is the smooth muscle, unitary? examples?
Gap junctions allow electrical communication between the cells → coordinated contraction of many cells as a single unit;
Example: GI tract, uterus, blood vessels
what are the invaginations on the smooth muscle called?
caveoli
what triggers muscle contraction?
increase in Ca2+, so the time that Ca2+ remains elevated determines the duration of muscle contraction
what is excitation contraction coupling?
The process by which “excitation” triggers the increase in [Ca2+] by removing the the inhibition of cross bridge cycling
once the calcium comes in, where do they bind?
the troponin molecule
what comprises the troponin?
heterotrimer consisting of Troponin C, Troponin T, Troponin I
what does troponin C do?
binds to calcium then moves the tropomyosin away from the binding sites, myosin binding can happen and can have the cycle
what does troponin T do?
binds to a single molecule of tropomyosin
what does troponin I do?
binds to actin and inhibits contraction
why is the “heterotrimeric” troponin molecule important?
it contains the key Ca2+ sensitive regulator troponin C.
what does the troponin C in the skeletal muscle have that helps in the binding of troponin C to the thin filament?
two high affinity Ca2+ binding sites
Ca2+ binding to the high affinity Ca2+ binding sites of the troponin C does not change during muscle contraction T/F?
T
in addition to the high affinity binding sites of the troponin C molecule, there are also two additional low affinity Ca2+ binding sites T/F?
T
what are the two effects of Ca2+ binding to these low affinity sites besides inducing a conformational change?
1) the C terminus of the inhibitory troponin I moves away from the actin/tropomyosin filament, thereby permitting the tropomyosin molecule to move
2) transmitted through troponin T, is to push tropomyosin away from the myosin-binding site on the actin and into the actin groove.
With the steric hindrance removed, the myosin head is able to interact with actin and engage in cross-bridge cycling.
Name the 5 steps of Cross Bridge Cycling
1) ATP binding
2) ATP hydrolysis
3) Cross Bridge formation
4) release of the Pi from the myosin
5) ADP release
describe step 1 of the cross bridge cycle
- ATP binds to myosin head → dissociation of actin-myosin complex
● ATP binds to heavy chain on myosin head → reduces the affinity of myosin for actin
● If all cross bridges in a muscle were in this state → muscle is fully relaxed
describe step 2 of the cross bridge cycle
- ATP is hydrolyzed causing myosin heads to return to resting position
● Breakdown of ATP occurs in myosin head
● After breakdown, myosin head is in “cocked position” → perpendicular to the filaments
● The change in position causes the myosin to move 11 nm along the thin filament
describe step 3 of the cross bridge cycle
- Cross-bridge forms and the myosin head binds to a new position on actin
● The “cocked” myosin head binds to a new position on the thin filament
● This binding shows that there is an increased affinity of the myosin-ADP-Pi complex for actin
describe step 4 of the cross bridge cycle
- P is released. Myosin heads change conformation → power stroke. The filaments slide past each other
● P leaves the myosin head → power stroke → myosin head bends 45 degrees → actin filament moves 11 nm towards the tail of myosin → force and motion is generated
describe step 5 of the cross bridge cycle
- ADP is released
● ADP leaves myosin head → actomyosin complex is left in a rigid state → myosin is stuck in the 45 degree position
● Myosin stays on the actin until another ATP binds to it
how far does each cycle move the myosin head?
two actin monomers or 11 nm
how much ATP do you need for the entire cycle?
1 ATP for the entire cycle and another ATP for release
the cross bridge cycling is what type of movement?
rowing movement
why is Rigor Mortis important to note? What happens? what stays attached?
■ A dead person can not make ATP → there is no ATP to separate the myosin from the actin → muscle is left in a rigid state
how does excitation-contraction coupling happen in smooth muscle?
1) Ca2+ enters the cytoplasm through channels located in caveoli
2) Ca2+ release from the sarcoplasmic reticulum can occur either via Ca2+ induced Ca2+ release or more importantly via IP3 activation of SR Ca2+ channels
3) When the SR Ca2+ store become depleted, the SR signals - by an unknown mechanism - a store operated Ca2+ channel to open allowing Ca2+ to enter
what is the cause of fatigue?
metabolic byproducts are the onset of fatigue
***what is not the result of fatigue?
depletion of energy stores
in terms of onset fatigue, what happens during intense exercise?
P and lactic acid buildup → decrease pH → inhibits actin-myosin interactions
are ATP levels decreased a lot during intense exercise?
not by a lot
because its noted that there is a decrease in pH during fatigue, what is this effect?
● The decrease in pH reduces the sensitivity of the actin-myosin interaction to Ca by altering Ca binding to Troponin C → decreased the number of actin-myosin interactions
Regardless of whether the muscle is fatigued as a consequence of high-intensity exercise or prolonged exercise, the myoplasmic ATP level does not decrease substantially, T/F?
T
why does fatigue happen?
a protective mechanism to minimize the risk of muscle cell injury or death.
whats isometric contraction?
when there is no change in muscle length and force generated in measured
whats isotonic contraction?
when there is no change in force and the length of muscle is measured
