Lecture 4-Skeletal Muscle Contraction Flashcards
A?
I-band
HI MA -> H-myosin only; I-actin only
B?
h-band
Contains both
C?
z-band
Anchors myosin (via titin)
Anchors actin filaments
D?
M-line: center of myosin filaments
E?
H-band: Myosin chain only
HI MA -> H-Myosin only; I-Actin only
What occurs (prior to the NMJ) to being contraction?
A.P. arrives at terminal bouton, opening voltage gated Ca2+ channels, Ca2+ influx causes NT vesicle to fuse with P.M.; ACh released into synaptic cleft
What is the result of N.T. binding to the motor end plate?
ACh binding to receptor causes ligand gated Na+ channel to open; Na+ influx produces A.P.
A.P. arrives at the ____ and opens the dihydropyridine-R (DHP-R)
T-tubule (invagination of the sarcolemma)
What occurs after depolarization of the DHP-R?
DHP-R interacts with Ca2+ channel on sarcoplasmic reticulum, prompting efflux of Ca2+ into the muscle cell
What is the f(x) of calcium release from the SR in muscle contraction?
Ca2+ binds to troponin-C; this produces a conformational changes to tryopomyosin, uncovering the active sites on actin
What “activates” the mysoin head?
ATP
ADP+Pi (after ATP hydrolysis) causes _______.
Conformational change to mysoin head; this results in binding of myosin head and A.S. on actin
When does the “powerstroke” occur?
With the loss of ADP from myosin head
What releases the myosin head from the actin active site?
Binding of ATP to myosin head
What is the change to cytosolic Ca2+ with opening of Ca2+ on the SR?
0.1 microMolar to 1 microMolar
How is the cytosolic [Ca2+] reduced?
Na/Ca2+ pump uses ATP to cause Ca2+ to be “taken up” into the SR
What is the conformation of DHP-R?
Voltage-gated L-type channel in quadruplets
What is the mechanism behind calsequestrin?
Binds Ca2+ in the SR, thereby setting up a favorable [Ca2+] gradient, making it easier to return Ca2+ to SR
How is myosin “reactivated?”
Hydrolysis of ATP-> ADP+Pi “cocks” the myosin head
Define “pre-load”
The load on a muscle while the muscle is relaxed (before contraction)
Define “afterload”
The load the muscle must work against
Isotonic contraction occurs when the muscle generates ____ force than the afterload
more
Isometric contraction occurs when the muscle generates ____ force than the afterload
less
Type of tension produced by preload
Passive
Type of tension produced by cross-bridge cycling
Active tension
What produces maximum tension in a muscle
Maximum overlap of actin/myosin (appx. 1.65-2.2 μm)
What is the relation between muscle load and contractile speed
Inverse (load proportional to 1/contractile speed)
3 places ATP required
- CROSS-bridge cycle
- pump Na/K out/in to reset RMP on SR
- pump Ca2+ back into SR
What molecule reconstituted ATP and can sustain contraction for 5-8 seconds?
Phosphocreatine
What E* stores are used in contraction (from shortest to longest duration)
- ATP (1-2 sec)
- Phosphocreatine (5-8 sec)
- glycolysis (1 minute)
- oxidative phosphorylation (long term contraction)
Isotonic contraction where the muscle lengthens is ______ contraction.
eccentric
Isotonic contraction where the muscle shortens is _____ contraction.
concentric
The _____ neuron determines the type of myofiber in the muscle.
Explain the mechanism behind the ability of white fibers to contract rapidy and fatigue easily
Fast twitch, or white fibers, lack large numbers of mitochondria. As a result, they are unable to undergo long term, oxidative metabolism. They primarily rely on ATP stores to contract. Because of their inability to undergo oxidative metabolism, they fatigue readily.
Fast, white fibers utilize what for energy?
Glycolytic reactions (rapidly depletes energy stores)
True or false: Muscle mass is increased by increasing the number of myofibers in the muscle.
False; muscle mass is increased by increasing the number of myofibrils
The neuron and the myofibers it innervates is known as a ______.
motor unit
Describe the mechanism behind “tetany.”
Prior to a muscle returning to a relaxed state, an additional action potential arrives prior to full Ca2+ sequestration. Additional Ca2+ is released, prompting more cross-bridging, which increases tension. Eventually, new APs arrive so quickly, that no relaxation occurs, resulting in “tetany.”
In a first class lever, the ____ is in the middle.
fulcrum
In a second class lever, the _____ is in the middle.
resistance (‘load’)
In a third class lever, the ____ is in the middle.
effort (‘in force’)
Type: 1st
A: out-lever
B: in-lever
Type: 2
A: out-lever
B: in-lever
Type: 3rd
A: in-lever
B: out-lever
