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Skeletal muscle


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what does skeletal muscle do?
causes the bones to which muscle is attached to move.
What does skeletal muscle allow?
-body movement
-heat generation - regulation of homeostatic systems.
how is skeletal muscle organised? describe...
in a hierarchical system.
Muscle > muscle fiber > myofibrils > thick/thin filaments
what is a muscle cell?
another name?

how long?
a myofibril

as long as the muscle.
what are each made of?
thick = myosin
thin = actin
what is the nucleation like in skeletal muscle?
multinucleated, from fusing of many cells
what is a
-Z line
-M line
Z = disc-shaped protein that holds thin filaments aligned.

M = disc-shaped protein that holds thick filaments aligned.
what are thick filaments made of?
several hundred myosin proteins
how is myosin arranged?
tail = light meromysin
head = heavy meromysin
What two parts compose Heavy meromysin?
S1 and S2 chains
S1 = light chains, head portion.
S2 = the hinge.
What are the S1 regions of heavy meromysin?
what is the light meromysin composed of?
2 myosins coiled together in a tail.
what are the two binding sites on the crossbridge of myosin?
-ATP binding site
-Actin binding site
What binding site does Actin have?
Myosin binding site.
What other two molecules are with Actin?
Troponin and Tropomyosin
what is the role of
Troponin: the coordinating molecule - binds everything!

Tropomyosin: covers the myosin bindng site in resting states.
what is the sliding-filament theory?
just the statement that during sarcomere shortening, filaments move past ea. other, but don't shorten.
which bands in the sarcomere get reduced during contraction?
-I band narrows
-H band disappears
how is the ATP binding site on myosin an enzyme?
-it hydrolyzes atp when bound
what is the crossbridge cycle?

how many steps?
the events that occur btwn cross-bridge binding to thin filament, its movement, and resetting to repeat.
-four steps
What are the four steps in the cross-bridge cycle?
1. Attachment - crossbridge to thin filament.
2. Movement - thin fil tenses
3. Detachment
4. Re-energizing
What is the myosin crossbridge like in a resting muscle?
-cocked and ready to go; ATP is split and ADP and Pi are still bound.

-Cytosplasmic Ca+ is low tho so no binding to actin.
What molecule initiates crossbrg cycling?
CALCIUM!!! in the cytosol
what is a Power Stroke?
the movement caused when energized myosin crossbrg (with ADP and Pi) binds actin. immediately releases energy and causes movement.
What breaks the link between myosin and actin? How?
ATP binding decreases myosin's affinity for actin.
What happens when ATP binds to myosin?
Hydrolyzes and re-energizes myosin.
-If Ca still present, another cycle.
What are ATP's two roles?
1. Hydrolysis provides energy for movement

2. Binding allosterically regulates myosin to decr. actin affinity.
What is rigor mortis?
stiffening of dead body because ATP is not present to unbind myosin from actin.
what is tropomyosin, and what does it do?
rod like protein that binds 7 actin molecules, blocking their myosin binding sites.
what is troponin, and what does it do?
small globular protein that binds tropomyosin and actin; when Ca+ binds actin, it drags tropomyosin away and allows actin to bind myosin.
What is excitation-contraction coupling?
the link between action potential excitation in a muscle fiber's membrane, and how it causes a crossbridge cycle.
In general, how does an action potential cause a crossbrg cycle?
by increasing cytosolic calcium
what is the cytosolic [Ca] in a resting muscle?
very low = 10-7
what is the source of cytoplasmic calcium when excited?
sarcoplasmic reticulum
what are lateral sacs?
what are they for?
-enlarged regions of the SR sheaths around myofibrils;
-store calcium for release during excitation.
what are T-tubules?
structures that are between SR sheaths around myofibrils; they circle at the A-I junction.
-continuous with plasma membrane; conducts Action potentials to the cell center.
what are the functions of the sarcoplasmic reticulum?
-take up Ca
-store Ca
-release CA
how does a T-tubule cause calcium release?
action potential conducts through the T-tubule, to where it contacts Lateral sacs of SR, which release calcium into the cytosol.
how is contraction stopped by the SR?
Calcium ATPase pumps take calcium back up - takes a while though, so contraction lasts a long time.
How can contraction strength of skeletal muscle be controlled?
(3 ways)
1. By motor unit recruitment
2. By summation and tetany
3. By changing the initial muscle fiber length.
what kind of contraction is caused by ONE action potential?
a twitch - very small contraction.
how do you get more than a twitch?
by motor unit recruitment:
-making more motor units fire.
-making more muscle cells contract.
what prevents fatigue over time in muscle?
asynchronous firing
What is asynchronous firing?
Variation in which muscle cells are firing during a contraction. 10 might be on, 10 off.
What is summation?
the concept that repetitive action potentials cause stronger and longer contractions.
How are contraction strengths varied?
By varying the number of AP within a given period of time; very fast/repetive firing causes summation so much that the cell doesn't even relax.
What is tetani?
fused contraction of muscle due to rapid firing of neuron - continuous action potentials - cytosolic Ca2+ is always high.
How does changing initial muscle fiber length control muscle strength?
If muscle is too short to begin with, less force will be generated. If too long, no crossbridges interact.
what is the optimal crossbrdg overlap for muscle contraction?
between 1.95 um and 2.25
-flat line because between these points, the H band will change no further.
what are two types of skeletal muscle contractions?
what is isometric contraction?
That when muscle length stays the same.
-Load > force developed
-Force increases but you're not moving anything yet.
what is isotonic contraction?
that when muscle length changes because force > load.
-Force stays constant as the muscle contracts/relaxes.
what are 3 things that affect skeletal muscle energetics?
-ATP source
-Oxygen debt
-Type of skeletal muscle
what are 3 sources of ATP?
-Creatine phosphate
-Oxidative phosphorylation
How does Creatine phosphate generate ATP?
-What is ATP supplied for?
-How fast?
by phosphorylating ADP to produce Creatine and ATP.
-Supplies for FAST movement.
-1 sec needed for reaction.
CP + ADP -> ?
Creatine + ATP
What generates ATP for endurance movement? Why?

(when O2 is present)
-Oxidative phosphorylation - aerobic metabolism.
-B/c 36 ATPs are generated from every 1 glucose.
-What can be used for oxydative phosphorylation to generate ATP?

-What is required for Oxid phos?
Fatty acids OR Glucose.

-Requires Oxygen and TIME -slow.
How much ATP is generated when no oxygen is present?
By what process?
Is this for fast or endurance movement?
-2 ATP + Pyruvate (breaks to lactic acid) from one glucose.
-Glycolysis - anaerobic metabolism.
-fast - no enzymatic steps.
What is better for ATP generation, Creatine phsosphate or glycolysis? Why?
Creatine phosphate - much higher yield, more efficient.
-Glycolysis generates 2 ATP but pyruvate has to be broken down to lactic acid.
what are 3 types of skeletal muscle based on their metabolic processes?
SOX - slow oxididative
FOX - fast oxidative
FGL - fast glycolytic
what is
-red meat
-dark meat
red = slow oxidative/fast oxid.
white = FGL - fast glycolytic.
where would fast oxidative muscle (type 2) be found?
in your arms.
which type of tissue resists fatigue the best?

whihc has least mitochondria?
slow oxidative

fast glycolytic
what type of meat do sprinters have more of?
white meat = fast glycolytic
what type of meat do cross country runners have?
slow oxidative
If a sprinter trained to run long distance, what type of meat would develop?
His type 3 fast glycolytic would become type 2 fast oxidative.
on the force vs. length curve, what is the Optimal length?
the length at which max isotonic force is developed - about 2 um.
what effect does stretching a muscle beyond optimal length have on force developed?
decreases it. b/c thick and thin filaments are overlapping and so fewer slide during the crossbridge cycle.
what effect does contracting a muscle shorter than its optimal length have on force development? what two reasons?
decreases it.
1. the z lines run into the thick filaments.
2. Thin filaments collide and no more contraction can occur - or they overlap and interfere with crossbg forming

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