Muscle Physiology


Sliding Filament Theory of Muscle Contraction: The theory that the thick and thin filaments of a myofibril slide past each other during muscle contraction, decreasing the length of the sarcomeres but maintaining their own initial lengths.

1. A myofiber, together with all its myofibrils, shortens by movement of the insertion toward the origin of the muscle.

2. Shortening of the myofibrils is caused by shortening of the sarcomeres-the distance between Z lines (or discs) is reduced.

3. Shortening of the sarcomeres is accomplished by sliding of the myofilaments-the length of each filament remains the same during contraction.

4. Sliding of the filaments is produced by asynchronous power strokes of myosin cross bridges, which pull the thin filaments (actin) over the thick filaments (myosin).

5. The A bands remain the same length during contraction, but are pulled toward the origin of the muscle.

6. Adjacent A bands are pulled closer together as the I bands between them shorten.

7. The H bands shorten during contraction as the thin filaments on the sides of the sarcomeres are pulled toward the middle.


The cross bridge cycle that causes sliding of the filaments and muscle contraction: Sliding of the filaments is produced by the action of numerous cross bridges that extend out from the myosin toward the actin. These cross bridges are part of the myosin proteins that extend from the axis of the thick filaments to form "arms" that terminate the globular "heads".

1. Resting fiber; cross bridge is not attached to actin.
2. Cross bridge binds to actin.
3. Pi is released, causing conformational change in myosin.
4. Power stroke causes filaments to slide; ADP is released.
5. A new ATP binds to myosin head, allowing it to release actin.
6. ATP is hydrolyzed, causing cross bridge to return to the original orientation.

Muscular System, Sliding Filament Theory

Muscular System, Sliding Filament Theory 2

Stages of A Muscle Contraction


actin and myosin
actin and myosin


legend of components in the animation
legend of components in the animation













Thick filament (MYOSIN) Thin Filament (Actin)







myosin
myosin

actin
actin




The 2 heads link the actin and myosin together during contraction.

The ends of a myosin filament contain the heads & there is a central bare area



The heads contain ATP binding sites.



Active sites are blocked when the muscle is relaxed


synapitic vessels
synapitic vessels




Initiation of Muscle Contraction

Step 1) Neuromuscular Control




The axons of the nerve cells of the spinal cord branch and attach to each muscle fiber forming a neuromuscular junction.

i). An action potential passes down the nerve.

ii). The nerve releases Ca++ that results in the release of Acetylcholine (ACh)



Step 2). ACh binds with the sarcolemma.
Step 3). Muscle Fiber Action Potential




i). ACh binds with receptors and opens Na+ channels

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Na+ Channels open and Na+ in
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There is a decrease in the resting potential

ii). Na + rushes in and the sarcolemma depolarizes.

iii). The regional depolarization spreads rapidly.

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The positive patch in the membrane changes the adjacent patch of the membrane.
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Thus depolarization spreads.

iv). The K+ channels open and the region repolarizes

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Immediately after the action potential passes the membrane permeability changes again.

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Na+ channels close and K+ channels open.

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K+ rushes out of the cell.

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Cell reploraizes



Step 4). Ca++ is released from the sarcoplasmic reticulum.




i). Ca++ is stored in thesarcoplasmic reticulum.

ii). Depolarization releases the Ca++.

iii). The Ca++ clears the actin binding sites.




stages of actin myosin crosss bridge formation
stages of actin myosin crosss bridge formation


Step 5). Sliding Filament Theory of Contraction



During muscle contraction the thin actin filaments slide over the thick myosin filament.

When Calcium is present the blocked active site of the actin clears.


Step A: Myosin head attaches to actin. (High energy ADP + P configuration)

Step B: Power stroke: myosin head pivots pulling the actin filament toward the center.
Step C: The cross bridge detaches when a new ATP binds with the myosin.
Step D: Cocking of the myosin head occurs when ATP à ADP + P. Another cross bridge can form.


Actin-Myosin Sliding Filament Theory
Actin-Myosin Sliding Filament Theory








The end result is a shortening of the sarcomere.

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The distance between the Z discs shortens
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The H zone disappears
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The dark A band increases because the actin & the myosin overlap more
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The light I band shortens.


Sarcomere shortening
Sarcomere shortening

sarcomere shortening
sarcomere shortening

sarcomere shortening
sarcomere shortening






Step 6). Ca++ is removed from the cytoplasm
Step 7). Tropomysin blocks the actin site



In the nursing profession, the physiology of the muscles is very important. As we know, we use muscles for every movement! Being able to know about the skeletal muscle actions, there contractions, and disorders of the muscles, make it a lot easier to know how they work together to form your muscular system. What an amazing body we have!

Resources:http://www.sci.sdsu.edu/movies/actin_myosin_gif.html
Human Physiology Stuart Fox Tenth Edition
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