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Muscular System:

Stages of A  Muscle Contraction

actin and myosin

legend of components in the animation




Thick filament (MYOSIN)                                Thin Filament (Actin)

myosin 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

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

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

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

iii). The regional depolarization spreads rapidly.

bulletThe positive patch in the membrane changes the adjacent patch of the membrane.
bulletThus depolarization spreads.

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

bulletImmediately after the action potential passes the membrane permeability changes again.
bulletNa+ channels close and K+ channels open.
bulletK+ rushes out of the cell.
bulletCell 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

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   ADP + P. Another cross bridge can form.

Actin-Myosin Sliding Filament Theory

The end result is a shortening of the sarcomere.

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


Step 6).  Ca++ is removed from the cytoplasm

Step 7).  Tropomysin blocks the actin site