Muscular System: Stages of A Muscle Contraction.........

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Thick filament (MYOSIN)                                Thin Filament (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

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

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

ii). Na + rushes in and the sarcolemma depolarizes.
iii). The regional depolarization spreads rapidly.

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

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

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

Na+ channels close and K+ channels open.

K+ rushes out of the cell.

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.

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.

The end result is a shortening of the sarcomere.

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

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

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