1. Sliding Filament Theory
Picture/ Description step by step

2. Sarcomere= Where the contraction occurs
I-band= space on sarcomere where only actin lie
A-band= space on sarcomere where myosin lie
H-zone= space between actin filaments (middle of sarcomere)
Actin= thin proteins
Myosin= thick proteins

3. Closer Look at Actin and Myosin
Calcium (released by the sarcoplasmic reticulum) binds to troponin. This change the shape of troponin and shifts tropomyosin out of the way, exposing myosin-head binding sites.

4. At Rest
When at rest and the calcium has exposed the binding sites, the myosin head will attach to the actin binding sites
Attached to each actin is an ADP + Pi

5. Stimulation + Contraction
In order for the “power stroke” to occur (pulling actin inwards), ADP + Pi are released as energy. This causes the myosin head to shift.
Another ATP molecule attaches to the head in order for it to release from the binding site.
Acting as an enzyme, myosin breaks down ATP into ADP + Pi for more useable energy to bind and shift again

6. Contraction  Relaxation
The previous steps will repeat to complete a full contraction or until calcium is depleted/ “called back” to the sarcoplasmic reticulum.
Once calcium is removed or levels become too low, troponin and tropomyosin go back to their original shapes + positions and myosin cannot bind to actin (no movement can occur)

7. ALL TOGETHER! Brain releases a nerve impulse to initiate movement
Neuro-cell body sends message nerve message (action potential) down the axon of the nerve
Action potential reaches axon terminal at the neuromuscular junction
Axon terminal releases acetylcholine (ACh)
ACh crosses the synaptic cleft to bind to receptors on the sarcolemma
Sodium (Na+) floods the T-tubules and the sarcolemma becomes depolarized
The action potential is transported to the interior of the muscle via the T-tubules
The sarcoplasmic reticulum releases calcium ions
Calcium binds to troponin
Tropomyosin slides revealing myosin binding sites on Actin
Myosin (already containing ADP, Pi and energy) attaches to Actin
Energy is released from myosin and power stroke occurs (causing Actin to slide over myosin)
ATP attaches to the head of myosin
ATP splits into ADP, Pi and energy
Steps repeat until contraction is completed or until calcium levels deplete
Calcium is removed and muscle returns to rest