Muscle Contraction

1. The brain sends an electrical message down a motor neuron.

2. The neuron branches into axons and the electrical message reaches an axon terminal (end of an axon)

3. Here the electrical message triggers the release of a neurotransmitter (acetylcholine) to enter the synaptic cleft

The phospholipid bilayer is the fundamental structure of the membrane The cell membrane contains both hydrophilic and hydrophobic regions

Two types of proteins in membranes – peripheral proteins and integral proteins

4. The Synaptic cleft is a tiny gap or space between the axon terminal and the cell membrane (or more specifically the sarcolemma)

5. The acetylcholine diffuses across the cell membrane and binds to receptor proteins on the cell membrane allowing the polar membrane to become permeable to sodium, depolarizing the membrane

6. The sudden influx of sodium and depolarization of the membrane creates another electrical impulse that sweeps down the cell membrane deep into the cell by way of the Transverse (T) Tubules.

7. The electrical impulse traveling through the T Tubules triggers the release of Calcium from the Sarcoplasmic Reticulum

8. This calcium diffuses into the interior of each myofibril and attaches to the Troponin-Tropomyosin Protein Complex located on each Actin (thin) filament

9. The Troponin-Tropomyosin complex was blocking the Myosin binding sites on Actin 10. The attachment of Calcium to Troponin-Tropomyosin changes its configuration (shape) and moves it off the Myosin binding sites on Actin allowing the Myosin heads to bind to Actin

11. Once the Myosin heads attach they use ATP to "flip" pulling the Actin filaments closer together, shortening the sarcomere Myosin in Action

12. The Shortening of the Sarcomere shortens the length of the myofibril, muscle fiber, fascicle, and then the actual muscle itself, pulling on the bone and causing body movement. Muscle Movement

Reuptake Transporters

Active Transport

Active Transport Summarized Molecules often move from low  high concentration Such movement requires energy: active transport Carrier protein used as cell membrane “pump” Example: Na+ and K+ pump