• Muscle
• Nerve (supplies muscle)
• Synapse (communicating junction inbetween)

Sarcomere Shortening: muscle contraction is based on shortening of sarcomeres throughout muscle; thick and thin filaments must overlap and slide past each other; greater overlap = shorter muscle

1. Thick Filament: made of myosin molecules; has hinged tail that can bend and 2 heads that can flex; heads have ATP binding site & Actin binding site

2. Thin Filament: made of actin, tropomyosin, and troponin; actin has myosin binding site

3. Calcium Ions: necessary for contractions to occur

4. ATP: also necessary for contractions to occur

1. Relaxed Muscle

2. AP along Motor Nerve

3. Cross Bridge Attachment

4. Power Stroke

5. Cross Bridge Detachment

6. Myosin Head Activation

7. Return to Relaxed State

• Minimal filament overlap between thick and thin filaments
• Low intracellular calcium (stored in term. cisternae)
• No AP coming from nerve
• Activated myosin heads (holding ATP as potential energy)
• Tropomyosin covers myosin binding site
• Troponin intact and holding tropomyosin in place (no calcium in cytoplasm surrounding myofibrils)

• Causes Ca to enter cytoplasm, surrounding myofibrils
• Troponin binds with Ca and changes shape
• Change in shape causes tropomyosin to be pulled away from myosin binding site on actin
• Binding sites exposed which allows for Cross Bridge Attachment

Myosin head binds to actin (joining of thick and thin filaments) creates Actomyosin Complex

• After Actomyosin Complex is formed, energy from ATP is used to cause flexion of myosin head and tail;
• Myosin head pivots and pulls actin along thick filament (sarcomere shortening)
• ADP + P released

CC: Cross Bridge Detachment

Requires ATP- enables myosin to bind to new location

• ATP binding to myosin
• Myosin head detaches from actin
• Calcium released from troponin

• Myosin head contains the enzyme Myosin ATPase
• This enzyme cleaves ATP into ADP + P
• Myosin returns to high energy state (potential energy stored in heads, ready for next cross bridge attach)

• Calcium ions reuptake (back to terminal cisternae)
• Troponin (back to original configuration??)
• Tropomyosin (back to original configuration/covering myosin binding stie on actin)

Healthy skeletal muscle does not contract on its own--> motor nerve tells what to do and when to do it.

1. Motor Nerve/Neuromuscular Junction: communicating synapse between 2 excitable cells; has an axon that expands into terminal boutons; Synapse- area where nerve joins muscle (without touching)

2. Action Potential Initiation and Propagation: Sodium in and potassium out causes voltage change/opening

3. ACH Hydrolysis: Acetylcholine hydrolized by Ach Esterase

Communicating synapse between 2 excitable cells;

3 components:

 1. Nerve Side: alpha motor neuron; terminal boutons contain synaptic vesicles which contain acetylcholine (neurotransmitter/ligand); Calcium channels in nerve plasma membrane (in addition to Na & K channels)

 2. Muscle Side: motor end plate (sarcolemma) thrown into juntional folds to increase surface area; Ach Receptors (proteins) within these folds in sarcolemma

 3. Synaptic Cleft: "space" between nerve and muscle; tissues do not touch; glycoproteins in between hold structures in place

1. Voltage change opens calcium channels; allows Ca to enter terminal bouton which causes:

2. Vesicle Migration (traveling from h-l concentration) to end of terminal boutons which causes:

3. Vesicle fusion where acetylcholine is released (excitosed out) and:

4. Ach crosses synapse where it binds with receptors on motor end plate causing:

5. Sarcolemma Graded Potential (local depolarization) followed by an  Action Potential (wave of depolarization as Na+ enters causing AP to travel down plasma membrane)

ACH Hydrolysis

Acetylcholine hydrolized by Ach Esterase; important because you want to remove Ach after it has been there for a short while (don't want continued APs); Only want the APs we need

When you excite a cell, a contraction will follow; these two events will always couple (can't have one w/out the other)

1. Alpha Neuron Action Potential -->
2. Terminal Bouton Calcium Entry -->
3. Synaptic Vesicle Migration/Fusion -->
4. Acetylcholine Release -->
5. Acetylcholine Binding/Sarcolemma Graded Potential -->
6. Sarcolemma Action Potential -->
7. T-Tubule Action Potential -->
8. Terminal Cisternae Calcium Release -->
9. Calcium binds with Troponin -->
10. Cross-Bridge Formation and Contraction