Section 4: Neuromuscular Junction (NMJ) & Excitation-Contraction Coupling

When does A skeletal muscle fiber contract

When stimulated by a motor neuron

Neuromuscular junction (NMJ)

site where motor neuron meets midpoint of muscle fiber

1 in each muscle fiber

one motor neuron can

can control many muscle fibers

Action potential

Electrical signal

Depolarization

muscle fiber becomes less negative (more positive) because Na+ enters

Repolarization

muscle fiber becomes more negative (less positive) because K+ exits

Excitation-Contraction Coupling

Excitation → signal traveling

coupling → actual contraction of muscle fiber

Steps of Excitation-Contraction Coupling

1. motor neuron sends action potential

2. action potential arrives at axon terminal of NMJ (neuromuscular junction

3. release acetylcholine (ACh) → binds to sarcolemma receptors

4. Sarcolemma ion permeability changes → depolarization (Na+ enters)

5. Action Potential propogates( travels) along sarcolemma

6. Action potential travels along t tubule

7. Sarcoplasmic reticulum (terminal cisternae) release Ca2+

8. Ca2+ binds to troponin on actin thin filament → removing blocking action of tropomyosin

9. Myosin head binds to actin → begins contraction

troponin and tropomyosin

troponin = protein binded to tropomyosin which blocks the binding site on actin thin filaments

when ca2+ binds → troponin pulls tropomyosin away

Latent period

Period between action potential initiation and contraction

When does repolarization happen

After action potential travels through sarcolemma and t tubules (sent the message)

What is required for cross bridge detachment between myosin and actin

ATP → hydrolysis

Rigor Mortis

3-4 hrs after death, muscles begin to stiffen with weak rigidity

Dying cells take in Ca2+ causing cross bridges to form but no ATP is being generated to break the cross bridges