Friday; Ch 10: Muscle Physiology (Part 2)

What are the three parts of the NMJ?

Axon terminal, synaptic cleft, motor end plate

What neurotransmitter is released at the NMJ?

Acetylcholine (ACh)

What enzyme breaks down ACh?

Acetylcholinesterase

What are the three phases of contraction?

Excitation, Excitation–Contraction Coupling, Contraction

Describe excitation.

ACh released → binds to motor end plate → Na⁺ enters → depolarization → end-plate potential

Describe excitation–contraction coupling.

AP travels down sarcolemma → T-tubule → Ca²⁺ released from SR

Describe contraction phase.

Ca²⁺ binds troponin → tropomyosin moves → actin and myosin form cross-bridges → sarcomere shortens

What stops muscle contraction?

ACh breakdown, closing of Ca²⁺ channels, Ca²⁺ pumped back to SR, tropomyosin blocks actin

What causes rigor mortis?

No ATP to detach myosin from actin; muscles remain stiff

What are the three energy sources for muscle contraction?

• ATP (few seconds)

• Creatine phosphate (~10s)

• Glycolysis and aerobic respiration (long-term)

What causes muscle fatigue?

ATP depletion, ion imbalance, buildup of metabolic byproducts

Difference between slow-twitch and fast-twitch fibers?

Slow = oxidative, fatigue-resistant; Fast = glycolytic, strong but fatigues quickly

What is recruitment?

Increasing the number of motor units activated

What is wave summation?

Increasing frequency of stimulation before relaxation

What are isometric and isotonic contractions?

• Isometric: Tension increases, muscle length constant

• Isotonic: Muscle changes length (concentric/eccentric)

What is the length-tension relationship?

Optimal overlap between actin and myosin produces maximum tension

What are characteristics of smooth muscle?

No sarcomeres or troponin, uses calmodulin & MLCK, involuntary, can stay contracted (latch state)