Muscles and Locomotion

Sarcomere

Smallest contractile unit of muscle; extends Z-disc to Z-disc

Thick Filament

Made of myosin molecules

Thin Filament

Made of actin plus troponin and tropomyosin

Role of Tropomyosin

Blocks myosin-binding sites on actin at rest

Role of Troponin

Binds Ca²⁺ and moves tropomyosin to expose actin sites

A-band

Region containing thick filaments; length does not change during contraction

I-band

Region of thin filaments; shortens during contraction

H-zone

Center of A-band where thick filaments lack heads; disappears during contraction

Titin

Elastic protein that stabilizes and aligns the sarcomere

Sliding Filament Theory

Thin filaments slide toward M-line as myosin pulls actin

ATP Role in Cross-Bridge Cycle

Binds myosin to detach it from actin

ATP Hydrolysis

Cocks myosin head into high-energy ADP+Pi state

Pi Release

Triggers the power stroke

ADP Release

Leaves myosin in low-energy rigor position

Rigor Mortis Cause

No ATP after death → myosin cannot detach

Neuromuscular Junction

Synapse where motor neuron releases ACh onto muscle fiber

ACh Function

Opens Na⁺/K⁺ channels and depolarizes sarcolemma

ACh Breakdown

Acetylcholinesterase removes ACh to stop stimulation

End Plate Potential

Local depolarization from ACh-gated ion channels

Action Potential Propagation

Voltage-gated Na⁺ channels carry AP along sarcolemma

T-tubules

Carry action potentials deep into muscle fiber

DHP Receptor

Voltage sensor in T-tubules that mechanically opens RyR Ca²⁺ channels

Ryanodine Receptor (RyR)

Releases Ca²⁺ from sarcoplasmic reticulum

Calcium's Role

Binds troponin and initiates cross-bridge cycling

SERCA Pump

Uses ATP to pump Ca²⁺ back into SR, ending contraction

Muscle Relaxation

Ca²⁺ removal → tropomyosin covers binding sites → no cross-bridges

Locomotion

Translation of muscle contraction into whole-body movement

Respiratory Quotient (RQ)

CO₂ produced / O₂ consumed; indicates fuel type

RQ for Carbohydrates

1.0

RQ for Fats

0.7

Highland Deer Mice Adaptation

Burn fats at high rates; Hb adapted for high affinity O₂ binding

Metabolic Fuel During Max Exercise

Carbohydrates (require less O₂ per kcal)

Metabolic Fuel During Endurance

Fats (more energy per gram)

Salmon Migration Fuel Use

Stored lipids first; glycogen spared until spawning