Sensory and Motor Mechanisms

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Skeletal Muscle

Bundle of muscle fibers

TEM

Muscle

Thick filaments myosin

M line

Single muscle fiber (cell)

Nuclei

Z lines

Plasma membrane

Myofibril

Sarcomere

Z line

Z line

Thin filaments actin

Sarcomere

0.5 µm

Slide 56

The Sliding-Filament Model of Muscle Contraction

According to the sliding-filament model, filaments slide past each other longitudinally, producing more overlap between thin and thick filaments.

Slide 57

The sliding-filament model of muscle contraction

Z

Relaxed muscle

M

Z

Fully contracted muscle

Contracting muscle

Sarcomere

0.5 µm

Contracted Sarcomere

Slide 58

The sliding of filaments is based on interaction between actin of the thin filaments and myosin of the thick filaments.

The “head” of a myosin molecule binds to an actin filament, forming a cross-bridge and pulling the thin filament toward the center of the sarcomere.

Glycolysis and aerobic respiration generate the ATP needed to sustain muscle contraction.

Slide 59

Myosin-actin interactions underlying muscle fiber contraction

Thin filaments

ATP

Myosin head (low- energy configuration

Thick filament

Thin filament

Thick filament

Actin

Myosin head (high- energy configuration

Myosin binding sites

ADP

P i

Cross-bridge

ADP

P i

Myosin head (low- energy configuration

Thin filament moves toward center of sarcomere.

ATP

ADP

P i

+

Slide 60

The Role of Calcium and Regulatory Proteins

A skeletal muscle fiber contracts only when stimulated by a motor neuron.

When a muscle is at rest, myosin-binding sites on the thin filament are blocked by the regulatory protein tropomyosin.

Myosin-binding sites exposed when Ca2+ released.

Slide 61

Myosin- binding site

Tropomyosin

(a) Myosin-binding sites blocked

(b) Myosin-binding sites exposed when Ca2+ released.

Ca2+

Ca2+-binding sites

Troponin complex

Actin

Slide 62

The synaptic terminal of the motor neuron releases the neurotransmitter acetylcholine.

Acetylcholine depolarizes the muscle, causing it to produce an action potential.

Slide 63

Regulation of skeletal muscle contraction