Sliding Filament Mechanism

Ch. 10-3 notes

Sliding Filament Mechanism

• Describes the cause and mechanism of muscle contraction

• Myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere

• The thin filaments are pulled toward the M line and meet at the center of the sarcomere

• The Z discs come closer together and the sarcomere shortens.

• This shortening shortens the entire muscle

Sliding Filament Mechanism

The Contraction Cycle

• Contraction– SR releases Ca2+ ions into the cytoplasm– Ca2+ binds to troponin to move tropomyosin out

of the way of the myosin-binding sites on actin– Contraction cycle – repeating sequence of events

that causes the filaments to slide• 4 steps

4 Steps to Contraction Cycle

1. ATP hydrolysis– Breaking ATP into ADP + P adds energy to the

myosin head so it can turn2. Attachment of myosin to actin to form cross-

bridges– Myosin head attaches to binding site on actin– Releases P– Crossbridge forms

4 Steps to Contraction Cycle

3. Power Stroke– Myosin crossbridges rotate

4. Detachment of myosin from actin– Myosin remains attached until another ATP binds causes

it to detach– * rigor mortis

Contraction cycle continues- with ATP- with Ca2+ levels high enough

Contraction Cycle

Contraction

• The myosin heads “walk” down the actin• They can attach and detach about 5 times per second

http://www.dnatube.com/video/4154/Muscle-Contractionhttp://www.dnatube.com/video/1305/Muscle-Contractionhttp://www.dnatube.com/video/1306/Muscle-contractionhttp://www.dnatube.com/video/1308/Muscle-Contraction

http://www.sci.sdsu.edu/movies/actin_myosin_gif.html

Neuromuscular Junction (NMJ)

• Somatic motor neurons – neurons that stimulate skeletal muscle fibers

• Place where action potentials arise• Synapse – region where 2 neurons or 1 neuron

and a target cell (muscle fiber) meet• Synaptic cleft – gap that separates two cells• Neurotransmitter – chemical released for the

2 cells to communicate

NMJ

NMJ

NMJ

NMJ Parts

• At the end of each axon are synaptic vesicles filled with acetylcholine (ACh)– ACh is the neurotransmitter

• Motor end plate – sarcolemma opposite the synapse– Contains acetylcholine receptors

• Action potential starts here with stimulation from neurons

Action Potential

1. Release of ACh– Vesicles do exocytosis– Diffuse to motor end plate

2. Activation of ACh receptors– 2 Ach bind to motor end plate and opens the

receptor– Na+ flows across membrane through channel

Action Potential

3. Production of muscle action potential– Na+ makes fiber more + charged– This causes release of Ca+– Contraction cycle!

4. Termination of ACh activity– Acetylcholinesterase (AChE) – breaks down ACh

so further muscle contractions don’t occur– Unless more ACh is released

Overview