The Sliding-Filament Mechanism

• The “sliding” of actin on myosin (thick filaments on thin filaments) can be broken down into a 4 step process

The Sliding-Filament Mechanism

• Step 1: ATP hydrolysis

• Step 2: Attachment

• Step 3: Power Stroke

• Step 4: Detachment

The Sliding-Filament Mechanism

Contraction and Movement OverviewInteractions Animation

• Contraction and Movement

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Limited contact between actin and myosin

Compressed thick

filaments

Length-Tension Relationship• Sarcomere shortening produces tension within a muscle

Excitation-Contraction Coupling• We will come back to the term excitation-contraction

coupling in a little bit

Before we can describe the

entire process, from

thinking of moving a

muscle to actual contraction

of sarcomeres, we must

first explore the processes

that occur at the neuromuscular junction

• Excitation-Contraction coupling (EC coupling) involves events at the junction between a motor neuron and a skeletal muscle fiber

Neuromuscular Junction

• An enlarged view of the neuromuscular junctionThe presynaptic membrane is on the neuron while the postsynaptic membrane is the motor end plate on the muscle cell. The two membranes are separated by a space, or “cleft”

Neuromuscular Junction

• Conscious thought (to move a muscle) results in activation of a motor neuron, and release of the neurotransmitter acetylcholine (AcCh) at the NM junction

• The enzyme acetylcholinesterase breaks down AcCh after a short period of time

Neuromuscular Junction

• The plasma membrane on the “far side” of the NMJ belongs to

the muscle cell and is called the motor end plate• The motor end plate is rich in chemical (ligand) - gated sodium

channels that respond to AcCh. Another way to say this: The

receptors for AcCh are on the ligand-gated sodium channels on

the motor end plate

Neuromuscular Junction

• The chemical events at the NMJ transmit the electrical events of a neuronal action potential into the electrical events of a muscle action potential

Neuromuscular Junction

Neuromuscular JunctionInteractions Animation

• Neuromuscular Junctions

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• The muscle AP is propagated over the surface of the muscle cell membrane (sarcolemma) via voltage (electrical)-gated Na+ and K+ channels

Muscle Action Potential

• By placing a micropipette inside a muscle cell, and

then measuring the electrical potential across the

cell membrane, the phases of an

action potential

(AP) can be

graphed (as in this

figure)

Muscle Action Potential