1. The Sliding-Filament Mechanism
The “sliding” of actin on myosin (thick filaments on thin filaments) can be broken down into a 4 step process

2. Step 1: ATP hydrolysis
Step 2: Attachment

3. Step 3: Power Stroke
Step 4: Detachment

4. The Sliding-Filament Mechanism
None of this occurs without intracellular Ca2+ release from its storage sites in the sarcoplasmic reticulum.

5. Contraction and Movement Overview Interactions Animation
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6. Length-Tension Relationship
Sarcomere shortening produces tension within a muscle
Limited contact between actin and myosin
Compressed thick filaments

7. 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

8. Neuromuscular Junction
Excitation-Contraction coupling (EC coupling) involves events at the junction between a motor neuron and a skeletal muscle fiber
A single muscle cell from a biceps may contain 100,000 sarcomeres. The myofibrils of smooth muscle cells are not arranged into sarcomeres.

9. Neuromuscular Junction
An enlarged view of the neuromuscular junction
The 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”

10. 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

11. 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

12. 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
The term “electrical events” is somewhat of a misnomer since the flow of Na+ and K+ ions which generate human action potentials is not the same as the flow of electrons which occurs in the copper wiring of household electrical systems.

13. Neuromuscular Junction Interactions Animation
Neuromuscular Junctions
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14. Muscle Action Potential
The muscle AP is propagated over the surface of the muscle cell membrane (sarcolemma) via voltage (electrical)-gated Na+ and K+ channels

15. 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)