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

Step 1: ATP hydrolysis Step 2: Attachment

Step 3: Power Stroke Step 4: Detachment

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

Contraction and Movement Overview Interactions Animation You must be connected to the internet to run this animation.

Length-Tension Relationship Sarcomere shortening produces tension within a muscle Limited contact between actin and myosin Compressed thick filaments

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

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.

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”

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 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.

Neuromuscular Junction Interactions Animation Neuromuscular Junctions You must be connected to the internet to run this animation.

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

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)