Chapter 11 Muscular Tissue - Excitation, Contraction, and Relaxation

A set of practice flashcards covering the step-by-step processes of muscle fiber excitation, excitation-contraction coupling, contraction (sliding filament mechanism), and relaxation.

What specific event occurs in the synaptic knob when the nerve signal arrives?

The action potential signal opens voltage-gated $Ca^{2+}$ channels.

What effect does the entry of $Ca^{2+}$ into the synaptic knob have?

It stimulates the release of acetylcholine (ACh) from synaptic vesicles into the synaptic cleft via exocytosis.

How do ACh molecules move across the synaptic cleft to reach the motor end plate?

ACh molecules diffuse across the cleft.

What happens when ACh binds to receptors on the motor end plate?

Ligand-gated channels open, allowing $Na^+$ to enter and $K^+$ to exit.

What is the name of the brief event where the membrane potential shifts from polarized to depolarized and back?

End plate potential (EPP).

What is the consequence of the voltage change caused by the end plate potential?

Nearby voltage-gated channels open, producing an action potential that spreads over the muscle surface.

Through what structure is the action potential propagated from the sarcolemma into the interior of the muscle fiber?

The T tubules.

What occurs when the action potential spreads down the T tubules?

Voltage-gated ion channels in the T tubules open, which subsequently opens mechanically gated $Ca^{2+}$ channels in the sarcoplasmic reticulum (SR).

Where does $Ca^{2+}$ go after leaving the sarcoplasmic reticulum?

$Ca^{2+}$ leaves the SR and enters the sarcoplasm to bind with troponin in the thin filaments.

How are the active sites on actin exposed during excitation-contraction coupling?

$Ca^{2+}$ binds to troponin, causing the troponin-tropomyosin complex to change shape and shift positions.

What role does ATPase play in the myosin head?

The enzyme ATPase hydrolyzes an $ATP$ molecule into $ADP + P_i$ to energize and "cock" the myosin head into a recovery stroke position.

What defines the formation of a cross-bridge?

The cocked myosin head binds to an active site on actin.

What happens during the power stroke phase of contraction?

Myosin releases $ADP + P_i$ and flexes, pulling the thin filament over the thick filament.

What causes the myosin head to release the actin active site?

The binding of a new $ATP$ molecule to the myosin head.

According to the lecture, how many power strokes does each myosin head perform per second?

$5$\,power strokes per second.

What enzyme is responsible for breaking down ACh in the synaptic cleft during relaxation?

Acetylcholinesterase (AChE).

What happens to the $Ca^{2+}$ ions during the relaxation process?

$Ca^{2+}$ ions are reabsorbed by the sarcoplasmic reticulum and lost from troponin.

What is the final state of the thin filament during relaxation?

Tropomyosin returns to its position blocking the active sites of actin.