Order the following events of excitation and excitation-contraction coupling. Put 1 by the first event, 2 by the second, and so on.
____ The motor end plate generates an end-plate potential.
____ The action potential spreads along the T-tubules, SR Ca²⁺ channels are pulled open, and Ca²⁺ floods the cytosol.
____ Acetylcholine binds to receptors on the motor end plate, and ligand-gated ion channels open.
____ Ca²⁺ bind troponin, which allows tropomyosin to move away from the actin active site, initiating a contraction cycle.
____ The action potential propagates through the sarcolemma and dives deeply into the cell along the T-tubules.

Verified step by step guidance

1. Identify the sequence of events in excitation and excitation-contraction coupling by understanding the physiological process of muscle contraction.
2. Step 1: Acetylcholine (ACh) is released from the motor neuron and binds to receptors on the motor end plate, causing ligand-gated ion channels to open. This allows sodium ions (Na⁺) to enter the muscle fiber, initiating depolarization.
3. Step 2: The motor end plate generates an end-plate potential, which is a localized depolarization that triggers an action potential in the sarcolemma.
4. Step 3: The action potential propagates along the sarcolemma and dives deeply into the muscle fiber through the T-tubules, ensuring the signal reaches all parts of the muscle cell.
5. Step 4: The action potential in the T-tubules causes the sarcoplasmic reticulum (SR) to release calcium ions (Ca²⁺) into the cytosol by opening SR Ca²⁺ channels.
6. Step 5: Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the actin active sites, allowing the myosin heads to bind to actin and initiate the contraction cycle.

Key Concepts

Excitation-Contraction Coupling

Excitation-contraction coupling is the physiological process that links the electrical excitation of a muscle fiber to its contraction. It begins with the generation of an action potential in the muscle cell membrane, which then triggers the release of calcium ions from the sarcoplasmic reticulum. This increase in intracellular calcium concentration is crucial for muscle contraction, as it initiates the interaction between actin and myosin filaments.

Role of Acetylcholine

Acetylcholine is a neurotransmitter that plays a vital role in muscle contraction. It is released from motor neurons and binds to receptors on the motor end plate of the muscle fiber, leading to the opening of ligand-gated ion channels. This influx of sodium ions generates an end-plate potential, which is essential for initiating the action potential that propagates along the muscle membrane and into the T-tubules.

Calcium Ion Functionality

Calcium ions (Ca2+) are critical for muscle contraction. When the action potential reaches the T-tubules, it causes the sarcoplasmic reticulum to release Ca2+ into the cytosol. The binding of Ca2+ to troponin causes a conformational change that moves tropomyosin away from the actin active sites, allowing myosin heads to bind to actin and initiate the contraction cycle.