he conversion of an action potential (AP) generated by a motor neuron to contraction of skeletal muscle fiber is called excitation-contraction coupling. This ctivity will test your understanding of the sequence of events that occur during excitation-contraction coupling. View Available Hint(s) $$ \begin{array}{c}\text { Voltage-sensitive proteins } \ \text { open } \mathrm{Ca}^{2+} \text { channels }\end{array} $$ $$ \begin{array}{c}\text { Sarcoplasmic reticulum } \ \text { releases } \mathrm{Ca}^{2+}\end{array} $$

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1. **Action Potential Arrival**

The motor neuron generates an action potential (AP) that travels to the neuromuscular junction, where it triggers the release of the neurotransmitter acetylcholine. This neurotransmitter diffuses across the synaptic cleft and binds to receptors on the muscle fiber membrane.

2. **Propagation and Depolarization**

The binding of acetylcholine causes depolarization of the muscle fiber’s sarcolemma. This depolarization propagates along the membrane and invades the transverse tubules (T-tubules).

3. **Activation of Voltage-Sensitive Proteins**

The depolarization in the T-tubules activates voltage-sensitive proteins. One key voltage-sensitive protein is the dihydropyridine receptor (DHPR). When these receptors sense a change in voltage, they undergo a conformational change.

4. **Opening of $$ \mathrm{Ca}^{2+} $$ Channels**

The conformational change in the DHPR is mechanically linked to the ryanodine receptor (RyR) on the sarcoplasmic reticulum (SR). As a result, the ryanodine receptors open, allowing $$ \mathrm{Ca}^{2+} $$ to flow from the SR into the cytosol.

5. **Release of $$ \mathrm{Ca}^{2+} $$**

With the opening of these channels, the sarcoplasmic reticulum releases $$ \mathrm{Ca}^{2+} $$, leading to an increase in the intracellular $$ \mathrm{Ca}^{2+} $$ concentration.

6. **Initiation of Contraction**

The released $$ \mathrm{Ca}^{2+} $$ binds to troponin, a regulatory protein on the actin filaments. This binding induces a conformational change in the troponin-tropomyosin complex, exposing the binding sites on actin. Consequently, the interaction between actin and myosin is initiated, leading to muscle contraction.
1. **Action Potential Arrival**: The motor neuron sends an action potential to the muscle fiber. 2. **Depolarization**: This causes the muscle fiber to become more positive. 3. **Opening of $$ \mathrm{Ca}^{2+} $$ Channels**: Special proteins open channels that let $$ \mathrm{Ca}^{2+} $$ into the muscle cell. 4. **Release of $$ \mathrm{Ca}^{2+} $$**: $$ \mathrm{Ca}^{2+} $$ is released from storage in the sarcoplasmic reticulum. 5. **Muscle Contraction**: The $$ \mathrm{Ca}^{2+} $$ triggers the muscle fibers to contract. This sequence connects the neuron's signal to the muscle's response.

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