Action Potential

Muscle contraction (specifically in skeletal muscle) follows a series of well-coordinated steps beginning with an action potential and ending with muscle relaxation. Here's a detailed breakdown of the **steps to muscle contraction and back**, starting at the **action potential**:

---

### **1. Action Potential Reaches Neuromuscular Junction**

- A motor neuron sends an **action potential** to the axon terminal at the neuromuscular junction.

### **2. Acetylcholine (ACh) Release**

- The action potential triggers the release of **acetylcholine** into the synaptic cleft.

### **3. Muscle Fiber Depolarization**

- ACh binds to receptors on the **sarcolemma** (muscle cell membrane), causing **Na⁺ to enter**, leading to **depolarization** and generation of a muscle **action potential**.

### **4. Action Potential Travels Along Sarcolemma and T-Tubules**

- The muscle action potential travels along the sarcolemma and down the **T-tubules**.

### **5. Calcium Release from Sarcoplasmic Reticulum (SR)**

- The signal causes the **sarcoplasmic reticulum** to release **Ca²⁺ ions** into the **cytosol**.

### **6. Calcium Binds to Troponin**

- Ca²⁺ binds to **troponin**, causing a conformational change in **tropomyosin**, exposing **myosin-binding sites** on **actin** filaments.

### **7. Cross-Bridge Formation**

- **Myosin heads** bind to exposed sites on **actin**, forming **cross-bridges**.

### **8. Power Stroke**

- Myosin heads pivot, pulling actin filaments inward (**power stroke**), and **ADP + Pi** are released.

### **9. ATP Binds to Myosin**

- A new **ATP molecule** binds to myosin, causing it to **detach** from actin.

### **10. ATP Hydrolysis**

- ATP is hydrolyzed to **ADP + Pi**, "re-cocking" the myosin head for another cycle (if Ca²⁺ is still present).

---

### **Muscle Relaxation Steps:**

### **11. Calcium Reuptake**

- Ca²⁺ is **pumped back** into the sarcoplasmic reticulum by **active transport (ATP-dependent)**.

### **12. Tropomyosin Re-Covers Binding Sites**

- As Ca²⁺ levels drop, **troponin** changes back, allowing **tropomyosin** to block myosin-binding sites on actin.

### **13. Cross-Bridges Break**

- Without Ca²⁺ and with ATP present, cross-bridges **detach**, and the muscle **relaxes**.

---

Would you like a visual flowchart of these steps?