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**:
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### **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).
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### **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**.
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Would you like a visual flowchart of these steps?