细胞生物.5 | Muscle
Muscle Tissue Summary / 肌肉组织总结
Three Types of Muscle Tissue / 三种肌肉组织
1. Skeletal Muscle / 骨骼肌
- Location / 位置: Attached to bones / 附着在骨骼上
- Control / 控制: Voluntary / 随意控制
- Appearance / 外观: Striated / 有条纹
- Structure / 结构: Long cylindrical fibers, multinucleated / 长圆柱状纤维，多核
- Function / 功能: Fast contraction, generates strong force / 收缩快，力量大
2. Smooth Muscle / 平滑肌
- Location / 位置: Walls of hollow organs (intestines, blood vessels) / 空腔器官壁（肠道、血管）
- Control / 控制: Involuntary / 不随意控制
- Appearance / 外观: Non-striated / 无条纹
- Structure / 结构: Spindle-shaped, single nucleus / 梭形，单核
- Function / 功能: Slow contraction, weak force / 收缩慢，力量弱
3. Cardiac Muscle / 心肌
- Location / 位置: Heart walls / 心脏壁
- Control / 控制: Involuntary / 不随意控制
- Appearance / 外观: Striated / 有条纹
- Structure / 结构: Branched fibers, usually single nucleus (1–2), connected by gap junctions / 分支纤维，通常单核（1–2个），由缝隙连接连接
- Function / 功能: Medium speed contraction, generates moderate force / 收缩速度中等，力量适中
Muscle Cell Organization (Big → Small) / 肌肉细胞层级结构（由大到小）
- Muscle / 肌肉
→ Fascicle / 肌束
→ Muscle Fiber / 肌纤维（单个长肌细胞）
→ Myofibril / 肌原纤维
→ Sarcomere / 肌小节 （收缩功能单位，位于两个Z线之间）
→ Myofilaments / 肌丝
- Actin / 肌动蛋白（细丝，细胞骨架的一部分）
- Myosin / 肌球蛋白（粗丝）
Special Terms / 专有术语
- Sarcolemma / 肌膜 = Cell membrane of a muscle fiber / 肌纤维的细胞膜
- SR (Sarcoplasmic Reticulum) / 肌质网 = Smooth ER, stores calcium / 平滑内质网，储存钙
- Sarcoplasm / 肌质 = Cytoplasm of muscle cells / 肌肉细胞的细胞质
Why are skeletal and cardiac muscles striated?
Because their myofibrils are organized into repeating units called sarcomeres.
Each sarcomere is made of precise, regular arrangements of actin (thin filaments) and myosin (thick filaments). This repeating pattern creates light and dark bands under a microscope — that’s what we see as striations.
Key Terms:
- Striated = 有条纹的
- Sarcomere = 肌节（收缩单位）
- Z-line = Z线，标志肌节的两端
- A-band = 暗带，myosin所在区域（包含部分actin）
- I-band = 明带，只有actin的区域
Why smooth muscle is not striated?
Smooth muscle doesn’t have sarcomeres. Actin and myosin are still present, but they’re arranged irregularly, not in repeating bands — so no striations are visible.
肌肉收缩放松过程：
1. Signal from CNS / 中枢神经系统的信号
- EN: The CNS sends a signal through a motor neuron towards a skeletal muscle.
- CN: 中枢神经系统通过运动神经元向骨骼肌发送信号。
2. Signal arrives at NMJ / 信号到达神经肌肉接头
- EN: The signal travels along the axon and reaches the axon terminal at the neuromuscular junction.
- CN: 信号沿轴突传导，抵达神经肌肉接头处的轴突末端。
3. Release of ACh / 释放乙酰胆碱
- EN: The signal opens voltage-gated Ca²⁺ channels on the neuron membrane. Ca²⁺ enters from extracellular fluid, triggering vesicles to release acetylcholine (ACh) into the synaptic cleft.
- CN: 信号触发神经元膜上的电压门控钙离子通道打开，Ca²⁺由细胞外进入细胞质，促使囊泡释放乙酰胆碱 (ACh) 到突触间隙。
4. ACh binds sarcolemma receptors / ACh 结合肌膜受体
- EN: ACh binds receptors on the sarcolemma, opening sodium channels. Na⁺ enters the muscle cell, generating a new action potential.
- CN: ACh 与肌膜上的受体结合，打开钠离子通道，Na⁺ 进入肌细胞，产生新的动作电位。
5. Action potential spreads / 动作电位扩散
- EN: The action potential spreads along the sarcolemma and into T-tubules, carrying the signal deep inside the muscle fiber.
- CN: 动作电位沿肌膜传导，并进入横小管，将信号传递至肌纤维内部。
6. Ca²⁺ released from SR / SR 释放钙离子
- EN: The signal triggers Ca²⁺ channels on the SR to open, releasing calcium into the cytoplasm.
- CN: 信号触发肌质网 (SR) 上的钙离子通道打开，Ca²⁺ 释放到肌细胞的细胞质中。
7. Troponin–tropomyosin shift / 调节蛋白复合物移动
- EN: Ca²⁺ binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.
- CN: Ca²⁺ 与肌钙蛋白 (troponin) 结合，使原肌球蛋白 (tropomyosin) 移动，暴露出肌球蛋白结合位点。
8. Cross-bridge cycling / 横桥循环（收缩）
- EN: Myosin binds to actin, forming cross-bridges. Using ATP, myosin pulls actin filaments inward, leading to muscle contraction.
- CN: 肌球蛋白与肌动蛋白结合形成横桥，利用 ATP 拉动肌动蛋白丝内滑，引起肌肉收缩。
9. Signal stops / 信号终止
- EN: When the signal ends, Ca²⁺ release stops. Calcium pumps actively return Ca²⁺ into the SR.
- CN: 当信号停止时，Ca²⁺ 不再释放，钙离子泵将 Ca²⁺ 主动运回肌质网。
10. Relaxation / 肌肉舒张
- EN: Troponin no longer binds calcium, tropomyosin covers the binding sites, cross-bridges break, and the muscle relaxes.
- CN: 肌钙蛋白失去 Ca²⁺ 结合，原肌球蛋白重新覆盖结合位点，横桥断开，肌肉舒张。