The Molecular Mechanism of Skeletal Muscle Contraction | 骨骼肌收缩的分子机制

📚 The Molecular Mechanism of Skeletal Muscle Contraction | 骨骼肌收缩的分子机制

Skeletal muscle contraction is one of the most elegantly coordinated processes in biology, involving the precise interaction of proteins, ions, and energy molecules. This article explores the molecular machinery behind muscle contraction, from the structure of the sarcomere to the biochemical events of the sliding filament mechanism.

骨骼肌收缩是生物学中最精妙协调的过程之一，涉及蛋白质、离子和能量分子的精确相互作用。本文将深入探讨肌肉收缩背后的分子机制，从肌节的结构到肌丝滑行机制的生化事件。

1. Structural Organisation of Skeletal Muscle | 骨骼肌的结构组织

Skeletal muscle is composed of muscle fibres (cells), each containing hundreds of myofibrils. Each myofibril is divided into repeating functional units called sarcomeres, which are the basic contractile units of muscle. The sarcomere extends from one Z-line to the next Z-line.

骨骼肌由肌纤维（细胞）组成，每条肌纤维含有数百条肌原纤维。每条肌原纤维被分为重复的功能单位，称为肌节，肌节是肌肉的基本收缩单位。肌节从一条Z线延伸到相邻的另一条Z线。

Within the sarcomere, two types of protein filaments are arranged in a highly ordered pattern:

在肌节内部，两种类型的蛋白丝以高度有序的方式排列：

Thick filaments — composed primarily of myosin, located in the A-band (anisotropic band).
粗肌丝 — 主要由肌球蛋白组成，位于A带（各向异性带）。
Thin filaments — composed of actin, tropomyosin, and troponin, located in the I-band (isotropic band) and partially overlapping the thick filaments.
细肌丝 — 由肌动蛋白、原肌球蛋白和肌钙蛋白组成，位于I带（各向同性带），并与粗肌丝部分重叠。

Sarcomere length = distance between two adjacent Z-lines

肌节长度 = 相邻两条Z线之间的距离

Key regions of the sarcomere include the I-band (actin only), the A-band (myosin plus overlapping actin), the H-zone (myosin only, in the centre of the A-band), and the M-line (protein scaffold anchoring myosin filaments).

肌节的关键区域包括I带（仅有肌动蛋白）、A带（肌球蛋白加上重叠的肌动蛋白）、H区（仅有肌球蛋白，位于A带中央）和M线（锚定肌球蛋白丝的蛋白质支架）。

2. The Sliding Filament Hypothesis | 肌丝滑行假说

The accepted model for muscle contraction is the sliding filament hypothesis, proposed independently by Andrew Huxley and Hugh Huxley in 1954. According to this model, muscle contraction occurs when the thin filaments slide past the thick filaments toward the centre of the sarcomere, causing the sarcomere to shorten.

肌肉收缩的公认模型是肌丝滑行假说，由Andrew Huxley和Hugh Huxley于1954年分别独立提出。根据该模型，肌肉收缩发生在细肌丝滑过粗肌丝向肌节中心移动时，导致肌节缩短。

Evidence supporting this model includes:

支持该模型的证据包括：

During contraction, the A-band width remains constant because myosin filament length does not change.
收缩期间，A带的宽度保持不变，因为肌球蛋白丝的长度没有改变。
The I-band and H-zone narrow or disappear, indicating that actin filaments are sliding inward.
I带和H区变窄或消失，表明肌动蛋白丝正在向内滑动。
The Z-lines move closer together, reducing sarcomere length.
Z线相互靠近，缩短了肌节长度。

The force for sliding is generated by myosin heads forming cross-bridges with actin, and pivoting to pull actin filaments toward the centre.

滑动的力由肌球蛋白头部与肌动蛋白形成横桥并枢转，将肌动蛋白丝拉向中心而产生。

3. Molecular Components: Actin and Myosin | 分子组成：肌动蛋白与肌球蛋白

Actin (thin filament): Globular actin (G-actin) monomers polymerise into a double helical filament called fibrous actin (F-actin). Each G-actin molecule has an active site that can bind to myosin heads. Along the groove of the actin helix lies tropomyosin, a rod-shaped protein that covers the myosin-binding sites in a relaxed muscle. Attached to tropomyosin are troponin complexes, each consisting of three subunits: troponin C (binds Ca²⁺), troponin I (binds actin), and troponin T (binds tropomyosin).

肌动蛋白（细肌丝）： 球状肌动蛋白（G-肌动蛋白）单体聚合成双螺旋丝，称为纤维状肌动蛋白（F-肌动蛋白）。每个G-肌动蛋白分子有一个活性位点，可与肌球蛋白头部结合。在肌动蛋白螺旋的沟槽中分布着原肌球蛋白，一种杆状蛋白，在肌肉放松时覆盖肌球蛋白结合位点。附着在原肌球蛋白上的是肌钙蛋白复合体，由三个亚基组成：肌钙蛋白C（结合Ca²⁺）、肌钙蛋白I（结合肌动蛋白）和肌钙蛋白T（结合原肌球蛋白）。

Myosin (thick filament): Myosin is a motor protein with a long tail and two globular heads. Each myosin head contains an actin-binding site and an ATPase enzyme site that hydrolyses ATP to provide energy for contraction. Thick filaments are formed by bundling approximately 300 myosin molecules, with heads projecting outward in a staggered arrangement.

肌球蛋白（粗肌丝）： 肌球蛋白是一种运动蛋白，具有长尾部和两个球状头部。每个肌球蛋白头部含有一个肌动蛋白结合位点和一个ATPase酶位点，可水解ATP为收缩提供能量。粗肌丝由约300个肌球蛋白分子聚集而成，头部以交错排列的方式向外突出。

4. The Cross-Bridge Cycle | 横桥循环

The cross-bridge cycle describes the cyclic interaction between myosin heads and actin filaments, driven by ATP hydrolysis. This cycle can be broken down into four distinct stages:

横桥循环描述了由ATP水解驱动的肌球蛋白头部与肌动蛋白丝之间的循环相互作用。该循环可分为四个不同的阶段：

Stage 1 — Attachment: In the presence of Ca²⁺, troponin undergoes a conformational change, shifting tropomyosin away from the myosin-binding sites on actin. The myosin head, already energised with ADP and inorganic phosphate (Pi), binds firmly to actin, forming a cross-bridge.

第一阶段 — 结合： 在Ca²⁺存在时，肌钙蛋白发生构象变化，使原肌球蛋白从肌动蛋白上的肌球蛋白结合位点移开。已经携带ADP和无机磷酸（Pi）的肌球蛋白头部牢固地结合到肌动蛋白上，形成横桥。

Stage 2 — Power Stroke: The myosin head pivots toward the centre of the sarcomere, releasing ADP and Pi. This pivoting motion pulls the thin filament inward, shortening the sarcomere. This is the force-generating step.

第二阶段 — 动力冲程： 肌球蛋白头部向肌节中心枢转，释放ADP和Pi。这个枢转运动将细肌丝向内拉动，缩短肌节。这是产生力的步骤。

Stage 3 — Detachment: A new ATP molecule binds to the myosin head, causing the head to lose affinity for actin and detach from the actin filament.

第三阶段 — 分离： 一个新的ATP分子与肌球蛋白头部结合，使头部对肌动蛋白的亲和力降低，并从肌动蛋白丝上解离。

Stage 4 — Re-cocking: ATP is hydrolysed by myosin ATPase into ADP and Pi. The energy released re-cocks the myosin head to its energised, high-energy conformation, ready for the next cycle.

第四阶段 — 复位： ATP被肌球蛋白ATPase水解为ADP和Pi。释放的能量将肌球蛋白头部重新扳回高能构象，为下一个循环做好准备。

Myosin + ATP → Myosin-ADP-Pi (energised) → Cross-bridge → Power stroke → Detach → Re-cock

肌球蛋白 + ATP → 肌球蛋白-ADP-Pi（高能）→ 横桥 → 动力冲程 → 解离 → 复位

Each cycle shortens the sarcomere by approximately 10 nm. Many cycles occur in rapid succession during a single contraction, with individual myosin heads working asynchronously to maintain tension.

每个循环使肌节缩短约10 nm。在单次收缩过程中，许多循环快速连续发生，各个肌球蛋白头部异步工作以维持张力。

5. The Role of ATP in Muscle Contraction | ATP在肌肉收缩中的作用

ATP serves four critical functions during muscle contraction:

ATP在肌肉收缩过程中有四个关键功能：

Providing energy for the power stroke of the myosin head.
为肌球蛋白头部的动力冲程提供能量。
Binding to myosin to cause detachment from actin (essential for relaxation).
与肌球蛋白结合使其从肌动蛋白上解离（对放松至关重要）。
Powering the Ca²⁺ pump (SERCA) in the sarcoplasmic reticulum to actively transport Ca²⁺ back, enabling muscle relaxation.
为肌浆网中的Ca²⁺泵（SERCA）提供能量，主动将Ca²⁺泵回，使肌肉放松。
Maintaining the Na⁺-K⁺ pump at the sarcolemma to restore resting membrane potential after action potentials.
维持肌膜上的Na⁺-K⁺泵，在动作电位后恢复静息膜电位。

Without ATP, the muscle would remain contracted — this explains rigor mortis, where ATP production ceases after death, preventing myosin detachment from actin.

没有ATP，肌肉将保持收缩状态——这解释了尸僵现象：死亡后ATP产生停止，阻止肌球蛋白从肌动蛋白上解离。

6. Excitation-Contraction Coupling | 兴奋-收缩耦联

Excitation-contraction coupling is the physiological process by which an action potential triggers muscle fibre contraction. This involves the following sequence of events:

兴奋-收缩耦联是动作电位触发肌纤维收缩的生理过程。这涉及以下一系列事件：

Neuromuscular junction: A motor neuron releases acetylcholine (ACh) into the synaptic cleft, which binds to nicotinic ACh receptors on the muscle fibre membrane (sarcolemma).
神经肌肉接头： 运动神经元向突触间隙释放乙酰胆碱（ACh），ACh与肌纤维膜（肌膜）上的烟碱型ACh受体结合。
Action potential: The binding triggers an action potential that propagates along the sarcolemma and into the T-tubules (transverse tubules).
动作电位： 结合触发动作电位，沿肌膜传播并进入T-管（横小管）。
Calcium release: The action potential in the T-tubules causes voltage-sensitive receptors (dihydropyridine receptors) to activate ryanodine receptors on the sarcoplasmic reticulum, opening Ca²⁺ channels and releasing Ca²⁺ into the sarcoplasm.
钙释放： T-管中的动作电位使电压敏感受体（二氢吡啶受体）激活肌浆网上的兰尼碱受体，打开Ca²⁺通道，将Ca²⁺释放到肌浆中。
Contraction: Ca²⁺ binds to troponin C, initiating the cross-bridge cycle.
收缩： Ca²⁺与肌钙蛋白C结合，启动横桥循环。

The close apposition of T-tubules to the sarcoplasmic reticulum ensures rapid and uniform delivery of Ca²⁺ to all parts of the muscle fibre.

T-管与肌浆网的紧密并置确保了Ca²⁺快速均匀地传递到肌纤维的所有部分。

7. The Role of Calcium and Regulatory Proteins | 钙离子与调节蛋白的作用

Calcium ions (Ca²⁺) act as the primary intracellular signal for muscle contraction. The regulation of contraction depends on the troponin-tropomyosin complex:

钙离子（Ca²⁺）是肌肉收缩的主要细胞内信号。收缩的调节依赖肌钙蛋白-原肌球蛋白复合体：

Relaxed state: When Ca²⁺ concentration is low (below ~10⁻⁷ mol dm⁻³), tropomyosin sits in the groove of the actin helix, physically blocking the myosin-binding sites on actin. No cross-bridges can form, and the muscle is relaxed.

放松状态： 当Ca²⁺浓度较低时（低于约10⁻⁷ mol dm⁻³），原肌球蛋白位于肌动蛋白螺旋的沟槽中，物理上阻断了肌动蛋白上的肌球蛋白结合位点。无法形成横桥，肌肉处于放松状态。

Excited state: When Ca²⁺ is released from the sarcoplasmic reticulum, the sarcoplasmic Ca²⁺ concentration rises to approximately 10⁻⁵ mol dm⁻³. Ca²⁺ binds to troponin C, causing a conformational change in the entire troponin complex. This change shifts tropomyosin deeper into the actin groove, exposing the myosin-binding sites, allowing cross-bridge formation.

兴奋状态： 当Ca²⁺从肌浆网释放时，肌浆中Ca²⁺浓度升至约10⁻⁵ mol dm⁻³。Ca²⁺与肌钙蛋白C结合，引起整个肌钙蛋白复合体的构象变化。这一变化将原肌球蛋白更深地推入肌动蛋白沟槽中，暴露肌球蛋白结合位点，允许横桥形成。

Relaxation: When the action potential ceases, the SERCA pump actively transports Ca²⁺ back into the sarcoplasmic reticulum. As sarcoplasmic Ca²⁺ falls, troponin releases Ca²⁺, tropomyosin returns to its blocking position, and cross-bridge cycling stops.

放松： 当动作电位停止时，SERCA泵将Ca²⁺主动转运回肌浆网。随着肌浆中Ca²⁺浓度下降，肌钙蛋白释放Ca²⁺，原肌球蛋白回到阻断位置，横桥循环停止。

8. Energy Supply for Muscle Contraction | 肌肉收缩的能量供应

Sustained muscle contraction requires a continuous supply of ATP. Muscle fibres have several strategies to regenerate ATP:

持续的肌肉收缩需要持续供应ATP。肌纤维有多种再生ATP的策略：

(1) Phosphocreatine (creatine phosphate) system: Phosphocreatine stores a phosphate group that can be rapidly transferred to ADP by creatine kinase, regenerating ATP within milliseconds. This provides immediate energy for the first 5-10 seconds of intense exercise.

（1）磷酸肌酸系统： 磷酸肌酸储存一个磷酸基团，可通过肌酸激酶快速转移到ADP上，在毫秒级时间内再生ATP。这为剧烈运动的最初5-10秒提供即时能量。

Phosphocreatine + ADP ⇌ Creatine + ATP (catalysed by creatine kinase)

磷酸肌酸 + ADP ⇌ 肌酸 + ATP（由肌酸激酶催化）

(2) Anaerobic glycolysis: Glucose is converted to pyruvate and then to lactate, producing a net of 2 ATP per glucose molecule. This pathway is rapid but produces lactic acid, contributing to muscle fatigue.

（2）无氧糖酵解： 葡萄糖被转化为丙酮酸，再转化为乳酸，每分子葡萄糖净产生2个ATP。该途径速度快，但产生乳酸，导致肌肉疲劳。

(3) Aerobic respiration: In the presence of oxygen, glucose, fatty acids, and amino acids are fully oxidised via glycolysis, the Krebs cycle, and oxidative phosphorylation, yielding approximately 30-32 ATP per glucose molecule and around 106-110 ATP per fatty acid molecule. This is the most efficient pathway but requires a continuous oxygen supply.

（3）有氧呼吸： 在有氧条件下，葡萄糖、脂肪酸和氨基酸通过糖酵解、Krebs循环和氧化磷酸化被完全氧化，每分子葡萄糖产生约30-32个ATP，每分子脂肪酸产生约106-110个ATP。这是最有效的途径，但需要持续供氧。

The type of energy system used depends on the intensity and duration of exercise. Short, intense activities rely on phosphocreatine and anaerobic glycolysis, while prolonged endurance activities depend primarily on aerobic respiration.

使用的能量系统类型取决于运动的强度和持续时间。短时间剧烈活动依赖磷酸肌酸和无氧糖酵解，而长时间耐力活动主要依赖有氧呼吸。

9. Muscle Fatigue and Oxygen Debt | 肌肉疲劳与氧债

Muscle fatigue occurs when a muscle can no longer contract despite continued stimulation. Causes include:

肌肉疲劳是指肌肉在持续刺激下无法再收缩的状态。其原因包括：

Depletion of phosphocreatine and glycogen stores.
磷酸肌酸和糖原储备的耗竭。
Accumulation of lactic acid, which lowers intracellular pH and inhibits enzymes involved in glycolysis and cross-bridge cycling.
乳酸积累，降低细胞内pH值，抑制参与糖酵解和横桥循环的酶。
Depletion of ATP, leading to failure of the SERCA pump and Ca²⁺-mediated regulation.
ATP耗尽，导致SERCA泵和Ca²⁺介导的调节功能失效。
Inhibition of ryanodine receptors by Mg²⁺ and reduced Ca²⁺ release.
Mg²⁺对兰尼碱受体的抑制以及Ca²⁺释放减少。

Oxygen debt (excess post-exercise oxygen consumption, EPOC) refers to the extra oxygen consumed after exercise to restore the body to its resting state. This oxygen is used to:

氧债（运动后过量氧耗，EPOC）是指运动后机体为恢复静息状态而额外消耗的氧气。这些氧气用于：

Convert lactate back to pyruvate, glucose, or glycogen in the liver (Cori cycle).
在肝脏中将乳酸转化回丙酮酸、葡萄糖或糖原（Cori循环）。
Re-synthesise phosphocreatine stores in muscles.
在肌肉中重新合成磷酸肌酸储备。
Restore elevated body temperature, heart rate, and breathing rate to normal.
将升高的体温、心率和呼吸频率恢复正常。
Replenish oxygen stores in myoglobin and haemoglobin.
补充肌红蛋白和血红蛋白中的氧气储存。

10. Structural Changes in the Sarcomere During Contraction | 收缩过程中肌节的结构变化

During isotonic contraction (shortening against a constant load), the following changes are observed in the sarcomere:

在等张收缩（克服恒定负荷的缩短）过程中，可以观察到肌节的以下变化：

Region \| 区域	Change during contraction \| 收缩时的变化
I-band \| I带	Narrows \| 变窄
A-band \| A带	Remains constant \| 保持不变
H-zone \| H区	Narrows or disappears \| 变窄或消失
Z-line separation \| Z线间距	Decreases (sarcomere shortens) \| 减小（肌节缩短）

These observations are consistent with the sliding filament hypothesis: the thick filaments remain fixed in length while the thin filaments slide past them toward the M-line.

这些观察与肌丝滑行假说一致：粗肌丝长度保持不变，而细肌丝滑过粗肌丝向M线方向移动。

11. Clinical Relevance and Applications | 临床意义与应用

Understanding muscle contraction has important medical applications:

理解肌肉收缩具有重要的医学应用价值：

Myasthenia gravis: An autoimmune disease where antibodies attack acetylcholine receptors at the neuromuscular junction, causing muscle weakness and fatigue.
重症肌无力： 一种自身免疫性疾病，抗体攻击神经肌肉接头处的乙酰胆碱受体，导致肌无力和疲劳。
Malignant hyperthermia: A genetic disorder where certain anaesthetics trigger uncontrolled Ca²⁺ release from the sarcoplasmic reticulum, causing severe muscle rigidity and hyperthermia.
恶性高热： 一种遗传性疾病，某些麻醉药触发肌浆网中Ca²⁺的失控释放，导致严重的肌肉僵硬和高热。
Muscular dystrophy: Genetic mutations affecting structural proteins such as dystrophin, which connects the sarcomere to the cell membrane, leading to muscle degeneration.
肌营养不良症： 影响结构蛋白（如肌营养不良蛋白）的基因突变，该蛋白连接肌节与细胞膜，导致肌肉退化。
Rigor mortis: Post-mortem muscle stiffening caused by ATP depletion, useful in forensic science for estimating time of death.
尸僵： 死亡后因ATP耗尽导致的肌肉僵硬，在法医学中用于估计死亡时间。

12. Summary: Key Points for A-Level Examinations | 总结：A-Level考试要点

For CIE A-Level Biology examinations, students should be able to:

对于CIE A-Level生物学考试，学生应能够：

Label and describe the structure of a sarcomere, including Z-lines, I-band, A-band, H-zone, and M-line.
标注并描述肌节的结构，包括Z线、I带、A带、H区和M线。
Explain the sliding filament hypothesis and cite the evidence supporting it.
解释肌丝滑行假说并引用支持该假说的证据。
Describe the four stages of the cross-bridge cycle in detail.
详细描述横桥循环的四个阶段。
Explain the roles of Ca²⁺, troponin, tropomyosin, and ATP in regulating contraction and relaxation.
解释Ca²⁺、肌钙蛋白、原肌球蛋白和ATP在调节收缩和放松中的作用。
Outline the sequence of events in excitation-contraction coupling, including the role of the neuromuscular junction and T-tubules.
概述兴奋-收缩耦联中的事件序列，包括神经肌肉接头和T-管的作用。
Compare the mechanisms of ATP regeneration: phosphocreatine system, anaerobic glycolysis, and aerobic respiration.
比较ATP再生的机制：磷酸肌酸系统、无氧糖酵解和有氧呼吸。

Mastering these concepts provides a solid foundation for understanding muscle physiology, energy metabolism, and related clinical conditions.

掌握这些概念为理解肌肉生理学、能量代谢及相关临床疾病奠定了坚实的基础。

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Region \| 区域	Change during contraction \| 收缩时的变化
I-band \| I带	Narrows \| 变窄
A-band \| A带	Remains constant \| 保持不变
H-zone \| H区	Narrows or disappears \| 变窄或消失
Z-line separation \| Z线间距	Decreases (sarcomere shortens) \| 减小（肌节缩短）

The Molecular Mechanism of Skeletal Muscle Contraction | 骨骼肌收缩的分子机制