Muscle Contraction | 肌肉收缩

📚 Muscle Contraction | 肌肉收缩

Skeletal muscle contraction is a coordinated process that converts chemical energy from ATP into mechanical force. In Cambridge A-Level Biology, you need to understand the structure of a sarcomere, the roles of actin and myosin, how a nerve impulse triggers contraction, and why ATP is essential throughout the cycle.

骨骼肌收缩是一个协调的过程，将 ATP 中的化学能转化为机械力。在剑桥 A-Level 生物中，你需要理解肌节的结构、肌动蛋白和肌球蛋白的作用、神经冲动如何触发收缩，以及 ATP 在循环中为什么必不可少。

1. Skeletal Muscle Organisation | 骨骼肌的结构层次

Skeletal muscle is organised into bundles of muscle fibres, each surrounded by connective tissue. A single muscle fibre is a long, cylindrical, multinucleated cell with a specialised plasma membrane called the sarcolemma and cytoplasm called the sarcoplasm.

骨骼肌由肌纤维束组成，每条肌纤维被结缔组织包围。单条肌纤维是一个长圆柱形多核细胞，具有特化的细胞膜，称为肌膜，细胞质称为肌浆。

Inside each muscle fibre are many parallel myofibrils, which show alternating light and dark bands under the microscope. The sarcoplasmic reticulum, a specialised endoplasmic reticulum, stores calcium ions and surrounds each myofibril.

每条肌纤维内部有许多平行的肌原纤维，在显微镜下呈现明暗交替的条带。肌浆网是一种特化的内质网，储存钙离子并包围每条肌原纤维。

Muscle → muscle fibre bundles → single muscle fibre → myofibrils → sarcomeres
肌肉 → 肌纤维束 → 单条肌纤维 → 肌原纤维 → 肌节

2. The Sarcomere: Functional Unit | 肌节：功能单位

The sarcomere is the repeating contractile unit of a myofibril, extending from one Z-line to the next Z-line. Its banding pattern explains how contraction occurs.

肌节是肌原纤维中重复的收缩单位，从一条 Z 线延伸到下一条 Z 线。它的带型可以解释收缩是如何发生的。

The A-band is the dark region that contains thick myosin filaments, including any overlapping thin actin filaments. The I-band is the light region that contains only thin actin filaments. The H-zone is the lighter area in the centre of the A-band where there is no actin overlap, and the M-line lies in the middle of the sarcomere.

A 带是深色区域，含有粗肌球蛋白丝，包括与细肌动蛋白丝重叠的部分。I 带是浅色区域，只含有细肌动蛋白丝。H 区是 A 带中央较浅的区域，没有肌动蛋白重叠，M 线位于肌节中央。

During contraction, the I-band shortens and the H-zone narrows or disappears, but the A-band remains the same length. This is a key observation supporting the sliding filament model.

收缩时，I 带缩短，H 区变窄或消失，但 A 带长度保持不变。这是支持滑丝模型的关键观察。

3. Thick and Thin Filaments | 粗肌丝与细肌丝

The thick filament is made of many myosin molecules. Each myosin has a long tail and a protruding head with an actin-binding site and an ATPase site. The heads project from the thick filament and can bind to actin to form cross-bridges.

粗肌丝由许多肌球蛋白分子组成。每个肌球蛋白有一个长尾和一个突出的头部，头部有肌动蛋白结合位点和 ATP 酶位点。头部从粗肌丝伸出，能与肌动蛋白结合形成横桥。

The thin filament consists mainly of actin, tropomyosin, and troponin. Actin is a globular protein polymerised into two twisted strands, each subunit containing a myosin-binding site. Tropomyosin is a fibrous protein that lies along the groove of actin and blocks these binding sites at rest. Troponin is a complex of three proteins that binds to actin, tropomyosin, and calcium ions.

细肌丝主要由肌动蛋白、原肌球蛋白和肌钙蛋白组成。肌动蛋白是一种球状蛋白，聚合成两条扭曲的链，每个亚基含有一个肌球蛋白结合位点。原肌球蛋白是一种纤维状蛋白，沿肌动蛋白凹槽排列，静息时封闭这些结合位点。肌钙蛋白是由三种蛋白质组成的复合体，能结合肌动蛋白、原肌球蛋白和钙离子。

Thin filament = actin + tropomyosin + troponin

4. Neuromuscular Junction and Excitation | 神经肌肉接头与兴奋

A motor neurone transmits an action potential to a muscle fibre at the neuromuscular junction. The axon terminal, or synaptic knob, contains vesicles filled with the neurotransmitter acetylcholine.

运动神经元在神经肌肉接头处将动作电位传递给肌纤维。轴突末梢即突触小结含有充满神经递质乙酰胆碱的突触小泡。

When the action potential arrives, voltage-gated Ca²⁺ channels open in the presynaptic membrane. Calcium ions enter the synaptic knob, causing vesicles to move to the membrane and release acetylcholine by exocytosis into the synaptic cleft.

当动作电位到达时，突触前膜上的电压门控 Ca²⁺ 通道打开。钙离子进入突触小结，促使突触小泡移动到膜上，通过胞吐作用将乙酰胆碱释放到突触间隙。

Acetylcholine diffuses across the cleft and binds to receptor proteins on the muscle fibre membrane. These receptors are ligand-gated Na⁺ channels; when opened, Na⁺ enters the muscle cell, depolarising the membrane and producing an end-plate potential. If the depolarisation reaches threshold, an action potential is generated. Acetylcholinesterase then breaks down acetylcholine to prevent continuous stimulation.

乙酰胆碱扩散穿过突触间隙，与肌纤维膜上的受体蛋白结合。这些受体是配体门控 Na⁺ 通道；打开时，Na⁺ 进入肌细胞，使膜去极化并产生终板电位。如果去极化达到阈值，就会产生动作电位。乙酰胆碱酯酶随后分解乙酰胆碱，防止持续刺激。

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

The muscle fibre membrane has deep invaginations called T-tubules that carry the action potential into the interior. The T-tubules lie close to the sarcoplasmic reticulum.

肌纤维膜有深度内陷，称为 T 管，能将动作电位传导到内部。T 管与肌浆网紧密相邻。

When the action potential spreads along the T-tubules, it causes the sarcoplasmic reticulum to release calcium ions into the sarcoplasm. The calcium ions then bind to troponin C, causing a shape change in the troponin-tropomyosin complex.

当动作电位沿 T 管传播时，促使肌浆网将钙离子释放到肌浆中。钙离子随后与肌钙蛋白 C 结合，引起肌钙蛋白-原肌球蛋白复合体构象变化。

Action potential in T-tubule → Ca²⁺ released from sarcoplasmic reticulum → Ca²⁺ binds troponin

This shape change moves tropomyosin away from the myosin-binding sites on actin, exposing them so that myosin heads can bind. Calcium ions are therefore the link between excitation and contraction.

这种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，暴露出结合位点，使肌球蛋白头部能够结合。因此钙离子是兴奋与收缩之间的连接纽带。

6. The Sliding Filament Model | 滑丝模型

According to the sliding filament model, contraction occurs when thin actin filaments slide over thick myosin filaments. The filaments themselves do not shorten; instead, the sarcomere shortens as Z-lines are pulled closer together.

根据滑丝模型，当细肌动蛋白丝滑过粗肌球蛋白丝时发生收缩。肌丝本身并不缩短，而是肌节缩短，因为 Z 线被拉得更近。

During contraction, repeated cross-bridge cycling causes the actin filaments to be pulled towards the centre of the sarcomere. This reduces the I-band and H-zone widths while the A-band remains constant.

收缩过程中，反复的横桥循环将肌动蛋白丝拉向肌节中央。这使 I 带和 H 区宽度减小，而 A 带保持不变。

The force for filament sliding comes from the power stroke of the myosin head, which changes angle while attached to actin. After the power stroke, the myosin head detaches, returns to its original position, and can bind further along the actin filament.

肌丝滑动的力来自肌球蛋白头部的动力冲程，它在与肌动蛋白结合时改变角度。动力冲程后，肌球蛋白头部解离，回到原来的位置，并能结合到肌动蛋白丝更远处。

7. The Cross-Bridge Cycle | 横桥循环

The cross-bridge cycle describes the repeated attachment and detachment of myosin heads to actin. Each cycle requires ATP and Ca²⁺. The main stages are:

横桥循环描述肌球蛋白头部与肌动蛋白反复结合和解离的过程。每个循环都需要 ATP 和 Ca²⁺。主要步骤如下：

A myosin head with ADP and Pi bound attaches to an exposed actin-binding site, forming a cross-bridge.
肌球蛋白头部结合 ADP 和 Pi 后，与暴露的肌动蛋白结合位点结合，形成横桥。
ADP and Pi are released, causing the myosin head to change shape. This is the power stroke, which pulls the actin filament towards the centre of the sarcomere.
ADP 和 Pi 被释放，导致肌球蛋白头部构象改变。这就是动力冲程，将肌动蛋白丝拉向肌节中央。
A new ATP molecule binds to the myosin head, causing it to detach from actin.
新的 ATP 分子与肌球蛋白头部结合，使其与肌动蛋白解离。
The ATP is hydrolysed to ADP and Pi by the ATPase activity of the myosin head. This hydrolysis releases energy and returns the myosin head to its original cocked position, ready for another cycle.
ATP 被肌球蛋白头部的 ATP 酶水解为 ADP 和 Pi。水解释放能量，使肌球蛋白头部回到原来的竖起位置，准备进行下一个循环。

ATP → ADP + Pi provides energy for myosin head re-cocking

If Ca²⁺ concentration remains high, the cycle repeats as long as ATP is available. If Ca²⁺ is removed, tropomyosin re-covers the binding sites and contraction stops.

如果 Ca²⁺ 浓度保持较高，只要 ATP 可用，循环就会重复进行。如果 Ca²⁺ 被移除，原肌球蛋白重新覆盖结合位点，收缩停止。

8. Role of ATP and Rigor Mortis | ATP 的作用与尸僵

ATP plays several essential roles in muscle contraction. It provides the energy for the myosin head to detach from actin after the power stroke, and its hydrolysis re-cocks the myosin head so it can attach again.

ATP 在肌肉收缩中发挥着几个关键作用。它为动力冲程后肌球蛋白头部与肌动蛋白解离提供能量，其水解还能使肌球蛋白头部重新竖起，以便再次结合。

ATP is also required by the sarcoplasmic reticulum’s Ca²⁺ pumps, which actively transport Ca²⁺ back into the sarcoplasmic reticulum against its concentration gradient. Without this, calcium would remain in the sarcoplasm and troponin would stay activated.

肌浆网的 Ca²⁺ 泵也需要 ATP，它能逆浓度梯度将 Ca²⁺ 主动转运回肌浆网。如果没有 ATP，钙离子会留在肌浆中，肌钙蛋白将保持激活状态。

After death, ATP production stops. Without ATP, myosin heads cannot detach from actin, and Ca²⁺ pumps stop working, so cross-bridges remain firmly bound. This causes muscle stiffness known as rigor mortis, which lasts until muscle proteins begin to break down.

死亡后，ATP 生产停止。没有 ATP，肌球蛋白头部无法与肌动蛋白解离，Ca²⁺ 泵停止工作，因此横桥保持牢固结合。这导致肌肉僵硬，称为尸僵，持续到肌肉蛋白开始分解为止。

9. Muscle Fibre Types | 肌纤维类型

Muscles contain different fibre types adapted to different functions. Slow-twitch or slow oxidative fibres contract slowly and are specialised for endurance activities such as posture and marathon running. They contain many mitochondria, a rich blood supply, and large amounts of myoglobin, which stores oxygen for aerobic respiration.

肌肉含有不同类型的肌纤维，适应不同功能。慢缩肌或慢氧化型纤维收缩较慢，专门用于耐力活动，如保持姿势和马拉松跑。它们含有许多线粒体、丰富的血液供应和大量肌红蛋白，肌红蛋白为有氧呼吸储存氧气。

Fast-twitch or fast glycolytic fibres contract rapidly and are adapted for short bursts of intense activity such as sprinting. They have fewer mitochondria, less myoglobin, and rely more on anaerobic respiration, so they fatigue more quickly.

快缩肌或快糖酵解型纤维收缩迅速，适应短时间高强度活动，如短跑。它们线粒体较少，肌红蛋白较少，更多依赖无氧呼吸，因此更容易疲劳。

Feature	Slow oxidative fibres	Fast glycolytic fibres
Contraction speed	Slow	Fast
Mitochondria	Many	Few
Myoglobin	High	Low
Respiration	Aerobic	Mainly anaerobic
Fatigue resistance	High	Low

At A-Level, you may also see fast oxidative fibres described as an intermediate type. They contract quickly but have more mitochondria and myoglobin than fast glycolytic fibres, giving them a greater aerobic capacity.

在 A-Level 中，你可能会看到快氧化型纤维被描述为中间类型。它们收缩快，但线粒体和肌红蛋白含量高于快糖酵解型纤维，因此有氧代谢能力更强。

10. Summary and Common Exam Pitfalls | 总结与常见考试易错点

Muscle contraction involves excitation at the neuromuscular junction, calcium release from the sarcoplasmic reticulum, exposure of actin-binding sites, cross-bridge cycling, and ATP-dependent detachment. The sarcomere shortens as thin filaments slide over thick filaments, but the filaments themselves do not change length.

肌肉收缩涉及神经肌肉接头的兴奋、肌浆网释放钙离子、肌动蛋白结合位点的暴露、横桥循环以及依赖 ATP 的解离。肌节因细肌丝滑过粗肌丝而缩短，但肌丝本身长度不变。

The A-band does not shorten during contraction; only the I-band and H-zone shorten.
收缩时 A 带不缩短，只有 I 带和 H 区缩短。
Calcium ions come from the sarcoplasmic reticulum, not directly from the neuromuscular junction.
钙离子来自肌浆网，而不是直接来自神经肌肉接头。
Tropomyosin moves away from binding sites; troponin remains bound to actin and tropomyosin.
原肌球蛋白从结合位点移开；肌钙蛋白仍与肌动蛋白和原肌球蛋白结合。
ATP is needed for myosin head detachment and re-cocking, not only for the power stroke itself.
ATP 用于肌球蛋白头部的解离和重新竖起，而不仅仅用于动力冲程本身。

Use precise terminology such as sarcomere, sarcoplasmic reticulum, troponin C, power stroke, and cross-bridge cycle. In exam answers, avoid vague phrases like ‘calcium allows contraction’ without linking Ca²⁺ to troponin and tropomyosin movement.

在答题中使用准确术语，如肌节、肌浆网、肌钙蛋白 C、动力冲程和横桥循环。考试答案中避免含糊表述，如 ‘钙允许收缩’，而要将 Ca²⁺ 与肌钙蛋白和原肌球蛋白的移动联系起来。

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Muscle Contraction | 肌肉收缩