Muscle Contraction | 肌肉收缩

📚 Muscle Contraction | 肌肉收缩

Muscle contraction is a fundamental physiological process that converts chemical energy into mechanical work, enabling movement, posture and vital functions such as breathing and circulation. This article follows the Cambridge International A-Level Biology syllabus to explain the hierarchical structure of skeletal muscle, the sliding filament theory, excitation-contraction coupling, the cross-bridge cycle and the regulation of force. A thorough grasp of these mechanisms is central to understanding locomotion and cellular energetics.

肌肉收缩是将化学能转化为机械功的基本生理过程，使运动、姿势以及呼吸、循环等关键功能得以实现。本文依据剑桥国际A-Level生物学大纲，讲解骨骼肌的分层结构、滑动丝模型、兴奋-收缩耦联、横桥循环以及力量的调控。透彻理解这些机制是掌握运动与细胞能量学的核心。

1. Structure of Skeletal Muscle | 骨骼肌的结构

Skeletal muscle is organised into fascicles of long, cylindrical, multinucleated muscle fibres. Each fibre is enclosed by a cell membrane called the sarcolemma and contains hundreds to thousands of myofibrils. Connective tissue layers – epimysium, perimysium and endomysium – bind fibres together, conduct blood vessels and nerves, and merge into tendons that transmit force to bones. The highly organised internal structure of myofibrils gives skeletal muscle its characteristic striated appearance.

骨骼肌被组织成由长圆柱形多核肌纤维组成的肌束。每条纤维由称为肌膜的细胞膜包裹，内部含有成百上千条肌原纤维。结缔组织层——肌外膜、肌束膜和肌内膜——将纤维束捆绑在一起，为血管和神经提供通路，并汇聚成肌腱将力传递至骨骼。肌原纤维内部高度有序的结构赋予骨骼肌特有的横纹外观。

Each myofibril is about 1–2 μm in diameter and is subdivided along its length into repeating contractile units called sarcomeres. The sarcomere is the smallest functional unit of a muscle fibre and is bordered by Z-lines. Under the light microscope, the arrangement of thick and thin filaments creates alternating dark A-bands and light I-bands, while the central H-zone and M-line add further detail to the banding pattern.

每条肌原纤维直径约1-2微米，沿纵向被细分为称作肌节的重复收缩单位。肌节是肌纤维中最小的功能单位，边界由Z线界定。在光学显微镜下，粗肌丝与细肌丝的有序排列形成明暗交替的A带和I带，而中央的H区和M线则为带型增添了更多细节。

2. Sarcomere: The Contractile Unit | 肌节：收缩单位

A sarcomere extends from one Z-line to the next. Thick filaments, composed mainly of myosin

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