Muscle relaxants

Japanese: 筋弛緩薬 - きんしかんやく

This refers to drugs that cause relaxation of skeletal muscles. There are those that act on the central nervous system and those that act peripherally at the neuromuscular junction. The former act on the spinal cord and brain stem to cause relaxation of skeletal muscles, but do not have a strong muscle relaxant effect and are used to relieve muscle stiffness and tension. The latter have a strong effect of completely paralyzing the muscles and are mainly used during general anesthesia.

When a nerve is stimulated, acetylcholine (a nerve fiber transmitter) is released from the nerve terminal at the neuromuscular junction, which acts on the endplate, causing depolarization, which causes the muscle to contract. The depolarized endplate immediately returns to its original state (repolarizes) and attempts to respond to the next acetylcholine. This is the mechanism of muscle contraction, but if the endplate is not depolarized by acetylcholine, or if the endplate that has been depolarized does not repolarize, the muscle will not contract even if the nerve is stimulated. The mechanism of action of peripheral muscle relaxants is either of these, and those that use the former are called non-depolarizing muscle relaxants. These muscle relaxants are used safely in anesthesia in a wide range of fields, including during endotracheal tube intubation, relaxation of abdominal muscles during abdominal surgery, when stopping the patient's breath and performing artificial respiration during anesthesia, and treatment of convulsions.

[Hideo Yamamura and Yoshitsugu Yamada]

Central muscle relaxants

These drugs relax the central nervous system that controls skeletal muscles, and include methocarbamol, chlorzoxazone, chlormezanone, tolperisone, carisoprodol, chlorphenesin carbamate, phenprobamate, dantrolene sodium, and pridinol mesilate. They are primarily used orally in orthopedic surgery for shoulder and lower back pain (painful spasticity).

[Fumiji Koho]

Peripheral muscle relaxants

These drugs block the binding of acetylcholine to acetylcholine receptors at the neuromuscular junction. There are competitive blockers such as tubocurarine chloride and alcuronium chloride, and depolarizing blockers such as suxamethonium chloride. They are mainly used by injection for the purpose of muscle relaxation during surgery and anesthesia.

[Fumiji Koho]

[Reference] | General anesthesia

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

骨格筋の弛緩をおこす薬をいう。中枢神経に作用するものと、末梢(まっしょう)で神経筋接合部に作用するものとがある。前者は脊髄(せきずい)や脳幹に作用し骨格筋の弛緩をおこすもので、筋の弛緩作用は強いものではなく、筋肉のこり、筋緊張を和らげるために用いられる。後者は完全に筋が麻痺(まひ)する強い作用を有しており、主として全身麻酔時に用いられる。

　神経に刺激を加えると、神経筋接合部において、神経末端からアセチルコリン（神経線維の伝達物質）が出され、これが終板に作用して脱分極を生じ、このため筋肉は収縮する。脱分極をおこした終板はすぐに元に戻り（再分極する）、次にくるアセチルコリンに反応しようとする。以上が筋の収縮の機序（メカニズム）であるが、アセチルコリンによって終板が脱分極をしなかったり、一度脱分極をおこした終板が再分極しない状態では、神経が刺激を受けても筋の収縮はおこらない。末梢性の筋弛緩薬の作用機序はこのいずれかによるものであり、前者によるものを非脱分極性筋弛緩薬とよぶ。これら筋弛緩薬の麻酔科での使用は、気管内チューブの挿管時、腹部手術における腹筋の弛緩、麻酔中に患者の呼吸を止めて人工呼吸をするとき、およびけいれんの治療など、きわめて広い分野にわたり、安全に行われている。

［山村秀夫・山田芳嗣］

中枢性筋弛緩薬

骨格筋を支配している中枢神経に作用して弛緩をおこさせるもので、メトカルバモール、クロルゾキサゾン、クロルメザノン、トルペリゾン、カリソプロドール、カルバミン酸クロルフェネシン、フェンプロバメート、ダントロレンナトリウム、メシル酸プリジノールなどがある。おもに整形外科で、肩や腰の痛み（有痛性痙縮(けいしゅく)）に内服で使用される。

［幸保文治］