ニューロン(神経細胞)がシグナルを伝える仕組みについて、わかりやすく簡単に解説したいと思います。
あやまって熱い鍋に触れてしまったとしましょう。そんなとき、わたしたちは「熱い！」と思い、わたしたちは手を引っ込めるという反応をします。これは、手の神経細胞が「熱さ」という刺激を受け取り、その情報が脳や脊髄に伝わったということなのです。
「熱い」と感じなければ、ずっと鍋に手を触れたままで、ひどいやけどをしてしまいます。そうならないのは、ニューロンが受け取った刺激が中枢神経(脳や脊髄)へすばやく伝えられ、中枢神経が受け取ったシグナルに対して反応を命令しているからなのです。
このような神経の仕組みについて、ステップに分けて、わかりやすく解説していきたいと思います。
ニューロンの形と名称
ニューロンは大きく分けると、核のある部分である細胞体と長く伸びた軸索からなります。細胞体には樹状突起という部分があり、何千ものほかのニューロンとつながっています。軸索の末端はほかのニューロンとつながっていたり、筋細胞や腺細胞とつながったりしています。ニューロンの種類によって異なります。
① 受け取った刺激をニューロンが伝導させる
まず、「熱い」という刺激を受容器(感覚器)が受け取ります。受容器は特定の刺激だけを感知できるように専門化しており、それぞれ特有の感覚細胞(受容細胞)を持っています。たとえば、光を感知する視細胞はヒトならば目だけにあります。足では光を感じる(＝ものが見える)ことはできません。
しかしミミズは目はありませんが体表に視細胞が散らばっているので、光を感じることができます。このように、鼻は嗅覚を完治する受容器、耳は聴覚を感知する受容器、肌は触覚を感知する受容器で、それぞれが特有の刺激を受け取ることができます。
視細胞のような受容細胞が刺激を受け取ると、ある特定の酵素が活性化させられます。これにより、ある特定の物質の濃度が上がり、細胞膜の電位変化が起こります。
細胞は通常は内側がマイナスの電荷、外側がプラスの電荷を帯びています。そこへ刺激が発生すると、ニューロンの軸索の細胞膜にあるNa⁺チャネルが開き、刺激が起こった部分の細胞膜内にNa⁺が流れ込みます。すると、内側がプラスになり、外側がマイナスになります。このように興奮したときに生じる電位の変化を活動電位といいます。
図の①のように活動電位が発生しました。すると、今度は隣の部分が興奮して、その部分のプラスとマイナスが入れ替わり、もともと活動電位が生じていた場所の内側はマイナスに戻ります(図②)。
さらに同じように、隣に移った活動電位はさらにその隣へと移動していき、「内プラス外マイナス」の部分がニューロンの両端へと移動していきます。これが興奮の「伝導」です。また、このように「内プラス外マイナス」に一時的に電位が逆転することを「神経インパルス」といいます。
活動電位 → 興奮によって生じる電位の変化
神経インパルス → 神経の中を伝わっていく活動電位のこと
興奮の伝導 → 「内プラス外マイナス」の状態になって活動電位がニューロンの左右に伝わっていくこと
興奮と活動電位はなにが違うんだ、と思った人がいるかもしれません。広義では違いがあるのですが、ニューロンや感覚細胞における興奮は、「活動電位が発生すること＝興奮」と思っていてＯＫです！
全か無かの法則
細胞膜の電位が内プラス外マイナスになることが神経インパルスですが、神経インパルスは刺激の強さには関係がなく、常に一定の大きさです。ある神経を起こさせるのに必要な最小の強さの刺激を閾値といいます。
この閾値を超えていれば興奮が起こり、神経インパルスは一定の常に一定です。強い刺激があったからといって、強い活動電位が発生するわけではありません。強い刺激を受け取っても弱い刺激を受け取っても同じ反応が起こることは「全か無かの法則」と呼ばれます。神経インパルスは全か無かの法則に従います。
なので、沸騰したお湯の入っているお鍋と、湯飲みにいれてしばらくたったお湯、どちらの刺激でも発生する活動電位の大きさは同じです。しかし、わたしたちは「熱い」と「温かい」を感じわけられます。
これは細胞ごとに活動電位が発生するために必要な刺激の閾値が異なるからです。「熱い」ときには多くの細胞がニューロンが興奮していますが、「温かい」と感じるときには「熱い」と感じるときよりも少ないのです。
跳躍伝導 – すばやく興奮が伝導できる画期的な仕組み
興奮が伝導する速度は神経によって異なります。太い神経ほど速いのですが、それでも１秒間に数メートルという速さです。ということは、首筋で受け取った刺激は、つま先で受け取った刺激よりも、脳に届いて感じられるまでに長い時間がかかりそうだとは思いませんか。しかし実際には、つま先と首を同時に触れば、同時に刺激を感じます。
長い距離、興奮を伝導させなければならないニューロンには髄鞘と呼ばれる部分があるからです。髄鞘は絶縁体で、電気を通しません。なので活動電位は、髄鞘がある部分を飛び越えて、髄鞘に覆われていない部分(ランビエ絞輪)だけを流れます。これにより、秒速100メートルもの速さで興奮は伝導することができ、わたしたちの体感では「同時」に感じられるのです。
このようにして、活動電位はときには1メートルもの長さのある軸索を「飛ぶように」すばやく伝導することができます。髄鞘と髄鞘の間の軸索の部分だけに飛び飛びに活動電位が流れる有髄神経の伝導は「跳躍伝導」と呼ばれます。
髄鞘を持つニューロンを有髄神経といいます。髄鞘がなかったら、脳までシグナルが届くまでにそれなりの時間がかかってしまうので、刺激を受け取ってから実際にそれを感じるまでにそれなりに長い時間がかかってしまっていたでしょう。なので、髄鞘のない無髄神経だけしかなかったら、動物の動きは極めてゆっくりなものになっていたと思われます。
髄鞘は脂質でできているので活動電位は流れません。髄鞘はシュワン細胞と呼ばれる細胞が軸索に絡みついてロールケーキのようにぐるぐる巻きになっています。なので、髄鞘にはシュワン細胞の核もあります。
② ニューロン同士での興奮の伝達
ニューロンの本体である細胞体には樹状突起という部分があり、何千ものほかのニューロンの末端とつながっています。このつながっている部分のことを「シナプス」といいます。しかし、シナプスでは直接、細胞と細胞はつながっていません。なので、神経インパルスのままでシグナルを伝えることはできません。なので、神経伝達物質という化学物質を用いてシグナルを伝達させるのです。
ニューロンの末端は少し膨らんでいます。そこにはシナプス小胞と呼ばれる小さな袋がたくさん存在し、その中には神経伝達物質が蓄えられています。興奮が神経の末端まで届くと、細胞膜にあるカルシウムチャンネルが開いて細胞内にCa²⁺が流れ込みます。それが合図となって、シナプス小胞は細胞の最末端へと押し出され、細胞膜と融合して、神経伝達物質を細胞と細胞の隙間(細胞間隙)に放出します。
受け取る側の細胞膜には、神経伝達物質の受容体分子(レセプター)が並んでいて、そこに神経伝達物質が結合します。それによって、神経伝達物質を受け取った側の細胞でも、興奮の伝導がはじまります。
このようなシナプス間でのシグナルの伝え方を興奮の「伝達」といいます。軸索でのバケツリレーのような伝え方は興奮の「伝導」でした。「伝達」「伝導」とまどろっこしいですが、それぞれはシグナルの伝えられる場所と方法が違います。間違わないように気をつけましょう。
興奮の伝導 → ニューロンの軸索で活動電位として興奮が伝わること
興奮の伝達 → シナプスでのニューロン間で興奮が伝わること
ちなみに、シナプスには、隣のニューロンを興奮させる興奮性シナプスと、次のニューロンの興奮を静める抑制性シナプスの二種類があります。
興奮性シナプスの主な神経伝達物質には、アセチルコリン、ノルアドレナリン、ドーパミンがあげられます。抑制性シナプスではGABA(γアミノ酪酸)が主な神経伝達物質となっています。
③ 情報が中枢神経で処理される
脳や脊髄などの中枢神経系は感覚ニューロンから入ってきた情報を記憶したり、記憶された情報と照らし合わせて運動ニューロンにどのように行動するか命令を与えたりします。このような情報処理を行うのが中枢神経の働きです。
反射は大脳を介さない単純な反応のことです。よく「反射神経がよい」というときの反射というもので、熱いものに触れたときに手を引っ込めるといったような、考える前に(大脳に情報が届くまえに)、無意識に行われる行動が反射です。大脳を介さないので複雑な動作は行えませんが、その代わり早く反応をすることができます。
以上が、高校生物レベルで知っておきたいニューロンに関する知識になります。これ以上の詳しい事項や、面白い雑学コラムは別記事をご覧ください！
・ 吉田邦久『好きになる生物学』
・『Essential 細胞生物学』(原書第4版)
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