跳躍伝導レース：有髄神経 vs 無髄神経Saltatory Conduction Race: Myelinated vs Unmyelinated Nerves

1予想しよう（触る前に）Predict (before you touch anything)

1. 髄鞘で包まれた有髄神経と、髄鞘のない無髄神経で同時に興奮を起こすと、脊髄から 30 cm 先の筋肉に先に届くのはどちらだと思いますか。どのくらい差がつくと思いますか。If an impulse starts at the same moment in a myelinated nerve (wrapped in myelin) and an unmyelinated nerve, which do you think reaches a muscle 30 cm from the spinal cord first? How big do you think the difference will be?💡 髄鞘は電気を通しにくい「さや」です。さやに覆われた部分で興奮が起こるかどうかを考えましょう。Myelin is a sheath that does not conduct electricity well. Think about whether an impulse can occur where the axon is covered by it.
2. 軸索を太くすると、伝わる速さはどうなると思いますか。有髄と無髄で同じように変わるでしょうか。What do you think happens to the speed when the axon gets thicker? Will it change the same way for myelinated and unmyelinated nerves?💡 画面の速度の欄に書かれている計算式に注目しましょう。Look at the formulas written next to the speed values on screen.

2動かしてみようTry it

1. 最初の状態(d1 = 10.0 μm、d2 = 1.0 μm、L = 30 cm)で「▶ スタート（同時に発火）」を押し、上段の有髄神経では光が「ランビエ絞輪」から絞輪へ跳び「髄鞘」の部分は光らないこと、下段の無髄神経では光が連続的に進むことを観察する。In the starting state (d1 = 10.0 μm, d2 = 1.0 μm, L = 30 cm), press "▶ Start (fire both)". Observe that in the top myelinated nerve the light jumps from node of Ranvier to node while the myelin sheath stays dark, and that in the bottom unmyelinated nerve the light moves continuously.
2. 両方の神経が「到達・筋肉収縮！」になったら、それぞれの「速度」と「到達」の値(60.0 m/s・5.00 ms と 1.00 m/s・300.0 ms)を記録する。必要なら「⏸ 一時停止」で止めて見る。When both nerves show "Arrived, muscle contracts!", record each "Speed" and "Arrival" value (60.0 m/s, 5.00 ms and 1.00 m/s, 300.0 ms). Use "⏸ Pause" to freeze the animation if needed.
3. 「有髄の軸索の太さ d1」を 4.0 μm、「無髄の軸索の太さ d2」を 4.0 μm にして、速度と到達時間の変化を記録する。Set "Myelinated axon diameter d1" to 4.0 μm and "Unmyelinated axon diameter d2" to 4.0 μm, and record how the speeds and arrival times change.
4. 「脊髄〜筋肉の距離 L」を 100 cm と 1 cm に変えて、2つの到達時間の差がどう変わるかを比べる。Change "Distance L (spinal cord → muscle)" to 100 cm and then 1 cm, and compare how the difference between the two arrival times changes.
5. d1 を 1.0 μm(最も細い)、d2 を 5.0 μm(最も太い)、L を 30 cm にして、それでもどちらが速いかを確かめる。Set d1 to 1.0 μm (thinnest), d2 to 5.0 μm (thickest) and L to 30 cm, and check which nerve is still faster.

3確かめようCheck

1. 有髄神経と無髄神経では、光(興奮)の進み方にどんな違いがありましたか。How did the way the light (impulse) moved differ between the myelinated and unmyelinated nerves?→ 有髄神経では髄鞘の部分は光らず、光はランビエ絞輪から次の絞輪へ跳ぶように進んだ(跳躍伝導)。無髄神経では光が端から端まで連続的に同じ速さで進んだ(連続伝導)。どちらも脊髄から筋肉への一方向だけに進んだ。In the myelinated nerve the myelin sheath never lit up, and the light jumped from one node of Ranvier to the next (saltatory conduction). In the unmyelinated nerve the light moved continuously at a steady speed from end to end (continuous conduction). In both, it moved only one way, from spinal cord to muscle.
2. 最初の状態で、2本の神経の到達時間はそれぞれ何 ms でしたか。In the starting state, what were the arrival times of the two nerves?→ 有髄神経は 5.00 ms(60.0 m/s)、無髄神経は 300.0 ms(1.00 m/s)で、無髄神経は 60 倍の時間がかかった。筋肉はそれぞれの興奮が届いた瞬間に収縮した。The myelinated nerve took 5.00 ms (60.0 m/s) and the unmyelinated nerve 300.0 ms (1.00 m/s), so the unmyelinated nerve took 60 times as long. Each muscle contracted the moment its impulse arrived.
3. d1 と d2 をどちらも 4.0 μm にしたとき、速度はそれぞれどうなりましたか。With both d1 and d2 set to 4.0 μm, what were the speeds?→ 有髄は 24.0 m/s(6×4)、無髄は 2.00 m/s(√4)。到達時間は 12.5 ms と 150.0 ms になった。有髄は太さに比例して、無髄は太さの平方根に比例して速くなる。Myelinated: 24.0 m/s (6×4); unmyelinated: 2.00 m/s (√4). Arrival times became 12.5 ms and 150.0 ms. Myelinated speed grows in proportion to diameter, unmyelinated speed only with the square root of diameter.
4. 距離 L を 100 cm と 1 cm にしたとき、到達時間の差はどうなりましたか。When distance L was 100 cm and 1 cm, what happened to the difference in arrival times?→ d1 = d2 = 4.0 μm のとき、100 cm では 41.7 ms と 500.0 ms で差は約 458 ms、1 cm では 0.42 ms と 5.00 ms で差は約 4.6 ms。到達時間は距離 ÷ 速度なので、距離が長いほど差は大きくなる。With d1 = d2 = 4.0 μm, at 100 cm the times were 41.7 ms and 500.0 ms (a difference of about 458 ms); at 1 cm they were 0.42 ms and 5.00 ms (about 4.6 ms). Arrival time is distance ÷ speed, so the longer the distance, the bigger the difference.

4考えようThink further

1. d1 を最も細い 1.0 μm、d2 を最も太い 5.0 μm にしても有髄神経の方が速かったのはなぜでしょう。体にとってどんな利点があるでしょうか。Even with d1 at its thinnest (1.0 μm) and d2 at its thickest (5.0 μm), the myelinated nerve was faster. Why, and what advantage does this give the body?→ 有髄神経では興奮が絞輪だけで起こり跳ぶように進むので、細くても速い(6.0 m/s 対 2.24 m/s、50.0 ms 対 134.2 ms)。無髄神経で同じ速さを出すには軸索をとても太くしなければならないが、髄鞘があれば細い神経をたくさん束ねても速く伝えられるので、体の中に多くの神経を通しながら素早く反応できる。In a myelinated nerve the impulse occurs only at the nodes and jumps along, so even a thin axon is fast (6.0 m/s vs 2.24 m/s; 50.0 ms vs 134.2 ms). An unmyelinated nerve would need a very thick axon to match it, but with myelin many thin, fast nerves can be bundled together, so the body can fit many nerves and still respond quickly.
2. ヒトの運動神経がもし無髄神経(1 m/s 程度)だったら、熱いものに触れて手を引く反射はどうなるでしょうか。If human motor nerves were unmyelinated (about 1 m/s), what would happen to the reflex of pulling your hand away from something hot?→ 脊髄から手の筋肉まで約 1 m あると、命令が届くのに約 1 秒(1000 ms)かかる。有髄神経なら 10〜20 ms ほどで届くので、無髄では反射が大きく遅れ、やけどなどのけがが大きくなってしまう。With about 1 m from the spinal cord to the hand muscles, the command would take about 1 second (1000 ms) to arrive. Myelinated nerves deliver it in about 10-20 ms, so with unmyelinated nerves the reflex would be greatly delayed and injuries such as burns would be worse.

よくある誤解Common misconceptions

• 「髄鞘の中を電気が速く流れるから速い」という誤解。髄鞘は電気を通しにくい絶縁のさやで、髄鞘の部分では興奮は起こらず、とぎれ目(ランビエ絞輪)だけで興奮が起こって跳ぶように伝わるから速い。The misconception that the impulse is fast because electricity flows quickly through the myelin. Myelin is an insulating sheath; no impulse occurs under it. The impulse happens only at the gaps (nodes of Ranvier) and jumps between them, which makes it fast.
• 「神経は太ければ太いほど、種類に関係なく同じように速くなる」という誤解。有髄神経は太さに比例して速くなるが、無髄神経は太さの平方根にしか比例しないので、太くしてもあまり速くならない。The misconception that any nerve speeds up the same way as it gets thicker. Myelinated speed is proportional to diameter, but unmyelinated speed grows only with the square root of diameter, so thickening it helps much less.

用語（日本語 / English）Key terms (Japanese / English)