Question
A nerve impulse travels along a myelinated axon at 120 m/s. If the axon is 1.8 m long (from spinal cord to big toe), how long does the signal take to reach the toe? Compare with an unmyelinated axon conducting at 2 m/s.
Solution — Step by Step
Myelinated speed m/s, unmyelinated m/s, length m. We want time for both.
s = 15 ms. That is faster than a blink.
s. Nearly one full second — your toe would feel pain way after you stepped on the nail.
. Myelination makes the signal 60× faster here. This is exactly why multiple sclerosis (which damages myelin) causes slow, uncoordinated movements.
Final answer: Myelinated: 15 ms. Unmyelinated: 900 ms. Ratio: 60:1.
Why This Works
Saltatory conduction lets the action potential ‘jump’ from one Node of Ranvier to the next instead of regenerating at every patch of membrane. The myelin sheath is a lipid insulator, so current leaks less and the signal travels further before needing a boost.
Alternative Method
If the question gives you internodal distance and time per node instead of bulk speed, use and multiply accordingly.
Most neural-system numericals come down to unit hygiene and remembering what each gate does at each phase. Draw the action potential graph before reading the question — it saves time.
Common Mistake
Students often forget to convert units — if length is in cm and speed in m/s, you’ll be off by 100×. Always line up SI units first.
Do not confuse passive channels (follow the gradient) with active pumps (fight the gradient). This single distinction clears half of all neural-system doubts.