A motor nerve is 0.5 m long. When healthy, all of it conducts at 60 m/s…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A motor nerve is m long. When healthy, all of it conducts at m/s. A disease strips the myelin from the last m. In that part the speed falls to m/s, while the other m still conducts at m/s. These are model values.
- How long does a signal take in the healthy nerve?
- How long does it take in the damaged nerve?
- What is the average speed over the damaged nerve?
Need a reminder? The key ideaShow
Time equals distance divided by speed. A thin non-myelinated fibre conducts at about , and a thick myelinated fibre at about to . An electrical synapse passes current directly. A chemical synapse uses a neurotransmitter, and it adds a delay of about . The new potential may be excitatory or inhibitory.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Length: m. Healthy speed: m/s. Damaged part: m at m/s. The rest: m at m/s.
Plan. Add the times of the two parts and divide the distance by the total time.
Step 1. Healthy. s ms.
Step 2. Good part of the damaged nerve. ms.
Step 3. Bare part. ms.
Step 4. Damaged total. ms.
Step 5. Average speed. m/s.
Answer: ms; ms; m/s.
Check. A tenth of the nerve has slowed the whole message by about three times. The slow part controls the total time.
Watch out. The average speed is not the average of and . It is the distance divided by the time.
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