Week 12 — Signed Steps and a Ruler Drop: A Nerve Impulse Report — Traditional alternative (no AI)

Instructors: publish this instead of the adaptive version if you prefer, and unpublish or delete the adaptive one.

Assignment 11 of the term — work the action potential's two signed differences, average a ruler drop, put the spike and the synapse in order, and explain to someone new why losing myelin slows a signal.

This week you learned how a neuron is built and supported by the neuroglia, why a resting neuron holds a charge of about −70 mV, how the action potential runs in a fixed order with sodium in and then potassium out, why every spike is all or none, how myelin lets a signal jump, and how a signal crosses a synapse as a chemical. Week 3 gave you the sodium-potassium pump and the difference between passive and active transport, and Week 10 showed a motor neuron releasing acetylcholine at the neuromuscular junction; this week you open the neuron itself. In this assignment you work four short problems, interpret each one, and explain myelin to someone who has never studied anatomy and physiology. This week's lecture tutorial, quiz, discussion, and A&P Lab & Inquiry practise the same ideas from other angles.

The task

This week's shared datasets are Dataset I, the action-potential values (conventional approximations: rest about −70 mV, threshold about −55 mV, peak about +30 mV), and Dataset J, the ruler drop (illustrative: five catches of 18, 22, 19, 21, and 20 cm). The spike runs rest → depolarization (Na⁺ in) → repolarization (K⁺ out) → hyperpolarization → rest, and a synapse runs action potential at the terminal → Ca²⁺ in → neurotransmitter released → neurotransmitter binds the next cell.

Work these four problems, showing your arithmetic:

1. A neuron rests at −70 mV and reaches threshold at −55 mV. How large is the rise from rest to threshold, in mV?
2. The same neuron climbs from rest at −70 mV to a peak of +30 mV. How large is the upstroke, in mV?
3. In the ruler drop, five catches land at 18, 22, 19, 21, and 20 cm. What is the sum of the five catches, in cm?
4. Using that sum, what is the average catch distance, in cm?

Beside each answer, add one brief interpretation:

• Problem 1: name the channels that open at threshold, and say which ion moves and in which direction once they open.
• Problem 2: name the phase that follows the peak with its ion and direction, and name the brief phase that comes after it.
• Problem 3: name the links in the chain a ruler catch measures, and put the four steps at one synapse along that chain in order.
• Problem 4: say in words about how long a 20 cm catch takes, and explain whether a longer average distance would mean a faster or a slower chain.

Explain it to a non-expert

A friend reads that a disease which strips myelin from nerve fibers in the brain and spinal cord can slow or block signals, and asks, "Why would losing a wrapping slow a signal down? Isn't the nerve still there?" Write a 3–5 sentence message that explains, in everyday words, what myelin is and which cells make it, how the signal jumps from gap to gap along a wrapped fiber, and why losing the wrapping slows or blocks the jumps. Name one limit, for example that this course describes speeds only in words, or that this message is about how nerves work and not about anyone's health. Put any technical term in everyday words. More than one good message is possible.

For a different, ungraded example, imagine a friend who asks why a neuron at rest still holds a charge. Model answer: "Think of a phone battery sitting in a drawer: it is not doing anything, but it is ready. A resting nerve cell is like that, sitting at about minus 70 millivolts, with the inside more negative than the outside. A pump in its outer layer keeps more sodium outside and more potassium inside, and that uneven spread is the stored charge. The battery is only a classroom picture, and it leaves out how the charge is spent when the cell fires."

Use that pattern for the message to your friend, with your own wording and evidence.

Show your arithmetic. A reader should be able to rebuild every value you report from the numbers given, without guessing which numbers you used or which operation you chose.

Deliverables

Submit a single report containing:

• the four answers, each with the arithmetic shown and the brief interpretation the task names for that problem;
• your message to your friend, explaining what myelin is and which cells make it, how the signal jumps from gap to gap, why losing the wrapping slows or blocks the jumps, and one limit; and
• a one-line restatement of the values you used, so your results are reproducible.

Keep it to roughly one page.

Choose how you show it. Present your report as written text, or as a recorded audio or video explanation of about three minutes with a transcript, or as a labeled sketch of one action potential with its phases and ions and a short note. Every form is graded by the same scoring criteria. Submit in any accessible file type or as a link; a recorded response may be submitted as audio or video with a transcript, or as text.

How you'll be graded

The full analytic rubric is attached to this assignment in Canvas.

Academic integrity and AI use

This version is designed to be completed without AI: do the arithmetic, the interpretations, and the writing yourself, without a chatbot or other AI tool. You may use a calculator or a spreadsheet for the arithmetic. Submitting AI-generated work as your own, or copying another student's work, is an integrity violation under the course policy.

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