Week 12 — A&P Lab / Scientific Inquiry · "Watch a Neuron Fire"

Course: Anatomy & Physiology I (BIOL 2301 + BIOL 2101) · Silver Oak University (fictional sample) · Prof. Navarro
Objective: Objective 6 — read membrane-potential values; relate the action-potential phases to their ions · SLO A (relate structure to function; reason about a sequence) · SLO B (read and record physiological values)
Worth 50 points · Labs group = 15% of the grade · Lab 12
Format: a guided exploration of a free PhET physiology simulation (no download, nothing to buy) — you'll stimulate a neuron and read its membrane-potential values through the action potential, then catch the AI's mistakes when it labels the phases. (An at-home reaction-time alternative is in Part 3B if you can't run the simulation.)

This is the course's signature weekly component. Every instructional week has one A&P lab. This week's uses a free PhET simulation of a firing neuron; earlier weeks used a virtual atlas, a virtual microscope, and other PhET physiology sims. All lab resources are links to external sites — nothing to buy or download.

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Part 1 — The Big Picture

This week you learned how a neuron fires: it rests at about −70 mV (inside negative), and when a stimulus reaches threshold (≈ −55 mV) it runs a fixed sequence — depolarization (Na⁺ in, peak ≈ +30 mV) → repolarization (K⁺ out) → hyperpolarization → back to rest. Today you'll watch that happen — stimulate a simulated neuron and read the voltage climb and fall as the ion channels open — and match what you see to the phases and ions from class.

The scientific habit this builds: observation → reading a quantitative trace → matching the data to a mechanism. In physiology, "what's happening" is written in the membrane-potential values and the ions moving — and the lab is where reading them becomes automatic.

Background (optional, ~5 min): OpenStax A&P §12.4, "The Action Potential" — keep it open as your answer key for the phases, the ions, and the voltages: 🔗 https://openstax.org/books/anatomy-and-physiology-2e/pages/12-4-the-action-potential

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Part 2 — Your Scientific Question & Hypothesis

Physiology labs start like any inquiry — with a question and a prediction you'll test against evidence (here, the simulation's voltage trace).

The question: When a neuron is stimulated, what happens to its membrane potential over time — and which ion movement drives each phase?

Before you start, write your hypothesis / prediction:

I predict that when the neuron is stimulated, the membrane potential will rise from about __ mV toward a peak near _ mV during depolarization (driven by moving ), then fall back toward negative during repolarization (driven by moving ) — and that when I ask an AI to label the phases, it will make at least ___ error(s) I can catch.

(There's no "right" number for the AI part — you're predicting how reliable it will be, then checking.)

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Part 3 — Materials & Procedure

Part 3A — PhET "Neuron" (primary procedure)

You need (all free, in a browser):
- The PhET "Neuron" simulation (free, no download): 🔗 https://phet.colorado.edu/en/simulations/neuron
- Optional second reference: OpenStax §12.4 (linked above).
- An approved chatbot (Gemini, Claude, or ChatGPT) for Part 6.

Procedure:
1. Open the Neuron simulation and let it load. Note the resting membrane potential displayed before you do anything.
2. Click Stimulate to fire the neuron. Watch the membrane-potential trace rise to a peak and fall back. Re-stimulate as needed; use the slow-motion / pause controls so you can read the values.
3. Watch the sodium (Na⁺) and potassium (K⁺) channels/particles: note which ion moves IN as the voltage rises, and which ion moves OUT as the voltage falls.
4. Fill in the Part 4 table from what you observe, using the class values where the sim's display is approximate. Keep OpenStax §12.4 open and check each answer before moving on.

No access to the sim? Use the at-home reaction-time protocol in Part 3B instead — it measures your own nervous system in action. Either path earns full credit; do one.

Part 3B — At-home reaction-time "ruler drop" (alternative)

You need: a partner, a standard 30 cm ruler, and a calculator.

Procedure:
1. Have your partner hold the ruler vertically, the 0 cm end at the bottom, just above your open thumb-and-finger.
2. Without warning, they release it; you catch it as fast as you can. Record the cm mark where you catch it (smaller = faster reaction).
3. Do 5 trials. Record each catch distance, then compute your average catch distance.
4. (Optional conversion, given in the key) a shorter catch distance means a shorter reaction time — your nerves carried the "drop!" signal and triggered your muscles faster. This is the same action-potential machinery you studied, end to end: eye → brain → motor neurons → hand.

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Part 4 — Data Table (fill this in)

If you did Part 3A (PhET):

Phase of the action potential	What the membrane potential is doing	Which ion moves, and direction	Approx. voltage
Resting	steady, inside negative	(pump maintains gradients)	______ mV
Threshold	the "trigger" point	—	______ mV
Depolarization	rising toward positive	__ moves ____	up to ______ mV (peak)
Repolarization	falling back toward negative	__ moves ____	returning toward −70 mV
Hyperpolarization	brief dip below resting	(K⁺ channels slow to close)	just below ______ mV

Use the class values where the display is approximate: resting −70 mV, threshold −55 mV, peak ≈ +30 mV.

If you did Part 3B (reaction time):

Trial	Catch distance (cm)
1	______
2	______
3	______
4	______
5	______
Average	______ cm

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Part 5 — Identify the Reasoning

Answer in a sentence each:
1. At rest, is the inside of the neuron positive or negative, and what holds it there? (Name the pump and its 3-out / 2-in action.)
2. During depolarization the inside becomes positive. Which ion enters, and why does that movement make the inside positive?
3. Pick one structure or feature and explain how it serves the neuron's function of fast signaling (e.g., why does myelin speed conduction? what would a loss of it — as in MS — do?). (This is the structure→function habit.)
4. (Reaction-time path only:) Your "drop!" signal traveled eye → brain → motor neurons → hand as a chain of action potentials. Why would a faster average catch distance suggest faster nerve-and-muscle conduction?

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Part 6 — AI-Critique Moment (required — this is the BYOAI step)

Now bring in your approved chatbot (Gemini, Claude, or ChatGPT) and be the physiologist who checks its work.

1. Paste this to the chatbot: "List the phases of the action potential in order, and for each phase tell me which ion moves and in which direction. Also give the approximate resting potential, threshold, and peak voltages, and state how many of each ion the sodium–potassium pump moves and in which direction."
2. Check everything it says against the simulation and OpenStax §12.4:
   - Did it keep the order: resting → depolarization → repolarization → hyperpolarization? (Chatbots scramble these.)
   - Did it say depolarization = Na⁺ IN and repolarization = K⁺ OUT (not the reverse)?
   - Did it give the resting potential as ≈ −70 mV (negative) — not +70?
   - Did it state the pump as 3 Na⁺ OUT / 2 K⁺ IN (not reversed)?
3. Write 2–3 sentences reporting what the AI got right and at least one error you caught and corrected (with the correct phase, ion, direction, or value). If it happened to get everything right, say how you verified each claim against the sim and OpenStax — that's the skill.

The habit all term: the tool drafts, you judge. A chatbot will confidently scramble the phases or flip an ion — catching it is the point, and here the numbers matter: −70 resting, −55 threshold, +30 peak.

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Part 7 — What to Submit

Submit a single document (or text entry) with: your hypothesis/prediction, your completed Part 4 table (PhET or reaction-time), your Part 5 answers, and your Part 6 AI-critique paragraph. Due Sunday, Nov 22, 11:59 p.m. (50 points).

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Instructor answer key — REMOVE BEFORE PUBLISHING TO STUDENTS

Every value below is verified against standard A&P (OpenStax §12.4; InnerBody Nervous System) and re-computed in /tmp/w12_check.py. The simulation's exact on-screen numbers vary slightly; grade against the class values and the correct ions/order.

Part 4 (PhET) — verified answer table:

Phase	Membrane potential	Ion & direction	Voltage
Resting	steady, inside negative	Na⁺/K⁺ pump maintains gradients (3 Na⁺ out, 2 K⁺ in)	−70 mV
Threshold	the trigger point	—	−55 mV
Depolarization	rising to positive	Na⁺ moves IN	up to ≈ +30 mV (peak)
Repolarization	falling to negative	K⁺ moves OUT	returning toward −70 mV
Hyperpolarization	brief dip below rest	K⁺ channels slow to close	just below −70 mV

• Useful checks: −70 → −55 = 15 mV to reach threshold; −70 → +30 = a 100 mV swing on the upstroke. (Both re-verified in Python.)

Part 4 (reaction-time) — worked sample (numbers will vary per student; method is what's graded):
- Sample trials: 12, 15, 11, 14, 13 cm → average = (12+15+11+14+13)/5 = 65/5 = 13.0 cm. (Verified in /tmp/w12_check.py.)
- Optional time conversion (not required of students; for instructor context): a 13.0 cm drop corresponds to a reaction time of about 0.16 s (≈ 163 ms) using t = √(2d/g) with g = 980 cm/s² — a typical human value. Full credit requires a correct average of the five trials, not the time conversion.

• Part 5: (1) inside is negative (≈ −70 mV), held by ion gradients and the Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in per ATP). (2) Sodium (Na⁺) enters; because Na⁺ is positive, an influx of positive charge makes the inside positive (depolarization). (3) Myelin speeds conduction by letting the signal jump node to node (saltatory conduction); losing it (MS) slows or blocks signals → weakness, numbness, vision/coordination problems — structure→function. (4) a shorter average catch distance means your eye-to-brain-to-hand action potentials and muscle response completed faster, so less distance was fallen before you caught it.
• Part 6 (AI-critique): full credit for a specific catch — most commonly the AI scrambling the phase order, saying depolarization is "potassium leaving," giving a positive resting potential (+70), or reversing the pump (3 in / 2 out). Full credit also if the student verified each claim against the sim and OpenStax.

Grading rubric — 50 points

Criterion	Full	Partial	None
Hypothesis / prediction — a clear prediction about the voltage trace/ions (or reaction time) and the AI's reliability (6)	6	3–4	0–2
Data table (Part 4) — phases/ions/voltages correct (PhET) or five trials + a correct average (reaction time) (18)	18	9–15	0–7
Reasoning (Part 5) — resting potential & pump, the depolarization ion logic, and a sound structure→function point (14)	14	7–11	0–5
AI-critique (Part 6) — names a specific error caught and corrected with the right phase/ion/value (8)	8	4–6	0–3
Physiological language & values — uses the terms and the −70/−55/+30 values correctly (4)	4	2	0–1

Quality gate (self-checked): every phase→ion→voltage pairing in the key is verified against standard A&P (OpenStax §12.4; InnerBody): resting −70 mV (inside negative), threshold −55 mV, peak ≈ +30 mV; depolarization = Na⁺ IN, repolarization = K⁺ OUT, in the order rest → depol → repol → hyperpol; the Na⁺/K⁺ pump moves 3 Na⁺ out / 2 K⁺ in; the synapse uses neurotransmitters across the cleft. No structure, ion, or phase is mislabeled. Anatomy-accuracy gate: PASS. All numbers are pre-computed and re-verified in Python (/tmp/w12_check.py): −70 → −55 = 15 mV; −70 → +30 = 100 mV; the reaction-time sample averages to 13.0 cm (≈ 163 ms). Overview-level only (no Nernst). Quantitative gate: PASS.

Provenance & links: all lab resources are links to free external sites (PhET, OpenStax), each verified live; no files are hosted or redistributed and no license/CC claims are made.