Week 12 — Lecture Outline · Nervous Tissue, the Neuron & the Action Potential

Course: Anatomy & Physiology I (BIOL 2301 + BIOL 2101) · Silver Oak University (fictional sample) · Prof. Navarro
Objective covered: Objective 6 — Describe nervous tissue (the neuron and neuroglia), explain the resting membrane potential and the action potential as an ordered sequence of ion movements, and describe synaptic transmission.
SLOs touched: A (relate structure to function; reason about a sequence) · B (anatomical/physiological literacy; read membrane-potential values)
Meeting pattern: 2 sessions × 75 min = 150 min. Segment minutes below total ~150; scale to your own pattern.

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Week at a Glance

The week's big question	"How does a single cell turn a touch into an electrical signal — and pass that message on to the next neuron?"
By the end of the week, students can…	(1) name the parts of a neuron and their functions and list the major neuroglia; (2) explain the resting membrane potential (≈ −70 mV, inside negative) and the Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in); (3) put the action potential in order — resting → depolarization (Na⁺ in) → repolarization (K⁺ out) → hyperpolarization — with the right ion and voltage for each, and state the all-or-none principle; (4) describe synaptic transmission across the cleft.
Key vocabulary	neuron, dendrites, cell body (soma), axon, axon hillock, axon terminal, myelin sheath, node of Ranvier, saltatory conduction, neuroglia (astrocyte, oligodendrocyte, microglia, ependymal, Schwann cell, satellite cell), membrane potential, resting potential (≈ −70 mV), ion gradient, sodium–potassium pump, voltage-gated channel, threshold (≈ −55 mV), depolarization, repolarization, hyperpolarization, peak (≈ +30 mV), all-or-none principle, synapse, synaptic cleft, neurotransmitter, calcium (Ca²⁺) trigger
Materials	slides (Deck 12), the week's readings + video links, one approved chatbot (Gemini / Claude / ChatGPT) for the AI-critique moment and the tutorial, the PhET "Neuron" simulation (or a ruler for the at-home reaction-time lab)
Timing note	8 segments, ~150 min total. Session 1 = Segments 1–4 (~75). Session 2 = Segments 5–8 (~75).

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Segment 1 — Hook & the Promise (8 min) · Session 1 opens

Hook. Put one line on a slide: "You touch a hot stove — and your hand pulls back before you feel the pain." Ask how that's possible. Let them guess, then land it: "A sensory neuron turned heat into an electrical signal and raced it down its axon faster than you could think. Generating that signal — the action potential — is the single most important thing a neuron does, and by the end of today you'll trace it ion by ion." Then the surprise: "The 'electricity' in a neuron isn't electrons in a wire — it's ions crossing a membrane."

The promise (write it on the board): "By Friday you'll name the parts of a neuron, explain why the inside sits at about −70 millivolts at rest, put the action potential's phases in order with the right ion for each, and describe how the signal jumps to the next neuron."

Why it matters line (memory hook): "Structure determines function — again. A neuron's long shape and its insulation exist for one job: receive a signal and send it on, fast."

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Segment 2 — The Neuron: Structure → Function (20 min)

Plain language first. A neuron (nerve cell) is built to receive a signal and send it onward. Information flows in one direction: in through the dendrites → across the cell body → out along the axon.

The parts (one slide — a labeled-figure description; signal order):

Picture a neuron from its branched end to its tip.
- Dendrites — the branched "antennas" that receive incoming signals from other neurons.
- Cell body (soma) — holds the nucleus and most organelles; integrates the inputs and decides whether to fire.
- Axon — the long fiber that conducts the impulse away from the cell body. Begins at the axon hillock, the trigger zone where an action potential is launched.
- Axon terminals — the branched ends that output the signal to the next cell.
- Myelin sheath — fatty insulation wrapping many axons; speeds conduction.
- Nodes of Ranvier — the bare gaps between myelin segments where the signal "jumps" ahead.

Memory hook: "Dendrites are the inbox; the soma is the decision; the axon is the outgoing cable."

The clarification students always need: the long, insulated axon isn't decoration — a signal has to travel from your spinal cord to your toe in a fraction of a second, and the shape is the whole reason it can. Name a part, then ask: what does it do, and how does its shape make that possible?

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Segment 3 — Neuroglia & the Resting Potential (22 min)

Plain language first — the support crew. Neurons can't work alone. Neuroglia (glial cells) outnumber neurons and feed, protect, and insulate them.

The neuroglia (one slide; CNS vs. PNS):
| Location | Cells | What they do |
|---|---|---|
| CNS | Astrocytes | support neurons; help form the blood–brain barrier |
| | Oligodendrocytes | make myelin in the CNS |
| | Microglia | immune cells; clean up debris |
| | Ependymal | help make cerebrospinal fluid |
| PNS | Schwann cells | make myelin in the PNS |
| | Satellite cells | support neuron cell bodies |

"The one pairing to keep straight: oligodendrocytes myelinate in the CNS, Schwann cells in the PNS. Multiple sclerosis attacks CNS myelin — so it's an oligodendrocyte-territory disease."

Now the resting potential — the starting line. Before it fires, a neuron sits at a resting membrane potential of about −70 mV. The minus sign is the point: the inside is negative relative to the outside. Two things hold this state:
1. Ion gradients — more Na⁺ outside, more K⁺ inside.
2. The Na⁺/K⁺ pump — uses ATP to push 3 Na⁺ out for every 2 K⁺ in, running constantly to keep the "battery" charged.

Quick interaction (~3 min): "Is the inside of a resting neuron positive or negative? By about how many millivolts?" (Negative; about −70 mV.) Make them say the minus.

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Segment 4 — The Action Potential, IN ORDER (22 min) · Session 1 closes (~75)

Set it up: "The resting neuron is a charged battery. A strong enough stimulus makes it fire — and firing is a fixed four-step sequence. Learn the order and the ions, and you've learned the action potential."

The phases (one slide — a labeled-figure description; this is the heart of the week):

Trace the voltage as it changes over a few milliseconds.
1. Resting — about −70 mV, inside negative, waiting.
2. Depolarization — a stimulus pushes the membrane to threshold (≈ −55 mV); voltage-gated Na⁺ channels fly open → Na⁺ rushes IN → the inside flips positive, peaking at about +30 mV.
3. Repolarization — Na⁺ channels close, K⁺ channels open → K⁺ flows OUT → the inside drops back toward negative.
4. Hyperpolarization — a brief overshoot dips just below −70 mV, then the cell settles back to rest.

One fully worked numeric example (do it out loud — keep the values consistent):

Start at the resting potential, −70 mV. A stimulus arrives. To fire, the membrane must climb to threshold, −55 mV — that's a change of −70 → −55 = 15 mV to reach threshold. Once it crosses, Na⁺ floods in and the inside shoots all the way up to the peak, ≈ +30 mV. So the full upstroke is −70 → +30 = a 100 mV swing. "Fifteen millivolts to trigger it; a hundred-millivolt swing once it fires. Those three numbers — −70, −55, +30 — are the ones to keep straight."

The all-or-none principle: once threshold is crossed, the neuron fires a full action potential every time — there's no partial spike. A stronger stimulus doesn't make a bigger action potential; it makes them more often.

Name the misconceptions out loud, then cure each:
- ❌ "At rest, the inside of the neuron is positive."
✅ Cure: it's negative, about −70 mV. Circle the minus. The inside only goes positive briefly, during depolarization.
- ❌ "Depolarization is potassium leaving."
✅ Cure: depolarization is Na⁺ entering (inside goes up/positive). Repolarization is K⁺ leaving (inside comes back down). Sodium in to fire, potassium out to reset.
- ❌ "The phases can happen in any order."
✅ Cure: the order is fixed — rest → depolarize → repolarize → hyperpolarize — every time.

Interaction — Think-Pair-Share (~5 min): put four statements on a slide; for each, students decide true or false: (1) the resting potential is about +70 mV; (2) during depolarization Na⁺ moves into the cell; (3) repolarization is K⁺ moving out; (4) the action potential follows the all-or-none principle. (Answers: false — it's −70; true; true; true.)

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Segment 5 — Na⁺ In, K⁺ Out — and the Pump vs. the Channels (24 min) · Session 2 opens

Hook back in: "Last session: what a neuron is and the four-step firing sequence. Today: the two ions in detail, then how the message gets to the next neuron."

Slow down on the two ions, two directions (one slide):
- Depolarization = Na⁺ IN. At threshold (≈ −55 mV), voltage-gated Na⁺ channels open, positive sodium rushes into the cell, and the inside flips from negative to positive (peak ≈ +30 mV).
- Repolarization = K⁺ OUT. Na⁺ channels close, K⁺ channels open, positive potassium leaves the cell, pulling the inside back toward negative.

The distinction students blur — the pump vs. the channels (build it on the board):

During the action potential, ions move passively through gated channels — fast, downhill, no ATP. The Na⁺/K⁺ pump is different: it works in the background, using ATP to actively haul 3 Na⁺ OUT and 2 K⁺ IN per cycle, against their gradients, to restore and maintain resting concentrations so the neuron can fire again.
"Notice the directions are opposite: during depolarization the channels let Na⁺ in; the pump later pushes Na⁺ back out. And because the pump moves 3 positives out for 2 in, it also helps keep the inside slightly negative."

Misconception + cure:
- ❌ "The Na⁺/K⁺ pump moves 3 in and 2 out."
✅ Cure: it's 3 Na⁺ OUT, 2 K⁺ IN. Mnemonic: "three sodiums out the door, two potassiums back." Out-numbers-in is why it helps keep the inside negative.

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Segment 6 — Myelin, Speed & the MS Connection (18 min)

Set it up: "Why bother wrapping an axon in fat? Speed — and the clearest A&P payoff we'll see all term: damage the structure, predict the lost function."

Plain language first (one slide — a labeled-figure description):

A myelinated axon conducts much faster than a bare one, because the signal doesn't have to regenerate along every inch — it jumps from one node of Ranvier to the next. That leaping mode is saltatory conduction. Insulation + gaps = speed.

Land the clinical case (and the discussion seed):

In multiple sclerosis (MS), the immune system strips CNS myelin. With the insulation gone, signals slow, scatter, or fail — causing weakness, numbness, vision problems, and loss of coordination. "This is pure A&P logic: myelin speeds conduction, so destroying it slows conduction. Predict the symptom from the lost structure."

Quick interaction (~3 min): "Before I show you the symptom list — if myelin's job is speed, what would you predict a patient notices when it's damaged?" (Slow, weak, unreliable signals: numbness, weakness, blurred vision.) They usually nail it — that's the structure→function habit working.

Misconception + cure:
- ❌ "Myelin slows the signal down (it's in the way)."
✅ Cure: myelin speeds conduction by enabling saltatory (node-to-node) conduction. Lose it and conduction suffers.

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Segment 7 — Across the Synapse (20 min)

Plain language first — the junction between neurons (one slide — a labeled-figure description):

The synapse is the junction where one neuron passes its message to the next. There's a tiny gap — the synaptic cleft — between them. The signal crosses like this, in order:
1. The action potential reaches the axon terminal.
2. Voltage-gated Ca²⁺ channels open → calcium enters the terminal.
3. Calcium triggers vesicles to release neurotransmitter molecules into the cleft.
4. The neurotransmitter diffuses across and binds receptors on the next neuron, which may start a brand-new action potential there.

Land the key idea: "The signal is electrical along the axon, becomes chemical to cross the gap, then becomes electrical again in the next cell. It does not spark across — chemistry carries it." Name one neurotransmitter they've heard of (dopamine, serotonin) and note this is where most drugs and medications act.

Misconception + cure:
- ❌ "The action potential jumps the synaptic gap as a spark."
✅ Cure: at a chemical synapse the impulse triggers neurotransmitter release; the chemical crosses the cleft. (Ca²⁺ is the trigger that links the arriving impulse to the release.)

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Segment 8 — Technology Workflow + AI-Critique, Callback & Hand-off (16 min) · Session 2 closes (~75)

Technology workflow — the PhET "Neuron" simulation:
1. Open the free PhET "Neuron" simulation linked in the module.
2. Stimulate the neuron and watch the membrane-potential trace climb and fall.
3. Note the resting value, the peak, and which channels open as it fires.
4. Match what you see to the four phases and the two ions from class.

AI-critique moment (students verify, not consume):

Paste this to an approved chatbot: "List the phases of the action potential in order, and for each phase say which ion moves and in which direction. Also give the approximate resting potential, threshold, and peak voltages, and state the Na⁺/K⁺ pump ratio."
Then check its work against today's class. Chatbots frequently scramble the phase order, say depolarization is 'potassium leaving,' claim the resting potential is positive (+70), or reverse the pump to 3 in / 2 out. Your job all semester: the tool drafts, you judge — and here the numbers matter (−70 resting, −55 threshold, +30 peak; 3 Na⁺ out, 2 K⁺ in). This is exactly how this week's lab AI-critique step works.

Callback + tease:
- Callback: "Three themes from earlier weeks all showed up: structure→function (the neuron's shape, the myelin), homeostasis (the pump keeping the resting potential steady), and even positive feedback (the Na⁺ upstroke — Na⁺ entry opens more Na⁺ channels)."
- Tease next week: "We built the single neuron and watched it fire. Next week we zoom out to the central nervous system — the brain and its regions, the spinal cord, the meninges and CSF that protect it, and the reflex arc that pulled your hand off that stove. Every one of those signals is built from the action potentials you learned today."

Hand-off (the week's graded work):
- Lecture Tutorial 12 (AI tutor, share-link submission) — neuron structure, the resting potential, the action-potential phases, and the synapse.
- Quiz 12 and Discussion 12 ("When the Insulation Fails") and Assignment 12 ("Trace the Signal").
- Lab 12 — "Watch a Neuron Fire" — stimulate a neuron on PhET and read membrane-potential values (or measure your own reaction time at home), then catch the AI's mis-labeled phases.

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Instructor FAQ — Common Stumbles

Student says / does	Quick cure
"The inside of a resting neuron is positive."	It's negative, about −70 mV. It goes positive only briefly during depolarization.
"Depolarization is potassium leaving."	Depolarization is Na⁺ IN (inside up/positive). Repolarization is K⁺ OUT. Sodium in to fire, potassium out to reset.
Scrambles the action-potential phases.	Fixed order: rest → depolarize → repolarize → hyperpolarize. Anchor each to its ion.
"The Na⁺/K⁺ pump moves 3 in, 2 out."	It's 3 Na⁺ OUT, 2 K⁺ IN per ATP. Out-numbers-in.
Confuses the pump with the channels.	Channels = fast, passive, no ATP (during the AP). Pump = slow, active, uses ATP, runs in the background.
"Myelin slows the signal."	Myelin speeds conduction (saltatory, node-to-node). MS damages it → signals slow.
"The signal sparks across the synapse."	At a chemical synapse, the impulse triggers neurotransmitter release; the chemical crosses the cleft (Ca²⁺ is the trigger).
Says a stronger stimulus makes a bigger action potential.	All-or-none: the AP is full-size every time. A stronger stimulus fires them more often, not bigger.

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Scope flag

This outline stays within Objective 6 (nervous tissue: the neuron and neuroglia; the resting and action potentials as an ordered overview; synaptic transmission). The membrane potential is taught at overview level — ion concentrations, the ≈ −70 mV resting value, the phase ordering, and the key voltages (−55 threshold, +30 peak) — not the Nernst/Goldman equations or full electrophysiology (deferred; not required first semester). The central nervous system (brain regions, cord, meninges, reflex arc) is Week 13 and only teased here. The PNS/ANS (sympathetic vs. parasympathetic) is Week 14. Neurotransmitters are named as the synaptic messengers and as where drugs act — endocrine/pharmacology is not taught here. Named structures and processes are referenced factually; the instructor and institution remain fictional.