The Neuron & Neural Firing | AP Psychology online course

1. Neuron Anatomy (Physical Layer)

2. Electro-Chemical Workflow (Firing Sequence)

1. Resting Potential (–70 mV)

   • Inside cell: high K⁺, large negative proteins

   • Outside: high Na⁺, Cl⁻

   • Maintained by the Na⁺/K⁺ pump (3 Na⁺ out : 2 K⁺ in, ATP-driven).

2. Threshold & Depolarization

   • Summation at axon hillock hits ≈ –55 mV.

   • Voltage-gated Na⁺ channels fling open → Na⁺ flood → spike to +30 mV.

3. Repolarization

   • Na⁺ channels close; K⁺ channels open → K⁺ exits → voltage heads back ↓.

4. Hyperpolarization & Refractory Period

   • Drops ~5–10 mV below resting.

   • Absolute refractory: no firing possible (Na⁺ channels reset).

   • Relative refractory: strong stimulus can trigger new AP.

5. All-or-None Law

   • Intensity = frequency, not spike size (every AP is ~+30 mV).

6. Saltatory Conduction

   • AP leaps node-to-node → energy- and time-efficient.

3. Synaptic Transmission (Application Layer)

1. Ca²⁺ Influx at terminal button → vesicles merge with membrane.

2. Neurotransmitter Release into synaptic cleft (≈ 20–40 nm).

3. Binding to postsynaptic receptors → opens ligand-gated ion channels.

   • EPSP: Usually Na⁺ influx (depolarize).

   • IPSP: Usually Cl⁻ influx or K⁺ efflux (hyperpolarize).

4. Termination

   • Re-uptake (e.g., serotonin transporter).

   • Enzymatic breakdown (e.g., ACh by acetylcholinesterase).

   • Diffusion away from cleft.

4. Key Neurotransmitters & AP-Worthy Disorders

5. Glial Support Crew

• Astrocytes: blood-brain barrier, nutrient supply

• Microglia: immune defense, cleanup

• Schwann / Oligodendrocytes: make myelin

• Ependymal cells: produce cerebrospinal fluid

10 AP-Style Multiple-Choice Questions

(Choose the best answer; detailed rationales follow.)

1. The sudden influx of sodium ions during an action potential is triggered when the membrane potential reaches approximately
   A. +40 mV B. –55 mV C. –70 mV D. 0 mV E. –90 mV

2. Which sequence correctly represents the flow of information within a typical multipolar neuron?
   A. Axon → soma → dendrites
   B. Terminal button → axon → dendrites
   C. Dendrites → soma → axon → terminal button
   D. Soma → dendrites → axon → nodes
   E. Myelin → dendrites → soma → axon

3. Multiple sclerosis primarily interferes with which aspect of neural signaling?
   A. Neurotransmitter synthesis
   B. Saltatory conduction along the axon
   C. Calcium influx at terminal buttons
   D. Ion pump function in the soma
   E. Receptor binding on dendrites

4. A neuron is unlikely to fire again during the absolute refractory period because:
   A. The Na⁺/K⁺ pumps are inactive.
   B. Neurotransmitters have been exhausted.
   C. Voltage-gated Na⁺ channels are inactivated and cannot reopen.
   D. The membrane is hyperpolarized beyond –90 mV.
   E. All K⁺ channels are closed.

5. Which statement best distinguishes graded potentials from action potentials?
   A. Only graded potentials obey the all-or-none law.
   B. Graded potentials travel without decrement along the axon.
   C. Action potentials occur only in dendrites.
   D. Graded potentials can be summed, whereas action potentials cannot.
   E. Action potentials vary in amplitude; graded potentials do not.

6. Selective serotonin re-uptake inhibitors (SSRIs) exert their effect primarily by:
   A. Blocking serotonin receptors on the postsynaptic membrane
   B. Inhibiting monoamine oxidase in the synaptic cleft
   C. Preventing presynaptic transporters from reabsorbing serotonin
   D. Increasing Ca²⁺ influx at the axon terminal
   E. Enhancing synthesis of tryptophan in the soma

7. An influx of chloride ions into a postsynaptic neuron will most likely produce a(n):
   A. Depolarizing EPSP
   B. Hyperpolarizing IPSP
   C. Excitatory action potential
   D. Absolute refractory period
   E. Release of glutamate

8. The strength of a stimulus is encoded in the nervous system primarily by the:
   A. Amplitude of each action potential
   B. Frequency of action potentials over time
   C. Duration of the refractory period
   D. Thickness of the myelin sheath
   E. Size of the synaptic cleft

9. Which glial cell type is correctly matched with its function?
   A. Microglia — forming myelin in the PNS
   B. Astrocytes — acting as neuronal immune defense
   C. Oligodendrocytes — producing cerebrospinal fluid
   D. Schwann cells — insulating axons outside the brain and spinal cord
   E. Ependymal cells — maintaining the blood-brain barrier

10. Long-term potentiation (LTP) is considered a neural basis for learning because it involves:
    A. Temporary blockage of Na⁺ channels
    B. Decreased release of glutamate at synapses
    C. Strengthening of synaptic connections through repeated activation
    D. Increased breakdown of acetylcholine
    E. Hyperpolarization of postsynaptic neurons

Detailed Answer Key & Explanations

Free-Response Practice

FRQ #1 — Action Potential & Refractory Period (7 pts)

Question:
Trace the ionic events that generate and terminate a single action potential in a myelinated motor neuron. Explain how the absolute and relative refractory periods ensure one-way propagation and encode stimulus intensity.

High-Scoring Answer Outline

1. Threshold Reached (1 pt): EPSPs depolarize axon hillock to ≈ –55 mV, opening voltage-gated Na⁺ channels.

2. Depolarization (1 pt): Rapid Na⁺ influx reverses polarity to +30 mV.

3. Propagation (1 pt): Charge spreads to next node; myelin enables saltatory conduction.

4. Repolarization (1 pt): Na⁺ channels inactivate; K⁺ channels open, K⁺ exits.

5. Hyperpolarization & Absolute Refractory (1 pt): Na⁺ channels remain inactive; neuron cannot fire, enforcing one-way flow.

6. Relative Refractory (1 pt): Membrane hyperpolarized; stronger stimulus can trigger AP, allowing frequency modulation for intensity coding.

7. Na⁺/K⁺ Pump Reset (1 pt): Active transport restores ionic gradients, readying next spike.

FRQ #2 — Synaptic Transmission & Drug Effects (7 pts)

Question:
Explain the steps of chemical synaptic transmission at a serotonergic synapse. Then discuss how (a) an SSRI and (b) an MDMA (Ecstasy) molecule each alter these steps, including their short-term psychological effects.

High-Scoring Answer Outline

(Total 7 pts: steps + drug mechanisms + psychological links)