Central nervous system physiology
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I now have comprehensive material from Guyton & Hall, Neuroscience (Bear), and Medical Physiology (Boron). Here is your exam-focused summary:
Neurons & Action Potentials — CNS Physiology (Exam Key Facts)
1. Resting Membrane Potential (RMP)
| Parameter | Value |
|---|---|
| RMP of large nerve fibers | −70 mV (inside negative) |
| Na⁺ outside / inside | 142 / 14 mEq/L |
| K⁺ outside / inside | 4 / 140 mEq/L |
| K⁺ Nernst potential | −94 mV |
| Na⁺ Nernst potential | +61 mV |
| Actual RMP | ~−70 mV (dominated by K⁺ permeability) |
How RMP is established:
- Na⁺-K⁺ ATPase (electrogenic pump): Pumps 3 Na⁺ out for every 2 K⁺ in → net deficit of positive charge inside
- K⁺ leak channels: High resting permeability to K⁺ allows K⁺ to diffuse out, pulling the membrane toward E_K (−94 mV)
- Net result: Na⁺ permeability (~0.01×) drags the potential slightly positive from −94 mV → actual RMP ≈ −70 mV
— Guyton and Hall Textbook of Medical Physiology
2. Action Potential — Stages
| Stage | Membrane Potential | Key Event |
|---|---|---|
| Resting | −70 mV | Membrane polarized; Na⁺ activation gate CLOSED |
| Depolarization | −70 → +35 mV | Threshold (~−55 mV) reached; voltage-gated Na⁺ channels open; Na⁺ rushes in |
| Repolarization | +35 → −70 mV | Na⁺ channels inactivate; delayed K⁺ channels open; K⁺ rushes out |
| Hyperpolarization (undershoot) | −70 → −80 mV | K⁺ channels remain open briefly too long |
| Return to RMP | −80 → −70 mV | K⁺ channels close; pump restores gradients |
— Guyton and Hall Textbook of Medical Physiology
3. Voltage-Gated Na⁺ Channel — 3 States (HIGH-YIELD ★)
| State | Activation Gate | Inactivation Gate | Ion Flow |
|---|---|---|---|
| Resting (−70 mV) | CLOSED | OPEN | None |
| Activated (≥ threshold) | OPEN | OPEN → closing | Na⁺ flows IN |
| Inactivated | OPEN | CLOSED | None |
- Inactivation gate closes milliseconds after the activation gate opens
- The inactivation gate cannot reopen until the membrane repolarizes back toward −70 mV → this is the molecular basis of the absolute refractory period
Voltage-Gated K⁺ Channel:
- Only one gate, slower to open
- Opens at the same time Na⁺ channels inactivate → drives repolarization
- Prolonged opening → hyperpolarization (undershoot)
— Guyton and Hall Textbook of Medical Physiology
4. Refractory Periods (HIGH-YIELD ★)
| Period | Timing | Mechanism | Implication |
|---|---|---|---|
| Absolute Refractory Period (ARP) | From initiation → near-complete repolarization | Na⁺ channel inactivation gates are closed; cannot open regardless of stimulus strength | No second AP possible |
| Relative Refractory Period (RRP) | After ARP until RMP restored | Some Na⁺ channels recovering; K⁺ channels still open (hyperpolarized) | Second AP possible only with suprathreshold stimulus |
- Refractory periods determine maximum firing frequency of neurons
- The ARP ensures unidirectional conduction of the AP along the axon
— Medical Physiology (Boron)
5. Conduction Velocity & Saltatory Conduction
Unmyelinated fibers
- AP spreads continuously, heel-to-toe along the entire membrane
- Slow conduction (0.5–2 m/s)
Myelinated fibers — Saltatory Conduction ★
- Myelin (Schwann cells in PNS; oligodendroglia in CNS) electrically insulates the axon, reducing current loss
- Voltage-gated Na⁺ channels are concentrated at nodes of Ranvier (1–2 µm gaps in myelin)
- AP "leaps" from node to node (saltatory conduction, from Latin saltare = "to leap")
- Nodes are spaced 0.2–2.0 mm apart (larger diameter → larger internodal distance)
- Conduction velocity in myelinated fibers is linearly related to axon diameter
| Fiber Type | Diameter | Velocity | Function |
|---|---|---|---|
| Aα (Ia) | 12–20 µm | 70–120 m/s | Muscle spindle afferents, motor |
| Aβ | 6–12 µm | 30–70 m/s | Touch, pressure |
| Aδ | 1–5 µm | 5–30 m/s | Fast pain, temperature |
| C (unmyelinated) | 0.2–1.5 µm | 0.5–2 m/s | Slow pain, autonomic |
— Neuroscience: Exploring the Brain, 5th Ed.; Guyton and Hall
6. All-or-None Law
- Once the threshold is reached, the AP is always the same amplitude and duration — it cannot be graded
- Subthreshold stimuli produce only local graded potentials that decay
- Stimulus intensity is encoded by frequency of APs (rate coding), not AP size
7. Strength-Duration Relationship
- Excitability depends on the product of stimulus strength × duration
- A brief, strong stimulus or a prolonged, weak stimulus can both reach threshold
- Rheobase = minimum current strength needed (at infinite duration)
- Chronaxie = time needed at 2× rheobase (clinically used to assess nerve excitability)
— Medical Physiology (Boron)
Quick-Reference Exam Mnemonics
| Concept | Mnemonic / Key Rule |
|---|---|
| RMP = −70 mV | "70 is the resting number" |
| Threshold ≈ −55 mV | "15 mV above rest" |
| 3 Na out, 2 K in (pump) | "3 out, 2 in" → net negative inside |
| Na inactivation = ARP | "Inactivated = absolutely refractory" |
| Saltatory = faster + efficient | "Jumps save space and speed" |
| AP travels unidirectionally | Because the region behind it is in ARP |
Sources: Guyton and Hall Textbook of Medical Physiology; Medical Physiology (Boron & Boulpaep); Neuroscience: Exploring the Brain, 5th Ed. (Bear, Connors, Paradiso)
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