Transmission of Neural Impulses
High-Yield Summary
- Resting potential ≈ −70 mV: K⁺ leaks out (toward −90 mV equilibrium), Na⁺ leaks in (toward +60 mV equilibrium); more K⁺ channels → resting potential lands closer to K⁺'s equilibrium. Na⁺/K⁺ ATPase restores gradients (3 Na⁺ out, 2 K⁺ in per cycle).
- Axon hillock integrates excitatory (depolarizing) + inhibitory (hyperpolarizing) input via temporal (same neuron, rapid succession) and spatial (multiple neurons, simultaneous) summation; threshold ≈ −55 to −40 mV triggers an all-or-nothing action potential.
- Action potential: rest → threshold (−55mV, Na⁺ channels open) → depolarization (Na⁺ in, peak ~+30mV) → repolarization (K⁺ out) → hyperpolarization (overshoot) → recovery (Na⁺/K⁺ ATPase).
- Absolute refractory period: no AP possible (Na⁺ channels inactivated). Relative refractory period: AP possible but needs stronger stimulus.
- Myelination → saltatory conduction (jumps node to node); larger axon diameter → less resistance → faster conduction. Stronger stimulus → higher firing FREQUENCY, not bigger AP.
- At the synapse: AP → Ca²⁺ influx → vesicle fusion → neurotransmitter release → binds postsynaptic receptors; cleared by enzymatic breakdown, reuptake, or diffusion.
Key Terms
- Depolarization
- Membrane potential becomes less negative (closer to 0); excitatory input causes this.
- Hyperpolarization
- Membrane potential becomes more negative; inhibitory input causes this.
- Temporal summation
- Multiple signals from the SAME neuron, close in time, adding together.
- Spatial summation
- Signals from MULTIPLE neurons arriving simultaneously at different dendrite points.
- Refractory period
- Post-AP window (absolute: no firing possible; relative: firing needs stronger stimulus) enforcing one-way signal propagation.
- Synaptic cleft
- Gap between presynaptic and postsynaptic neurons that neurotransmitters diffuse across.
The Action Potential
- 1Rest — membrane at ~−70 mV; voltage-gated channels closed.
- 2Threshold reached — stimulus brings membrane to ~−55 mV, voltage-gated Na⁺ channels open.
- 3Depolarization — Na⁺ floods in, membrane spikes to ~+30 mV.
- 4Repolarization — Na⁺ channels inactivate/close; voltage-gated K⁺ channels open, K⁺ flows out.
- 5Hyperpolarization — K⁺ efflux overshoots, membrane briefly more negative than resting.
- 6Recovery — Na⁺/K⁺ ATPase restores gradients (3 Na⁺ out, 2 K⁺ in), returns to −70 mV baseline.
Na⁺/K⁺ ATPase Stoichiometry
1 ATP → 3 Na⁺ out, 2 K⁺ in
- Na⁺ out = 3 sodium ions pumped out of the cell per cycle
- K⁺ in = 2 potassium ions pumped into the cell per cycle
- Active transport against both ions' concentration gradients — requires ATP.
- Maintains the resting potential and restores gradients after an action potential.
Absolute vs. Relative Refractory Period
| Absolute | Relative |
|---|---|
| No AP possible, any stimulus strength | AP possible, but needs stronger-than-normal stimulus |
| Na⁺ channels inactivated/resetting | Cell still hyperpolarized, farther from threshold |
Common MCAT Trap
- Stronger stimulus increases FIRING FREQUENCY, not action potential size/magnitude — APs are always all-or-nothing, same magnitude every time.
- Resting potential is closer to K⁺'s equilibrium potential (−90 mV) than Na⁺'s (+60 mV) because there are more K⁺ leak channels — not because K⁺ "wins" some other way.
- Depolarization = excitatory (more likely to fire); hyperpolarization = inhibitory (less likely) — easy to invert under time pressure.
Quick Recall
What ion channel opens first at threshold, and what does it cause?
What triggers neurotransmitter release at the presynaptic terminal?
What does a stronger stimulus change about neural signaling?
Name the 3 mechanisms that clear neurotransmitter from the synaptic cleft.