Unit 01 — Cell Physiology · Question Bank

Local (graded) potential

• Produced by a SUBthreshold stimulus.
• Amplitude is GRADED — varies with stimulus strength.
• Can SUMMATE (temporal and spatial).
• Conducted with DECREMENT (dies out over distance, electrotonic spread).
• NO threshold, NO refractory period.
• Few ion channels involved (e.g. partial Na⁺, or ligand-gated). Examples: EPSP, IPSP, receptor potential, end-plate potential.

Action potential

• Produced by a threshold or supra-threshold stimulus.
• ALL-OR-NONE — constant amplitude and shape.
• Cannot summate.
• Propagated WITHOUT decrement (regenerated along the membrane).
• Has a threshold and a refractory period (absolute + relative).
• Voltage-gated Na⁺ (depolarization) then K⁺ (repolarization). Examples: nerve impulse, muscle AP.

Effect on resting potential

Raising ECF [K⁺] reduces the K⁺ concentration gradient → E_K becomes less negative → the resting membrane potential DEPOLARIZES (rises toward 0, e.g. −90 → −70 mV).

Biphasic effect on excitability

• Mild hyperkalaemia: the RMP moves closer to threshold, so a smaller stimulus is needed → excitability INCREASES.
• Severe / sustained hyperkalaemia: the persistent depolarization keeps voltage-gated Na⁺ channels INACTIVATED → the cell cannot generate an AP → excitability DECREASES (the cell becomes inexcitable).

Clinical relevance

This is why hyperkalaemia is dangerous to the heart — it can cause arrhythmias and cardiac arrest in diastole.

Ionic gradients (set by the Na⁺/K⁺ pump)

K⁺ is high inside (~140 mM) and low outside (~4 mM); Na⁺ is high outside (~142 mM) and low inside (~14 mM). Large anions (proteins) are trapped inside.

K⁺ diffusion potential — the main cause

At rest the membrane is far more permeable to K⁺ than to Na⁺ (K⁺ leak channels). K⁺ diffuses out down its gradient, leaving the inside negative, until the electrical pull inward balances the chemical push outward — the potential then sits close to E_K (≈ −90 mV, by the Nernst equation).

Na⁺/K⁺ pump contribution

The pump (3 Na⁺ out : 2 K⁺ in) is electrogenic and adds a few mV of negativity, and it maintains the gradients that make the diffusion potential possible.

Why not E_Na?

A small resting Na⁺ permeability makes the real RMP (e.g. −70 to −90 mV) slightly less negative than E_K — described quantitatively by the Goldman equation.

Resting (polarized)

−70 to −90 mV, maintained by K⁺ leak + Na⁺/K⁺ pump.

Depolarization (upstroke)

A threshold stimulus opens voltage-gated Na⁺ channels → regenerative Na⁺ INFLUX (positive feedback) drives V_m toward E_Na (~+30 mV).

Repolarization

Na⁺ channels INACTIVATE and delayed voltage-gated K⁺ channels open → K⁺ EFFLUX returns V_m toward rest.

After-hyperpolarization

K⁺ channels close slowly, so V_m briefly dips below the RMP.

Restoration

The Na⁺/K⁺ pump restores the small amounts of Na⁺/K⁺ that moved (it does NOT cause repolarization).

All-or-none & refractoriness

Once threshold is reached the AP is full-sized; the absolute refractory period (Na⁺ inactivated) limits firing frequency and ensures one-way conduction.

Passive (no ATP, down the gradient)

• Simple diffusion — lipid-soluble substances through the bilayer: O₂, CO₂, alcohol, steroids.
• Facilitated diffusion — carrier-mediated (glucose via GLUT) or channel-mediated (ions, water via aquaporins); saturable.

Active (energy required, against the gradient)

• Primary active — pump hydrolyses ATP directly: Na⁺/K⁺-ATPase, Ca²⁺-ATPase, H⁺/K⁺-ATPase.
• Secondary active — uses the Na⁺ gradient: symport (Na⁺-glucose, SGLT) or antiport (Na⁺-Ca²⁺, Na⁺-H⁺).

Vesicular (bulk) transport

• Endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis — for large particles and secretion.