Physiology - Unit 2 Graded & Action Potentials

Membrane Potential

The difference in electrical charge between the inside and outside of a cell. Measured in millivolts (mV).

Equilibrium Potential

The membrane voltage at which the electrical force pushing an ion one way exactly balances the chemical gradient pushing it the other way → no net ion movement.

Resting Membrane Potential (RMP)

The membrane potential of a cell at rest, determined by all ions and their permeabilities. Neurons typically sit around –70 mV.

Graded Potentials

Local, short‑distance changes in membrane potential caused by ion flow through ligand‑gated channels.
Key features:

• Graded: magnitude varies with stimulus strength

• Decremental: fade with distance

• Can be depolarizing OR hyperpolarizing

• Initiated by ligand‑gated channels, not voltage‑gated channels

• First electrical event in all neurons

Depolarizing Graded Potentials

Membrane becomes less negative (moves toward 0).
Usually caused by Na⁺ entry.
These bring the neuron closer to threshold → more likely to fire an action potential.

Hyperpolarizing Graded Potentials

Membrane becomes more negative than RMP.
Usually caused by K⁺ leaving or Cl⁻ entering.
These move the neuron farther from threshold → less likely to fire an action potential.

Why can graded potentials be either depolarizing or hyperpolarizing?

Because different ligand‑gated channels open depending on the stimulus:

• Open Na⁺ channels → depolarization

• Open K⁺ or Cl⁻ channels → hyperpolarization

Receptor Potentials

Graded potentials produced by sensory receptors (e.g., touch, temperature).

Synaptic Potentials

Graded potentials produced when neurotransmitters bind to postsynaptic receptors.

Action Potentials (APs)

Large, rapid, long‑distance electrical signals.
Require voltage‑gated Na⁺ and K⁺ channels.

Key features:

• All‑or‑none

• Same size and shape every time

• Do not decrement

• Triggered only if graded potentials reach threshold

Depolarization

Membrane becomes less negative.
Voltage‑gated Na⁺ channels open → Na⁺ rushes in.

Overshoot

Membrane potential becomes positive (>0 mV).

Repolarization

Membrane potential returns toward RMP.
Na⁺ channels inactivate, K⁺ channels open → K⁺ leaves.

Hyperpolarization (Afterhyperpolarization)

Membrane becomes more negative than RMP.
K⁺ channels stay open longer than needed → extra K⁺ leaves.

Excitable Cells

Cells capable of generating action potentials:

• Neurons

• Muscle cells

• Endocrine cells
  Any cell with voltage‑gated Na⁺ channels is considered excitable.

Voltage‑Gated Sodium Channels

• Open very fast

• Have an inactivation gate that closes shortly after opening

• Repolarization resets the channel

• Once opened, they allow enough Na⁺ to guarantee an AP

Voltage‑Gated Potassium Channels

• Open slowly

• Responsible for repolarization and hyperpolarization

Permeability Changes During an Action Potential

Step 1 — RMP:
Pk > Pna

Step 2 — Depolarizing Stimulus:
Pna increases

Step 3 — Rapid Depolarization:
Pna ≫ Pk

Step 4 — Overshoot:
Pna at maximum

Step 5 — Repolarization:
Pna decreases, Pk increases

Step 6 — Afterhyperpolarization:
Pk ≫ Pna

Step 7 — Return to RMP:
Pk decreases but still > Pna

Threshold Stimulus

Just strong enough to open voltage‑gated Na⁺ channels → triggers AP.

Subthreshold Stimulus

Too weak to open Na⁺ channels → no AP.

All‑or‑None Principle

If threshold is reached → full AP.
If not → no AP.
AP size never changes.

Drugs Affecting Action Potentials

Local anesthetics (e.g., lidocaine, novocaine) block voltage‑gated Na⁺ channels.
→ Graded potentials cannot trigger APs
→ Pain signals never reach the brain.

Action Potential Frequency

Stronger stimuli do not make bigger APs.
They make more frequent APs.
Frequency encodes stimulus intensity.

Absolute Refractory Period

No second AP possible.
Na⁺ channels are either:

• Already open

• Inactivated

Relative Refractory Period

Second AP possible only with a stronger stimulus because:

• Membrane is hyperpolarized

• Some Na⁺ channels have reset, but not all
  AP amplitude is smaller.

Why Action Potentials Travel in One Direction

Because the region behind the AP is in the absolute refractory period, so it cannot fire again.
The region ahead is at RMP and ready to fire.
→ AP moves away from the cell body.

Action Potential Speed

• Larger diameter axons: less resistance → faster

• Myelinated axons: less charge leakage → much faster

Myelin & Nodes of Ranvier

APs occur only at nodes (where Na⁺ channels are).
APs “jump” node to node → saltatory conduction.

Demyelination

Damage to myelin (e.g., MS):

• AP conduction slows

• Signals may fizzle out (conduction block)

• Timing becomes inconsistent → brain receives “blurry” information