The excitable membrane and action potential

What is the resting membrane potential and what are the two factors determining it?

At rest, Vm ≈ −65 mV (inside is ~65 mV more negative than outside).

Determined by:
1. Concentration gradient of ions across the membrane
2. Selective permeability of the membrane to different ions

Why do ions need membrane proteins to cross the lipid bilayer?

The lipid bilayer is highly impermeable to polar species and ions. Thus, for ions to cross the barrier, integral membrane proteins are required to facilitate this transport.

This includes:

1. Passive transport through channel-mediated or transporter-mediated proteins

2. Active transport through “pump”-proteins

Describe the Na/K pump cycle and its importance.

Ionic gradient under normal conditions:

• Na⁺: high outside, low inside

• K⁺: high inside, low outside

The Na/K pump is an active transporter that is important in setting the membrane potential of neurones.

1. Inside-facing opening binds 3 Na⁺

2. 1 ATP hydrolysed → conformational change opens outward

3. 3 Na⁺ released outside; pump loses Na⁺ affinity, gains K⁺ affinity

4. 2 K⁺ bind outside (no ATP needed)

5. Conformational change returns pump to original shape; 2 K⁺ released inside

Net per cycle:

• 3 Na⁺ out

• 2 K⁺ in

• 1 ATP hydrolysed

• Net outward positive charge

What does the figure tell us about Nernst’s equation?

• Shows a KCl gradient (10 mM vs 1 mM) with K⁺-selective channels.

• Initially V=0, so K⁺ flows down its concentration gradient (left to right).

• As K⁺ moves, charge builds up, creating a voltage that opposes further flow.

• At equilibrium, V₁₋₂ = −58 mV —> the Nernst potential for K⁺ at this gradient.

  • AKA Equilibrium potential (E_ion)

• Net ion movement = 0.

Nernst equation?

Describes the voltage that arises because of the difference in concentration of a specific ion type.

• E_ion = equilibrium potential for that ion

• R = gas constant, T = temperature (Kelvin)

• z = valence of ion (e.g. +1 for K⁺, +2 for Ca²⁺, −1 for Cl⁻)

• F = Faraday's constant

• [ion]_out / [ion]_in = concentration ratio outside/inside

How does Ohm's law apply to ion channels?

• The movement of ions is described as the current (I)

• At equilibrium, the current = 0

• Ohm’s law predicts the direction and amplitude of the current flow

• Conductance (G) is the specific permeability; represent the portion of channels that are open

I-V plot?

• X-axis: voltage (V)

• Y-axis: current (I)

• Slope: conductance (G)

  • More open channels → higher G → steeper slope on I-V plot

  • 0 channels open → no G → flat line along x-axis

  • Concentration gradient → G (slope) shifts so that x-intercept is the E_ion

Positive and negative currents.

Cations (+):

• Positive current = outward flow

• Negative current = inward flow

Anions (-):

• Positive current = inward current

• Negative current = outward current

What is the reversal potential (V_rev)?

The voltage at which the net current of a system is 0.

• V_rev can be used interchangeably with E_ion for one type of ion

Why is the membrane more permeable to K⁺ than Na⁺ at rest?

Leak channels are open at rest and are mostly selective for K⁺. These keep the membrane potential close to the K⁺ equilibrium potential (EK ≈ −90 mV), pulling Vrest negative.

What is the concentration gradient of Na⁺, K⁺, Ca²⁺, and Cl⁻ relative to the cell?

High outside: Na⁺, Ca²⁺, Cl⁻
High inside: K⁺

Describe the structure and gating mechanism of a voltage-gated K⁺ channel.

A tetramer of 4 symmetrical subunits, each with segments S1–S6:

• S1–S4: voltage-sensing domain (VSD). S4 has positively charged residues every 3rd amino acid

• At rest (negative inside): S4 is pulled inward, keeping pore closed

• Upon depolarisation: S4 is repelled upward (outward), changing pore conformation → K⁺ can flow

• Conductance follows a sigmoidal relationship with voltage

What are the 3 main voltage clamp techniques?

1. Cell-attached recording: pipette on membrane surface with suction; records single channel currents

2. Whole-cell recording: stronger suction breaks membrane; records currents from all channels in the cell

3. Two-electrode voltage clamp (TEVC): used with Xenopus laevis oocytes; channel gene injected, two electrodes control Vm and record current

How did the voltage-gated Na⁺ channel evolve and how does it differ structurally from K_V?

• NaV evolved from KV via 2 gene duplication events, making it a tandem concatemer

• 4 domains in one polypeptide (pseudo-tetrameric)

• Each domain has accumulated different mutations → different functional properties per domain

• Eukaryotic CaV channels share the same architecture

How does a NaV channel open and inactivate? What is the IFM motif?

Opening (m-gate): VSDs I–III sense depolarisation > at −40 mV, pore opens → Na⁺ flows in

Inactivation (h-gate): VSD IV controls inactivation. The IFM motif (Isoleucine-Phenylalanine-Methionine) on the III–IV intracellular loop physically blocks the open pore from the cytosolic side

Open probability: pO = m³ × h
h = 1 → not inactivated; h = 0 → fully inactivated (no current despite open pore)

Synaptic signalling

• Glutamate / ACh / Serotonin (cation channels): Vrev ≈ 0 mV — permeable to Na⁺, K⁺ (sometimes Ca²⁺); depolarising → excitatory

• GABA / Glycine (Cl⁻ channels): Vrev ≈ −50 to −70 mV — permeable to Cl⁻; near or below resting potential → inhibitory/stabilising

Describe the phases of the action potential.

1. Initiation: Integration at soma; Axon initial segment decides whether AP is carried out or not → if V_m > −40 mV (threshold), NaV channels open → Na⁺ influx → depolarisation → more Na_V open (Hodgkin cycle / positive feedback)

2. Peak (30–40 mV): V_m approaches E_Na; peak is reached where net current = 0 momentarily; Na_V inactivation begins; K_V channels start opening

3. Termination: K_V open + Na_V inactivated → repolarisation toward E_K → undershoot (hyperpolarisation) because K_V slow to close → leak channels restore V_rest

What are the different refractory periods?

• Absolute refractory: no AP possible regardless of stimulus (NaV inactivated)

• Relative refractory: need stronger-than-normal stimulus (some NaV inactivated, some KV still open)

Why do refractory periods exist and why do we need them?

Refractory periods exist because:

1. NaV channels are inactivated (need time to recover)

2. KV channels are still open and opposing depolarisation

Function: prevents backward propagation of the AP — the region behind the AP is refractory, so the signal can only travel forward along the axon.

What solution do vertebrates use to propagate signals faster?

Myelination: myelin sheath (from oligodendrocytes in CNS, Schwann cells in PNS) wraps the axon and provides electrical insulation, thereby decreasing membrane capacitance and increasing resistance, leading to faser signal propagation.

Voltage-gated ion channels are only at nodes of Ranvier (gaps between myelin). The AP 'jumps' from node to node → saltatory conduction, which is much faster and more energy efficient.