L8 - Neurons and neuronal communication - establishing electrochemical potentials and action potentials

What are the three primary learning outcomes of Lecture 8?

Describe the basic structure and function of a neuron, explain how neurons establish resting membrane potential, and understand neuronal communication via action potentials.

What is the fundamental difference between intracellular and extracellular electrophysiological recording?

Intracellular recording places the electrode inside the cell to measure large potentials (~100 mV), whereas extracellular recording places the electrode outside the cell (~0.1 mV).

What is the patch clamp recording technique?

An electrophysiological technique where a glass pipette is sealed onto the cell membrane forming a high-resistance giga-ohm seal to measure ion currents.

How is a whole-cell patch clamp configuration formed from a cell-attached seal?

Suction is applied through the patch pipette to rupture the small membrane patch beneath the pipette tip.

Which scientists conducted pioneer intracellular and extracellular nerve recording experiments on the giant squid axon in 1939?

Alan Hodgkin and Andrew Huxley.

What electrical charge state characterizes the neuronal membrane at rest versus during activation?

At rest, the inside of the membrane is negatively charged relative to the outside (hyperpolarised); when activated, the inside becomes positively charged (depolarised).

What three factors establish and maintain the electrochemical gradient across a neuronal membrane?

Active ATP-dependent ion pumps, relative impermeability of the lipid bilayer, and the membrane acting as a capacitor storing ionic charge differentials.

What ion pump maintains the resting membrane potential, and what is its stoichiometry per ATP hydrolysed?

The Na+/K+-ATPase pump, which actively pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell per ATP molecule.

What is the typical value of a neuron's resting membrane potential (Vm)?

Approximately -70 mV.

Which two primary ions establish the resting membrane potential of a neuron?

Sodium (Na+) and potassium (K+) ions.

What is the definition of an ion's equilibrium potential?

The membrane voltage required to generate an electrical force that balances and prevents net diffusion of that ion down its concentration gradient.

What formula defines the concentration gradient ratio for positive versus negative ions?

[C]out / [C]in for positive ions (+ve), and [C]in / [C]out for negative ions (-ve).

What is the Nernst equation formula for calculating an ion's equilibrium potential at physiological temperature?

E = 58 mV x log10([C]out / [C]in).

What are the physiological equilibrium potentials for potassium (EK) and sodium (ENa)?

EK is -90 mV to -96 mV, and ENa is +52 mV to +65 mV.

Why is the resting membrane potential (-70 mV) much closer to EK than to ENa?

Because the resting neuronal membrane has approximately 50 times more open K+ channels than Na+ channels, making it far more permeable to K+.

Why is net ion flow zero across the membrane at a constant resting membrane potential?

The passive efflux (leakage) of K+ out of the cell is exactly matched by the passive influx (leakage) of Na+ into the cell.

What electrical change occurs when voltage-gated sodium (Na+) channels open?

Rapid influx of Na+ into the cell causes depolarisation (the cell interior becomes more positive).

What electrical change occurs when voltage-gated potassium (K+) channels open?

Rapid efflux of K+ out of the cell causes hyperpolarisation (the cell interior becomes more negative).

What two physical forces act on ions across the plasma membrane?

Electrostatic force (electrical charge attraction/repulsion) and force of diffusion (movement down concentration gradients).

What equation calculates resting membrane potential by accounting for multiple ion species and their relative permeabilities?

The Goldman-Hodgkin-Katz (GHK) equation.

What are the typical intracellular and extracellular concentrations of Na+ and K+ in a neuron?

Na+ is 10 mM intracellular and 140 mM extracellular; K+ is 140 mM intracellular and 4 mM extracellular.

What calculated Vm values are obtained when ignoring permeability versus factoring in a 50:1 K+:Na+ permeability ratio?

Ignoring permeability yields -1.09 mV, while factoring in permeability yields -85.41 mV.

What is optogenetics?

A technique using light-sensitive ion channels and pumps expressed in specific neurons to control their electrical activity using light.

Which optogenetic channel is activated by blue light and what effect does it have on neuronal potential?

Channelrhodopsin (ChR); opens cation-selective channels causing Na+ and Ca2+ influx to depolarise the neuron.

Which optogenetic channel is activated by yellow light and what effect does it have on neuronal potential?

Halorhodopsin (HR); opens chloride channels causing Cl- influx to hyperpolarise the neuron.

What are four core principles of an action potential?

Triggered by threshold depolarisation, all-or-nothing event, propagates without decrement along the axon, and peak voltage approaches ENa.

What are the three functional states of voltage-gated sodium (NaV) channels?

Closed (resting), Open (active), and Inactive (refractory).

How do voltage-gated potassium (KV) channels differ from NaV channels in their functional states?

KV channels lack an inactivation state, possessing only closed and open states.

How do voltage-sensing paddles in KV channels open the channel gate during depolarisation?

Positively charged paddles attracted to the negative interior at rest are repelled by the positive interior during depolarisation, mechanically pulling the channel gate open.

What sequence of events occurs when a depolarising stimulus reaches threshold?

Voltage-gated Na+ channels open rapidly, permitting Na+ influx that rapidly depolarises the neuron toward +30 mV.

What two molecular events occur at the peak of an action potential to stop depolarisation?

Voltage-gated Na+ channels undergo inactivation and slower voltage-gated K+ channels (KV) open (~0.5 ms delay).

What ionic movement causes the repolarisation phase of an action potential?

Efflux of potassium (K+) ions out of the neuron through open KV channels.

What causes the after-hyperpolarisation phase following repolarisation?

KV channels remain open, allowing extra K+ to leave the cell and driving membrane potential temporarily near -90 mV.

What causes the absolute refractory period, and what is its physiological significance?

Inactivation of voltage-gated Na+ channels; prevents any second action potential from firing until NaV channels reset to the closed state at resting Vm.

What causes the relative refractory period, and how does it affect AP generation?

Ongoing hyperpolarisation due to open KV channels; requires a stronger-than-normal depolarising stimulus to reach threshold.

How does action potential propagation differ between non-myelinated and myelinated axons?

Non-myelinated axons conduct continuously and slowly along the axolemma, whereas myelinated axons conduct via rapid saltatory conduction jumping between nodes of Ranvier.