Cellular Neurophysiology - Lecture 1

Battery

• Something that stores electrical energy through the separation of charge (Q)

• RMP: concentration gradient of ions across the membrane

Q (charge)

• Can be electrons in a wire or ions in solution

• Charge (Q) separation creates a potential difference or voltage (V) across the charges

Voltage (V)

• makes a separated charge (Q) want to flow (it’s the electrical energy)

• increasing the separation distance of Q increases V (increases the electrical energy)

• increasing the density of Q increases V (increases the electrical energy)

• what makes the charge flow

Conductor (g)

• Separated charge flows through g, creating a current (I)

• I = Q/t

• RMP: Selectively permeable ion channels

Ohm’s law

• The amount of current (I) that flows through a conductor (g) is proportional to the voltage (V)

• V = l/g

• V = IR

• I = V/R

Current (I)

• Flow of charge over time

• Current can flow onto conducting plates, but across a dielectric

Capacitors

• Separate charges of opposite signs

• Store charge (similar to battery)

• Made up of 2 conducting plates separated by an insulating layer called a dielectric

• RMP: Membrane that can separate ions

Capacitance

• Ability of a capacitor to store Q (measured in farads, F = 1 coulomb of charge per volt)

Increasing capacitance means

more Q stored on the plates to maintain the same voltage (Q density or electric field) across the plates

Increasing the capacitor’s area (A) increases

capacitance (C) —> more charge required to maintain the same charge density/electric field & therefore voltage

Increasing distance (D) between plates decreases

capacitance (charge density needed to maintain the same voltage is less)

Greater density of Q on plates

greater voltage across it

Neuronal membranes are capacitors because

• They separate charge much like an electronic capacitor —> create a voltage across the membrane

• Only right along the membrane is charge unequal —> creates a voltage

• Charge further from membrane is in equilibrium

Phospholipids

Self-assemble into a bilayer in water due to the hydrophilic (water-loving) phosphate heads and hydrophobic (water-hating) lipid tails

Specific capacitance

• The specific properties of phospholipid bilayer give it a _____ of ~1 µF/cm²

Neurons membrane capacitance

• c_m is a lot smaller than 1 µF

• c_m = specific capacitance x Area

1 pC

charge needed to create a voltage of -100 mV across this cell’s membrane

E_ion

• Point where chemical driving force is in equilibrium w/electrical driving force

Each ion species contributes to resting membrane voltage based on

• The equilibrium ptl for that ion & relative permeability of the membrane to that ion

Goldman-Hodgkin-Katz (GHK) equation

RMP formula (form of Nernst equation)

• P is relative permeability of the membrane to a specific ion species

• Typical permeability ratios at rest might be PK : PNa : PCl = 1.0 : 0.03 : 0.1

How do neuronal membranes behave like electronic circuits?

• Equilibrium potential for each ionic species acts like a battery

• Conductance (or permeability) of the membrane to specific ionic species acts like resistors in parallel

• The membrane acts like a capacitor in parallel with resistors.

• Together, these properties determine the voltage across the membrane

Primary active transport

• Uses energy from ATP hydrolysis —> ADP

• Transports ions against their concentration gradients

• 3 bound Na⁺ ions move up their concentration gradient from inside —> outside cell at cost of ATP energy

• 2 bound K⁺ ions are also pumped against their concentration gradient to cell

Secondary active transport

• Takes advantage of ptl energy of ionic concentration gradients

• 3 Na⁺ ions move down their concentration gradient from outside to inside cell at no energy cost, taking 1 bound Ca²⁺ up it’s concentration gradient to outside the cell

What determines membrane ptl

The concentration gradient that these pumps set up and the selectively permeable ion channels

Active transport

Creates concentration gradients and selectively permeable ion channels set up RMP according to GHK equation

Ion pumps set up

Ionic concentration gradients across membrane

Selectively permeable ion channels set up

Ptl difference across the membrane