Electrical properties of neurones

How is the membrane potential measured

• intracellular glass microelectrodes

• Glass can be heated and pulled to a fine filament

What are the requirements of a resting membrane potential

• intact semi permeable CM

• Ion concentration gradients

• Ion permeabilities

• Metabolic processes over the long term

Why doesn’t the Na ion conc affect resting potential

• the CM is impermeable to Na

How does K+ move through the membrane at equilibrium

• balance between moving in and out

• In = electrical gradient

• Out = concentration gradient

Resting potential

Inside the cell is more negative than the outside

What happens if extracellular K+ increases

• membrane depolarises

• Less concentration gradient as a driving force

• Electrical gradient reduces since balances with conc grad

Why is Em (membrane potential) usually less -ve than calculated by Nerst equation

• leakage of K+

• Some Na+ can cross

How was the Nerst equation improved

• includes membrane permeability to K, Na and Cl

• Includes intracellular and extracellular concentrations of each

How are ionic gradients maintained

• Na/K ATPase

• Moves K into cell and Na+ out

• Against conc gradients

• Using ATP

What is the action potential

• mechanism of neurone communication

• Triggers neurotransmitter release

What does the action potential depend on

• Na+

What are VG Na+ channels

• transmembrane proteins

• Activated by depolarisation

• Each VG channel is selective for an ionic species

What happens when VG Na+ channels open

• Na+ moves in the cell

• Conc and electrical gradient drives this

How is the membrane initially depolarised

• synaptic transmission

• Generator potential

• Intrinsic properties

• Experimental stimulation

What is the process of VG Na+ channels opening

• membrane depolarises

• Channels open

• Na+ influx

• Repeats

• EXPLOSIVE, quick generation of AP

Absolute refractory period

• due to inactivation of Na+ channels

• Harder to generate an AP

Relative refractory period

• immediately after the absolute

What is the mechanism of VG Na+ channels inactivation

• ball and chain

• 1. +ve charged gate in the channel keeps it closed

• 2. Upon depolarisation, the gate moves out the way

• 3. To close, a ball swings in to physically block the movement

Axon hillock

• specialised site at the top of the axon

• Site of AP conductance

• Identified by presence of specific proteins

How does axon diameter affect action potential Conduction

• larger Diameter = faster conduction

Is ATP needed for action potentials

• not short term

• Long term, Na+ builds up internally and K+ is lost

• ATP needed to run pumps to maintain membrane potential