Nerve Cell Potentials
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Describe the ion distribution between inside and outside of a cell
Outside = High Na+ Cl- (150mM), low K+ (4mM) Inside = High K+ (140 mM), low Na+ (and Cl-) (15mM)
What creates the concentration gradient for the membrane ions/
Na+ K+ ATPase pump - 2 sodium out, 3 potassium in
What generates the resting membrane potential?
The leakage of K+ ions down their concentration gradient, from inside to outside.
What the is Equilibrium Potential?
The potential difference that would precisely balance the concentration gradient
Why is the RMP never as large at the Eq P?
Membrane isn’t perfectly impermeable to substances that aren’t K+, so other substances can pass sometimes - meaning RMP is -70 and not -95 (as expected). Mainly due to Na+ diffusing INTO cell. All depends on relative permeability of membrane to K+ and Na+
What are the 3 types of relevant ion channels?
• ‘leak’ • ligand gated • voltage gated
What is an electrotonic potential?
Graded potential where there is small potential/charge change - no ion change, the size can tell us about strength of signal causing it. Naturally decays as no mechanisms allow repolarisation.
What is the key difference in usage between voltage-gated and ligand-gated Na+ channels?
Voltage gated sodium channels used in action potentials Ligand-gated sodium channels used in electrotonic potentials
Describe the electrotonic potential mechanism
• Sodium enters via ligand gated ion channels • patch of depolarisation • forms electrotonic current across membrane • depolarisation spreads along membrane Carry signals for SHORT distances (300 microns)
Describe the path of an electrotonic potential
Dendrite -> soma -> axon hillock Progressively decays and gets weaker but if threshold (-50mV) met, then triggers AP along axon
Key differences between electrotonic potential and action potential?
small (15mV) vs large (100mV) slow depo vs rapid depo exp decay vs rapid repo GRADED vs ALL-OR-NOTHING
Describe the mechanism of an Action Potential
• RMP • – K+ MORE permeable than Na+ (more K+ leaving than Na+ entering) • Rapid Depolarisation • – VGNa activation gate opens in response to threshold • – More channels open due to positive feedback (x100) • – Na+ MORE permeable than K+ (more Na enters than K leaves) • Repolarisation • – VGNa activation gate remains open/cannot close so.. • – Channel INACTIVATION gate is now pulled shut (at +50mV) - UNEXCITABLE • – Opening of extra VGK+ channels to repolarise cell • RMP • • Both gates ‘reset’ to resting state (activation closed, inactivation open) - EXCITABLE!
How do local anaesthetics abolish APs? Examples?
Interfere with VgNa channels -> no AP -> no nerve impulse -> no movement/response eg. lidocaine, neurotoxins
What is the absolute and relative refractory period?
Absolute = repolarisation, VgNa channels inactivated (unexcitable) Relative = hyperpolarisation, VgNa Channels de-inactivate so excitable, but less excitable than at rest due to stronger signal required for threshold