Establishing electrochemical potentials and action potentials Flashcards
3 ways of measuring electrical potentials
-Extracellular recording (electrode outside cell)
-Intracellular recording (electrode inside cell)
-Patch clamping (electrode sealed to cell surface)
Resting and active state of neurone
-At rest inside of membrane is more negatively charged than the outside (hyperpolarised)
-When cells become activated, the inside of the membrane becomes more positively charged (depolarised)
Examples of extracellular recordings
-ECG - electrocardiogram
-EMG - electromyography
-EEG - electroencephalogram
Resting membrane potential features
-Typically around -70mV
-Mainly determined by Na+ and K+ ions
-If inside of cell is very negative (at rest), K+ will be prevented from leaving
-If the inside of cell is very positive (when active) , Na+ will be prevented from entering
What is the equilibrium potential of an ion?
-Equilibrium potential of an ion is the membrane voltage required to prevent movement of an ion down its concentration gradient
Membrane features
-Surrounds the entire neurone providing a hydrophobic relatively impermeable barrier
-Composed of lipids and proteins, with ion channels and pumps providing entry/exit routes for ions
Na+/K+ ATPase pump
Na+/K+ ATPase pump uses ATP to actively pump:
-3 Na+ ions out of cell
-2 K+ ions into the cell
This maintains a more depolarised internal environment
Na+ channels
-Sodium (Na+) channels permit the rapid influx of sodium into the cell upon opening, with resultant depolarisation (more positive)
K+ channels
-Potassium (K+) channels permit the rapid efflux of potassium out of the cell upon opening, with resultant hyperpolarisation (more negative)
Forces acting on ions in membrane
-Electrostatic force
-Force of diffusion
What is the equilibrium potential for Sodium and Potassium at physiological concentrations?
Potassium = -90mV
-Cell needs to be at -90mV to stop K+ leaving
Sodium = +50mV
-Cell needs to be at +60mV to stop Na+ entering
-Resting membrane potential is much closer to E(k) than E(Na) because membrane has many more K+ than Na+ channels
Driving force on potassium at rest
Driving force on sodium at rest
Driving force of ions against each other inside and outside of membrane
High conductance vs Low conductance
-Drugs can block channels for lower conductance
What can Nernst equation be used for
-To calculate resting membrane potential
Action potential principles
-Triggered by a depolarising stimuli
-There is a specific threshold of depolarisation required to trigger an action potential
-It is an all or nothing event, you don’t get 1/2 an action potential
-Propagates without decrement (faster in larger myelinated axons)
-At its peak: membrane potential approaches E(Na)
-After action potential membrane is unexcitable during its absolute refractory period
When is membrane potential closer to E(K) and E(Na)
At rest: Closer to E(K)
During action potential: Closer to E(Na)
Action potential phases
1) Resting membrane potential
2) Depolarising stimuli
3) Depolarisation reaches threshold: voltage-gated sodium channels (NaV) open and Na+ ions enter neurone
4) Rapid Na+ entry depolarises the neurone further
5)NaV channels inactivate and slower potassium channels open
6)Potassium ions (K+) move out of the neurone, repolarising the neurone
7)Kv channels remain open and more K+ leaves the neurone, hyperpolarising it
8)Kv channels close, some K+ enters cell through leak channels
9)Normal membrane potential
Absolute vs relative refractory period
-Absolute results from the inactivation of NaV channels, and lasts until the resting membrane potential is restored
-Relative results from the after hyperpolarisation phase, during which a greater stimuli is needed to reach the triggering threshold
3 functional states of ion channels
-Closed (resting)
-Open (active)
-Inactive (refractory)
-V-gated Na channels have all 3
-V-gated K channels have no inactivation site
How do Na and K combine to form action potential?
Non-myelinated action potential conduction
-Slow propagation
-Dull, aching pain
What makes up myelination in PNS and CNS?
PNS: Schwann cells
CNS: Oligodendrocytes
Myelinated action potential conduction
-Sodium and Potassium channels only expressed at nodes of Ranvier
-Action potential appears to jump from cell to cell
Demyelination diseases
-Channels are randomly distributed as barriers have broken down
-Less myelin means current can leak out
-Signal is not strong enough to trigger action potential
Conduction in non-myelinated and myelinated axons
