Membrane Potentials + Action Potentials Flashcards
Voltage
Current
Resistance
Definitions + Equation
Voltage - generated by ions to produce charge gradient
Current- movement of ions due to PD
Resistance -barrier that prevents movement of ions
Voltage = Current x Resistance
Properties of Ion Channels
Lipid cell membrane acts as a barrier to ion movement and separates ionic environments
Cell membrane can selectively change permeability to specific ions
Permeable pores in membrane (ion channels) open and close depending of transmembrane voltage, presence of activating ligand or mechanical forces
Movement across membrane will occur when there is a concentration gradient between either side of membrane
Electrochemical Equilibrium
Electrical forces (+/-) balance diffusional forces so stable transmembrane potential is achieved
Equilibrium Potential
Potential at which electrochemical equilibrium has been reached which also prevents the diffusion of the ion down its concentration gradient
How to calculate Equilibrium Potential (E)
Nernst Equation E= RT/zF lnX2/X1 R - gas constant T - temp in kelvin (310K) Z = ion charge F = Faraday’s no° (charge per mol of ion) ln = natural log X2 = intracellular ion concentration X1 = extracellular ion concentration
Why is the Nernst equation limited?
As equilibrium potentials of K+ and Na+ are based on theoretic values but biological membranes aren’t uniquely selective for any one ion at a time and membranes have variable permeability to all ions.
Each ion’s contribution to MP is proportional to how permeable the membrane is to the ion at any time.
GHK is proposed instead
Explain the Goldman-Hodgkin-Katz equation
Describes the Em more accurately by including P
This stands for He permeability or channel open probability ( 0=100% closed 0.5=50% time open 1=100% open)
Subscript on P indications the ion (K/Na/Cl) and subscript on conc of ion indicates inside or outside the cell (i/o)
Define: Depolarisation Repolarisation Overshoot Hyperpolarisation
D - membrane potential becomes more positive towards 0mV
R - membrane potential decreases towards resting potential
O - membrane potential becomes positive
H - membrane potential decreases beyond resting potential
What are graded potentials?
External stimulations or neurotransmitters causes change in membrane potential
This change is graded in response to the type or strength of stimulation
Graded potentials produce the initial change in membrane potential that determines what happens next (initiate/prevent APs)
Why do graded potentials decrease from one end to the other?
Moving down the axon from the site of initial depolarisation, charge ‘leaks’ from the axon into the extracellular fluid and so the size of the potential change decreases along the axon
When do APs occur?
APs occur when a graded potential reaches a threshold for the activation of Na+ channels which occur in excitable cells aka nerve impulses allowing transmission of info reliably and quickly over long distances
Also used for intracellular processes
What causes APs to form?
Permeability depends on conformational state of ion channels (open/closed/inactivated)
When membrane permeability of an ion increases it crosses the membrane down its electrochemical gradient
Movement changes the membrane potential towards the equilibrium potential for that ion
These changes cause APs
Name 5 phases of AP
Phase 1: Resting Membrane Potential Phase 2: Depolarising Stimulus Phase 3: Upstroke Phase 4: Repolarisation Phase 5: After-hyperpolarisation
Describe phase 1 RMP
Permeability for K+ is more than that for Na+
Membrane potential is also nearer equilibrium potential (-90mV vs +72mv respectively)
Describe Phase 2 Depolarising Stimulus
The (graded potential) stimulus depolarises the membrane potential
Moved it in the positive direction towards threshold
Describe phase 3 upstroke
Starts at threshold potential
Na+ potential increases as VGNC open quickly so Na+ enters the cell down the electrochemical gradient
K+ potential increases as VGPC start to open slowly so K+ leaves the cell down the electrochemical gradient (less K+ leaves than Na+ that enters)
Membrane potential moves towards the Na+ equilibrium potential
Describe phase 4 Repolarisation
Na+ potential decreases as VGSC close so Na+ entry stops
K+ potential increases as more VGPC open and remain open so K+ leaves the cell down its electrochemical gradient
Membrane potential moves towards the K+ equilibrium potential
Describe the absolute refractory period
Absolute refractory period is where Na+ channel activation gate is open but the inactivation gate is closed
New AP cannot be triggered as Na+ channels are closed
Absolute refractory period continues and so activation AND inactivation gates are closed
Describe phase 5 after-hyperpolarisation
At rest VGPC are still open
K+ continues to leave the cell down the electrochemical gradient
Membrane potential moves closer to K+ equilibrium so then some VGPC close
Membrane potential returns to the resting potential
Describe the relative refractory period
Some Na+ channels have recovered from inactivation and so the gate is open
An AP can be triggered at this stage but a stronger than normal stimulus is required
How do APs move along an axon?
Internal and membrane resistance, diameter of axon alters propagation distance and velocity
Local current flow depolarises adjacent region towards threshold causing that area to activate from resting potential while the old active region returns to resting potential and this continues along the axon
Voltage gates channels are mostly located at nodes allowing Saltatory conduction down nerve fibres
Describe factors that affect conduction velocity
Both axon diameter and myeloma toon influence conduction velocity
Small diameter + non myelinated axons - 1m/s
Large diameter + myelinated axons - 120m/s
Velocity decreases with:
reduced axon diameter and reduced myelination
Cold
Anoxia (absence of O2)
Compression
Drugs
