01 Neurophysiology Flashcards
Active Transport
ATP protein pump needed to transfer across membrane.
Diffusion
The movement of solute from an area of higher concentration to an area of lower concentration
Facilitated Diffusion
Passive process where a facilitator (channel protein) is needed. Eg. Ions
Equilibrium Potential
When [] Gradient = electric repulsion
Different for K and Na.
Ek = -90mV
ENa = +60mV
Resting membrane potential (RMP)
~ -70mV
Established by the Na+/K+ATPase.
Need selectively permeable membrane and [ion] difference between inside/outside of cell.
Depolarization
More positive (+)
- Open more Na+ channels (INward gradient)
- Decrease #K+ channels open
Generator Potential
Electrical event generated by stimuli. If large enough will produce an AP.
GRADED - vary in size (amplitude): larger stimulus will generate larger depolarization
What is the role of a Na+/K+ATPase in the generation and maintenance of a RMP?
ONLY MOVES IONS. Selectively permeable membrane separates charges. ATPase regulates the [K+] and [Na+] inside and outside the cell which affects equilibrium potential.
What is the [K+] inside and outside the cell? Which direction does the gradient favour?
Inside: [150mM]
Outside: [5mM]
Gradient is OUTward
Always tries to push Vm to Ek (-90mV)
What is the [Na+] inside and outside the cell? Which direction does the gradient favour?
Inside: [15mM]
Outside: [150mM]
Gradient is INward
Always tries to push Vm to ENa (+60mV)
At rest, why is RMP closer to Ek?
At rest, more K+ channels are open so the RMP is closer to the Ek value (-90mV)
Action potential (AP)
All or nothing electrical event triggered when membrane potential reaches threshold.
What are the components of Axonal Action Potential? (Think about the graph)
Threshold: AP elicited by suprathreshold (more than threshold) or threshold stimuli
Rising Phase: Rapid depolarization of the membrane caused by increased permeability of Na+ (PNa+)
Falling Phase: Rapid return to RMP because of increased K+ permeability (PK+)
After Hyperpolarization: Vm is closer to Ek than at rest because K+ channels are open and gK (change in conductance - movement of charged ions - of K+ ions) is greater than at rest.
Explain the role of voltage-gated Na+ and K+ channels in the generation of an axonal AP. (Think of images. Slide 36)
Na: Activation gate & Inactivation gate
K: Activation gate
Voltage-gated channels have a voltage sensors (AA with charged residues) that moves in response to membrane voltage changes. This movement is coupled to the activation gate.
Depolarization: Activation gate probably open. Inactivation gate probably closed.
How is action potential a REGENERATIVE EVENT?
Positive feedback depolarization?
AP in one part of the membrane will initiate an AP in a more distant part of the cell
Depolarization opens SOME voltage-gated Na+ channels and the influx of Na+ further decreases the membrane potential and recruits neighbouring channels (opens them)
Absolute Refractoriness
Inactive Na+ channels ensures that signals only go one way. A second stimulus will not do anything yet.
Relative Refractoriness
When AP closer to being fully recovered, we can generate a second AP by using a more intense stimulus. Why? because some channels have not recovered from inactivation yet (fewer channels available)
What is the mechanism of depolarization block of AP firing?
Some Na+ channels are inactivated (Inactivation gate closed) and cannot conduct current.
Recovery from inactivation opens Na+ inactivation gate (Channel available again) and closes K+ activation gate (return Vm to RMP)
Electrotonus
Newton’s cradle.
Process by which electrical events propagate - Movement of positive charge axially.
[Analogy]: Water in a hose
- Current enters axon through ion channels in a region of membrane and depolarizes that region.
- Intracellular (+) charge attracted to adjacent (-) charged regions of membrane
- Electrotonic spread of current (passive process)
Electrotonic Decay & Length Constant
Electrotonic Decay: As a current travels through the axon, charge leaks outward across the membrane. (Leakiness)
Length Constant: distance a passive electrical event can propagate along a neuronal process. (How long it travels before leaking to nothing)
Active Propagation of Electrical Potentials
Regenerative process where APs activate voltage-gated channels along axonal membrane to REGENERATE the depolarization. (Combats leakiness)
Voltage-gated channels BOOST and regenerates inward current and counteracts outward current leak.
What factors influence the active propagation of an AP along an axon?
- Diameter of the fibre: Increase diameter = decreased Ra (Axial resistance)
- Amount of membrane capacitance: Less membrane capacitance = greater rate of AP propagation.
Resistance
Rm - resistance of membrane
Ra - axial resistance
Resistors can change voltage instantly.
Capacitor
Two conducting plates separated by an insulating layer. Has the ability to STORE electrical charge. Takes time to change charge on capacitors. Resistors can change voltage instantly.