ME01 - Membrane Electrophysiology Flashcards
Characteristics of Ion Channels
Ion Channel Selectivity
Ion Channel Gating
Conductance of Ion Channel
Ion channels permit the passage of some ions, but not other
Ion Channel Selectivity
Ion Channel Selectivity is based on:
Channel size
Distribution of charges that line it
Size and charge of ions
How much water the ion attracts and holds around it
Ion channels are either GATED or NON-GATED
Ion Channel Gating
What are Gated ion channels and it’s types
Gated ion channels have gates that can open or close the channel
Voltage-gated - gates are controlled by voltage (difference in membrane potential)
Ligand-gated - open or closed by hormones, second messengers or neurotransmitters
Ion channels that are always open
Also called as leakage channels
Non-Gated Ion Channels
Conductance of Ion Channels depend on ______
If the channel is OPEN
The higher the probability that a channel is open, the higher the conductance or permeability
Location of ION CHANNELS
Non-Gated: Cell membrane on dendrites, Cell Body and Axon
Ligand-Gated: dendrites, cell body
Voltage-Gated: Axon hillock, unmyelinated axons, along the nodes of Ranvier in myelinated axons
Functions of ION CHANNELS
Non-Gated : RMP
Ligand-Gated: Synaptic potentials (incoming signals to neurons)
Voltage-Gated: Generation and Propagation of action potential (outgoing signals from neurons)
Types of Membrane Potentials
Diffusion Potential
Equilibrium Potential
Potential difference generated across a membrane because of a concentration difference of an ion
DIFFUSION POTENTIAL
Can be generated only if the membrane is permeable to the ion
Factors related to the Diffusion Potential
Size
Sign
Size depends on the size of the concentration gradient
Sign depends on whether the diffusing ion is positively or negatively charged
Diffusion Potential do not result in changes in concentration of the diffusing ions
TRUE.
Diffusion potential that exactly balances (opposes) the tendency for diffusion caused by a concentration difference
Equilibrium Potential
Chemical and Electrical Driving forces that act on an ion are equal and opposite
No net diffusion of the ion occurs
Electrochemical Equilibrium
In the presence of a nondiffusable ion, diffusable ions distribute themselves so that at equilibrium their concentration ratios are equal
For any pair of cation/anion
Gibbs-Donnan Equilibrium
Formula for Gibbs-Donnan Equilibrium
[K+]in x [Cl-]in = [K+]out x [Cl-]out
Implications and Consequence of Gibbs-Donnan Equilibrium
(1) Presence of charged proteins in cells, there are more osmotically active particles in cells than in IF
C: Animal cells have flexible walls; Osmosis make cells swell and rupture; Prevented by Na/K ATPase
(2) At equilibrium, distribution of ions across membrane is assymetric
C: Electrical potential difference exists across membrane (MP)
Magnitude can be determined by Nernst equation
(3) There are more proteins in the plasma than in the IF
C: Gibbs-Donnan equilibrium will affect ion movement across capillary walls
Calculates the equilibrium potential at a given concentration difference of a permeable ion across cell membrane
Nernst equation
EMF = 61+log concentration inside/conc. outside
Signs to remember in Nernst equation
sign of potential is (+) if diffusing ion is a negative ion
sign of potential is (-) if diffusing ion is a positive ion
Calculates the membrane potential in the inside of a membrane when a membrane is permeable to several different ions
Goldman-Hodgkin-Katz Equation
Measured potential difference across the cell membrane in millivolts (mV)
RMP
Intracellular potential relative to the extracellular potential
RMP
RMP of -70 means 70mV cell negative
Characteristics of EMP
Established by diffusion potentials that result from concentration differences of permeable ions
Each permeable ion attempts to drive the membrane potential toward its equilibrium potential
ORIGIN OF RMP
Contribution of K+ Diffusion Potential
Contribution of Na+ Diffusion
Contribution of Na/K ATPase
If potassium ions were the only factor causing the resting potential, the resting potential INSIDE THE FIBER would be equal to ________
-94 millivolts
Potassium vs Sodium in Diffusion and Permeability
Diffusion of K contributes far more to membrane potential than diffusion of Na
Permeability to K is about 100 times as great as its permeability to Na
Contribution of Na/K ATPase to the Original RMP
Direct electrogenic contribution of the pump (3Na2K) is small
This creates an additional degree of -4mV
Short range change in voltage
Incoming Na ions diffuse for short distances inside the plasma membrane
Produce a current that travels from the point of stimulation toward the cell’s trigger zone
LOCAL POTENTIALS
Any change in which membrane voltage shifts to a less negative value
Depolarization
Two possible effects of Depolarization
Local potential - generation of a nonpropagated response
Action potential - generation of a propagated response
Characteristics of LOCAL POTENTIAL
Graded. (Stronger the stimulus, stronger the response)
Decremental. (Gets weaker as they spread from point of stimulation)
Reversible. (If stimulation ceases, K diffusion out of cell to return membrane voltage to its resting potential)
Either Excitatory or Inhibitory.
Excitatory - produce an action potential
Inhibitory - less likely to produce an action potential
Types of Local Potential
Synaptic Local Potentials
Generator/Receptor Potentials
Electrotonic Potentials
Alteration in the Membrane Potential of cell resulting from activation at the synapse
Synaptic Potential
Types of Synaptic Potential
Excitatory Post-Synaptic Potential - Intracellular voltage increases
Inhibitory Post-Synaptic Potential - Intracellular voltage decreases
Transmembrane potential difference produced in sensory receptor
Occurs as depolarization resulting from inward current flow
Generator/Receptor Potential
Influx of current will often bring the membrane potential of the sensory receptor towards the threshold for triggering an action potential
Generator/Receptor Potential
Passive spread of charge inside a neuron due to local changes in ionic conductance
Ionic charge enters in one location and dissipates to others, losing intensity as it spreads (graded response)
Electrotonic Potential
Rapid changes in the membrane potential that spread along the nerve fiber
Action Potential
How do you conduct a nerve signal
The action potential moves along the nerve fiber until it comes to the fiber’s end
Characteristics of Action Potential
Follow All-or-None Law. If stimulus depolarizes neuron to threshold, neuron fires at its maximum voltage. Above threshold, stronger stimuli do not produce stronger action potential.
Nondecremental. AP do not get weaker with distance
Irreversible. AP goes to completion. AP cannot be stopped once it begins.
Fill the table. Parameter LOCAL POTENTIAL ACTION POTENTIAL Mediator Voltage Change Intensity Reversibility Distance of Effect Progression
Parameter LOCAL POTENTIAL ACTION POTENTIAL
Mediator Ligand-gated ion channels Voltage-gated ion channel
Voltage Change Depolarizing/Hyperpolarizing Depolarizing
Intensity GRADED ALL-OR-NONE
Reversibility Reversible Irreversible
Distance of Effect Short (Local) Long (distant)
Progression Decremental Non-decremental
Steps in the Generation of Action Potential
Resting Stage - RMP, before AP begins, -90mV
Depolarization - Threshold potential is reached (-65mV)
Voltage-gated Na overwhelm K+, permeable to Na ions
Overshoot - great excess of positive Na ions causes MP to overshoot
beyond the zero level
Ionic Basis of Depolarization
Necessary factor in both depolarization and repolarization of the nerve membrane
Voltage-Gated Sodium Channel
Feedback Control of Depolarization
Positive Feedback
Sodium channels begin to close and the K channels open more than normal
Rapid diffusion of K ions to the exterior reestablishes resting membrane potential
Repolarization
Ionic Basis of Repolarization
Also plays an important role in increasing the rapidity of repolarization of the membrane
Voltage-Gated Potassium Channel
Feedback Control of Hyperpolarization
Negative Feedback
Afterhypolarization
K+ conductance remains higher than at rest after closure of the Na+ channels
Membrane potential is driven very close to the K+ equilibrium potential
Undershoot
Effects of Ion Fluxes on Action Potential
Structure Contribution
K+ leak channel
Na+/K+ ATPase pump
Voltage-gated Na+channels
Voltage-gated K+ channels
Structure Contribution
K+ leak channel Resting Membrane Potential
Na+/K+ ATPase pump Maintenance of the Concentration gradient
Voltage-gated Na+channels Depolarization and Hyperpolarization
Voltage-gated K+ channels Hyperpolarization
Effect on
DECREASE extracellular Na+ concentration
Reduce the size of AP
No effect on RMP
Effect on:
INCREASE extracellular K+ concentration
Decrease RMP
Effect on:
DECREASE extracellular Ca2+ concentration
INCREASE excitability
DECREASE threshold
Effect on:
INCREASE extracellular Ca2+ concentration
DECREASE excitability
INCREASE threshold
Effect on:
INCREASE intracellular Cl- concentration
INCREASE intracellular Protein concentration
DECREASE RMP
How are AP propagated?
By the spread of local currents to ADJACENT areas of membrane, which are then depolarized to threshold and generate action potentials
Direction of AP Propagation
No single direction of propagation
An AP travels in both directions away from the stimulus until the entire membrane has become depolarized
Depolarization process travels over the entire membrane if conditions are right, or it does not travel at all if conditions are not right
All-Or-Nothing Principle
Factors Affecting Conduction Velocity
Fiber Size - Increase diameter, faster
Myelination - myelin acts insulator; saltatory conduction (5-50 fold)
Action potentials can be generated only at gaps in the myelin sheath called as
Nodes of Ranvier
Importance of Saltatory conduction
It conserves energy for the axon
Only the nodes depolarize, allowing 100 times less loss of ions than would otherwise be necessary
Time periods after an action potential during which new stimulus cannot be readily elicited
Refractory Period
Another AP cannot elicited, no matter how large the stimulus is
Coincides with almost the entire duration of the action potential
Absolute Refractory Period
Ionic basis of Absolute Refractory Period
Inactivated gates of Na+ channel are closed when membrane potential is depolarized and remain closed until repolarization occurs
No AP can occur until the inactivations gates open
Begins at the end of the Absolute refractory period and continues until the membrane potential returns to the resting level
AP ca be elicited only if a larger than usual inward current is provided
Relative Refractory period
Ionic Basis of Relative Refractory Period
K+ conductance is higher than the rest
MP is closer to the K+ equilibrium potential and farther from the threshold
More inward current is required to bring the membrane to threshold
Integral proteins that span the cell membrane
When open, permit the passage of certain ions
Ion Channels