Lecture 3: Electrical Potential of Cells Flashcards
Potential difference
The ability of separated opposing charges to do work, aka electrical potential
Current
Movement of charge
Resistance
Hindrance to electrical charge movement
Resting membrane potential of neurons
-70 mV
Contributors to membrane potential
Na+ outside, K+ inside (salty bananas); chloride somewhat as well. Negligible amounts compared to total compartment concentrations.
Equilibrium potential
Potential at which net flux due to concentration balances net flux due to charge. A larger concentration gradient means a larger equilibrium potential. Differs between ions.
Nernst Equation
61 combines R, physiological temperature, Faraday constant
Goldman-Hodgkin-Katz Equation
= -70 mV. Expanded Nernst that factors in permeabilities; note that Cl- is reversed because it’s an anion.
How do different ions’ equilibrium potentials influence membrane potential?
The ions with highest permeability influence the resting potential the most via their equilibrium potentials. K+ has a high permeability due to leak channels, which is why membrane potential is so close to E_K (-90 mV).
Electrogenic pump
A pump that moves net charge and thus directly contributes to membrane potential (small effect in most cells).
How are equilibrium potentials created?
Concentration gradients across semipermeable membranes.
Excitable cell
Cells that can produce electrical signals along their membranes via gated ion channels
Graded potential
Potential of variable amplitude/duration conducted decrementally. Triggered by chemical stimulus opening ion channels.
Types of graded potentials
- Receptor: produced at receptor cells/peripheral ends of afferent neurons in response to stimulus.
- Synaptic: produced in post-synaptic neuron in response to neurotransmitter release and binding.
- Pacemaker: spontaneously occurring, occurs in specialized cells
Driving force of an ion
= Membrane potential - ion equilibrium potential. Depends on electrochemical gradient.
Ionic current
= g (Vm - E). Ions move only when there is a driving force AND a conductance/permeability (g) for that ion.
Depolarization
Potential moving from resting membrane potential to less negative value
Overshoot
Reversal of membrane polarity (inside becomes more positive than out)
Repolarization
Potential returns to RMP from depolarized state
Hyperpolarization
Potential becomes more negative than RMP.
What defines the conductance of the membrane?
Open and closed ion channels.
Time constant τ
τ = Rm * Cm (membrane resistance and capacitance). Defined as amount of time post-current injection for potential to rise to 63% of its final value OR drop to 37% of its initial value. Shows how quickly membrane depolarizes and affects temporal summation.
Length constant λ
λ = sqrt(r_m / r_i)
(membrane/internal or axonal resistance per unit length). Defined as distance from sit of injection where potential falls to 37% of its initial value. Indicates how far a depolarizing current will spread/how leaky the membrane is. Determines decremental propagation.
How does length constant λ affect neurons?
λ determines synaptic efficiency. Synapsing to a larger dendrite is more likely to initiate an AP.
