Week 5 Cellular Excitation

Gibbs-Donnan Equilibrium

with a semipermeable membrane and an impermeant ion, the other ions in tend to distribute themselves to resolves concentration and charge differences, which may result in differences of concentration but still a charge difference of zero

Nernst Equation

used to calculate the equilibrium potential of a single ion

equilibrium potential

when at equilibrium, the charge difference of the ion at the membrane

resting membrane potential

NOT equal to the equilibrium potentials of individual ions (K+, Cl-, or Na+), may differ some for different cell types, excess of positive charge on the outer surface of the membrane and negative charge on the inner surface, reflects the interactions of all three ions (more strongly potassium and sodium though, respectively)

Equation of membrane potential

measured in voltage

resting ‘leak’ ion channels

movement of Na+, K+, Cl- passively through the membrane due to concentration gradients, Na+ is 10x greater outside, K+ is 30x greater inside, Cl- is 10x greater outside but is more variable

intracellular

where impermeant organic anions (proteins) are typically found

Na+/K+ Pump

maintains the concentration gradient needed for the resting membrane potention, overall charge imbalance of the pump activity only lowers Vm slightly (-4 mV), must burn ATP, pumps 3 Na+ out and 2 Na+ in

electrochemical gradient

differences in ion concentrations, chemical/diffusion forces, electrical forces, no net movement at equilibrium

Goldman- Hodgkin- Katz Equation

a more accurate equation to represent resting membrane potential, takes into account conductance and permeability factors of the ions, if the g is very much greater for one ion than the others the eq will reduce to the nernst equation (also g = p essentially)

Resting membrane potential of muscles, nerves in the PNS, and nerves in the CNS

-90 mV, -75 mV, -65 mV

relative permeabilities at rest PK:PNa: PCl

1.0: 0.04: 0.45, this means that K+ contributes most to the resting membrane potential at rest

gated ion channels

the action of these can alter the resting membrane potential, if altered enough that the change is enough to reach the threshold potential, an action potential will be generated, may be voltage gates or ligand gated receptors

action potential

basic signaling unit in nervous system, a binary signal, all or nothing!, self propagating

what would not cause an AP

weak stimuli, small, temporary change in Vm (local potential, passive)

what would cause an action potential

sufficiently strong stimulus achieves threshold of depolarization (typically a change in +20 mV), either a large stimulus or sufficient # of subthreshold stimuli

depolarizing

Na+ moves rapidly into cell, causes a positive increase in membrane potential due to the opening of voltage gates Na+ channels, until reaches around +35 mV, at this time frame Na+ has the greatest conductance

local, graded, generator, or receptor potential

environmental stimuli produces a local, graded change in the membrane potential, different that an action potential, examples are mechanical, pressure sensing, olfactory, retina

receptor potential characteristics

produces by opening of ion channels, depolarizing(excitatory) of hyperpolarizing (inhibitory), magnitide of potential proportional to magnitude of stimulus, AP are a result of local potential

local potential summation

can be temporal or spatial, stimulatory or inhibitory and may come from receptors or synapses with other neurons, outcome is an AP or no AP, typically occurs on the axon Hillock, whcih is on the soma, has a lower threshold for depolarization, and a higher density of voltage gates Na+ channels

Voltage gates K+ channels

open and close more slowly than Na+ channels, at the peak of the action potential, g of it has increase to near that of gNa+

Na+ inactivation gates

start closing soon after the activation gates open, reduces Na+ current, does not appear to be related to the duration of stimulus

absolute refractory period

during the period in which the Na+ inactivation gates are closed, another action potential cannot be generated (also encompasses the rising phase of the action potential)

hyperpolarization

caused by the opening of the K+ channels, afterpotential, after the Na+ activation and inactivation gates have closed

relative refractory period

when the membrane potential is hyperpolarized still, even when the Na+ inactivation gates are open, a greater stimulus is needed to produce an action potential

AP travels to…

AP may be sent to another neuron, muscle, or gland, requires signal propagation, voltage change from initial AP will initiate others!