cell excitability Flashcards
action potential vs graded potential
action:
fixed size, all-or-nothing, travel along/propagate axon - can go either way, tends to go one way
graded:
variable size, local signals not propagated long distances, go both ways along neuronal membrane
information coding of action and graded potentials
action = coded by frequency - continuous stimulation produces a train of action potentials, more intense = more frequent
graded = coded by size - vary according to strength of stimulus
membrane potential
Vm
absolute requirement for a functioning nervous system
measuring potential across a neuron
connect a voltmeter by inserting a glass microelectrode full of KCl (carries charge) into the neuron
put electrode (silver chloride) into solution surrounding the outside of a neuron
PD of -65 - -90mV = uneven distribution of charge across neuronal membrane –> distribution of ions
resting potential
inevitable consequence of:
selectively permeable membrane
unequal distribution of charged molecules/ions
membrane is selective (channels and pumps) and unequal (maintain the ion concentrations)
physical forces
chemical and electrical gradients
2 main ion pumps in neurons
sodium potassium pump -> Na+ out of cell and K+ into cell
calcium pump -> Ca2+ is important for propagating action potentials across synapses
equilibrium potentials
Eion
membrane potential that would be achieved in a neuron if the membrane were selectively permeable to that ion
net movement = 0
electrical and chemical forces balance out
ionic driving force - consider Vm and Eion
Nernst equation - what does R, T, z, F stand for
calculate equilibrium potential
use ratio of ions outside and inside of cell
R = universal gas constant
T = temperature (in kelvin)
z = valence of ion (e.g. Na+ = +1)
F = faraday’s constant
permeability of neuronal membranes to ions at rest
very permeable to K+ –> Vm is close to Ek at rest
only slightly permeable to other ions
Goldman equation
calculates membrane potential using concentrations of multiple ions
relative permeability of membranes to ions - more permeable to potassium than sodium
action potential maximum frequency
500Hz
action potential stages
threshold = enough Nav channels open so permeability of Na+ is greater than K+
rising phase = rapid depolarisation drives Na+ into neuron
overshoot = Vm approaches ENa
falling phase = Nav channels inactivate, Kv channles open, drives K+ out of neuron
undershoot = Kv channels add to resting K+ membrane permeability and reduced Na+ permeability so Vm = Ek (back to resting potential)
structure of Nav and Kv channels
change in PD across membrane causes confirmational change of pore to open
-65mV = closed, -40mV = open
ion channels are open but blocked so ions cannot get through
channels are then closed before they can be activate again
poisons that affect voltage gated channels
tetraethylammonium (TEA) = Kv channels
lidocaine = Nav channels
tetrodotoxin (TTX) in puffer fish = Nav channels
saxitoxins (STX) in dinoflagellates = Nav channels
graded potentials - GABA and Clv channels
GABA receptors are Cl- channels
binding of GABA causes hyperpolarisation of membrane as Cl- enters the cell
