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
summation of graded potentials
temporal summation = many in quick succession from same neuron
spatial summation = many neurons connect to same axon and all are added up
EPSP
excitatory postsynaptic potential
EPSPs can be shunted by inhibitory inputs
inhibitory synapses can act near the soma to stop other signals as they come through before reaching the soma
electrical synapses
ions travel freely through connexins between neurons - gap junctions
much faster than molecular synapses with neurotransmitter diffusion
