RMP + AP Flashcards
what establishes the RMP of a cell?
the presence of large stationary anions within the cell
phosphate groups + amino acids
fixed -ve charge attracts +ve ions from extracellular material, establishes inflow of charge
which is more concentrated, the cell or extracell? what does this mean for osmotic flow?
cell as many proteins + soluble mols, with relatively small cell volume
osmotic pressures cause ions to efflux through CSM
what is the RMP in excitable cells e.g. nerve cells?
approx -90mV
what are the ions that contribute to RMP?
primarily Na+ and K+
also Cl- and HCO3-
what did the Hodgkin and Katz experiments establish?
1ry ion influencing RMP is K+
experimentally altered conc of K+ inside/outside cell and when the gradient across the membrane for K+ was 0, the RMP became approx 0mV
membrane is most permeable to K+
how much more permeable is the cell to K+ than Na+? what does this mean for membrane potential (Em)?
100x
lies closer to equilibrium potential for K+ than Na+
what forces apply to K+ to move in/out of the cell? what is the equilbrium potential?
insoluble anions within the cell attract K+, causes influx down ECG
K+ leak out of cell down the osmotic gradient
equilibrate at -93mV
what is the equilibrium potential of Na+?
+58mV
what does the membrane potential equilibrate at?
-87mV
what equation describes the interactions between the ions that move across the membrane?
Goldman
takes into account external and internal conc of each ion, giving RMP
why does K+ have a greater effect on RMP than Na+?
more K+ leak channels
membrane 100x more permeable to K+
Na+ can enter cell down its conc grad but there are less channels, so less influence.
what maintains the intra + extracellular ion concs?
Na/K ATPase
pumps 3 Na+ out and 2 K+ in using energy from hydrolysis of ATP
ions stay on the side they were pumped to unless there is a leak channel, bc membrane is impermeable to them, maintaining conc grad
what is some pathology associated with RMPs?
in anorexia, when refeeding, can develop hypokalemia
as during fasts, intracellular K+ used to maintain serum K+ levels, refeeding can cause rapid uptake of K+ leaving insufficient K+ in blood + ECF
hyperpolarisation of neurons = muscle weakness + hyporeflexia
what causes an action potential?
a significant enough depolarisation of the membrane potential
why are neurons excitable?
due to voltage-dependent ions in their plasma membranes with ion selectivity
how does positive feedback of Na+ work in an AP?
if neuron sufficiently depolarises to threshold potential, Na+ channels change shape allowing ions to flow into neuron down ECG
local influx causes depolarisation which drives other channels to open
+ve feedback in Na+ permeability
do action potentials lose amplitude or velocity along the axon?
no
what factors determine conduction velocity?
degree of myelination
axon diameter
what is the axon in non-myelinated neurons?
region of the axon that is depolarised and +ve compared to the outside
how is a local circuit created in the axon?
ahead of the active zone the cell has a negative potential, creating potential difference along the axon and local circuit currents which flow between the portions of the axon
define an action potential
a transient electrical impulse in which Em is displaced by up to 100mV, becoming more positive due to ion influx
how can you initiate an action potential?
ACh binding to and opening ligand-gated Na+ channels on post-syn membrane
if enough ACh excreted, enough ligand-gated ion channels open to depol the membrane past threshold potential -55mV
once past threshold, Na+ v.g. channels are activated and they start to initiate AP
what is the difference between action potential and synaptic potential?
SP: the electrical potential difference across the post-synaptic membrane
AP: occurs as a result of summation of many synaptic potentials across the membrane of a neuron
describe the course of an AP
stimulus occurs, if it causes membrane to depolarise to threshold i.e.
enough of a sensory change e.g. in stretch channels to open enough channels to let enough Na+ in to reach threshold
then v.g. Na+ channels open, depol occurs, Na+ slowly close, K+ are slowly opening the whole time
Na+ shut, become inactivated, K+ floods in, repols the cell towards Ek, absolute refractory period, Na+ channels stop being inactivated.
then hyperpolarises, K+ channels shut, relative refractory period: AP can be initiated but large stimulus is needed, Na/K returns cell to RMP
