action potential Flashcards
what is resting membrane potential?
the potential difference that exists across the membrane of all cells
what is the range of resting membrane potential?
20-90mV
which cellular fluid is more negative?
ICF
what causes the polarised ion/charge distribution?
the charge at the very edge of membranes
- inside
+ outside
which is the predominant cation in ECF?
sodium
what is the principal anion in ECF?
chloride
which is the predominant cation in ICF?
potassium
which is the principal anion in ICF?
phosphate
which directions do Na/K go?
Na into cell
K out of cell
which ion is the resting membrane very permeable to?
potassium
what causes the resting membrane potential?
diffusion of K leaves excess neg change, potential gradient arising= RMP
how is the small amount of Na that leaks into the cell expelled?
the Na/K pump
what other factor contributes to RMP?
Na/K pump by exchanging unequal numbers of Na(3 out) and K(2 in)
electrogenic
what is the AP threshold?
-55mV
what is the rising phase due to?
Na influx
voltage gated Na channels
what is the falling phase due to?
K efflux
voltage gated K channels
when does Na enter the neuron?
during upstroke
when does K move out of the axon?
during downstroke
what are ion channels?
transmembrane protein
describe the opening of a voltage-gated sodium channel
channel closed -M-gate closed, h-gate open channel open -m&h open channel closed (refractory) -m open, h closed
describe an AP
at threshold -voltage gated Na open -na diffuse in-> further depolarisation -+ feedback at peak -Na channels close; voltage-gated K channels open -K diffuse out-> repolarisation return to resting membrane potential
why is AP an all or none event?
amplitude is independent of stimulus
what are the consequences of the refractory period?
limits max firing frequency
ensures unidirectional propagation of AP’s
prevents summation of APs
prevents summation of contractions in cardiac muscle
-cardiac AP lasts as long as ventricular contraction
describe propagation of an AP
AP in one section of axon sets up longitudinal current flow
depolarises adjacent resting parts of axon
AP regenerated further along axon
more current flows and next region of axon activated
AP travels along axon as waves of depolarisation
how does current flow in ICF & ECF?
from + to - regions
how does the AP move with current?
current flow alters membrane potential in adjacent regions
what increases the speed of AP propagation?
axon diameter
myelin
what is myelin?
many layers of cell membranes wrapped around the axon
what cells lay down myelin?
glial-shwann
what does myelin do?
forms an insulating layer- reducing leakage of current from axon
what are nodes of ranvier?
myelin sheath interrupted at intervals
axon membrane exposed to ECF and ion flow can occur
what is saltatory conduction?
in myelinated nerve, the passive currents spread further along the axon
fewer regeneration steps per unit length of axon
so AP propagates more rapidly than in unmyelinated
how do peripheral nerve axons differ?
size
-diameter abd cond. velocity
function
-sensory/motor
what is the function and myelination of A beta fibres?
myelinated
mechanoreceptors
what is the myelination and function of a delta fibres?
myelinated mechanoreceptors thermoreceptors(cold) nociceptors chemoreceptors (taste)
what is the myelination and function of C fibres?
unmyelinated
mechanoreceptors
thermoreceptors (hot)
nociceptors
is majority of tooth pulp fibres myelinated or not?
unmyelinated
what is the rate and direction of diffusion determined by?
potential and concentration gradients
what is the potential difference in a resting state?
inside of cell 70mV more negative than outside
what happens if the MP of a neuron is changed from RMP -70mV to -60mV?
depolarisation
what is it called if the MP of a neuron is changed from RMP of -70 to -80mV?
hyperpolarisation
what is the stimulus for an AP?
depolarisation
what kind of feedback is involved when ions enter the cell, depolarisation and more ion channels open?
positive feedback
how does the MP return to resting value after the AP peak?
ions move by diffusion
K ions passing out of cell
what causes the period of inexitability in the refractory period?
inactivation of sodium channels
