Action Potentials Flashcards
what is the resting membrane potential
about -70mV
the ICF or the ECF more negative?
ICF
which ion is most abundant in the ECF?
Na
which ion is most abundant in the ICF?
K
which ion can move through the membrane more?
K
how is the resting membrane potential brought about?
potassium diffuses OUT of the cell which leaves an excess of -ve charges inside the cell. The potential gradient that arises from this is the resting membrane potential
what does the sodium potassium pump do?
move 3Na out and 2K in
does the sodium potassium pump require energy?
yes as it moves the ions from a low to high concentration
what is the threshold voltage during the AP?
-55mV
what is the rising phase during the action potential due to?
Na+ influx through voltage gated Na channels
what is the falling phase during the action potential due to?
K+ efflux through voltage gated K channels
what type of protein is an ion channel?
transmembrane protein
how many gates does the sodium channel have?
2 - H and M
what is the action of the sodium gates?
the channel opens when the M gate opens as H gate is already open. The channel closes when the H gate closes (refractory period)
at resting membrane potential what is the state of the Na gate?
H gate open, M gate closed
at the threshold until hyperpolarisation what is the state of the Na gate?
M gate and H gate open
at the refractory stage what is the state of the Na gate?
M gate open, H gate closed
how many gates are there for K
1
what is the first step of the AP?
stimulus applied - depolarisation - MP moves towards threshold, gated ion channels closed
what is the second step of the AP?
MP reaches threshold (-55mV) - Na channels start opening, Na influx, more depolarisation, K+ channels still closed
what is the third step of the AP?
all Na channels open, maximum Na influx, MP overshoots 0mV
what is the fourth step of the AP?
MP reaches +35mV - Na channels shut, inactivation, H gate closes, K+ channels open and K+ efflux begins
what is the fifth step of the AP?
AP downstroke (recovery phase) - Na+ channels shut, refractory period, K+ channels open, K+ efflux continues
what is the sixth step of the AP?
MP returns to resting level, ion channels return to resting state (H gate open, M gate closed), excitability restored
what happens at the threshold of the AP?
voltage gated Na channels open, Na+ diffuse in, further depolarisation, positive feedback involved
what happens at the peak of the AP?
Na+ channels close, voltage gated K+ channels open, K+ diffuse out - repolarisation
what is the refractory period
after an AP is initiated the neuron cannot generate another AP until the first one has ended (period of inexcitability)
what is the refractory period due to?
inactivation of the voltage gated sodium channels (H gate closed)
what are the consequences of the refractory period?
limits maximum firing frequency, ensures unidirectional propagation of action potentials, prevents summation of AP, prevents summation of contractions in cardiac muscle
describe AP propagation
AP set up in one section of axon, depolarises adjacent part, AP regenerated further along, it moves along the axon as waves of depolarisation
what increases AP propagation?
myelination and large axons
what does myelination do?
increases speed of AP propagation for given diameter of axon, forms an insulating layer which reduces leakage of current from axons
what is myelin laid down by?
glial cells
what are the spaces where the myelin sheath is interrupted called?
Nodes of Ranvier
what occurs at the nodes of Ranvier?
axon membrane is exposed to ECF and ion flow can occur
what is saltatory conduction
AP jumps from one node of Ranvier to the next which increases speed so fewer regeneration steps meaning more rapid AP propagation
what is the tissue layer surrounding axons?
endoneuron
what layer protects a bunch of axons?
perineuron
what layer protects lots of bunches of axons?
epineuron
what do A beta fibres do?
mechanoreceptors
what do A delta fibres do?
mechanoreceptors, thermoreceptors, nociceptors, chemoreceptors
what do C fibres do?
mechanoreceptors, thermoreceptors, nociceptors
are tooth fibres myelinated or unmyelinated?
most are unmyelinated as when myelinated fibres move closer to odontoblasts they lose myelination