ET : N - Action Potentials Flashcards
is potential inside neurons constant
no
when does potential inside neurons change
when ion concs or membrane permeability changes
hyperpolarisation occurs when it becomes more
negative
in hyperpolarisation, the potential inside the cell moves closer to the EP of
potassium
depolarisation occurs when it becomes more
positive
in depolarisation, the potential inside the cell moves closer to the EP of
sodium
what is an action potential (technically speaking)
a brief fluctuation in membrane potential caused by a transient opening of voltage-gated ion channels, which spreads, like a wave, along an axon
when does an action potential occur
when the membrane has been depolarsied to threshold
information is coded in the…
frequency of action potentials
what are action potentials a key element of
the process of signal transmission along axons
look at diagram of three stages of action potentials: what is the * representing
a slow and graded depolarisation evoked by a stimulus
look at diagram of three stages of action potentials: what is the 1 representing
after the MP reaches threshold: fast depolarisation to approx 30mV (overshoot)
look at diagram of three stages of action potentials: what is the 2 representing
repolarisation
look at diagram of three stages of action potentials: what is the 3 representing (and what happens in this)
after-hyperpolarisation (AHP) (becomes slightly more negative than RMP before returning to RMP)
look at diagram of three stages of action potentials: 1+2 together represent what period
or
what period does stimulus evoking and rapid depolarisation represent
absolute refractory period
look at diagram of three stages of action potentials: what period is 3
relative refractory period
in the refractory period the neuron is not ________. if you applied a second stimulus what would happen?
excitable
nothing (usually)
absoulte refractory period means (and compare to relative)
even if you introduced a second stimulus which was powerful and strong, no action potential will be evoked where as in stage three (relative) it will regain its excitability but a much stronger stimulus is required to depolarise it to threshold
when MP reaches threshold, there is a sudden activation (opening) of…
voltage-gated Na+ channels
when MP reaches threshold and voltage gated Na+ channels are activated what is the Pk:PNa+ (used to be 40:1)
1:20
thus in overshoot the MP shifts towards…
E(Na)
do the voltage gated Na channels open for long
no it is short lasting as these channels inactivate quickly
what happens when the Na+ channels are inactivated
followed by transient opening of voltage-gated K+ channels, leading to repolarisation and AHP
when K+ channels are opened what happens to MP and Pk:PNa
shifts towards E(K) and 100:1
when the voltage threshold is reached… (sodium channels and Na+ ions)
sodium channels open and Na+ ions move into the cell along both the conc and electrical gradient
influx of Na+ slows down and stops when..
1) the inside potential becomes positive (moves towards E(Na)) and thus attracts Na+ ions less
2) Na+ channels inactivate
if the activation gate is closed, is the channel permeable
no
why is the first phase of action potential so fast
because when threshold is met, Na+ channels open and due to conc grad, sodium ions rush into the cell, thus rapidly bringing positive charge into the cell
permeability is solely determined by
leak channels
inactivation gate sense depolarisation and
changes conformation to block etc
briefly state the steps of how voltage-gated Na+ channels activate and inactivate
RMP = gate closed
Depolarisation = gate opens
After fraction of a second = inactivation gate blocks the channel
back to initial state when membrane repolarises (gate closed, inactivation gate no longer blocking channel)
each action potential is an all or none event, which contrasts…… which are …..
contrasts small (sub threshold) depolarisations or hyperpolarisations which are graded
the amplitude of action potentials is usually ___________ and does not depend on the ________ _________ (provided that this ________ is _________)
constant
stimulus intensity
stimulus
suprathreshold
what does suprathreshold mean
stimulus causes depolarisation which just crosses the threshold
what does graded mean
the amplitude depends on the amplitude of the stimulus while the amplitude of the whole action potential does not depend on the stimulus intensity (provided that the stimulus causes depolarisation to threshold)
what is one way to induce an action potential
applying an electrical stimuli
when switch is closed, one electrode becomes _____ and one electrode becomes _____, this causes a _________ ___________
positive
negative
potential difference
current follows the path of
least resistance
what are the two main paths that current follows
1) outside from + to -
2) across membrane and inside axon
which path can change RMP
the second path, across membrane and inside axon
What happens when the current generated by an outside source flows through the cell membrane from outside to inside
hyperpolarisation (MP becomes more negative)
what happens when the current generated by an outside source flows through the cell membrane from inside to outside
depolarisation (MP becomes less negative)
flow goes from
positive to negative
in electrolytes current is carried by
ions
what is the largest component
the movement of cations (positively charged ions)
what affects excitability and the potential inside
the smaller component of the current where current goes across cell membrane, along cytoplasm and back to cathode (negative charged electrode)
along this path, current flows outside > inside which causes
accumulation of negative charge along this path
due to accumulation of negative charge, anode expects to see
increase in negativity of MP, known as local hyperpolarisation
under cathode (negative electrode) we have accumulation of
cations
if postivie charge shifted to this area, this would cause
local depolarisation
if the local depolarisation reaches threshold, the next event would be
activation of the voltage gated sodium channels and initiation of action potential
where are AP’s first generated
in the axon initial segment
why do AP’s generate in the axon initial segment
because it has the lowest threshold and thus serves as the ‘trigger zone’ for APs
what is depolarisation to threshold evoked by
excitatory postsynaptic potentials (EPSPs) which spread mainly passively from dendrites
what happens to APs once generated
APs are transmitted actively along the axon away from the cell body
what EPSPs evoked by
synaptic transmission from presynaptic axons to dendrites and, to a smaller degree, cell bodies
the current loop flows through the cell membranes in the axon initial segment region from ______ to ________
inside to outside
once generated, APs also spread from axon initial segment back to the
cell body
what are the two types of axons
unmyelinated axons and myelinated axons
diameter difference in unmyelinated axons and myelinated axons
unmyelinated = small diameter myelinated = larger diameter
difference in transmission of APs in unmyelinated axons and myelinated axons
unmyelinated = slow, continuous myelinated = fast, saltatory (in large steps)
what are the two stages of action potential transmission in both types of axons
1) passive spread
2) generation of action potentials
what are the three steps of passive spread of current
1) (subthreshold) depolarisation at one region of the membrane
2) passive current flow (inside and outside the axon)
3) depolarization of adjacent parts of membrane
what is subthreshold
hasn’t reached threshold at which voltage gated sodium channels kick in
when one section is depolarised it has
higher potential than the regions next to it
potential difference always leads to
flow of current
will there be flow of current in the extracellular space through the interstitial fluid? why?
yes because depolarisation is normally associated with loss of positive charges outside, there will also be a potential graident outside the axon, thus a potential difference and a flow of current in the extracellular space through the interstitial fluid
how can current move through the membrane
through the leak potassium channels as current is carried by ions
current can spread passively only over…
short distances
current quickly _________ as it flows along the axon
dissipates
when the circuit is enabled, you have highest potential at the _____ compared to the _______ and due to this ___________ ___________ current will flow
+
ground
potential difference
what are the five specific steps of action potential transmission in unmyelinated axons
1) action potential
2) passive current flow
3) depolarisation of adjacent parts of membrane to threshold
4) activation of voltage-gated Na+ channels
5) new (full size) action potentials generated in adjacent parts of membrane
if current is strong enough we expect hyperpolarisation under the ________ and depolarisation under the _______ which can cause action potential / opening of voltage gated sodium channels
anode
cathode
can passive current generated by action potential depolarise other adjacent parts of membrane to threshold
yes
in myelinated axons, action potential conduction velocity is
increased
how does myleination increase speed of action potential conduction
by increasing the effciency of passive spread
myelinated: where are APs generated
at nodes of ranvier
myelinated: current flows passively between _______
nodes
in myelinated axons, do AP need to be regenerated at every part of the axonal membrane
no
why is speed of AP transmission in unmyelinated axons slower than in myelinated axons
passive current flow between two adajcent points is fast, however AP must be regenerated at every point on the membrane in unmyelinated, where as it doesn’t in myelinated. This takes time and therefore conduction is slow
AP propogate in which axon faster
myelinated
in myelinated axons, what is the myelin sheath formed by in CNS and PNS
oligodendrocytes in CNS and schwann cells in PNS
what types of cells are oligodendrocytes and schwann cells
glia cells
in myelinated axons, myelination is ________________; interrupted at _________ ____ _____________
discontinuous
nodes of ranvier
myelination increases…
passive spread of current
how does myelination increase passive spread of current
due to the insulating properties of myelin, there is less current dissipation as it flows across the axon
what direction does passive conduction occur
in both directions
if myelinated fibres are much more effcient, why do we still have unmyelinated fibres
because unmyelinated fibres have a much smaller diameter and we have limited space in CNS due to skull, so by using unmyelinated, even though slower, yu can have more of them.
axons are able to conduct AP in both directions except…
under physiological conditions (ap one direction but passive conduction still goes both ways)
why does AP conduct in only one direction under physiological conditions
due to the absolute refractory period - by the time the absolute refracty period is over, AP has already moved down the axon
PNS contains axons of ___________ _______ as well as axons of _______________ and the ‘______________ ____________ ____________’
sensory neurons
motorneurons
Autonomic Nervous System
explain the process of AP generation in sensory neurons
- When stimulus acts on receptors in sensriy neurons, it does not immediately evoke APs
- first it evokes a graded depolarisation, known as the receptor potential
- the receptor potential spreads passively to more distally located trigger zone, where APs are generated
- APs then spread along the axon (myelinated or unmyelinated) towards the CNS
where is information about the strength of the stimulus coded (in sensory neurons?)
in the amplitude of the receptor potential and the frequency of APs
go to page 231 of course guide, label the bottom diagram
okay
