Lecture 6: Action potentials Flashcards
local potentials
- generated by conductance changes on dendrites and cell body (or sensory nerve endings)
- are not actively propagated
- can summate in space and time
- can be depolarising (excitatory) or hyperpolarizing (inhibitory)
- synapses are the sites where one neuron does something to generate a local potential
neurons 4 morphological types
- multipolar
- multiple processes emanate from the cell body - bipolar
- two processes emanate from the cell body - unipolar
- one process emanates from the cell body
- then branches into dendrite and axon - anaxonic (axonless)
- no distinct axon
- all processes look alike
important parts of the neuron
- neuron cell bodies are located within the CNS, in peripheral ganglia and in the walls of the intestine
dendrites = increase the SA of the cell body and are specialised for receipt of incoming signals
axon = single process, that is the output structure that allows nerve cells to communicate with others and to form networks
inital segment and axon hillock = base of the axon, that is the site of action potential generation
what is the magnitude of the voltage change across the membrane related to
the strength of the stimulus
are neurons organised into networks
- neurons are typically organised into networks
- the output of one neuron is often the input to another
- they communicate with on another through synapses
synapses
are the sites where one neuron does something to generate a local potential in the next neuron in the chain
what is the effect of local potentials on cell membrane potential summed over?
time (temporal summation) and space (spatial summation)
process of integration
- at each moment in time most neurons are likely to be receiving both inhibitory and excitatory inputs
- these inputs (EPSPs and IPSPs) will summate
- at each moment in time neurons are integrating the sum of their excitatory and inhibitory inputs so their MP reflects the timing and relative strength of their drive
- if there is enough EPSPs then an action potential will be fired
what happen if the majority of the inputs open K channels
- the cell membrane potential will be pushed towards Ek
- in this scenario the cell is less likely to generate an output
what happens if the excitatory inputs exceed inhibitory inputs
- this is depolarising, causing more sodium channels to open
- so the cell membrane potential is likely to become less negative (depolarised)
what happens at the initial segment
- the base of the axon has a very high density of voltage gated sodium channels
- these channels are opened by a depolarisation of the membrane
- at each moment in time, if the summed inputs to the neuron cause a depolarisation, then V-gated Na channel opening is made more likely
- as v-gated Na channels open, the cell becomes even more depolarized
- if a depolarisation is significant, it may cause enough v-gated Na channels to open that a major depolarising Na influx results, this voltage = threshold for initiation of an action potential
threshold
if sufficient Na channels open (at the initial segment from local potentials) the depolarisation reaches a point at which large numbers of channels open resulting in a sudden large increase in Na influx
the generation of action potentials
- if the membrane is depolarized to threshold at the initial segment, an action potential will be generated
voltage gated sodium channels
- V gated sodium channels are gated by the voltage across them
- when the cell is at rest (RMP) the gate is closed, the cell is relatively impermeable to sodium
- as the cell becomes depolarized (EPSPs) the gate opens (activated), allowing Na into the cell
- the increased Na conductance allows Na entry down its electrochemical gradient which drives the MP towards Ena
- as the polarity of the membrane becomes more positive inside, a subunit of the channel protein swings into the pore blocking it
- the channel is then said to be inactivated
- sodium movement through the channel then ends
the depolarization phase
- depolarisation phase of the AP coincides with an increase in sodium conductance
- the end of the depolarisation phase coincides with the decline in Na conductance
the repolarization phase
- at the peak of the action potential the MP has reversed, the cell is now inside positive
- sodium conductance is declining very quickly, so the cell is becoming impermeable to sodium
- there is a delayed increase in the permeability of the cell to potassium due to later/slower opening of V-gated K channels
- at the peak of the AP, potassium is being driven out
electrical gradient = driving outwards
chemical gradient = driving outwards - makes it more negative inside
how do voltage gated potassium channels work
- they typically have a charged subunit whose configuration changes when the voltage across the protein changes
- the V gated K channel changes its shape to open and close the selective K pore as a function of the voltage across it
- when the voltage is inside positive (the peak of the AP) the channel opens, allowing increased K conductance
- K flows out of the cell down its electrochemical gradient the MP is driven back towards Ek ie it is made more negative
- like VG Na channels, VG K channels are inactivated before they can be opened again
what causes the undershoot (MP is initially repolarized beyond the RMP)
- this “undershoot” is caused by the slow kinetics of the V-Gated K channel
- K conductance stays high for a relatively long time, pushing MP towards Ek
action potential, the sequence
- RMP
- Depolarisation to threshold
- opening of vg Na channels = rapid depolarisation
- peak of AP
Na channels inactive - opening vg OK channels = repolarisation
- K channels still open, higher Pk, closer to Ek = hyperpolarisation
inactivation of Na channels ends - RMP
what is inhibited between the peak of AP and hyperpolarization
impossible to have another AP = absolute refractory period, bc the voltage gated sodium channels are inactivated
what is slightly inhibited between hyperpolarization and RMP
harder to trigger AP = relative refractory period
- potassium channels are still open
- not impossible to generate another, but the depolarizing influence has to be much higher than normal
refractory
when Na channels are inactivated and cannot immediately reopen, another AP cannot be initiated until the membrane is substantially repolarized
absolute refractory period
no matter how large the stimulus, another AP cannot be generated since VG Na channels are inactivated
relative refractory period
an AP can be generated, but only in response to a very large stimulus because depolarization of stimulus dampened by rel. high K permeability at this time
