Topic 2 Flashcards
functional polarity
often dendrites on one side and axon on the other
- Dendrites receive signals post synaptically
- Soma integrates the signal
- Axons send the signal and are pre synaptic
Axon hillock
where all action potentials coming into the cell get summated ⇒ when they reach a certain voltage threshold an action potential is activated
properties of the axon hillock (2)
- There is a high density of voltage gated sodium channels at this hillock area where the action potential is initiated
- This is sent to the axon terminal leading to the release of NT at the synapse
(excitatory or inhibitory)
how does communication between neurons work? (9)
- synaptic potential from dendrites are summed up at the axon hillock
- axon initial segment is enriched in voltage gated Na+ and K+ channels
- if membrane potential is above threshold, voltage gated channels in axon will fire on AP
- AP travels down the axon away from the soma
- Pre-synaptic changes in Vm trigger release of neurotransmitter
- NT binds receptors on the postsynaptic side
- binding of NT causes ion channels to open
- ions flow through the channels (current) leading to change in the resting membrane potential in the dendrites
- dendrites generally do not have the voltage gated Na+ and K+ channels required for an action potential
saltatory conduction
when you have a continuous axon segment, the action potential jumps from each node of ranvier
- Increases speed and fidelity (probability of it happening)
where do receptor and synaptic potentials occur?
in the dendrites
- Relatively small changes in Vm
- Get summed in the axon hillock
when will an action potential occur?
start in the axon and occur only if the sum of the synaptic/receptor potential gets above a threshold to open voltage gated channels
- if the summed stimulus is below threshold, there is a passive change in membrane potential in the axon, but not action potential is generated
passive change
does not involve the opening of voltage gated channels
active response
takes energy to open voltage gated channels which results in an action potential
- with injection of sufficient depolarizing current (activation of enough synaptic potentials), the Vm reaches threshold and voltage dependent changes in membrane permeability change the nature of the ion current generating an AP
what happens at the point the summed stimulus is above threshold?
opening of voltage gated channels causes active response and generates an action potential
- Sodium will flood into the cell and depolarize it very quickly because sodium channels open
T/F the action potential mV can go past ENA+?
False
- The action potential mV cannot go past the equilibrium potential of sodium ⇒ you cannot get a higher membrane potential than this
what happens after sodium flows in?
Sodium stops flowing in soon after their channels open and the channels inactivate so the cell can repolarize
- The potassium channels open next and do not inactivate ⇒ inside to outside flow
T/F there are really only changes in ion concentrations near the membrane?
True because the cytoplasm is so large
Passive response
small injections of current (synaptic/receptor potentials) cause small changes in membrane potential (Vm)
T/F an action potential is all or none?
True
with increasing depolarization there is no change in the AP amplitude and there is increase in the frequency of AP’s because it is easier to go above threshold than when at a very negative Vm
explain Vm > Ex
ons move to make Vm = Ex in an outward current (hyper-polarization)
- cations move out or anions move in
explain Vm = Ex
DFx = 0, Ix = 0
i.e no driving force and no current
explain Vm < Ex
ions move to make Vm = Ex in an inward current (depolarization)
- cations move into the cell or anions move out of the cell
in hyperpolarization is DF + or -?
positive
DFx = Vm - Ex, so DF = +
in depolarization is DF + or -?
negative
DFx = Vm - Ex, so DF = -
Vrest
before the stimulus when the neuron is at rest
rising phase
the neuron depolarizes and Vm increases above Vrest
overshoot phase
Vm goes above zero
falling phase
the neuron repolarizes and Vm decreases to Vrest
undershoot phase
the neuron is transiently hyperpolarized
how does changing the extracellular concentration of an ion change the resting membrane potential in a manner predicted by the Nernst equation?
measure changes in membrane potential as the outside K+ concentration is increased and when you increase [K+]o stepwise ⇒ Vm increases
- the neuron is permeable to K+ at rest
- very close to the 10 fold rule but not quite because other ions are permeable to the membrane too
if we used sodium and increased its outside concentration, would we get the same results as incrementally increasing potassium concentration?
No, because the cell is less permeable to sodium and quite permeable to potassium
what is a voltage clamp?
a machine that holds the Vm constant in order to measure the current generated when ion channels open
- Normally, changes in current would cause changes in membrane potential but the machine injects equal and opposite current (machine current) to that caused by the opening of ion channels (biological current), keeping the membrane potential constant
what is the point of a voltage clamp?
an action potential will not be generated and we can determine how much current is being generated at a specific voltage
voltage clamp
you are maintaining voltage to measure current as you inject the equal and opposite current back into the cell
current clamp
a passive recorder monitoring the changes in the membrane potential and allowing the current to flow
what major role does sodium play in neurons?
determines the amplitude of the action potential (rising phase)
- at rest, Vm is very low and increasing its outside concentration stepwise has almost no effect
what happens when you lower the amount of sodium concentration outside coming into the cell during an AP?
the amplitude of the cell is lower
inward current
flow of cations (+) into or anions (-) out of the cell
outward current
flow of cations out or anions into the cell
why is there almost no current change when the neuron is hyperpolarized?
Because you are not reaching a voltage level that would trigger an action potential (aka voltage gated channels opening) in a normal neuron at rest
why does depolarization cause first a transient inward, then a delayed sustained outward current?
because of the timing of opening and closing of voltage gated channels
- potassium stays open longer causing the outward but delayed current that lasts longer
Voltage sensitive ion channels
These channels are usually only open when the neuron is depolarized
- voltage gated sodium (Na+) and potassium (K+) channels re responsible for the AP
- the membrane potential must change to alter their conformation stage
what happens as Vm increases? (2)
- Inward current first increases, then decrease until it reverses due to Na+ channels and ENa = early and transient
- Outward current increase linearly due to K+ channels and Ek = delayed and sustained
at what current is the reversal of sodium from inward to outward?
50 mV
- The Vm is closer to the ENa+ of sodium which would force potassium to flow outward
Tetrodotoxin (TTX)
blocks voltage gated Na+ channels
- cell won’t depolarize but potassium will flow still
Tetraethyl-ammonia (TEA)
blocks voltage gated K+ channels
- only sodium will flow inward
T/F conductance gets greater at each depolarized voltage step for sodium?
True
T/F potassium conductance also increase with depolarization?
True
- Potassium also increases monotonically ⇒ when we reach a threshold voltage and the channels open, we get an increase in conductance
T/F increase in conductance equates with increased current for that ion
False
- Increase in conductance does not necessarily equate with increase current for that ion
T/F potassium channels are not activated or inactivated but they open and close at certain points
True
real life conductance timing? (3)
- Both Na+ and K+ currents are transient because g is voltage dependent
- Na+ channels open and inactivate fairly rapidly = early and transient
- K+ channels open more slowly and stay open much longer (don’t inactivate = delayed and transient
how do sodium channels work?
- At rest, the VG Na+ channel is closed and inactivating domain is open
- After a depolarizing pulse, the channel opens and ions go through
- After a short time, the inactivating domain blocks the pore
Absolute refractory period
the interval during which a second action potential absolutely cannot be initiated no matter how large a stimulus is
- Coincides with nearly the entire duration of the action potential
what causes absolute refractory?
the inactivation of the Na+ channels that originally opened to depolarize the membrane
- The channels remain inactivated until the membrane hyperpolarizes
- The channels then close, de-inactivate and regain the ability to open in response to stimulus
Relative refractory period
the interval immediately following during which initiation of a second action potential is inhibited but not impossible (later in time)
- immediately follows the absolute ⇒ as voltage gated potassium channels open to terminate the action potential by depolarizing the membrane, the potassium conductance of the membrane increases dramatically
- Until potassium conductance returns to the resting value, a greater stimulus will be required to reach the initiation threshold for a second depolarization
- The return to the equilibrium resting potential marks the end of the relative refractory period
Role of refractoriness (3)
- refractory periods limit the number of APs that a neuron can produce per unit time
- Prevent re-excitation of the same membrane segment that was just excited
- Prevents APs from propagating backward toward their point of initiation
how does the AP feedback loop work?
The action potential is self-regenerating because of the positive feedback loop between Na+ channels opening ⇒ increased Na+ current ⇒ depolarization ⇒ slow opening of K+ channels ⇒ increased K+ current ⇒ hyperpolarization ⇒ etc.
what 2 things give rise to shapes and frequencies?
- Concentration of ions
- Types of ion channels present
passive conduction
VG channels open at the sight of depolarization and then the current diffuses along the axon
sub threshold depolarization
current diffuses along the axon
- Current gradually decays due to leakage of ions
active propagation
the external stimulus causes VG channels to open and current passively diffuses a short distance ⇒ each part of the membrane has an action potential
- Diffusing current activates opening of VG channels in nearby regions of the axon
types of resistance (3)
- membrane resistance
- capacitance
- axial resistance
membrane resistance
the more channels along the membrane, the less resistance it has compared to no channels where it is impermeable
capacitance
how well the membrane stores charge across the membrane ⇒ small diameter axons have little membrane and not much capacitance
- the ability to collect and store energy in the form of an electrical charge
axial resistance
the resistance along the inside of the tube (in cytoplasm) ⇒ larger diameter has lots of space to push things through so the charges flow through better
what is special about the axon hillock
it has enriched amounts of VG Na+ and K+ channels
- Synaptic inputs onto dendrites and cell bodies are summed at the axon hillock
- due to lack of VG channels in soma, passive diffusion cannot cause AP in soma
how is current leak overcome? (2)
- increasing axon diameter
- Myelination
oligodendrocytes
wrap multiple different axons which generates the myelin sheath in the CNS
Schwann cells
wrap axons but each individual schwann cell makes one wrap on an axon
what are in between the myelin sheets?
enrichment of voltage gated channels ⇒ mostly sodium
what does the myelin sheath do?
- Making sure the charge doesn’t flow out of the membrane (even if there are leak channels)
- The action potential is able to hop from one point to the next
nodes of ranvier
specialized regions in myelinated axons enriched in VG Na+ and K+ channels
- passive diffusion of current spreads until it reaches the closest node, which is full of VG channels
T/F the largest diameter axons are the fastest?
True
what is the equation for velocity?
v = sqrt(d/(8pC^2R))
- d= diameter
- p = resistivity of axioplasm
- C = membrane capacitance/unit area
R = membrane resistance/unit area
What does increasing diameter do?
increases AP velocity
- greater area for charge to escape through its membrane, and therefore the lower the membrane resistance
- more membrane available to store charge, and therefore greater the capacitance
T/F membrane resistance and membrane capacitance work against one another
True
- the effect of diameter on decreasing resistance must outweigh the effect on increasing capacitance
what does myelination do?
increases AP velocity
- decreases membrane capacitance
- increases membrane resistance
