Antons Excitable Cell Lectures

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  • Created by: Sarah
  • Created on: 16-05-17 11:07
what must neurones do?
collect, integrate and output
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what does the NS do?
system of communication, allows an organism to react rapidly and modifiably to changes in environment
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how does the NS provide communication?
electrical activity, chemical messengers and receptors
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what does electrical activity of the NS provide?
rapid, reliable and flexible means for neurones to receive, integrate and transmit sigs
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what provides much more flexbility in the NS?
diff receptors and diff chemical messenger
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what can electrical signals be divided into?
action potentials and graded potentials
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what are APs coded by?
frequency as they are a unit of size
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what are graded potentials coded by?
size and strength of stimulus
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properties of APs?
fixed size, all or nothing sigs that travel along the axon
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how do neurons have a resting potential?
selectively permeable membrane, unequal distribution of charge/ions, physical forces
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what are the properties of graded potentials?
variable size, local sigs not propagated over long distances
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which kind of potential can pass both ways along the neuronal membrane?
graded potentials
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when a nuerone has no action potentials or graded potentials its said to be what?
at rest
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what gives selectivity in neurons?
channels
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what gives an unequeal charge in a neuron?
pumps
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what 2 forces control movement of ions in aqueous solution?
1) diffusion 2) electrical activity
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what provides a barrier so we end up with different concentrations of ions on either side of the M?
lipid biayer makes concentration gradient
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what do channels provide a route for if ions move down their conc gradient?
passive diffusion
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why is there an electric current?
ions are charged, they can be attracted or repelled by electric fields, opposite charges attaact, like charges repel
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what is current measured in?
Amps
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what affects on how much curretn will flow?
1) electrical potential (voltage) 2) electrical conductance- how easy for charge to move 3) Ohms law- potential, conductance and how much current will floq
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what is the equaion for Ohms law?
I=gV so no current flows if conductance (g) = 0
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why do you need a potential difference and ion channels to drive ions across the membrane?
bilayer barrier, no channels open conductance is 0 so need potential difference and channels for ions to flow
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what do neuronal membranes separate?
electrically charged ions
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how can ions cross the membrane?
channels and pumps
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movement of ions depends on what?
conc gradient and the difference in electrical potential
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what is potential difference?
difference in electric potential, therefore difference in voltage across M
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what is an absolute requiremtn for functioning of the NS?
Negative Vm (negative resting potential)
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what is the Vm (resting potential) normally?
-65mv
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how do we measure the elctric potential?
connect neuron to voltmeter = insert a microelectrode (filled with Kcl) into the neurone+another electrode (silver chloride) into solution surrounding neurone
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what maintains a negative resting potential + sets up concentration gradients of ions? what ones?
ion pumps, Na+/K+ ATPase, Ca2_ pumps
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what do pumps move ions against?
their concentration gradient- nees ATP
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how much of the ATP in the brain is used up by the Na/K ATPase?
70%
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where does the Ca pump transport calcium to?
pumps calcium outside of neurons
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why does it pump calcium out of neurons?
to maintain low intracellular Ca
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why is it important to maintain low intracellular calcium?
1) Ca sign ion changes in conc detected by pros/enxymes that control cell functions 2) high intracellular Ca is toxic, kills neurons
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if you work out Eions why is it not exactly EK( K equilibrium)?
membrane is highly permeable to K a rest, and a little permeable to Na
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what is Eion?
equilibrium potential- M potential achieved in a neuron if the M were selectiely permeable to that ion
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when do you get equilibirum? Vm = Ek (when resting potential is K equilibrium potential)?
when electrostatic forces = diffusional forces
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what happens if you have an impermeable M and insert a K channel?
K channel inserted -> K will diffuse down its conc gradient -> build up of +ve on one side, -ve on other, electrostatic forces build up so K+ attracted to -ve force on other side
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when does the equilibrium potential occur?
when electrostatic forces = diffusional forces
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what does nerst equation calculate?
the equilibrium potential for that ion
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what do you need to know to be able to use the nerst equation?
concentration difference across M for ion
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how can large changes in membrane potential be caused?
by miniscule changes in ionic concentration
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why do net difference in electrical charge occur at the inside and outside surfaces of the membrane?
the bilayer is so thin that cjharges can attract or repel through it electrostaic forces
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where is trhe net negative charge in the cell localizede to?
the inner surface of the plasma membrane
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what is used to calculate the equilibrium potential for an ion?
nerst equation
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what temp is used in the nerst equation?
37 degrees
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ions are driven across the membrane a a rate proportional to what?
the difference between the M potential and equilibrium potential
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bigger the difference between what the more ions are driven across the M?
equilibrium potential and membrane potential
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what is the resting M potential close to but not?
Ek
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at rest the neuronal membrane is very permeable to what?
K+
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why are potassium channels a key determinant of restring M potentials?
M is highly permeable to K at rest, changes in K conc has big effects
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how can the membrane potential become depolarised? (not real theoritical if only permeable to K)
increasing extracellular K (as permeable to K at rest, cuases shift in K equilibrium potential, increase it, Ek more positive = depolarised)
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why do neuronsnot have resting M potential at K+ equilibrium potential (Eion)?
resting M is permeable to other ions eg Na (but much less)
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what equation is used to estimate the resting m potential?
Goldmann equation
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at rest the membrane is highly permeable to what? why?
K+ ions, open K+ channels
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what does goldman take into account?
other ions permeability as well as K+
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ion movement deps on what?
conc gradient and electrical potential
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what maintains a large K+ gradient across the neuronal membrane?
Na/K ATPase
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what does the resting membrane potential approach?
equilibrium potential of Ek
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Nerst for what?
equilibrium potential
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goldman for what?
resting membrane potential (combined potentials)
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what is the ionic drive force proportional to?
Vm (resting potential) - Eion (equilibrium potential
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action potentials are what?
all of the same size + duration, do not decrease as conducted down the axon
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does an AP decrease as its conducted down the axon?
no it does not
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what is a neuron AP usually triggered by?
Na sodium permeability inccrease
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can different types of excitable cell have different types of action potential?
yes
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what is the properties of APs?
transient, rapid and reversible change in M potential from -ve to +ve, fixed size all or nothing sigs propagated along axon
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what is the undershoot in an AP?
membrane potential gradual goes back to resting
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what is the rising phase?
rapid depolarization of the M caused by opening of vg Na channels
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what is the overshoot?
where the membrane potential is above zero (usually +40mV)
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how are aps generated?
stimulus (activation ofstrecth/thermal rec or a NT) causes channels in M permeable to Na to open (with or without Ca2+) -> depolarizes if threshold reached then AP
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what is the threshold?
usually +40mv, it is the potential you need to trigger an AP
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what happens to APs if stimulus is prolonged?
then a train of APs generated
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after how long do the Na channels snap shut?
after 1ms
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why is there a driving force of Na into the neuron?
large potential difference between membrane potential and sodium equilibrium
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what is threshold?
the value at which VG Na channels open
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why do VG Na channels open in response to?
depolarisation
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what are the voltage sensors on the VG Na channels responding to?
concentration of charge near plasma M, 5nm affects them
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what manner do Na channels inactivate in?
Voltage dep and time dependent manner
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how long is the channel inactivation for Na?
1ms - fast
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what must happen before VG Na channels can be activated again?
channels must be de-activated before activated again
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why is the AP so brief?
rapid inactivation of VG Na channels
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what is another name for VG K channels?
delayed rectifying channels
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what leads to repolarisartion?
opening of VG K+ channels
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how slowly do VG K+ channels open?
with a delay of 1ms
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when the VG channels remain open what period happens?
the relative refractory period
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what happens when the threshold is reached?
sufficient VG Na+ channels open so permeability of Na is more than K+, Na+ > K+
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what is the rising phase caused by?
rapid depolarisation caused by large influx of Na ions
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what causes the overshoot?
resting membrane potential appraches Na equilibrium potential
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what happens during the falling phase?
VG Na channels nactivate, VG K+ channels open, large force drives K+ out of neuron
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what causes the undershoot?
VG K+ channels (delayed rectifiers) add to restinf K+ membrane permeability and red Na+ permeability
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what makes the undershoot go
K+ VG channels closing
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when is the Na/K ATPase working to pump Na out of the cell?
All the time but slowly compared with channels
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how is the AP spread down the neuron?
spread of charged particles (Na)
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what stops the AP going both directions and keeps Na spreading in one direction?
Na channels behind become inactivated so only Na channels ahead open
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how fast can conduction velocity of an AP be?
10 m/sec
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what factors affect the conduction velocity?
diamter of axon, leaky (permeability) of M,
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what increases conduction velocity?
larger axons, less leaky (hose analogy), insulate the leaks by myelin
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why does myelin increase conduction velocity
nodes of ranvier, saltatory conduction
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what is lost in MS?
myelin
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why does diamter influence the Cv?
resistance to current flow is inversely proportional to cross-sectional area of the axon
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why does myelination increase cv?
prevents current loss and increases space constant
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what is the space constant?
the distance from the site of depolarisation to where it has fallen to 37%
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why is there so many unmyelinated small neurons?
1) because the space constant proportional to Rm/Ri so benefit of high M resistance is reduced by high internal resistance, 2) metabolic and volume costs
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what is the cv of the 1000um squid axon?
25m/s
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what is the cv of small unmyelinated neurons around 0.2-1.5um?
0.5-2m/s
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most axons bigger than 1.0um are myelinated, 1-20um cv?
5-120m/s
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what poisons are useful for K channels?
TEA = Tetraethylammonium
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what blocks Na channels?
lidocaine, saxitoxins and dinoflagellate
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what does tetrodoxin, TTX block?
Na channels
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where is tetrodoxin from?
the puffer fish
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saltatory conduction causes ion channels to have restricted distributions, where are focal accumulations of Na channels?
at the nodes of ranvier
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what ius the difference with dendrites in comparison to axons?
dendrites have Voltage-sensitive channels but don't usually produce APs
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what affects conduction speed?
diameter, myelination and permeability
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how is intensity shown by AP?
the frequency of AP- continuous stimulation produces a train of AP
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in an AP the stronger the stimulus the what?
rhe higher the frequency
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what is the relative refractory period?
you can fire another AP but needs a stronger stimulus because the threshold is raised
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what is the absolute refractory period?
no action potentials can be generated, Na channels are inactivated
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what is the stimulus intensity encoded by?
AP frequency
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what can graded potetnials be?
excitatory- depolarising or inhibitory-hyperpolarizing
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what are graded potentials caused by?
1)opening of NT gated ion channels 2) opening/closing of K channels
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what can graded potentials do to reach the threshold and trigger APs?
summate, by integrating info from multiple inputs to reach threshold and trigger APs
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what canESPs be shunted by?
inhibitory inputs
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what blocks esps?
inhibitory inputs (they have inhibitory synapses)
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how does an inhibitory input shunt esps?
making the M leky sesp is dissipated this shunting effect can be caused by opening on non-selective cation channels in the axon M
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whatuses electrical synapses?
retinal neuronal, few other adult CNS neurons (glial junctions),cardiac muscle and smooth muscle
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