the excitatory cell Flashcards
action potential
increase in membrane potential
fixed size, all or nothing
travel along (propagate) the axon
travel long distance very quickly
graded potential
variable size
local signals
not propagated over long distances
v short
the graded AP can pass either way along axon
pass both ways along the neuronal membrane
information coding
APs coded by frequency (unit of size)
graded potential are coded by size and vary according to the strength of the stimuli
the resting potential is inevitable because
selectively permeable membrance unequal distribution of ions physical forces (diffusion and electrical force)
ions channels confer
selectivity passive (down a gradient) usually in one direction
pumps assist
unequal charge distribtuion
active against conc grad
use ATP
two forces control movement of ions in aqueous soltions
diffusion
electrical field
electrical fields
opposide charges attract and like repel
ions are charged and gives rise to an electric current
how much current will flow depends on:
1) electrical potential (voltage exerted on an ion),
2)electrical conductance, relative ability it is for care to move from one point to another (symbol g, measured in siemens s), electrical resistance is the relative inability of an electrical charge to migrate represented as R and measured in Ohms (R=1/g)
Ohms Law describes the relationship between potential, conductance and the amount of current that will flow, I=gV….so no current flows if g or v=0!
bilayer as a barrier- potential difference
In a cell, bilayer provides a barrier to ion movement, thus if no channels are open, conductance will be zero and I = 0. Therefore to drive ions across the membrane electrically requires the membrane to have channels and a potential difference
measuring electrical potential
connect the neurone to a voltmeter (a device that measures the potential difference between two electrodes)
To do this, we insert a glass microelectrode (filled with KCl, to carry charge) into the neurone and another electrode (usually made of silver chloride) into the solution surrounding the outside of the neurone…before the glass electrode enters the neurone, the voltmeter reads Zero! Ie. There is no potential difference within the extracellular solution…but as soon as the electrode enters a ‘resting’ cell, this value changes to somewhere between -65 and -90 mV (symbol commonly used is Vm) ie. There is an uneven distribution of charge across the neuronal membrane
Negative Resting Potential is an ABSOLUTE requirement for a functioning nervous system.
2 main neurotransmitters in NV
gluctamate (most abundant) excitatory
Gaba
2 important pumps
sodium potassiom pump (ATPase) calcium pump (also found in membranes of other cells)
NA/K pump
The Na-potassium pump exchanges internal sodium for extracellular potassium, notice it is moving these ions against their concentration gradients and therefore it requires energy (provided by the breakdown of ATP) to do this….this pump probably uses up ~70% of ATP in the brain!!!
Ca pump
Ca pump transports Ca out of neurones, maintaining low intracellular ca is important because (1) Ca is a signalling ion, changes in ca concentration are detected by many proteins/enzymes and are used to control various cellular functions, (2) high intracellular Ca is toxic, kills neurones.
at rest permeability
membrane is highly permeable to K at rest, and a little permeable to Na…so we end up with a resting potential between the two.
nernst equation
considers the charge of the ion, the temperature, and the ratio of external and internal ion concentrations
calculates the value of equilibrium potential for any ion
the nernst equation formula
Eion= 2.303 RT/zF log [ion]o/[ion]i
R= gas constant T= absolute temperature z= charge of ion F= Farday's constant log= base 10 logarithm [ion]o= ionic conc outside cell [ion]i= ionic conc inside cell
the Goldman equation
takes into account the relative permeability of the membrane to different ions at REST
for multiple ions
neuron membrane is —- permeable?
selectively
polar centre
unequal distribution of charged molecules: water, sodium chloride (Na+ cl-) and K+ and Ca+
Channels confer
selectivity
pumps assist
unequal charges across membrane
what does voltmeter measure
the membrane potential
with tiny glass (microelectrode) electrode, filled with potassium chloride
important ion pumps
Na+/K+ ATPase
Ca2+ pumps (not just in plasmamembrane
without these the resting potential would not exist
equilibrium potentials
Eion
different for different ions depending on the concentration between the inside and outside the cell
Eion is the membrane potential that would be achieved in a neurone if the membrane were selectively permeable to that ion
what is an equilibrium potential
when electrical and diffusion forces become equal and opposite and there is no net movement of ions
why is Eion proportional to T?
increasing the thermal energy of each particle increases the diffusion and will therefore increase the potential difference at equilibrium
why is Eion inversely proportional to z?
increasing the electrical charge of each particle will decrease the potential difference needed to balance diffusion
Ek
Ek= 61.54mV log (K+)o/(K+)i
Ena
Ena = 61.54mV log (Na+)o/(Na+)i
Ecl
Ecl= 61.54mV log (Cl-)o/(Cl-)i
Eca
Eca= 30.77mV log (Ca2+)o/(Ca2+)i
ionic driving force
the rate at which the ions get across the membrane
which is proportional to the difference between membrane potential and equilibrium potential
IDF (proportional to) Mv - Eion
how fast is an action potential
about 1 ms
properties of AP
transient rapid and reversible change in membrane potential from -ve to +ve
different types of excitable ell may have different types of action potential
neuron AP often triggered by Na + permeability increase
AP’s or spikes generated by a cell
all the same size and duration
do not decrease as conducted down the axon
na+ channels inactivate how?
in a time-and voltage dependent manner
initially open but then close even if the membrane potential is high
leads to repolarisation
important to propagation of AP
to the channel to be active again the membrane potential needs to be hyper polarised (below the resting potential)
useful poisons
tetrathylammonium TEA= inhibits K+ channels
Lidocaine inhibits Na+ channels
Tetrodotoxin TTX- puffer fish (sp.) (fugu) inhibits Na+ channels
threshold summary
sufficient voltage gated Na+ channels open so that permeability to Na+»_space; K+
rising phase summary
rapid depolarisation caused by large forces drives Na+ into the neurone
overshoot summary
Vm approaches Ena (equilibrium potential of sodium)
Falling phase summary
voltage gated Na+ channels inactivate,
voltage gated K+ channels open
large force drives K+ out of the nerone
undershoot summary
voltage gated K+ channels (delayed rectifiers) add to resting K+ membrane permeability and reduced Na+ permeability to Vm is proportional to Ek
factors influencing speed of conduction
1) diameter of the axon (bigger diameter- the faster the speed)
2) myelination
why does diameter affect conduction velocity?
because the resistance to current flow is inversely proportional to correctional areas of the axon
why myelination affect conduction velocity
it prevents current loss length axon by more Rm and increase the space constant
increases the resistance of membrane and decreases the leakage of the membrane
number of ions required to depolarise membrane is reached much quicker
space constant is distance from site of depolarisation wherein has fallen to 37%
why so many unmyelinated axons in the brain?
because the space constant is proportional so Rm/Ri so the benefit of a high membrane resistance is reduced by the high internal resistance
short neurones don’t need to be myelinated
mostly on long axons e.g motor and sensory nerves that travel long distances
loligo peali
massive axon diameter- about 1mm
faster
unmyelinated
a human myelinated neurone can achieve similar velocity with less than 1micrometer
smallest unmyelinated axons diameter and conduction velocty (Cv)
- 2-1.5 micro meters
0. 5-2m/s
most axons
larger than 1 micro meter diameter t about 20
5-120m/s
squid giant axon
1000 micro meters =
squid giant axon
1000 micro meters =
nodes of ranvier
contain lots of ion channels
AP only fired at these nodes if myleinated
dendrites
dont generate AP
but have voltage sensitive channels
but the number of them is not enough to stimulant an action potential
mostly encode information with graded potentials
GABA tend to active
channels that are more permeable for chloride than sodium
why have graded potentials
allow neurones to summate
spatial summation
receive input from multiple other neurones at once
temporal summation
requires only one neutron but multiple action potentials generated
neurones can have how many synapses
200,000
electrical synapse
gap junction
important because they are really fast
signal goes into both directions
retinal neurons
few other adult CNS neurones e.g. glial junctions
