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
