Cell Excitability 1 - 3 Flashcards
nervous system
system of communication that allows an organism to react rapidly and modifiably to changes in environment
neurons must:
collect
integrate
output
electrical activity provides:
rapid, reliable and flexible means for neurons to receive, integrate and transmit signals
chemical messengers and receptors between and within cells provide much more flexibility e.g. for inhibition
action potential
fixed size
all or nothing
propagate along axon
can pass either way along axon but tend to go one way
coded by frequency as they are a unit of size
graded potenital
variable size
local signals not propagated over long distances
go both ways along neuronal membrane
coded by size (amplitude) and vary accoridng to strength of stimulus
what Vm is required for a functioning nervous system
negative
resting potential
-65mv
why - inevitable consequence of: selectivley permeable membrane, unequal distribution of charged molecules (uses ATP) and physcial forces
channels confer selectivley
pumps assist unequal charge distrubution
physical forces in neurons
diffusion - naturally move to balance concentrations via conc gradient
electrical - current makes cl- move to positive electrode and na+ move to negative, via difference in electrical potential
ion pumps in neurons
set up ionic concentration gradients in neurons
e.g. Na/ATPase, Ca pumps
without ion pumps, resting potenital wouldnt exist so brain wouldnt function
equilibrium potenitals
ionic gradients influence membrane potential by determining eqm pot Eion
Eion = mem pot that would be achieved in neurone if membrane was selectively permable to that ion
different permeablilites of membranes
impermeant - no ion channels, net charge = 0, potenital difference across membrane = 0
add k channels…
k diffuses down conc gradient so will get build up of positive on one side and negative on other = creates potential difference
as charges build, electrostatic forces are created, eventually eqm is established in which electrostatic forces exactly counteract diffusional force = Eion
calculating Eion
only if conc difference across membrane is known either: Nernst or Goldman
Nernst equation
only permable to one ion
assumes membrane is selectively permeable for this ion
at rest neuronal membrane is very permeabe to K but real membrane potenital is close to but not at Ek
Goldman equation
if membrane is permeable to lots of ions, neurons dont have resting Vm at Eion for K, to estimate real Vm - need Goldman
generating action potentials
1 - rapid depolarisation
2 - membrane potenital above 0 = +40mv
3 - rapid repolarisation
4 - gradually returns to rest
properties of action potentials
transient, rapid and reversible change in membrane potential
often triggered by Na permeability increase
dont decrease as conducted down axon
Na and K channels
rest = Na shut, K open
depolarisation to +mv = both open
repolarisation = Na shut
structure of Na channels
6 transmembrane domains
one positivley charged, when mem becomes more positive, charges in domain start to shift causing it to move from inside channel to outside = conformational change = pore widens - ions can move
they inactivate in a time dependant manner,
hyperpolarisation leads to removal of block from pore so it can reopen when needed
useful poisons
TEA - blocks K channels
lidocaine - blocks Na channels
TTX - puffer fish, blocks Na channels
STX - dinoflagellates, blocks Na channels
action potential conduction
occurs by spread of charged particles (Na+) although they spread in both directions, Na channels behind are inactivated so only channels in front are able to open - which is why aps usually only travel in one direction
axons can generate potenitals along entire length and propagted in non-decremental manner
factors affecting conduction velocity (Cv)
diameter
myelination
unmyelinated small axons
diameter on Cv
resistance to flow of current is inversely proportional to cross sectional area of axon
myelination on Cv
prevents loss of current by increased membrane resistance and increased space constant
space constant
distamce from site of depolarisation where it falls 37%
unmyelinated small axons on Cv
space constant is proportional to Rm/Ri sp benefit of high membrane resistance is reduced by high internal resistance
there are metabolic and volume costs of myelination
axon type - smallest unmyelinated
diameter
velocity
- 2-1.5 micrometers
0. 5-2 m/s
axon type - most axons
diameter
velocity
1-20 micrometers
5-120 m/s
axon type - squid giant axon
diameter
velocity
1000 micrometers
25 m/s
saltatory conduction
nodes of ranvier
where ion channels are located
focal accumulations of Na channels
axon hillock
where action potential is generated
spike initation zone
at sensory nerve ending
dendrites - aps or gps?
have voltage sensitive channels but dont usually produce aps
mostly encode info with graded potentials
coding by action potentials
frequency of ap depends on size of depolarising stimulus
stronger stimulus = higher frequency , provides way to encode stimulation intensity in nervous system
absolute refractory period
1ms
ap is generated no matter what
neuron is incapable of generating another ap, often followed by releavtive rp
relative refractory period
a few ms
during it can fire another ap but would require a stronger stimulus because the threshold is raised
graded potentials
not all or nothing
can be:
excitatory - depolarising
inhibitory - hyperpolarising
caused by opening of neurotransmitter - gated ion channel or opening/closing K channels
summation = temporal and spatial, integrate info from multiple neuronal inputs
if reach threshold —-> trigger ap
excitatory post synaptic potentials EPSPs
can be shunted by inhibitory inputs , leads to brief hyperpolarisation of membrane, makes membrane ‘leaky’ so EPSP is dissopated
can be caused by opening of non selective cation channels in axon membrane
