Cell Excitability 1 - 3

Question 1

nervous system

Answer 1

system of communication that allows an organism to react rapidly and modifiably to changes in environment

Question 2

neurons must:

Answer 2

collect
integrate
output

Question 3

electrical activity provides:

Answer 3

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

Question 4

action potential

Answer 4

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

Question 5

graded potenital

Answer 5

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

Question 6

what Vm is required for a functioning nervous system

Answer 6

negative

Question 7

resting potential

Answer 7

-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

Question 8

physical forces in neurons

Answer 8

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

Question 9

ion pumps in neurons

Answer 9

set up ionic concentration gradients in neurons
e.g. Na/ATPase, Ca pumps
without ion pumps, resting potenital wouldnt exist so brain wouldnt function

Question 10

equilibrium potenitals

Answer 10

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

Question 11

different permeablilites of membranes

Answer 11

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

Question 12

calculating Eion

Answer 12

only if conc difference across membrane is known
either:
Nernst
or
Goldman

Question 13

Nernst equation

Answer 13

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

Question 14

Goldman equation

Answer 14

if membrane is permeable to lots of ions, neurons dont have resting Vm at Eion for K, to estimate real Vm - need Goldman

Question 15

generating action potentials

Answer 15

1 - rapid depolarisation
2 - membrane potenital above 0 = +40mv
3 - rapid repolarisation
4 - gradually returns to rest

Question 16

properties of action potentials

Answer 16

transient, rapid and reversible change in membrane potential
often triggered by Na permeability increase
dont decrease as conducted down axon

Question 17

Na and K channels

Answer 17

rest = Na shut, K open
depolarisation to +mv = both open
repolarisation = Na shut

Question 18

structure of Na channels

Answer 18

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

Question 19

useful poisons

Answer 19

TEA - blocks K channels
lidocaine - blocks Na channels
TTX - puffer fish, blocks Na channels
STX - dinoflagellates, blocks Na channels

Question 20

action potential conduction

Answer 20

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

Question 21

factors affecting conduction velocity (Cv)

Answer 21

diameter
myelination
unmyelinated small axons

Question 22

diameter on Cv

Answer 22

resistance to flow of current is inversely proportional to cross sectional area of axon

Question 23

myelination on Cv

Answer 23

prevents loss of current by increased membrane resistance and increased space constant

Question 24

space constant

Answer 24

distamce from site of depolarisation where it falls 37%

Question 25

unmyelinated small axons on Cv

Answer 25

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

Question 26

axon type - smallest unmyelinated
diameter
velocity

Answer 26

0. 2-1.5 micrometers

0. 5-2 m/s

Question 27

axon type - most axons
diameter
velocity

Answer 27

1-20 micrometers

5-120 m/s

Question 28

axon type - squid giant axon
diameter
velocity

Answer 28

1000 micrometers

25 m/s

Question 29

saltatory conduction

Answer 29

nodes of ranvier
where ion channels are located
focal accumulations of Na channels

Question 30

axon hillock

Answer 30

where action potential is generated

Question 31

spike initation zone

Answer 31

at sensory nerve ending

Question 32

dendrites - aps or gps?

Answer 32

have voltage sensitive channels but dont usually produce aps

mostly encode info with graded potentials

Question 33

coding by action potentials

Answer 33

frequency of ap depends on size of depolarising stimulus

stronger stimulus = higher frequency , provides way to encode stimulation intensity in nervous system

Question 34

absolute refractory period

Answer 34

1ms
ap is generated no matter what
neuron is incapable of generating another ap, often followed by releavtive rp

Question 35

relative refractory period

Answer 35

a few ms

during it can fire another ap but would require a stronger stimulus because the threshold is raised

Question 36

graded potentials

Answer 36

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

Question 37

excitatory post synaptic potentials EPSPs

Answer 37

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