The excitable cell (1) (2) (3)

Question 1

nervous system definition

Answer 1

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

Question 2

neurones must … (3 functions)

Answer 2

collect, integrate, output

types: sensory, interneurone, motor

Question 3

How do neurons work?

Answer 3

electrical activity provides a rapid, reliable and flexible means for neurons
chemical messengers and receptors between and within cells provide much more flexibility

Question 4

Electrical properties
action potentials - size, signals, travel,direction, coded
graded potentials - size, signals, travel, direction, coded

Answer 4

AP -
fixed size, all-or-nothing response, signals that travel along axons(propagate)
tend to pass one way, coded by frequency
GP -
variable size, local signals (in dendrites of post synaptic cell), not propagated over long distances
can pass both ways along axon, coded by size

Question 5

Resting potential is what?
why have them?
Why are they selective and unequal?

Answer 5

-60mv
because selectively permeable membrane, unequal distribution of charged molecules(ions) and physical forces
because lipid bilayer= permeable for water NOT ions..so, have ion channels, due to unequal distribution = 3Na+ out, 2K+ in (pump causes net charge to change) – pump uses energy

Question 6

2 physical forces controlling ion movements in solution?

Answer 6

1) diffusion (high to low conc)

2) electrostatic (generated by electrical field + and - electrodes)

Question 7

Ionic basis of membrane potential
neurons have a + or - Vm?
name 2 ions pumps?

Answer 7

neurons have negative vm
ion pumps set up conc gradient:
Na+/K+ = ATPase
Ca2+ pumps (not just in plasmamembrane)

Question 8

Equilibrium potentials(Eion)?

What happens when electrostatic force and diffusion counteract?

Answer 8

ionic gradients influence membrane potentials by determining EPs(Eion)
Eion = the membrane potential that would be achieved in a neurone if the membrane were selectively permeable to that ion
INSIDE CELL (-) | OUSTIDE CELL (+)
This generates an electrical field.
When electrostatic force and diffusion counteract
(Vm = Ek) === EQUILIBRIUM POTENTIAL
so, no net movement despite potential difference

Question 9

THE NERNST EQUATION

what is it used to calculate?

Answer 9

used to calculate equilibrium potential for an ion

Practice a question from lecture slide

Question 10

THE GOLDMAN EQUATION

what is it used to calculate?

Answer 10

to estimate real Vm, as it takes into account other ions

practice a question

Question 11

Properties of an action potential?

AP triggered by?
Threshold reached, what happens?

Answer 11

transient, rapid and reversible charge in membrane from- to +
Ap triggered by Na+ permeability
AP = same size and duration
Threshold reached = Na+ channels opened = increase Na+ inside the axon = depolarisation
repolarisation = Na+channels closed, K+ leaves

Question 12

Structure of voltage- gated channels?
why do they open?
what affects voltage sensors?
how many polypeptide chains in protein?
(how many transmembrane domains in each +their role?)
How does it stay inactive until activated?

Answer 12

open in response to depolarisation
conc of charge near plasma membrane affects voltage sensors
protein has 4 polypeptide chains and 6 transmembrane domains in each( allows Na+ through)
Voltage sensor detects charges
stays inactive due to blockage until resting potential

Question 13

Overshoot

undershoot

Answer 13

overshoot - Vm approaches Ewa

undershoot - voltage-gated K+ channels add to resting K+ membrane permeability and reduced Na+ permeability

Question 14

Axon potential conduction - what happens? and how

why does it travel in 1D?

Answer 14

Na+ in axon depolarises adjacent part of axon
Na+ channels blocked in refractory period = only travels in one direction (also, undershoot can cause it to only travel in 1D)

Question 15

Factors influencing conduction velocity?

What is the space constant?

Answer 15

Leaks a lot = slower diffusion
diameter of axon - larger= less resistance = faster
– resistance to current flow is inversely proportional to cross-sectional area of the axon
myelination - insulates, also prevents leaking
– prevents current loss along the axon by increasing Rm(membrane resistance) and increases the space constant
Space constant = distance from the site of depolarosation to where it has fallen by 37%

Question 16

Why have so many small unmyelinated axons then?

Answer 16

because the space constant is proportional to the membrane potential /intracellular resistance (so the benefit of a high membrane resistance is reduced by the high internal resistance!) – as it moves down the axon is decays/decreases
metabolic and volume costs of myelination(energy use)

Question 17

Smallest unmyelinated axon length?

most axons >1.0 um are myelinated.. length?

Answer 17

0.2-1.5um    =    0.5-2m/s
1-20um        =    5-120 m/s

Question 18

Useful poisons and effect?
Tetraethylammonium (TEA)
Lidocaine
Tetrodoroxin (puffer fish)

Answer 18

affect K+ channels
affect Na+ channels
affect Na+ channels

Question 19

Dendrites -
do they have channels? if so, what are they?
spike - initiation zone?

Answer 19

yes, they have voltage-sensitive channels but don’t usually produce APs.
mainly encode info with graded potentials
SI zone - axon hillock = AP starts here and travels away from dendrites

Question 20

Coding by action potentials
continuous stimulation of neurone causes?
stimulus intensity is encoded by?

Answer 20

continuous stimulation of neurone causes a ‘train’ of APs
stimulus intensity is encoded by action potential frequency in the nervous system
Graded potentials = EPSP, GABA receptors mainly Cl- when they open = hyperpolarised
– can summate (temporal or spatial)
dendrites can be 99% of the surface area of a neuron.
1 neuron can have 200,000 synpases

Question 21

EPSPs can be shunted by..?

Answer 21

they can be shunted by inhibitory inputs

Question 22

Electrical synapses between neurons

Answer 22

+ - rapid, 2 way?

- - not flexible, can’t really regulate