Action Potentials

Question 1

what are different types of intercellular communication? (distance)

Answer 1

paracrine: local signalling. includes synaptic communication (can be neurotransmitters (NT), gap junctions)

endocrine: long distance. hormones released into bloodstream

Question 2

describe the intra and extracellular ion concentrations that sets up the cells resting membrane potential.

Answer 2

- Na+ greater outside cell

- K+ greater inside cell

- A- (proteins) greater inside cell

= creates a resting membreane potential: +ve outside, -ve inside

Question 3

how is resting membrane potential set up?

Answer 3

- passive ionic diffusion

• active ionic diffusion (e.g. Na / K pump)

- Gibbs-Donnan equilibrium (effect):

a) objective: obtain electroneutrality
b) impermeable protein ions: too big to diffuse through: causes asymmetrical distribution of charged ions (Na/K)

Question 4

how do cells get over asymmetrical ionic charge distribution caused by proteins not being permeable?

Answer 4

• *1. Active Na/K diffusion**
• 3Na+ from intracellular to extracellular
• 2K+ from extracellular to intracelluar

effects:
- high Na+ conc in extracellular space, low intracellular
- high K+ conc in intracellular space, low extracellular
32- results in +ve extraceullar space c.f. intracellluar space: sets up resting membrane potential

• *2. membrane permeability:**
• • K+ (50:1 difference): more +ve charged ions move out of the cell: sets up more -ve charge inside cell. neuron plasma membrane is 50-100 times more permeable to K+ than Na=
• resting membrane potential of cell: approx. -70mV

Question 5

(what does Na/K pump contribute charge to resting membrane potential?)

Answer 5

(contributes -4mV)

Question 6

which ion Na+ or K+ has more passive leaking out / in cell?

what is resting membrane potential of cell?

Answer 6

- K+ (50:1 difference): more +ve charged ions move out of the cell: sets up more -ve charge inside cell. neuron plasma membrane is 50-100 times more permeable to K+ than Na=

-resting membrane potential of cell: approx. -70mV

Question 7

what is the nernst equation?

Answer 7

the potential across the cell membran at which the net diffusion of ions across thecell membrane due to conc. gradient stops

Question 8

how would you work out charge, using nernst equation of Na which is 150mV outside cell and 15mV inside the cell

Answer 8

equation: E_m= 61.5 log ([C]₀/[C]_I

Na outside = 150mV

Na inside = 15mV

= 61.5 log (150/15)

= 61.5 x log10

= 61.5mV inside the postive

Question 9

work out membrane potential for K:

Koutside = 5mM

K inside= 150 mM

Answer 9

-91mV inside negative

Question 10

what is the resting membrane a calculation of?

Answer 10

The resting potential is determined by concentration gradients of ions across the membrane and by membrane permeability to each type of ion.

usually around -70mV

(value is closer to resting potenial of K)

Question 12

* what is the resting membrane potential a consequence of? *

Answer 12

resting membrane potential consequence of:

- concentration gradients of ions across the plasma membrane

• *AND
• relative ion impermeabilities of the membrane**

Question 13

what are excitable cells?

what are action potentials? what from ? (basic)

Answer 13

- excitable cells: have electrical activity - such as nerve and muscle cells

- action potential: rapid changes in voltage across the membrane due to prescence of voltage-dependent ion channels

Question 14

whats important to note about -ve and +ve charge of cells?

Answer 14

the +ve and -ve charge effects only small area: inside and outside of membrane. NOT the whole cytoplasm

Question 15

which cell types are voltage dependent ion channels found in?

Answer 15

nerve cells

muscle cells (skeletal, cardiac, smooth)

Question 16

where are voltage dependent ion channels in nerve cells?

what are the two type of ion channels (in nerve cells?)

Answer 16

voltage dependent ion channels:

cell body and axon terminals of nerve cells:

• axonal, especially highly concentrated at the axon hillock

types:

- Na+ channels

- K+ channels

Question 17

describe the structure of Na+ voltage sensitive channels

Answer 17

2 channels:

• activation gate: (in middle of channel)
  a) closed in resting state
  b) bridge in middle of channel stops Na+ being able to enter cell
• inactivation gate: (located intracellularly)
  a) open in resting state

Question 18

how does Na+ voltage channel work?

Answer 18

open in response to depolarisation:

• activation gate
  a) v fast opens due to depol
• inactivation gate
  a) closes due to depol
  b) closes slowly after depol

= Na+ go into cell - makes cell more postivie

after repolarisation, channels return to resting

Question 19

how do VD K+ channels work ?

Answer 19

(movement = inside to outside of cell)

• no AT occurs. K+ goes down conc gradient

during depolarisation:

• voltage-sensitive channels slower to open, but stay open longer
• more K moved out of cell than required (hyperpolarisation). but 3 mechanisms restore potential. takes a bit of time

Question 20

how does an AP work?

Answer 20

• resting potential starts at -70mV
• Na+ ions enter cell: depolarisation (more +ve mV). Na+ channels open
• leads to action potential
• 1ms after AP (3), inactivation gate of Na+ close, K+ channels open up: repolarisation (more -ve mV)
• K+ stay open longer than neccessary (4)
• K+ channels close

- Na/K ATPase pump, leak channels and proteins re-establish resting membrane potential

Question 21

explain whats going on at resting of AP

Answer 21

• all Na and K+ pumps closed (activation gate of Na ion channels closed, inactivation gate of Na channels are open)
• Na/K ATPase pump, leak channels and proteins establish resting membrane potential

Question 22

explain whats going on at depolarisation of AP

Answer 22

- Na+ channels open: activation gate opens and Na+ go through = increase positive charge of cell.

• (K+ ions stay close: they wait for significant positive charge to activate them)

Question 23

explain whats going on at repolarisation of AP

Answer 23

• after depol: fires AP
• Na channel ions close (blockage of inactivation gate: ball on chain)
• K channel opens (repolarisation)

= restores intracellular -ve charge

Question 24

explain whats going on at hyperpolarisation of AP

Answer 24

• K+ channels stay open
• Na+ channels go back to resting state

Question 25

explain whats going on after hyperpolarisation of AP

Answer 25

3 mechanisms of Na/K ATPase pump, protein and diffuse restore back to normal

Question 26

explain all or nothing principle of AP

Answer 26

all or none principle:

• needs high enough Na+ channels to open to reach a depolarisation threshold
• strong stimuli triggers enough of a depolarisation threshold to cause a postive feedback loop

Question 27

do suprathreshold stimulus cause a supreathreshold AP?

Answer 27

No: AP is limited by no. of Na / K voltage sensitive channels

Question 28

which part of AP is postive feedback and whch is negative feedback?

Answer 28

postive feedback: depolarisation

negative feedback: repolarisation

Question 29

explain what absolute v relative refractory periods are

Answer 29

refractory periods:

period time where cell membrane is resetting itself.

- absolute refractory period:

the period of time during which a second action potential absolutely cannot be initiated.

• occurs due to inactivation of Na channels

- relative refractory period:

• period after firing of nerve when partial repolirasation has occurred a greater than normal stimulis / depolarisation can stimulate a second response
• peak of AP will be lower (but still goes over threshold)
• recovering Na channels and open K channels

Question 30

what does suprathreshold stimuli cause to do with regards to AP?

Answer 30

due to the r_elative refractory period, suprathreshold stimuli_ can cause greater frequency of AP. (but never at same time due to absolute refractory period). each of AP is the same size

Question 32

what do local anaesthetics do?

Answer 32

• bind to open Na+ channel: become inactivated
• physically prevent Na+ reopening and generating AP: drugs stablises inactive state
• cant depolarise cell
• pain fibres cant send pain to brain