L19 - nerve cells & excitability: action potential

Question 1

how do nerve cells communicate?

Answer 1

via dendrites, axons and terminals

Question 2

what does it mean by neurones care polarised?

Answer 2

resting membrane potential

Question 3

what is RMP?

Answer 3

electrical charge across plasma membrane where the inside is more negative compared to the outside. -70mV

Question 4

what are action potentials?

Answer 4

change in voltage across membrane

Question 5

describe depolarisation of membrane

Answer 5

• excitatory signals at dendrites cause ligand gated sodium channels to open
• causing Na+ ions to diffuse into the cell
• this decreases the negative charge inside the cell and makes the membrane less negative
• so cell becomes less polarised

Question 6

why is axon hillock known as trigger zone?

Answer 6

• influx of Na+ diffuses inside neurone and produces current that travels towards axon hillock
• a.p usually starts at the axon hillock
• as a.ps are produced by voltage gated ion channels which are most concentrated at axon hillock

Question 7

all or none law

Answer 7

• if stimulus is strong enough A.P occurs
• frequency of A.P rather than strength
• all A.P same size
• no weak or strong A.P

Question 8

hypOpolarisation

Answer 8

initial increase of membrane potential to the threshold potential (-55mV)

Question 9

depolarisation

Answer 9

membrane potential goes from resting membrane potential (-70mV) to LESS negative values

Question 10

overshoot (peak)

Answer 10

peak of A.P at +40mV

Question 11

repolarisation

Answer 11

membran potential going back to more negative value (resting memb potential)

Question 12

hyperpolarisation/ undershoot

Answer 12

potential becomes even more negative than RMP

Question 13

voltage gated ion channels

Answer 13

• closed at RMP
• open when memb is depolarised (more positive)
• change in voltage triggers opening
• close when rmp is restored
• responsible for initiaiton of Na+ and termination of K+ of action potentials

Question 14

for A.Ps to be generated what must there be?

Answer 14

a signal strong enough to bring membrane voltage to threshold (-55mV), as this is the minimum needed to open voltage gated ion channels

Question 15

describe how sodium and potassium channels open at threshold

Answer 15

• Na+ channels open quickly
• K+ channels slowly
• so initial effect is due to sodium influx
• causes cell to become even more positive, leading to further depol

Question 16

describe in depth how A.P is triggered, talk about the channels and how they relate to the graph

Answer 16

• sodium channels open causing influx of Na+ ions to enter cell
• memb even more depol
• A.P triggered once men is positive enough
• by the time sodium channels start to close, K channels are fully opened
• so K+ ions rush out of cell
• rmp restored
• potassium gates slow to close so there is a undershoot called hyperpolarisarion (more neg)

Question 17

absolute refractory period

Answer 17

A.P can’t be evoked as voltage gated sodium channels are inactivated and cannot be activated again until memb is depolarised and resting state returns (this is from depol to rmp)

Question 18

relative refractory period

Answer 18

memb potential hyperpolarised by potassium ions, AP an be generated only if stimulus is strong enough to overcome the hyperpol to reach the threshold (during hyperpol)

Question 19

action potential propagation

Answer 19

• sodium influx spreads along axon
• sodium currents diffuse in both directions
• but refractory properties of ion channels make sure a.p is propagated in ONE DIRECTION
• as the part that just fired is unresponsive

Question 20

what does the speed of propagation depend on?

Answer 20

• depends on axon diameter (size)
• and myelination

Question 21

speed of propagation for a large axon

Answer 21

• less resistance to current
• larger diameter = faster propagation

Question 22

myelin sheath

Answer 22

• electrically insulated axon so currents can jump from node to node
• as voltage gated sodium and potassium channels are only at nodes of ranvier

Question 23

pain is one of the…

Answer 23

slowest sensations bodies can send as they are sent from smaller fibres without myelin

Question 24

faster signals are sent by?

Answer 24

larger, myelinated neurones

Question 25

Guillain-barre syndrome

Answer 25

destruction of schwann cells in PNS

Question 26

ms

Answer 26

loss of oligodendrocytes in brain and spinal column

Question 27

what do ms and Guillain barre syndrome cause?

Answer 27

• muscle weakness
• numbness
• tingling

Question 28

when myelin coating of nerves degenerates and the nerves are afferent (sensory)

Answer 28

• signals are diminished
• or destroyed completely
• causes numbness & tingling
• as sensations are not travelling the way they should

Question 29

when myelin coating of nerves degenerates and the nerves are efferent (motor)

Answer 29

leads to weakness as brain gives out lots of energy but still can’t actually move the affected limbs. limbs are especially affected as they have longest nerves, longer nerve = more myelin to be destroyed

Question 30

graded potentials

Answer 30

-changes in membrane potential in small region of membrsne
- size of potential changes with the strength of stimulus
- if the threshold for sodium channels is reached A.P is generated
- 2 types: excitatory & inhibitory

Question 31

excitatory graded potentials

Answer 31

increases likelihood of A.P by depolarising the membrane

Question 32

inhibitory graded potentials

Answer 32

decrease likelihood of A.P via hyperpol

Question 33

no summation

Answer 33

2 stimuli at separate times

Question 34

temporal summation

Answer 34

2 excitatory stimuli occur close in time

Question 35

spatial summation

Answer 35

2 stimuli occur at sae time at different locations

Question 36

spatial summation of EPSPs and IPSPs

Answer 36

changes in membrane potential can cancel each other out