Nerves 4

Question 1

what is the RMP?

Answer 1

-70mV

Question 2

what is the usual treshold needed to be reached to fire an AP?

Answer 2

-55mV

Question 3

what is the potential at sensory neurone called?

Answer 3

generator potential

Question 4

what is the potential at the skeletal muscle which determines whether the AP will be fired or not?

Answer 4

endplate potential

Question 5

describe steps of the AP when stimulus is activated (4)

Answer 5

1. stimulus causes sudden depolarisation and an overshoot to around +30 in a short space of time
2. repolarisation then occurs (AP falls back down to below RMP)
3. refractory period occurs
4. cells is at RMP again

Question 6

at rest, what is the Na and K permeability like?

Answer 6

Na permeability is low

K permeability is high (due to leaky K channels)

Question 7

Describe ionic basis of AP when treshold is reached. (4)

Answer 7

when threshold is reached..

1. Na channels open immediately and Na floods in depolarising the cell until its equilibrium potential is reached
2. this causes a massive increase in permeability (decrease in resistance)
3. K permeability slowly rises as more K channels open (voltage gated/ dependent ones NOT the leaky ones)
4. K channels opening and Na channels closing causes repolarisation and later hyperpolarisation and then we are back to where we started at RMP

Question 8

what is the refractory period?

Answer 8

occurs when there is no response to further stimulation (unresponsive tissue)

Question 9

what sort of effect occurs if the Na/K pump is poisoned?

Answer 9

delayed effect (it takes time for gradients to run down)

Question 10

what do local anaesthetics block? (procaine/ lidocaine)

Answer 10

block voltage dependent Na channels and no APs can be transported (some toxins do the same e.g. tetrodoxin)

Question 11

what happens at depolarisation phase?

Answer 11

• Na channels open and Na floods the cell very quickly

Question 12

what happens at repolarising phase?

Answer 12

• Na permeability drops
• VOLTAGE gated K channels open (more gradual, slow and stays open for longer), making cell more negative (drop back down)

Question 13

what are main properties of APs?

Answer 13

1. all or none treshold
2. can’t encode stimulus in their amplitude, only in FREQUENCY
3. they self propagate
4. once AP is made, they are all the same size
5. mediated by voltage gated channels
6. have a refractory period

Question 14

what is meant by APs being self-propagated?

Answer 14

• non-decremental (good for long distances)
• depolarisation can only sweep in ONE direction (forward) along the axon
• signal goes on until it reaches the muscle

Question 15

what can conduction velocity be improved by?

Answer 15

myelination

Question 16

what effect do large axons have on depolarisation?

Answer 16

-evokes a bigger AP as depolarisation evoked at one channel can spread further

Question 17

why do larger axons give a bigger AP?

Answer 17

depolarisation can happen in wider space since Na channels are more spread out and can reach their “neighbour” channels to reach treshold. The bigger the passive spread, the signal travels for longer

Question 18

what takes longer, the signal travelling or opening/closing of channels?

Answer 18

opening/closing of channels

Question 19

what is myelin sheath wrapped around?

Answer 19

the Schwann cells which wrap around axons

Question 20

in which part of the axon, do the GPs summate (integrate) which trigger an AP?

Answer 20

at the axon hillock/ initial axon segment (by reaching the treshold)

Question 21

what is saltatory conduction?

Answer 21

propagation of APs along myelinated axons from one node to the next, increasing conduction velocity of APs

Question 22

what is the site of APs on a neurone?

Answer 22

at nodes of Ranvier (AP decays as it moves along the axon)

Question 23

what glia bodies help with myelination?

Answer 23

oligodendrocytes

Question 24

why is myelination good for AP signal?

Answer 24

• makes axon less leaky
• increases action resistance (allows AP to spread like a local current)
• depolarisation spreads quicker and increases velocity of conduction

Question 25

what disease is assocaited with demyelination?

Answer 25

multiple sclerosis (APs are not effectively passed as it decays quicker and leaks out, no depolarisation occurs at next node and treshold is not reached (conduction fails).

Question 26

what is a compounda action potential?

Answer 26

mammals have big and small myelinated and big and small unmyelinated axons. Therefore an extracellular recording of axons (nerve trunk) evokes a compound AP

Question 27

what is the biggest peak AP on a graph?

Answer 27

An AP that travels the fastest (smaller ones arrive slightly later)

Question 28

which type of axons give the BIGEGST peak?

Answer 28

big myelinated axons

Question 29

which type of axons give the SMALLEST peak?

Answer 29

small unmyelinated axons

Question 30

What is the order of fibres from: (Most sensitive to anoxia and least sensitive to local anaesthetics) to (Least sensitive to anoxia and most sensitive to local anaesthetics)

Answer 30

1. proprioception, motor neurones 2. touch, pressure receptors 3. touch,cold, fast pain, 4. preganglionic, autonomic fibres (unconscious actions) 5. heat, slow pain

Question 31

which fibres are more sensitive to anaesthetics?

Answer 31

small ones are affected first by anaesthetics

Question 32

which fibres are most sensitive to absence of oxygen?

Answer 32

myelinated fibres

Question 33

Summary of graded potentials

Answer 33

1. graded 2. encoded by AMPLITUDE 3. can summate 4. no treshold 5. no refractory period 6. decremental 7. de or hyper polarising 8. ligand gated channels 9. "decides" if cell will fire AP 10. occurs in cell body and dendrites 11. initiated by entry of ions through the channels 12. usually Na,Cl and CA ions involved 13. input signal

Question 34

Summary of Action potentials

Answer 34

1. all or none 2. encoded by FREQUENCY 3. cannot summate 4. are evoked at treshold 5. has refractory period 6. self-propagating 7. depolarising 8. voltage gated channels 9. sends electrical signal over long distances 10. occurs in initial/trigger segment of the axon 11. initiated by above treshold GP at the trigger zone 12. usually Na and K ions involved 13. conduction signal

Question 35

what gives limits on APs?( so they happen one at a time)

Answer 35

refractory periods

Question 36

does depolarising lead towards the treshold?

Answer 36

yes

Question 37

does hyperpolarising lead away from treshold?

Answer 37

yes