Action potentials and nerve conduction

Question 1

graded potentials vs action potentials advantages:

Answer 1

• faster than APs (electrotonic spread)
• signal state w finer resolution (continuous analogue nature) vs binary/ digital nature of APs
• potentially higher info transfer than APs

Question 2

graded potentials vs action potentials disadvantages:

Answer 2

• rapidly dissipate -> only short distances (few mm most)
• not regenerated along nerve/ axon
• susceptible to noise than APs

Question 3

summation at axon hillock:

Answer 3

• graded electrical potentials generated by changes in membrane permeability (ligand-gated ion channels) on dendrites
• potentials propagate electrotonically from dendrites and cell body
• potentials are summed (spatial/ temporal) or integrated in axon hillock/ initial segment

Question 4

initial segment=

Answer 4

trigger zone

Question 5

define axon hillock:

Answer 5

where axon joints cell body

Question 6

define initial segment:

Answer 6

thick unmyelinated section of axon that joins cell body at axon hillock,
- site of action potential generation

Question 7

if depolarisation too weak and Vm doesn’t reach excitation threshold, how is it quickly dissapated?

Answer 7

• increase flow of K+ through leak channels returns cell to resting state

Question 8

how is a cell like a capacitor?

Answer 8

• device which separates and stores charges
• but are ions (Na, K) rather than electrons
• can estimate no. of ions that have to move across membrane to take it from RMP to threshold

Question 9

total charge on capacitor formula:

Answer 9

q = c x V

Question 10

define capacitor formula:

Answer 10

q- total charge (coulombs)

V- voltage across membrane bilayer

c- capacitance in Farads (F)

Question 11

1. rapid depolarisation phase: voltage-gated Na channels have 2 gates

Answer 11

• activation gate (m gate) closed at rest

- inactivation gate (h gate) open at rest

Question 12

1. rapid depolarisation phase: depolarisation above threshold causes which gate to open?

Answer 12

m gate (activation)

Question 13

1. rapid depolarisation phase: m gate opens causing

Answer 13

influx of positive Na ions causes cell to depolarise more coz inward Na more than outward K

Question 14

1. rapid depolarisation phase: depolarisation causes more voltage gated..

Answer 14

Na channels to open

• more Na flows in, positive feedback
• Vm rapidly driven to Na equilibrium potential (E Na)

Question 15

2. repolarisation phase: after 1-2ms opening m gate..

Answer 15

inactivation (h) gate shuts

- voltage gated Na channels cannot be reactivated yet for short time afterwards

Question 16

2. repolarisation phase: absolute refractory period

Answer 16

where Na channels can’t be reactivated by depolarising impulses for short time

• eventually m gate closes, h gate reopens and voltage gated sodium channels (VGSC) can be reactivated

Question 17

2. repolarisation phase: which electrochemical force larger? Na or K

Answer 17

K+ driving it out of the cell larger

• Na channels closing, voltage gated K+ channels in membrane start to open

Question 18

2. repolarisation phase: K+ channel gates and features

Answer 18

only n gate

• triggered at threshold but open after short delay
• repolarises

Question 19

3. after-hyperpolarisation phase (undershoot):

Answer 19

• cell repolarise close to resting Vm
• but pK elevated as K+ channels slow to close
• Vm more negative than RVm
• h gate on Na channels open again and another action potential can be initiated

Question 20

3. after-hyperpolarisation phase (undershoot): relative refractory period

Answer 20

• as pK still elevated and some Na channels still inactive, will need slightly stronger stimulus to trigger anoterh action potential during this period

Question 21

all or none principle:

Answer 21

• unlike graded potentials, action potentials can’t sum

- overlap is prevented by absolute refractory period

Question 22

frequency (rate) coding of stimulus strength:

Answer 22

• stimulus strength is coded in frequency (firing rate) of action potentials
• no action potentials during absolute refractory period
• but stronger stimuli can during relative r.p.

Question 23

local current flow:

Answer 23

electrotonic spread, same as how graded potential spreads

- activated nearby Na channels and ‘regenerates’ action potential in new part of membrane

Question 24

unidirectional propagation:

Answer 24

• depolarisation causes opening of voltage gated Na channels ahead of action potential
• propagation back towards axon hillock prevented in absolute refractory period

Question 25

action potential propagates along axon by:

Answer 25

• contiguous conduction (unmyelinated axon) = slower

- saltatory conduction (myelinated) = faster

Question 26

oligodendrocytes:

Answer 26

• CNS
• forms several myelin sheaths
• myelinates sections of several axons

Question 27

schwann cells:

Answer 27

• PNS
• forms one myelin sheath
• myelinates only 1 section of axon

Question 28

myelination:

Answer 28

• schwann/ oligodendrocyte wraps around axon
• cytoplasm eventually squeezed out
• forms 50-150 layers of membrane

Question 29

myelin sheath: features

Answer 29

• lipids
• insulator
• restricts movement of hydrophilic ions in/out of cell
• ions only leave in gaps called Nodes of Ranvier
• voltage gated Na and K channels concentrated there
• action potentials only at nodes (jump) along axon

Question 30

unmyelinated axons: cons

Answer 30

• positive charges leak across membrane via ion channels
• but if too may +ve charges leak out, depolarisation too weak to trigger action potential in next part of membrane
• VGSCs must be distributed evenly along axon membrane to constantly regenerate action potential and allow to continue along axon - slow

Question 31

myelinated axons: pros

Answer 31

• sheath increases electrical resistance of membrane (rm)
• reduces membrane capcitance (Cm)
• prevents +ve charges leaking out -> depolarisation can spread electrotonically further
• very fast

Question 32

cable theory:

Answer 32

voltage decreases exponentially as it spreads along membrane electrotonically

Question 33

membrane capacitance is:

Answer 33

measure of how strongly charges on either side of membrane attract each other

Question 34

myelin decreases capacitance why:

Answer 34

• increase distance btw ICF and ECF

- not need as many ions to alter voltage = reduced time constant

Question 35

nerve conduction: effect of diameter

Answer 35

large diameter:
- less resistance to movement of charges (ions) = faster (ri low)

smaller diameter: more resistance to movement of ions = slower (ri high)

Question 36

nerve fibre classified by:

Answer 36

• axon diameter

- whether myelin is present