Origin and propagation of Action Potentials

Question 1

Define resting potential

Answer 1

The potential difference across the cell membrane when no nervous impulse is conducted.

Question 2

What is the resting membrane potential?

Answer 2

high conc of Na ions outside axon of than inside

Higher conc of K ions inside axon than out

Conc gradient created by sodium/potassium pump- enzyme called Na/K ATPase uses ATP to move NA ions out of axon & k ions in

Pumps 3 Na ions outside of axon, lowering conc of Na ions- they cannot diffuse back in due to impermeability of membrane to Na ions

K ions diffuse out of cell down con gradient, making outside +ve & inside -ve

Electrical gradient pulls 2 K ions back into cell

Causes inside of axon to be slightly -ve relative to outisde- membrane is polarised !

Electrical potential across cell membrane is -70mV

Question 3

How does an action potential originate?

Answer 3

Axon is stimulated- voltage-dependant Na ion channels open- Na ions flow into axon down their conc gradient & electrochemical gradient

Threshold- the point when sufficient Na ion channels open to allow Na ions into axon is greater than K ions leaving- -55mV

Cell becomes +ve on the inside relative to outside- becomes DEPOLARISED
- pd is +40 mV

Na ion channels close, excess Na ions pumped out by sodium pump- absolute refractory period

Permeability of membrane to K increases as voltage-dependent K ion channels open
- membrane is REPOLARISED
- p.d returns close to -70mV

k ions diffuse down electrochemical gradient out of axon attracted to +ve charge outside of membrane
- inside of axon becomes -ve relative to outside again
- p.d is - 90mV
- Cell is hyper polarised!

Voltage gated K ions now close & sodium-potassium pump cause sodium ions to move out of cell
- Axon returns to normal potential now

Releative refractory period: a period where a greater than normal stiumlus is needed for another action potential.

NOTE: view graph on notes- very important- LEARN this!

Question 4

How is an action potential propagated along nerve?

Answer 4

Na ions moving laterally, depolarise adjacent section of the axon

This opens VgNa channels so a new action potential (30+ mv) is stimulated in the new part of the axon.

This reversal of potential energy causes local currents to form, due to opposite charges being on the same side of the membrane.

as each consecutive region is depolarised- action potential moves along the axon membrane, away from the cell body.
- Propagation is in 1 direction due to absolute refractory period of VgNa channels behind the AP.

Myelin sheath is wrapped around axon so that there are a few gaps in between- called theNodes of Ranvier.
- impulse jumps from 1 node to other hence called Saltatory Conduction

NOTE: The propagated impulse does not vary in amplitude

Question 5

What affects nerve conduction velocity and explain how have vertebrates adapted?

Answer 5

1. Temperature - ions move faster at higher temps due to more KE.
2. Diameter of axon - greater volume & SA so more channels for exchange if ions.
3. Myelination:Saltatory conduction
   - Nachannels are clustered at Nodes of Ranvier so axon only depolarises at the nodes.
   - The axon is insulated by Schwann cells made of myelin
   - the myelin forces local currents to jump a 1 mm distance.
   - Reduces loses, speeds conduction & saves energy.

NOTE: view notes for images of nerve structure

Question 6

Why does the axon have: hyperpolarisation, absolute and relative refractory periods?

Answer 6

Absolute refractory period - shuts VgNa channels so the action potential can only be propagated in 1 direction.

Hyperpolarisation:
- Prevents any stimulus already sent up an axon from triggering another action potential in the opposite direction
- Does this by increasing the size of stimulus needed to change the membrane potential to the threshold for an action potential.

Relative refractory period - this occurs to encourage the diffusion of Na+ one way along the neuron in order to depolarise the next region.