Action potential propagation

what are the 2 types of AP propagation

• contiguous conduction

• saltatory conduction

briefly describe contiguous conduction

-Conduction in unmyelinated fibers = slower
– AP spreads along every portion of the membrane

briefly describe saltatory conduction

– Rapid conduction in myelinated fibers = faster
– Impulse jumps over sections of the fibre covered with insulating myelin

-the propagation of action potentials along myelinated axons from one node of Ranvier to the next node

describe the process of contiguous conduction (5) - Explain how the passage of an action potential along non-myelinated axons results in nerve impulses

• AP passes as a wave of depolarisation - AP needs to be generated in every part of the membrane

• Influx of Na+ in one region increases permeability of adjoining region to Na+ by causing voltage gated Na+ channels to open so adjoining region depolarises

• however: a lot of ions would not be concentrated in specific areas - instead spread across the membrane

• therefore ions may be more likely to diffuse out of the axon

• and the AP is not propagated as quickly as in a myelinated axon

describe the process of saltatory conduction - how is an action potential propagated along a myelinated axon? (6)

1) Na+ enter the axon by facilitated diffusion into/at the first node of Ranvier through the Na+ voltage gated channels, causing axon to become more positive inside
2) the influx of Na+, repels positive ions already present in the axon and pushes them towards the next node of ranvier - (At nodes of Ranvier, there is a buildup of + charges inside and – charges outside the axon)
3) this causes threshold potential to be reached at the next node of ranvier
4) TP causes voltage gated Na+ channels to open and more Na+ diffuse in my facilitated diffusion
5) this process repeats down the axon : saltatory conduction

6) Layers of myelin insulate the axon - As a result, action potentials “jump” from node to node, increasing the speed of conduction.

why is saltatory conduction propagate APs faster than contiguous

because the AP does not have to be generated
underneath the myelin

state the speed of impulses in myelinated vs unmyelinated

• Myelinated fibers conduct impulses about 50x faster than
  unmyelinated fibers of comparable size:
  • large myelinated motor nerve - 120m/s
  • small unmyelinated autonomic neurone - 0.7m/s
  • squid giant axon (1mm diam. & unmyelinated) - 100m/s

explain the properties of the myelin (sheath) (3)

• Formed by oligodendrocytes in CNS

• Formed by Schwann cells in PNS

• primarily composed of lipids

explain the all or non law / all or nothing principle of an AP and frequency coding (4)

• If the stimulus is too low (below the threshold potential), there is no AP (this is the "none" part) - for an AP to be produced depolarization must exceed TP (-55mv)

• If the stimulus is above a threshold an AP is generated and is always the same size - it is not larger for stronger stimuli (this is the “all” part)

• action potentials produced are always same magnitude / size / peak at same potential

• Bigger/stronger stimuli instead increase the frequency of action potentials NOT the size/amplitude - (APs are independent of stimulus strength)

describe nature of refractory period (3)

● Time taken to restore axon to resting potential when no further action potential can be generated
● As Na+ channels are closed / inactive / will not open

• the unresponsive period of a neuron that follows the AP

importance of refractory period (4)

-ensures discrete impulses are produced (action potentials don't overlap)
-harder for an AP to be generated in this period - axon is even more depolarised, requires greater stimulation
-limits the frequency of impulse transmission at a certain intensity (prevents over reaction to stimulus)
- also ensures action potentials are in one direction - can't be propagated in a refractory region

Explain the relation between strength of stimulus and the refractory period (4)

-Rf: It allows us to see how strong / weak a stimulus is
- strength is dependent on the frequency of APs
- stronger/higher intensity stimulus causes a higher frequency of APs
- until a certain intensity

2 types of refractory periods and explain (2)

During the absolute refractory period, a neurone cannot respond to another stimulus as it is already generating an AP

During the relative refractory period the neurone’s threshold is elevated because repolarization is ongoing.