the action potential and its propagation

the action potential and its propagation

1. the main event in the action potential

   • 1 threshold
     • the level of depolarisation required to cause an increase in Na+ permeability (pNa). Beyond this point the action potential is an ‘all-or-none’ event.
   • 2 the rising phase
     • . the rapid depolarisation occurs as the large driving forces causing Na+ entry (-ve potential inside and concentration gradient) come
   • overshoot
     • the inside of the neurone becomes +ve to the outside, as the system is driven towards ENa
   • falling phase
     • the switch from high pNa to high pK allows the driving forces causing K+ exit (+ve potential inside and concentration gradient) to come into play
   • undershoot
     • the system is driven towards EK, before returning to its resting state

2. the resting potential is mainly dictated by K+

3. functional elements of a bootleg gated ion channels

   • Selectivity filter (for Na+ or K+ ions)
   • Voltage sensor (Na and K channels)
   • Activation gate(s) (Na and K channels)
   • Inactivation gate (Na channel only)
   • voltage gated ion channel states
     • Na+
       • 1. Closed 2. Open 3. Inactivated 4. Closed
     • K+
       • 1. Closed 2. Open 3. Closed

4. The role of the Na+/K+ exchange pump in the Action Potential

   • The Na+ / K + exchange pump only acts in the background to move Na and K ions back to their original places.
   • it plays no direct role in the events of an individual AP.
   • a nerve can support about 10,000 Action Potentials even when the Na + / K + exchange pump is poisoned.

5. how the action potential moves

   • By recording from 2 points along an axon (Electrodes 1 & 2), it can be seen that an Action Potential (AP) moves away from the point of electrical stimulation (Stimulus electrode)

6. cable properties of the membrane

   • Current flowing passively along an axon decays with distance beyond the block.
     • This is called an electrotonic current or or a local current.
       • he cable properties of the membrane dictate the rate of decay of this current.

7. propagation of the action potential

   • At the peak of the AP the membrane is +ve inside to the outside. As a result there is a longitudinal electrical gradient inside the axon.
     • This gradient causes an electrotonic current to flow away from the point of the AP. As a result, the membrane ahead of the AP (Point 2) becomes depolarised.
       • When the depolarisation reaches the threshold, an AP now exists at Point 2
         • Repetition of this process represents a process of AP propagation. These processes occur rapidly, so that the AP also conducts rapidly (rates in the range 1-10 m s-1 are typical).
   • A nerve has no intrinsic directionality of conduction. If a point is electrically depolarised using an electrode, electrotonic currents flow in both directions. As a result, an AP propagates along the axon in both directions. ie patch lamping
     • However, due to the refractory state, an AP in a real axon propagates in only one direction

8. factors affecting AP conducion velocity

   • the effect of acon diameter
     • larger diameter axons present less internal resistance to current flow (before it eventually leaks out).
       • This is the basis for the high conduction velocities in giant axons
   • the effect of melioration
     • Conduction velocity also depends on the extent of electrical insulation around the axon. This increases the rate of spread of electrotonic current.
     • Myelin is formed by Schwann cells, which wrap tightly around the axons (the internodes) and leave only small uninsulated patches (the nodes of Ranvier)
     • In a myelinated axon the electrotonic currents under an AP flow rapidly along the axon from one node to the next, and in fact the AP is formed only at the nodes (Saltatory Conduction)

9. the density f ion channels

   • in summary        In summary, the density of voltage-gated channels in a patch of membrane determines its electrical excitability. Thus the membrane of an unmyelinated axon is electrically excitable. Patches with even greater densities of channels – eg the Node of Ranvier - are particularly sensitive to electrotonic currents. However the membrane of the internode is electrically inexcitable and cannot transmit an AP    
   • The saltatory mechanism relies on voltage-gated channels at the nodes
   • The density of channels at nodes is extremely high (3000 μm-2), compared with an unmyelinated nerve (300 μm-2).
     • This high density is also helps increase conduction velocity, since the membrane comes to threshold at a greater rate.
   • In contrast, there are virtually no voltage-gated channels along the internode (as they are not involved in AP production). This saves energy.
     • However, it has severe consequences in demyelinating diseases such as Multiple Sclerosis (MS), since the APs move extremely slowly (if at all) along the denuded internodal regions

10. the excitability of neuronal membranes

    •        In contrast, the dendritic membrane and soma of nerve cells are electrically inexcitable. This makes these membranes the sites for summing local currents before they reach the SIZ - where in essence a decision is made whether or not to initiate an AP.    
    • Such extremely sensitive patches with high densities of channels are also found at those points where an AP is initiated naturally in a neurone, for example:
      • The Axon hillock or spike initiation zone - SIZ
      • The Sensory ending of a mechano-sensory neurone (which is also a SIZ)