Collision Block of Action Potentials.

An electrically initiated action potential (AP) can be sent towards an oncoming naturally generated AP to neutralize it and stop propagation, collision block. To effect 100% collision block of Action Potentials (AP) from a source, the stimulation frequency will depend on
• the distance of the stimulation site from the action potential generator (APG),
• the conduction velocity of the axons,
• and the refractory period.
In the left side of the figure is shown an APG, such as the soma of a motor neuron. On the right is the target that receives the signals.
The arrow at fs is the input from stimulation at frequency of fs Hz.

The conduction time (Tp) of an AP from the generator to the stimulation point is d/v where d is the distance between the generator and the stimulation point and v is the conduction velocity.
Let fs = 0 for t < 0
Any AP initiated by the generator between zero and Tp
will be annihilated by the stimulator initiated AP occurring at t=0.
Any AP initiated by the generator between t = Tp and 2Tp will be annihilated by the stimulation initiated AP occurring at t=2Tp
Therefore the collision window is Tc = 2Tp
If we include the refractory period, Tc =2Tp + r
Then the escape window is Te=Ts-Tc
For 100% block of the incoming APs, Te= 0, so Ts = Tc,
or Ts = 2Tp + r = 2d/v + r
Which means that the frequency fs needs to be (2d/v + r)-1
For example, if v=100 m/s, d = 10 cm and r = 2 ms then fs = 250 Hz.
If the stimulator frequency were less than 250 Hz, there is a possibility that a naturally occurring AP would not be annihilated by a stimulator produced AP.

Action Potentials (AP) propagation initiated by electrical stimulation with a nerve electrode can be stopped by applying an arresting current.
• To arrest the incoming AP, the current has to be applied at the time the propagating AP arrives at the arrest site.
• The arresting current has to persist for the time that excitation currents from the preceding node are flowing in to depolarize the adjacent node.
• When the distance between the location providing the arrest current and the location initiating the AP initiation is small, the temporal dispersion of arriving APs on axons of different diameters will be small. This suggests that the distance between the initiation site and the arrest site should be small.
A nerve electrode with multiple contact surfaces for injecting currents can be placed around a nerve trunk to initiate APs and arrest propagation in one direction.