Optical manifestations and collisions of Action Potentials in nerves

Abstract

This work is concerned with electrical and optical measurements of neural activity. The first part of my work describes extracellular electric measurements of colliding Action Potentials (APs) in nerve chords from earthworms. The electromagnetic interaction (ephaptic coupling) between APs and their surroundings is particularly strong when APs are generated or annihilate (ephaptic discharge). I show that APs annihilate when they collide. This offers a previously undiscovered way to measure the ephaptic discharge. These measurements show that the well accepted Hodgkin-Huxley model underestimates the ephaptic discharge by one order of magnitude, while the very simple but comparably unpopular Tasaki-Cable model precisely reproduces the effect. The effect on neighboring cells depends on their position and orientation, as well as the timing of the APs, and can be excitatory or inhibitory. I make the hypothesis that a strong ephaptic discharge is a universal property of APs and therefore a major component of intercellular commu- nication, especially at synapses. It is also to be expected that repeated discharges will cause morphological changes in the cells. However, these processes cannot be observed by electrical recordings. In the second part of my work, I am developing a new optical method with which such non-electrical processes can be observed. APs cause structural changes and deformations and thereby cause an optical signal, the fast Intrinsic Optical Signal (fIOS). The fIOS is usually detected either in the depolarization of transmitted light or in fluctuations of the scattered light. First, I examine the depolarization of transmitted light of electrically stimulated nerve lobster with an extremely sensitive method. An fIOS could only be detected under very small scattering angles (θ > 0.3°), but never in unscattered light (θ < 0.2°). This subtle difference was not resolved by previous measurement systems and contradicts the common interpretation that the fIOS is caused by a change in the birefringence. The final optical experiments use this knowledge in combination with a concept re- cently proposed by F. Amblard. Fluctuations in the scattered light are amplified accord- ing to the principle of multiple scattering by embedding the sample in a white cavity. I call this method Cavity Amplified Speckle Spectroscopy (CASS). Several prototypes were designed, optimized and integrated into the electrophysiological setup. CASS can even detect the fIOS of a single myelinated axon in the earthworm’s turbid ventral nerve chord. In addition, a slower process of neural activity is revealed, which I call slow IOS (sIOS). A connection between the sIOS and APs is clearly recognizable, but the origin of the sIOS has not yet been clarified.