PhD defence: Observing earthworm neuronal activity using phase sensitive SD-Oct microscopy

In this thesis, we aim to develop an optical method for detecting small changes in biological and non-biological tissues. One example of such changes occurs in the nervous system when it transmits signals. The nervous system is composed of neurons, which consist of a body and an axon, which is a tail-like structure that transmits signals to the next cell. This signal, known as an action potential, induces a minute change in the axon’s diameter. The axon diameter is about 1 μm - which is 1,000 times smaller than 1 mm - and the expected diameter change is less than 0.5 nm. This change is not only small - about 2,000 times smaller than the axon itself – it also occurs very fast, in just 1 ms.

To detect this deformation, we used Spectral-Domain Optical Coherence Tomography(SD-OCT), an interferometry-based technique similar to LIGO, which detects gravitational waves through tiny deformations in the distance. In our experiments, we used earthworm axons, as they are relatively easy to extract and can remain functional over extended periods. Using SD-OCT, we successfully observed a 0.5 nm deformation in the earthworm’s axon, demonstrating that, with further development, this method could be applied more broadly to detect similar changes in other specimens. Detecting this tiny deformation could potentially lead to a better understanding of nervous system disorders, as structural changes relate to the mechanical properties of neurons.