How does the myelin structure impact water diffusion? The answer is still not clarified but is important for interpreting diffusion MRI in conditions with altered myelin structure such as neurological disorders or developing brain. Myelin is sometimes modelled as permeable to explain exchange between compartments. This work investigates the impact of the spiralling nature of myelin on water exchange, until now only indirectly explored in one case. Findings emphasized that small axons and low number of myelin wraps lead to exchange times shorter than a second, which can be assessed at clinical scanners.