This research investigates elementary mathematics teachers' competencies to implement neuroscience theory (NS) of mathematical cognition into their classroom practices during and after, participation in a professional development (PD) course. We examine the teachers' familiarity with Approximate Number System (ANS), relevant to elementary school level, and build their competencies to implement this NS concept into teaching. Data for this qualitative study were collected from the reflective reports written by nineteen female elementary mathematics teachers (aged 25–40) and were analysed using two complementary methodologies: content and linguistic analyses. Our findings indicate that the teachers acquired NS knowledge, and their competencies evolved from listing the facts and analysing their meanings to weighing possible applications and attempts to integrate newly acquired knowledge into their teaching. This study seeks to contribute to the field of professional teacher development by demonstrating how teachers can lend meaning to theoretical knowledge they obtain in a PD course, in meaningful ways by connecting it with their classroom teaching practices.