Subsequently, he was an assistant to Eduard Hitzig (1839-1907) at the insane asylum in Nietleben as well as at the psychiatric and nerve clinic in Halle.
He also studied in Paris (under Jean Charcot) and England, later returning to his hometown of Hanover, where in 1903 he became a professor of neurology.
Bruns was the first director of the Gesellschaft Deutscher Nervenärzte (German Society of Neurologists).
In 1906 he published Die Hysterie im Kindesalter (Hysteria in Childhood), in which he explains that abnormal behaviour in children is due to internal conflicts, being largely caused by overbearing parents who favor harsh punishment.
His most significant work was Handbuch der Nervenkrankheiten im Kindesalter (Handbook of Nervous Diseases of Childhood), a textbook he co-authored with August Cramer (1860-1912) and Theodor Ziehen (1862-1950).