Annual medical checks up untill your 40th birthday. From then on, every 6 months.
Brain scans? Never done one, never on a renewal. Need a brain to scan I s'pose.
Hearing, eyes, cholesterol and ECG are done every X number of years. It's on the form.
Medicals are done by AME. Doctors who've done approved aviation medical courses and are recognised by your licensing authority. eg UK CAA.