BDNF and multiple sclerosis: (HRV): heart rate variability; (MS): multiple sclerosis; (MI): myocardial infarction; (CNS): central nervous system; (ECG): electrocardiogram; (NSM): neurogenic stunned myocardium; (TC): takotsubo cardiomyopathy; (BNP): brain natriuretic peptide; (BDNF): brain-derived neurotrophic factor.