sentence: However , there appears to be some resilience associated with DA cell death , as humans must lose 48 – 68 % of the dopaminergic neurons of the substantia nigra pars compacta ( SNc ) and about 70 – 80 % of the DA content of the striatum to experience symptomatic PD ( Bernheimer et al ., 1973 ; Fearnley and Lees , 1991 ) .