abnormal substantia nigra morphology abnormal substantia nigra pars reticulata morphology MP:0013218 MPheno.ontology abnormal SNPR morphology abnormal pars reticularis substantiae nigrae morphology abnormal reticular part of substantia nigra abnormal substantia nigra pars reticularis morphology any structural anomaly of the anterior (ventral) part of grey matter of the substantia nigra which contains smaller and loosely packed cells than the pars compacta, only some of which contain a small amount of pigment; the cells of pars reticulata bear a strong structural and functional resemblance to the inner segment of the globus pallidus; many cells in this region use GABA as a neurotransmitter