The only difference between the axonal connectomes of the striosomes and the axons of those neurons in the matrix, is in the numbers of their branching axons. Striosomal axons cross the extent of the SN, and in macaques emit 4 to 6 vertical collaterals that form vertical columns which enter deep into the SN pars compacta (SNpc); the axons from those in the matrix are more sparsely branched. This pattern of connectivity is problematic. The main mediator of the striatopallidonigral system is GABA and there are also cotransmitters. The GPe stains for met-enkephalin, the GPi stains for either substance P or dynorphin or both, and the SN stains for both. This probably means that a single axon is able to concentrate different co-mediators in different subtrees, depending on the target.