The morphological relationships between substance P immunoreactive processes and ventral horn neurons in the human and monkey spinal cord.

The distribution of substance P (SP)-containing neural processes in the ventral horn region of the human and monkey spinal cord was studied by staining for SP by the indirect antibody peroxidase-antiperoxidase technique and by light and electron microscopic examination of stained material. Immunoreactive fibers and terminals were found in all major cell columns of the ventral horn in the human and monkey spinal cord, with the immunoreactive profiles in close apposition to motoneurons. The immunoreactivity was contained in synaptic knoblike (bouton) structures, in which the label was associated with small agranular vesicles, dense-core vesicles, and mitochondria. The immunoreactive synaptic knobs formed synaptic junctions predominantly with dendrites. The labeled knobs were similar to "S" (agranular spheroid vesicles) and "G" (dense-core vesicles) type bulbs described previously in the monkey ventral horn. Their location, mainly on the neuron dendrites and soma supports the physiological evidence that SP-produced membrane depolarization is mediated primarily by a direct action of the neurochemical on the neuron rather than by a transynaptic mechanism.