The modular construction of the neuropil in the substantia gelatinosa of the cat's spinal cord. A computer aided analysis of Golgi specimens.

Golgi impregnated spinal cord sections cut in the sagittal plane were used to study the branching pattern, terminal structure and orientation features of terminal axon arborizations in the substantia gelatinosa (lamina II) of the cat. The histological description of the arborizations was supplemented with computer aided quantitative three-dimensional analysis. Six types of axon arborizations could be distinguished among the 31 arborizations collected from C4 and L4 spinal segments. One type was tentatively identified as terminals of non-myelinated primary afferent fibres (Class I arborizations), while the others (Class II arborizations) were suggested to belong to thin myelinated primary afferent fibres, to descending tract fibres and to spinal interneurons. Arborizations differed in branching pattern and in the thickness of the branches. They possessed several terminal and en passant thickenings (up to 75) which were interpreted as presynaptic sites. The arborizations were confined to long and narrow sagittal sheets of the neuropil. These sheets were tilted from the sagittal plane but their rostro-caudal axis lined up exactly parallel to the axis of the spinal cord. The dimensions of the elongated brick-shaped territories encompassing the presynaptic sites of the arborizations were measured. The average values of 10 Class I trees were 526 micrometer in length, 53 micrometer in dorso-ventral direction and 17 micrometer in width (medio-lateral dimension). Class II trees were generally shorter but wider. Trees were found occasionally to arborize in two adjacent sheets. Arborizations with partially or totally overlapping branches were frequently seen.