The fine structure of neurons in the dorsal horn of the cat spinal cord

The fine structure of the dorsal horn of the cat cord has been compared with the light microscope cytoarchitectural studies. Several distinct laminae may be seen with the electron microscope. Lamina I contains marginal cells of Waldeyer and has its processes oriented in a horizontal direction. Lamina II contains small neurons and processes oriented vertically, and may be distinguished from lamina III by the rich content of myelinated axons in the latter. In laminae I–III there are numerous non‐myelinated axons and many axodendritic and axoaxonal synapses but few axosomatic ones. Complex synaptic arrays made up of a central axon synapsing with other axons and dendrites are commonly seen. Laminae IV, V and VI are distinguished by their content of large neurons. They exhibit many axodendritic and axosomatic synapses but few axoaxonal ones and no complex synaptic arrays. Synaptic knobs containing rings of neurofilaments are present only in laminae IV–VI and synapse usually with somata or dendrites of neurons. There are differences in synaptic vesicle configuration and in the density of subsynaptic membranes associated with them. Differentiation of the dorsal horn into horizontal laminae is demonstrated by both light and electron microscope studies. The regional differences in neuronal organization may represent differences in functional organization as well.