Dorsal root projections to dorsal horn neurons in the cat spinal cord

The projection of dorsal root fibers to the dorsal horn of the spinal cord of the cat has been studied by electron microscopy. Two distinct types of synaptic degeneration are seen with the electron microscope: small knobs which exhibit a marked increase in electron density, and large knobs which fill with neurofilaments. Variations in the distribution of degenerating knobs to the six laminae of the dorsal horn are observed: dense degenerating knobs are found throughout the horn, but are quite rare in laminae I and II. Lamina III exhibits the most dense knobs, and a great many are also present in laminae IV and VI with a somewhat lesser number seen in lamina V. Knobs undergoing neurofilamentous degeneration are found only in laminae V and VI, and are much Jess frequently seen than are dense knobs. The types of synaptic contacts made by degenerating dorsal root fibers also vary from one region of the dorsal horn to another. Dense degenerating knobs synapse primarily with small dendrites in laminae I, II and III, but not at all with large dendrites or nerve cell bodies. In lamina III, dense knobs are seen in axoaxonal synapses, and are always the pre‐synaptic component of the synapse. In laminae IV, V and VI dark knobs are commonly seen to synapse upon cell bodies of large and medium sized neurons and their proximal dendrites, but not upon small cell bodies Dense knobs upon small dendrites are common. In degenerating axoaxonal synapses, the dense knob is always the post‐synaptic component. Degenerating neurofibrillar knobs are only seen to synapse with cell bodies or larger dendrites in V and VI and not at all other synaptic knobs. The results are correlated with findings by light microscopy. Comparisons are made with some of the known physiological properties of dorsal horn neurons.