Neuronal organization in the dorsal horn of the spinal cord.

Two major output systems from the dorsal horn, the spinocervical tract (SCT) and the post-synaptic dorsal column (PSDC) pathway, have been characterized by electrophysiological methods and the injection of horseradish peroxidase into single neurones. Neurones of both systems have somata in laminae III, IV and V but they differ in their physiology and anatomy. SCT neurones have few inputs and their receptive fields are simple. PSDC neurones show several patterns of excitatory convergent input and have more complex receptive fields. Structurally, SCT cells are fairly homogeneous with dendrites mainly in laminae III and IV; input is from two major types of boutons - 2/3 contain round clear vesicles and 1/3 contain flattened clear vesicles and these latter are located more proximally on the dendritic tree. PSDC cells have a more varied anatomy: the dorsal cells send their dendrites through lamina III into II where they receive a variety of contacts including the central element of the glomerular complexes; deeper neurones have dendrites in laminae V - VII and these receive large synaptic boutons, often on long-necked spines. Thus even though SCT and PSDC neurones occupy the same parts of the dorsal horn they constitute separate populations of cells differing in axonal projection, receptive field organization, light microscopical appearance and the ultrastructure of the synaptic contacts they receive.