Cells in laminae III and IV of rat spinal dorsal horn receive monosynaptic primary afferent input in lamina II.

In order to determine how information conveyed by fine primary afferent fibres might reach the deeper laminae of the spinal dorsal horn, 5 Golgi-stained neurones with somata in lamina III or IV and dendrites that entered lamina II were examined by electron microscopy. Three of the cells were from animals in which 2 or 3 dorsal roots had been cut 26 or 30 hours previously. These cells received numerous synapses in lamina II, and between 13 and 16% of these (24-31% of asymmetric synapses) were from degenerating axons. Synapses with degenerating axons were found throughout the depth of lamina II, including the dorsal part, which receives primary afferent input from myelinated nociceptors and from unmyelinated axons. In addition, all 3 cells were postsynaptic to degenerating axons within lamina III. The 2 cells from unoperated animals also received many synapses within lamina II and at some of these the presynaptic axon was the central terminal of a glomerulus. Only one example of a dendrodendritic synapse involving a stained dendrite was seen. Cells of laminae III and IV may therefore not be a major target for presynaptic dendrites of cells in lamina II. It is concluded that one way in which information carried by primary afferents passes from the superficial dorsal horn to the deeper laminae is through monosynaptic contacts between these afferents and the dorsal dendrites of some cells whose somata are situated in laminae III and IV. If the axons of these cells generate local collaterals, this may account for some of the activation of cells whose dendrites do not enter lamina II.