Structural and functional characteristics of commissural neurons in the superior colliculus of the hamster

Abstract

Intracellular recording and horseradish peroxidase (HRP) injection techniques were employed to delineate the structural and functional properties of superior collicular (SC) neurons in the hamster that were antidromically activated by electrical stimulation of the contralateral tectum. A total of 39 such cells were completely characterized, injected, and recovered. In ten of these, the axonal filling allowed us to reconstruct at least a portion of the terminal arborization in the SC contralateral to the labelled cell. Two of the recovered neurons were located in the stratum griseum superficiale (SGS), three were in the stratum opticum (SO), ten were in the stratum griseum intermediale (SGI), 11 were in the stratum album intermedium (SAI), 11 were in the stratum griseum profundum (SGP) and two were located in the stratum album profundum (SAP). The recovered cells were highly varied in both their morphological and their physiological characteristics. Somal areas ranged between 74 μm² and 364 μm², and the sample of recovered neurons included horizontal cells, narrow field vertical cells, and a variety of other multipolar neurons. Over one‐third (38.5%) of the recovered cells were unresponsive, 2.6% were exlcusively visual, 33.3% responded only to innocuous cutaneous stimuli, 10.2% were bimodal, 7.7% were specifically nociceptive, and 7.7% had complex (Rhoades, Mooney, and Jacquin: J. Neurosci. 3:1342–1354, ′83) somatosensory receptive fields. We observed no clear‐cut correlations between the structural and functional characteristics of these neurons. The conduction latencies of the commissural SC neurons ranged between 0.8 and 14.0 ms. The most rapidly conducting cells were located in the SGP and SAP. Conduction latency had a significant negative correlation with soma area.Labelled axons, in many cases, had at least one terminal arbor in a portion of the SC that was mirror symmetric with the location of the cell from which it originated. In several cases, however, commissural axons gave off a number of collaterals across the mediolateral extent of the tectum. Commissural axonal terminations were visible only in the laminae ventral to the SO. Several commissural SC neurons also had extensive ipsilateral axon collaterals. Both the ipsilateral and commissural axon branches of these cells gave off en passant and terminal swellings.