Laminar distribution of the cells of origin of the descending tectofugal pathways in the pigeon (Columba livia)

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Abstract
The horseradish peroxidase (HRP) technique was used to investigate the laminar distribution of the cell bodies of origin of the two major descending pathways from the avian tectum to the hindbrain, the crossed tectobulbar pathway (CTB) and the ipsilateral tectopontine‐tectoreticular pathway (ITP). The CTB, which projects to the contralateral paramedian reticular formation of the hindbrain, was found to arise mainly (80–95%) from the large multipolar neurons characteristic of layer 13 of the tectum. In addition, multipolar neurons of layers 14 and 15 of the dorsal tectum contribute to rostrally terminating portions of the CTB. Neurons of layers 11–12, particularly at rostroventral tectal levels, were also found to contribute to the CTB. The ITP, which projects to the ipsilateral lateral pontine nucleus and the overlying reticular formation, was found to arise chiefly from neurons of layers 9–10, 13, and 14–15, with some slight contributions from neurons of tectal layers 8, 11, and 12. Cells of layers 8–10 that contributed to the ITP tended to be of two general types: (1) small‐mediumsized fusiform neurons with vertically disposed processes, and (2) medium‐large‐sized multipolar neurons whose processes showed no consistent orientation from neuron to neuron. The lateral pontine nucleus apparently receives input from tectal layers 8–15 inclusive, while the reticular formation overlying the lateral pontine nucleus appears to receive input only from tectal layers 13–15. Injections of HRP into the CTB and ITP also resulted in labeled cells in subtectal sites within the optic lobe (nucleus intercollicularis and the lateral mesencephalic reticular formation).The present results indicate that the major descending projections of the avian tectum arise from neurons located in tectal layers 8–15. Previous studies have shown that neurons of tectal layers 10 and 13–14 also give rise to the ascending projections of the avian tectum to the ventral lateral geniculate nucleus and to the nucleus rotundus, respectively (Benowitz and Karten, '76; Hunt and Kunzle, '76b). Thus, the major ascending and descending tectofugal pathways in birds share the same layers of origin. The present results in birds are in contrast to those in mammals, in which the ascending projections of the midbrain roof to diencephalic “visual” structures arise from superficial layers of the superior colliculus (layers I–III) while the major descending projections arise largely from the deeper layers of the superior colliculus (Harting et al., '73; Holcombe and Hall, '81). The decending pathways of the midbrain roof are presumed to be involved in motor functions while the ascending pathways of the midbrain roof are thought to be more involved in the sensory processing of visual information. The similarity in the laminar sources of the ascending and descending tectofugal pathways in birds, as reflected by the present study, indicates that the asending and desending tectofugal pathways of birds may convey similar types of visual information.