A crossed projection from the optic tectum to craniocervical premotor areas in the brainstem reticular formation. An anterograde and retrograde tracing study in the mallard (Anas platyrhynchos L.)

The optic tectum in birds receives visual information from the contralatera l retina. This information is passed through to other brain areas via the d eep layers of the optic tectum. In the present study the crossed tectobulba r pathway is described in detail. This pathway forms the connection between the optic tectum and the premotor area of craniocervical muscles in the co ntralateral paramedian reticular formation. It originates predominantly fro m neurons in the ventromedial part of stratum,griseum centrale and to a les ser extent from stratum album centrale. The fibers leave the tectum as a ho rizontal fiber bundle, and cross the midline through the caudal radix oculo motorius and rostral nucleus oculomotorius. On the contralateral side fiber s turn to ventral and descend caudally in the contralateral paramedian reti cular formation to the level of the obex. Labeled terminals are found in th e ipsilateral medial mesencephalic reticular formation lateral to the radix and motor nucleus of the oculomotor nerve, and in the contralateral parame dian reticular formation, along the descending tract. Neurons in the medial mesencephalic reticular formation in turn project to the paramedian reticu lar formation. Through the crossed tectobulbar pathway visual information c an influence the activity of craniocervical muscles via reticular premotor neurons.