A projection from the mesencephalic tegmentum to the nucleus isthmi in the frogs, Rana pipiens and Acris crepitans.

The nucleus isthmi is a prominent part of the frog's visual system. Each nucleus isthmi receives input from the ipsilateral tectum and sends output to both tecta. Until now, no non-tectal inputs to the nucleus isthmi of amphibians have been demonstrated. Anterograde and retrograde tracing with horseradish peroxidase in Rana pipiens and Acris crepitans now reveal that a diffuse group of cells in the mesencephalic tegmentum projects to the caudal region of the contralateral nucleus isthmi. These cells are primarily within the nucleus anterodorsalis tegmenti. This same group of tegmental cells may also project to the caudal region of the ipsilateral nucleus isthmi. A similar investigation of the brain of another frog, Xenopus laevis, has not revealed any evidence of this tegmento-isthmic projection.