TANGENTIAL MEDULLA NEURONS IN THE MOTH MANDUCA-SEXTA - STRUCTURE AND RESPONSES TO OPTOMOTOR STIMULI

In Manduca sexta, large tangential cells connect the medulla via the lobula valley (LoV) tract to the midbrain and the contralateral medulla. Tract neurons have been stained and recorded to determine their responses to optomotor stimulation. Neurons in the LoV-tract comprise a physiologically and anatomically heterogeneous population: 1. Motion insensitive medulla tangential (Mt) neurons arise from cell bodies in the ventral rind. Heterolateral cells arborize massively in both medullae and one or both halves of the midbrain. Mt-neurons respond to changes in light intensity. Physiological and anatomical evidence argues for their monocularity and transmission from the medulla on the side of the soma to the central brain and the contralateral medulla. 2. Motion sensitive neurons with cell bodies behind the protocerebral bridge connect the midbrain to the ipsior contralateral medulla. Direction-selective responses are characterized by excitation to motion in the preferred and inhibition in the opposite direction with maxima either in a horizontal or vertical direction. Peak values appear at contrast frequencies of appr. 3/s. The results suggest that these neurons are binocular and relay information from the midbrain to the medulla. They have been labelled as centrifugal medulla tangential (cMt) neurons. The possible roles for tract neurons in visually guided behaviour are discussed.