Connections of the mesencephalic, thalamic and telencephalic auditory centers in turtles. Some structural bases for...

The organization of auditory projections at the mesencephalic, thalamic and telencephalic brain levels was studied utilizing the method of horseradish peroxidase (HRP) transport in two species of the turtle--Emys orbicularis and Testudo horsfieldi. It was shown that the torus semicircularis receives bilateral afferents from the brain stem auditory centers. They arise predominantly from the contralateral cochlear nuclei, the ipsilateral superior olive, the dorsal and ventral nuclei of the lateral lemniscus and from the symmetrical torus semicircularis. These connections appear to be reciprocal. After the enzyme injections correspondingly into the torus semicircularis and n. reuniens anterograde and retrograde HRP transports show that the central nucleus of the torus semicircularis projects to n. reuniens throughout its rostro-caudal extent mainly ipsilaterally. In turn, n. reuniens projects to the medioventral part of the dorsal ventricular ridge. A following common principle of the organization of the auditory system was revealed at the three brain levels explored. Auditory relay centers occupy the most medial positions at every level (n. centralis of the torus semicircularis, n. reuniens, the medioventral part of the dorsal ventricular ridge). Immediately lateral to them are somatic centers (correspondingly, n. intercollicularis, n. ventralis, the central part of the dorsal ventricular ridge). These together with the auditory centers form united functional complexes at every level. In these complexes auditory and somatic projections overlap, thus constituting a basis for the interaction between auditory and somatic afferent inputs. Mesencephalic and thalamic auditory centers were shown to receive direct somatic (cervical spinal) projections and non-direct from the underlying somatic center as well as from the adjacent somatic center at the same level (n. intercollicularis in the mesencephalon, n. ventralis in the thalamus). Somatic centers in the complexes described get no direct auditory projections. Auditory impulses however can enter them via two pathways: along neuron axons from the neighbouring auditory center reaching the adjacent somatic center and along somatic neuron dendrites which pass into the adjacent auditory center. The morphological basis for the auditory-somatic interactions primarily in the auditory center and also in the somatic center was demonstrated in Golgi-like HRP labeled and Golgi-impregnated neurons of these centers. The organization of the auditory-somatic projections at the three brain levels in turtles in to a degree comparable to the auditory system in mammals which is structured according to the core-belt principle.(ABSTRACT TRUNCATED AT 400 WORDS)