LAMINAR ORGANIZATION OF FREQUENCY-DEFINED LOCAL AXONS WITHIN AND BETWEEN THE INFERIOR COLLICULI OF THE GUINEA-PIG

We present a comprehensive description of the local (intrinsic and com missural) connections in the central nucleus of the inferior colliculi (CNICs) in guinea pig. Focal injections of the anterograde tracer bio cytin were made into physiologically identified loci of the CNIC and t he spatial organisation of the labeled fibres was revealed with comput er-assisted three-dimensional (3-D) reconstruction. The intrinsic fibr es form a series of V-shaped laminar plexuses composed of fibres beari ng both terminal and en passant boutons. Each laminar plexus has a cen tral wing located in the CNIC that extends into the dorsal cortex and an external wing located in the external cortex. The edge where the tw o wings intersect delimits the lateral border of the central nucleus w ith the external cortex. The density of labeled terminals was consiste ntly lower in the cortices than in the CNIC. The laminar plexus connec ts points of similar frequency within the CNIC. Seen in 3-D, the locat ion, orientation, shape, and area of the laminar plexus vary as a func tion of best frequency. The commissural fibres ending in the contralat eral IC to the injection also form a laminar plexus which is symmetric al to the ipsilateral plexus. Electrolytic lesions placed in the contr alateral IC at sites with best frequencies corresponding to those of t he injection coincided with the terminals of the commissural fibres in most instances. Possible patterns for the organisation of these conne ctions (point-to-point and diverging) are discussed. Three systems of peripheral axons to the laminar plexus are described: parallel, obliqu e, and perpendicular to the central wing. The novel parallel system ha s terminals in both ICs that run parallel to the central wing. It migh t constitute the anatomical basis for across-frequency interactions. T he oblique and perpendicular systems are fibres of passage projecting to the commissure and brachium of the IC, respectively. (C) 1995 Wiley -Liss, Inc.