SEVERAL NEURONAL AND AXONAL TYPES FORM LONG INTRINSIC CONNECTIONS IN THE CAT PRIMARY AUDITORY CORTICAL FIELD (AI)

Intrinsic connections in the cat Primary auditory field (AI) as reveal ed by injections of Phaseolus vulgaris leucoagglutinin (PHA-L) or bioc ytin, had an anisotropic and patchy distribution. Neurons, labelled re trogradely with PHA-L were concentrated along a dorsoventral stripe th rough the injection site and rostral to it; the spread of rostrally lo cated neurons was greater after injections into regions of low rather than high characteristic frequencies. The intensity Of retrograde labe lling varied from weak and granular to very strong and Golgi-like. Out of 313 Golgi like retrogradely labelled neurons 79.6% were pyramidal, 17.2% multipolar, 2.6% bipolar, and. 0.6% bitufted; 13.4% were putati vely inhibitory, i.e. aspiny or sparsely spiny multipolar, or bitufted . Individual anterogradely labelled intrinsic axons were reconstructed for distances of 2 to 7 mm. Five main types were distinguished on the basis of the branching pattern and the location. of synaptic speciali sations. Type 1 axons travelled horizontally within layers II to VI an d sent collaterals at regular intervals; boutons were only present in the terminal arborizations of these collaterals. Type 2 axons also tra velled horizontally within layers II to VI and had rather short and th in collateral branches; boutons or spine-like protrusions occurred in most parts of the axon. Type 3 axons travelled obliquely through the c ortex and formed a single terminal arborization, the only site where b outons were found. Type 4 axons travelled for some distance in layer I ; they formed a heterogeneous group as to their collaterals and synapt ic specializations. Type 5 axons travelled at the interface between la yer VI and the white matter; boutons en passant, spine-like protrusion s, and thin short branches with boutons en passant were frequent all a long their trajectory. Thus, only some axonal types sustain the patchy pattern of intrinsic connectivity, whereas others are involved in a m ore diffuse connectivity.