Distribution and Morphology of Cortical Terminals in the Cat Thalamus from the Anterior Ectosylvian Sulcus

Abstract

Two main types of cortical terminals have been identified in the cat thalamus. Large (type II) have been proposed to drive the response properties of thalamic cells while smaller (type I) are believed to modulate those properties. Among the cat's visual cortical areas, the anterior ectosylvian visual area (AEV) is considered as one of the highest areas in the hierarchical organization of the visual system. Whereas the connections from the AEV to the thalamus have been recognized, their nature (type I or II) is presently not known. In this study, we assessed and compared the relative contribution of type I and type II inputs to thalamic nuclei originating from the AEV. The anterograde tracer BDA was injected in the AEV of five animals. Results show that (1) both type I and II terminals from AEV are present in the Lateral Posterior- Pulvinar complex, the lateral median suprageniculate complex and the medial and dorsal geniculate nuclei (2) type I terminals significantly outnumber the type II terminals in almost all nuclei studied. Our results indicate that neurons in the AEV are more likely to modulate response properties in the thalamus rather than to determine basic organization of receptive fields of thalamic cells.

Figure 1

(A) Representation of the brain showing the location of the subregions of the AES cortex. (B) Cortical injection sites of the illustrated cases. (C,D) Example of a coronal section stained with AChE used to identify thalamic subregions.

Figure 2

Topographical representation of axon terminals in the thalamus after injection of AES. Injection sites are presented in insets. Blue dots: type I axon terminals, Green dots: Singletons, Red dots: type II axon terminals. In Figs 1A and 2B, one dot represent one terminal. In Figs 1B and 2A one dot represents 5 terminals. Scale 1 mm.

Figure 3

Topographical representation of axon terminals in the thalamus after injection of AES. Injection sites are presented in insets. Blue dots: type I axon terminals, Green dots: Singletons, Red dots: type II axon terminals. In Figs 1A and 2B, one dot represent one terminal. In Figs 1B and 2A one dot represents 5 terminals. Scale 1 mm.

Figure 4

Photomicrograph of corticothalamic terminals. (A) Reconstruction of a typical type I axon bearing small sparse terminals (indicated by arrows) found in the MGm. (B) Example of en-passant boutons along a corticothalamic axon of the MGm. (C) Typical terminal field containing mostly type I in the LM-Sg. (D) Type I axon terminals in the dendritic field of a thalamocortical cell in LM-Sg. (E) Example of a large cluster of type II axon terminals (black arrow). For comparison, note the size of type I axon passing in the background (white arrow). Scales: A: 50 µm; B, D and E: 10 µm; C: 100 µm.

Figure 5

Example of terminal fields found in the thalamus following injection in the AES (from case No 3). (A) A focus containing mostly type I axon terminal in the LPm thalamic region. (B) A focus containing both type I and II axon terminals in the MGd. Scale in panels A and B is 100 µm.

Figure 6

Proportion (%) of terminal types according to their cortical origin and thalamic targets.

Figure 7

A large injection of WGA-HRP in the thalamus reveals retrograde labeled cells in the AES. These neurons were mainly found in layer VI. Scale 100 µm.