Morphology and physiology of single neurons in the medial, interlaminar nucleus of the cat's lateral geniculate nucleus

Abstract

Physiological studies have shown that the cat's reticulocortical system is comprised of at least three parallel and independent pathways, the W-, X-, and Y-cell pathways. The morphological correlates of the constituent W-, X-, and Y-cells have been determined both in the retina and in the A and C laminae of the lateral geniculate nucleus. The aim of this study was to extend these structure/function relationships to neurons in laminae 1 and 2 of the medial interlaminar nucleus (MIN), which is a division of the cat's dorsal lateral geniculate nucleus. We used intracellular injection of horsradish peroxidase (HRP) into individual, physiologically identified MIN neurons. Since this procedure may yield an unrepresentative sample of MIN neurons, two controls were performed. First Nissl staining showed that the soma size of intracellularly labeled cells were representative of those of all MIN cells. Second, retrograde labeling following HRP injections into the optic radiations or specific visual cortical areas showed that the intracellularly labeled MIN cells were representative of MIN relay neurons. Many of the retrogradely labeled cells were so well filled that their entire dendritic arbors were revealed. Of 70 MIN neurons recorded physiologically, 22 were injected with HRP and successfully recovered. We also completely labeled the somata and dendrites of 114 MIN neurons from HRP injections into the optic radiations and retrogradely labeled 165 MIN neurons by injection of HRP into visual cortical areas. Our sample of intracellularly injected neurons, which were all Y-cells, were morphologically representative of all MIN relay cells. We thus conclude that laminae 1 and 2 of the MIN contain a nearly homogeneous population of Y-cells with properties essentially identical to these of Y-cells in the A and C laminae of the lateral geniculate nucleus. When viewed in the coronal plane, MIN projection neurons typically exhibited oval or elongated somata. In the medial and ventral parts of the MIN, these somata were smaller and more flattened. MIN soma sizes extended over the full range of those seen in the A laminae. Dendritic arbors of most MIN relay neurons radiated in a fairly spherical fashion. In the medial and ventral parts of the MIN, however, dendrites were oriented in a more bipolar fashion, but intermediate forms between spherical and bipolar arbors were also common. Dendrites of MIN projection neurons were typically smooth; most primary dendrites were straight, but secondary dendrites were more variable in structure. Some dendrites crossed borders between laminae 1 and 2; some crossed from the MIN to the A or C laminae or to the lateral posterior-pulvinar complex. Dendritic appendages were rare and usually simple, spine-like processes. Occasionally, more complex appendages clustered near dendritic branch points. Axons of these MIN neurons followed a direct trajectory out of the nucleus, although intrageniculate collaterals were occasionally seen. Only one axon in our sample innervated the perigeniculate nucleus. We found no physiological or morphological evidence for more than the one neuronal Y-cell class in laminae 1 and 2 of the MIN.