Golgi studies of the neurons in layer I of the dorsal horn of the medulla (trigeminal nucleus caudalis)

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Abstract
Two kinds of pyramidal neuro ns and two kinds of multipolar neurons have been identified in layer I in the adult cat on the basis of thier dendritic morphology. The spiny pyramid emits an apical dendrite which beings dividing within 50 μm of the cell body and gives rise to an extensive dendritic arbor in which some second and third order branches extend for over 100 μm. The primary branches of the basal dendrites divide into secondary branches within 20 μm of the cell body. At each of these divisions, one daughter secondary branch will turn 180° and course in the direction of the apical dendrite. It then undergoes further branching and becomes intermingled in the apical dendritic arbor. These recurrent basal dendrites and the apical dendrites are charcterized by numerous long‐necked spines. The other secondary basal branch either courses at right angles to the apical dendrite or droops basally away from it. Many of these basal dendrites are not as robust and extensive as the other dendritic branches. They may end without further branching or when they do branch they tend to be thinner and shorter with some showing signs of stunting. Smooth pyramids differ from the spiny pyramids in several respects. Their dendritic arbors contain fewer, more widely spaced spines and are more expansive than those of the spiny pyramids. While they still send branches into the apical arbors many branches run basally away from the cell body for considerable distances to give the entire dendritic field a more circular appearance. The compact multipolar neuron is found in the inner half of layer I. The long axis of its compact dendritic arbor (∼ 50 μm wide) is oriented in the rostrocaudal axis of the layer. Most of its dendritic branches lie to one side of the cell body squeezed between the axon bundles of the inner half of layer I. The loose multipolar neuron in found throughout the layer. Its loose dendritic arbor is at least 200 μm wide and, like the spiny pyramid, gives rise to several dendrites which terminate abruptly with little branching and without much change in diameter. The dendrites of all four layer I neurons are confined within layer I and its extensions into the spinal V tract. These four layer I neurons are considered to be Golgi type I projection neurons on the basis of the morphology of their initial axonal segment. Golgi type II inteneurons are not found in layer I.

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