The density, distribution and topographical organization of spinocervical tract neurones in the cat

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Abstract
In acute experiments, detailed grids of micro-electrode recordings were made from spinocervical tract (s.c.t.) cells in the lumborsacral cord of anesthetized cats. These grids provided electrophysiological data on the location, distribution, density and somatotopic organization of s.c.t. neurons. In acute experiments lasting up to 48 h, retrograde labeling of s.c.t. cells was carried out by injecting horseradish peroxidase into the lateral cervical nucleus in anesthetized cats. The aim was to flood the nucleus with the enzyme so that all s.c.t. neurons would be labeled in order to provide an independent check on the location, density and distribution data obtained in the electrophysiological experiments. The electrophysiological and the anatomical experiments were sometimes performed on the same animal. The electrophysiological and anatomical results were in excellent agreement. S.c.t. cells are located mainly in ipsilateral laminae III, IV and V. About 25% are in lamina III, 60% in lamina IV and 10% in lamina V. There are a few cells in laminae I, II and VI making up the remaining 5%. There are about 550-800 s.c.t. cells in the lumbosacral enlargement (L4-S2 inclusive) on each side of the cord. Most cells are in L7-S1 where there are 20 to 40 s.c.t. neurons in each millimeter length of cord. Many marginal (lamina I) cells were labeled with the retrograde horseradish perioxidase method and a few cells on the contralateral side in laminae III-V and VII-VIII were also labeled. The marginal cells formed 12-18% of labeled cells on the side of injection and, in addition, a similar absolute number of marginal cells was labeled on the side contralateral to the injection. The possible identity of these neurons is discussed. S.c.t. cells form a sheet of neurons across the dorsal horn. The sheet is organized somatotopically in a way which resembles the representation of the hind limb dermatomes in the dorsal columns. There is a relative enlargement of the L6-S1 dermatomes which encroach upon their neighboring cord segments. The gradient of the map is very steep in the mediolateral direction but gradual in the rostrocaudal direction. S.c.t. cells apparently are arranged so that their receptive fields form longitudinal columns and cells within the columns apparently have overlapping fields.