The ultrastructure of the external basilar region (EBR) of the spinal cord has been investigated in normal cats and after ablation of the contralateral sensorimotor cortex. This region (lateral part of lamina V of Rexed) is the main site of termination of descending pyramidal fibres.
The EBR contains neurons with light and dark cytoplasm and correspondingly light and dark dendrites. Axon terminals in the EBR can be divided into four types on the basis of the following structural criteria: (1) spheroid synapt vesicles, (2) flattened synaptic vesicles, (3) spheroid clear vesicles and large dense-core vesicles, (4) electron-dense plasma matrix and numerous spheroid vesicles. Types (1) and (2) prevail, forming mainly axondendritic contacts, generally with thin protrusions and spine-like appendages of the dendrites. A few larger synaptic arrangements of glomerular structure were observed.
Signs of terminal degeneration can be recognized in the EBR as early as 30 hours after cortical ablation. The structural elements of these terminals are fused into a compact electron-dense material corresponding to the known “dark” type of degeneration. Three to five days after the lesions a second set of degenerating terminals become recognizable, in which the lysis of cytoplasmic elements is prevalent. This slower modus of degeneration might be distinguished as the “light” type. Degenerating terminals are located mainly on dendrites of the EBR neurons and are of small size. The shape of the synaptic vesicles can be recognized in some terminals undergoing degeneration according to the “light” type; they are, as a rule, of flattened type. Synaptic terminals do not degenerate in the glomerular complexes.
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Dyachkova, L.N., Kostyuk, P.G. & Pogorelaya, N.C. An electron microscopic analysis of pyramidal tract terminations in the spinal cord of the cat. Exp Brain Res 12, 105–119 (1971). https://doi.org/10.1007/BF00234309
- Pyramidal tract
- Spinal cord
- External basilar region (EBR)
- Terminal degeneration