Abstract
Introduction: The purpose of this study is to clarify the projection to forelimb motoneurones from medulla oblongata in adult mice.Methods: The results were obtained from 15 mice (BALB/C) with age of 2-4 months. The animals were anaesthetized with a mixture of midazolam and fentanyl. Corticofugal fibres were stimulated in the ipsi- and contralateral pyramids at 0.5mm rostral to the obex. Results: The termination of the corticospinal fibres in C7 was assessed by recording the extracellular field potentials. The area with negative field potentials is covered the dorsal horn, lamina V, lamina VI and the medial part of lamina VII. Intracellular recording have been made from 30 forelimb motoneurones in the C6-C7 segments. The pyramidal EPSPs had latencies of 3.5 to 5.8 ms, which suggest minimally a trisynaptic linkage. Small pyramidal EPSPs could still be evoked after corticospinal transection in the rostral C2 segment. The EPSP amplitude increased almost twice 0.5 to 1.0 mm dorsal to the pyramid (the reticular formation) and the latency shortened to 2.3 ms. In the contralateral MLF, about 1.5 mm dorsal to the pyramid, the EPSP amplitude remained rather unchanged, but the latency decreased to 1.3 ms. Discussion: The major termination of the corticospinal fibres in the mouse is in the dorsal horn and medially in the intermediate layers, but not laterally in the intermediate layers and not in the motor nuclei. A reticulospinal pathway may provide for the fast and strong excitatory input to forelimb motoneurones in the mouse. [Jpn J Physiol 55 Suppl:S173 (2005)]
