Supraspinal and segmental input to lumbar epaxial motoneurons in the rat.

Inputs to medial longissimus (ML) and lateral longissimus (LL) motoneurons were studied in urethane or urethane-chloralose anesthetized rats by recording from ML and LL nerves while stimulating ipsilateral lumbosacral dorsal roots, medial medullary reticular formation (RF), vestibular nuclei (VN), dorsal midbrain (MDBR), or ventromedial hypothalamus (VMH). Stimulation of appropriate dorsal roots produced short-latency responses (1.5-3.0 ms) in nerves to medial longissimus or lateral longissimus. The connections underlying these responses, which could be monosynaptic, are weak, since generally two or more stimuli were necessary for a response to occur. Short-latency LL nerve responses required more dorsal root stimuli than did ML nerve responses and stable LL responses sometimes could not be obtained, suggesting that segmental reflexes to a back muscle (LL) could be weaker than those to a proximal tail muscle (ML). Trains of conditioning stimuli delivered to the RF, VN, and MDBR facilitated segmental responses in ML nerves or LL nerves. Temporal profiles of facilitation of ML differed for the three regions. On one extreme, the facilitation produced by RF conditioning required few stimuli (median, 3 shocks) and peak facilitation occurred at short condition-test intervals (median, 1.5 ms). On the other extreme, facilitation produced by MDBR conditioning required long trains (median, 14 stimuli) and peak facilitation occurred at longer condition-test intervals (median, 10 ms). Stimulation within the VMH never facilitated ML or LL nerve activity. These results demonstrate excitatory connections from reticular formation, vestibular nuclei and the dorsal midbrain to medial longissimus and lateral longissimus. Such connections could be involved in behaviors mediated by midbrain, and in postural regulation through brain stem control of axial musculature. Motoneuron cell bodies for LL, ML and lumbar transversospinalis (TS) muscles were localized by ejecting dye at sites where unitary antidromic responses to muscle nerve stimulation were recorded extracellularly. ML cells were found ventrolaterally in the L6-S1 ventral horn. LL and TS cells were found medially in the ventral horn of the lumbar enlargement.