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A prominent feature of lordosis, the sexual posturing of female rats, is lumbar dorsiflexion (rump elevation). This movement exposes the perineum and permits the male to intromit. Experiments were done to characterize the muscles used in lumbar dorsiflexion, to locate their motoneurons, and to describe some physiologic inputs to these muscles. First, the dorsal and ventral muscles of the back and tail of the female Norway rat were described. Of these muscles, the lumbar epaxial muscles lateral longissimus and the transversospinalis muscles are both trunk muscles and capable of producing dorsiflexion: they are suited for producing the rump elevation of lordosis. To see if they are used during lordosis, they were selectively ablated in hormone-primed female rats that were subsequently tested in mating encounters with male rats. Lordosis was rated as amount of rump elevation. Compared to control animals, animals with ablations of lumbar transversospinalis muscles or lateral longissimus were considerably impaired in their performance of lordosis. Ablations of medial longissimus, a lumbar epaxial muscle which produces proximal tail movements, did not affect lordosis. Motoneurons to lumbar epaxial muscles were localized following intraxonal retrograde transport of horseradish peroxidase (HRP) injected into muscle. Both lateral longissimus and transversospinalis motoneurons were found medially in the ventral horn throughout the lumbar enlargement, and continued anterior to the enlargement. Following HRP injections into more posterior regions of transversospinalis muscles, motoneurons were also found caudal to the enlargement. Medial longissimus motoneurons were found posterior to the lumbar enlargement, ventrolaterally in the ventral horn. Neuronanatomical localization was confirmed by identifying sites in the spinal cord at which low-level stimulation (mainly (LESSTHEQ) 20 (mu)A) produced visible twitches of transversospinalis, medial longissimus or lateral longissimus. Additionally, cells responding antidromically to stimulation of medial longissimus, lateral longissimus or transversospinalis nerves were identified (extracellular recording) and marked by dye deposition. For each muscle, dye spots fell within the region depicted neuronanatomically. Responses of lateral longissimus or medial longissimus nerves to stimulation of ipsilateral lumbosacral dorsal roots, medial medullary reticular formation, vestibular nuclei, or ventromedial hypothalamus were studied in urethane-anesthetized female rats, some of which were also hormone-primed and had shown lordosis to manual stimulation. Stimulation of appropriate dorsal roots, using with two or more shocks, evoked short-latency (re effective shock), probably monosynaptic, compound potentials in medial longissimus or in lateral longissimus nerves. Supraspinal influence was studied mainly by using a condition-test paradigm. Conditioning stimulation of the vestibular nuclei facilitated occurrence of short-latency responses to single dorsal root shocks in medial longissimus nerves (13/14) and in lateral longissimus nerves (5/10). Reticular formation conditioning stimulation also facilitated segmental responses recorded in medial longissimus nerves. Stimulation within the reticular formation or vestibular nuclei could also evoke responses in medial longissimus and lateral longissimus nerves. Stimulation in the area of the ventromedial nucleus of the hypothalamus had no facilitatory effect in these preparations. No differences associated with hormone treatment were apparent. Vestibular nuclei and reticular formation can influence lumbar back muscles and tail muscles as they do other axial muscles. Possibly, either or both pathways to lumbar back muscles could be active during lordosis. |
