Timothy Cope (Committee Member), David Ladle (Advisor), Mark Rich (Committee Member)
Master of Science (MS)
Muscle spindles (MS) and Golgi tendon organs (GTOs) are encapsulated, stretch-activated sensory receptors housed within skeletal muscles, yet their functional contributions to proprioceptive monitoring of limb movements are quite different. For example, MS are responsible for detecting stretch of muscle fibers, and GTOs provide feedback regarding muscle tension during contraction. Peripheral axons extending from proprioceptive sensory neurons (PSNs) in the dorsal root ganglia supply MS and GTOs. MS are innervated by Group Ia and II sensory fiber endings, while GTOs are innervated by a single Group Ib sensory fiber. The developmental processes guiding axons from PSNs to their targets and the maturation of distinct functional identities in skeletal muscle, however, remain largely unknown. To study PSNs and their peripheral endings more closely, we took advantage of existing transgenic mouse models using Cre-lox recombination technology in which all proprioceptive afferents are labeled with a red fluorescent protein, tdTomato (Parvalbumin-Cre/+; Rosa-CAG-LSL-tdTomato-WPRE). Using this model, we present results from a quantitative confocal microscopy analysis of proprioceptive receptors and their associated afferent axons taken from whole-mount neonatal mouse soleus muscle preparations. We focused a portion of our analysis on the unique extramuscular bifurcation of the soleus nerve, which gives rise to stereotypic thin and thick nerve branches. Our results show that the proprioceptor-enriched thin branch of the soleus nerve always contains Ib afferents and usually a mix of Ia and II spindle afferents, but occasionally lacks MS afferents altogether or contains MS afferents of only one type. Heterogeneity in the thin branch may provide a useful tool in analysis of MS and GTO maturation during embryonic and early postnatal development. Additionally, we optimized a technique for retrieving single fluorescently labeled neurons from wild-type and PV-Cre/+; Rosa-tdT/+ dorsal root ganglia and utilized qRT-PCR to screen those neurons for expression of established and putative markers of PSNs.
Department or Program
Department of Neuroscience, Cell Biology & Physiology
Year Degree Awarded
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