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This Article Full Text Full Text (PDF) All Versions of this Article: 582/2/525    most recent jphysiol.2007.130120v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Kim, G. H. Articles by Kanda, K. Search for Related Content PubMed PubMed Citation Articles by Kim, G. H. Articles by Kanda, K. Related Collections Neuroscience

¹ Graduate School of Human Sciences, Waseda University, Tokorozawa, 359-1192, Japan
² Motor and Autonomic Nervous System Integration Research Group, Tokyo Metropolitan Institute of Gerontology, Tokyo 173-0015, Japan

Age-related physiological and morphological changes of muscle spindles were examined in rats (male Fischer 344/DuCrj: young, 4–13 months; middle-aged, 20–22 months; old, 28–31 months). Single afferent discharges of the muscle spindles in gastrocnemius muscles were recorded from a finely split dorsal root during ramp-and-hold (amplitude, 2.0 mm; velocity, 2–20 mm s^–1) or sinusoidal stretch (amplitude, 0.05–1.0 mm; frequency, 0.5–2 Hz). Respective conduction velocities (CVs) were then measured. After electrophysiological experimentation, the muscles were dissected. The silver-impregnated muscle spindles were teased and then analysed using a light microscope. The CV and dynamic response to ramp-and-hold stretch of many endings were widely overlapped in old rats because of the decreased CV and dynamic response of primary endings. Many units in old rats showed slowing of discharge during the release phase under ramp-and-hold stretch and continuous discharge under sinusoidal stretch, similarly to secondary endings in young and middle-aged rats. Morphological studies revealed that primary endings of aged rat muscle spindles were less spiral or non-spiral in appearance, but secondary endings appeared unchanged. These results suggest first that primary muscle spindles in old rats are indistinguishable from secondary endings when determined solely by previously used physiological criteria. Secondly, these physiological results reflect drastic age-related morphological changes in spindle primary endings.

(Received 8 February 2007; accepted after revision 3 May 2007; first published online 10 May 2007)
Corresponding author G. H. Kim: Graduate School of Human Sciences, Waseda University, Tokorozawa, 359-1192, Japan. Email: jihee{at}moegi.waseda.jp