Motor innervation of intrafusal fibers in rat muscle spindles: Incomplete separation of dynamic and static systems

Abstract

Distributions of 53 motor axons to different types of intrafusal fibers were reconstructed from serial 1-micron-thick transverse sections of 13 poles of spindles in the rat soleus muscle. The mean number of motor axons that innervated a spindle pole was 4.1. Approximately 60% of motor axons lost their myelination prior to or shortly after entry into the periaxial fluid space of spindles. Motor innervation to the juxtaequatorial portion of nuclear bag fibers (particularly the bag1) consisted of groups of short, synaptic contacts that were terminations of thin, unmyelinated axons. In contrast, motor endings on both the bag1 and bag2 fibers were platelike in the polar intracapsular region. Chain fibers had a single midpolar platelike ending. The ratio of motor axons that innervated the bag1 fiber exclusively to axons that innervated bag2 and/or chain fibers was 1:1. However, one-fourth of motor axons coinnervated the dynamic bag1 fiber in conjunction with static bag2 and/or chain fibers. Thus the complete separation of motor control of the dynamic bag1 and static bag2 intrafusal systems observed in cat tenuissimus spindles is neither representative of the pattern of motor innervation in all other species of mammals nor essential to normal spindle function.

Afferent-inherent regulation of myosin heavy chain isoforms in rat muscle spindles

“…The distribution of sensory and motor endings was similar to that described previously for rat spindles. 59,60 Altogether the patterns of MHC expression of 62 intrafusal fibers in 15 spindles were examined.…”

Stability of myosin heavy chain isoforms in selectively denervated adult rat muscle spindles

“…Chain fibers in rat muscle spindles exhibit a simple pattern of motor innervation; typically, one relatively large endplate per pole located in the B or C region of the fiber (Walro and Kucera, 1985;Walro, unpublished data). No small ectopic endplates or multiple endplates were observed on chain fibers.…”

“…Indeed, these are the isoforms expressed by chain fibers in the absence of sensory innervation. Likewise, bag 1 fibers express type 1 MHC and occasionally possess motor endplates in their extracapsular region, which is suggestive of ␤-innervation (Kucera et al, 1978;Walro and Kucera, 1985). They can be coinnervated with type 1 extrafusal fibers by dynamic ␤-motorneurons in the cat (Jami et al, 1982;Kucera and Walro, 1986).…”

Transient expression of a slow-tonic MHC isoform by extrafusal fibers in the developing rat

“…The type of myotube contacted by sensory or motor neurons may also be a factor in expression of slowtonic MHC. Only nuclear bag fibers but not nuclear chain express slow-tonic MHC (Pierobon-Bormioli et al 1980), even though a single primary afferent innervates both bag and chain intrafusal fibers (Walro and Kucera 1985).…”

“…It has therefore been suggested that the afferent innervation may upregulate expression of slowtonic MHC in the myotubes that give rise to nuclear bag 2 and nuclear bag I fibers (Kucera and Walro 1991). However, the relationship between afferent innervation and expression of tonic MHC is not simply cause-and-effect because nuclear chain intrafusal fibers do not express slow-tonic MHC even though a single primary afferent innervates both nuclear bag and nuclear chain fibers (Pierobon-Bormioli et al 1980;Walro and Kucera 1985).…”