The Development of Fusimotor Innervation in the Cat: An Ultrastructural Study

Sutherland, Fiona I (1994) The Development of Fusimotor Innervation in the Cat: An Ultrastructural Study. PhD thesis, University of Glasgow.

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1. This study was carried out to investigate if the degree of selectivity of motor innervation to hind limb muscle spindles which occurs in the adult cat is present in the post-natal kitten. Selective axons innervate one type of intrafusal fibre only, while non-selective axons innervate more than one type in the same spindle (Boyd and Gladden 1985). 2. Tenuissimus and peroneal muscle spindle poles from kittens of 50df (n=7, 1 dpn (n=14), 12dpn (n=13), 21 dpn (n=12) and 28dpn (n=13) were serially sectioned for light and electron microscopy. Ultrathin sections were taken every 2-5 pin and semi-thin (1pm) sections collected in-between. If it was suspected that information was lost in the 1pm thick sections these were re-embedded and subsequently ultrathin sectioned. Electron micrographs were taken of the ultrathin sections and from these the spindles were reconstructed so that the lengths of primary and sensory endings, and capsule and intrafusal fibre lengths were estimated. Numbers and degree of myelination of the motor axons were noted. The distribution of motor axons to the intrafusal fibres was determined by following both myelinated and unmyelinated axons through serial sections: this has not been possible previously in such young material. Numbers, lengths, types and distance from the equator of motor endings were calculated. 3. Results of each of the ages studied were compared statistically for each feature studied in order to assess any significant change during this period of development. 4. Bag fibres were distinguished from chain fibres by size and arrangement of central nuclei under the primary sensory endings. The bag fibres were approximately twice the diameter of chain fibres and had their central nuclei arranged in a group whereas the chain fibre nuclei were arranged in a line. Many intrafusal fibres were found to decrease in cross-sectional area in the equatorial region. 5. Bagl fibres were distinguished from bag2 fibres by a) the grouping of the bag2 fibre with chain fibres in the equatorial region, and b) the more granular appearance of the bag2 fibre (due to more mitochondria and glycogen inclusions) than the bagl fibre. Bag fibre cross-sectional area measurements were not consistently different from each other. 6. A formula was constructed in order to determine when any chain fibres should be included as long chain fibres. This was necessary because although the majority of chain fibres were shorter than bag fibres (typical chain fibres), long chain fibres were also found; these can be of a typical chain or bag fibre diameter. Since the total length of spindles increased with age, the absolute length of chain fibres could not be used to discriminate between long and typical chains. 7. Endings were identified as sensory when the muscle basement membrane encompassed the axon terminal. Motor endings were distinguished from sensory endings by the presence of the basement membrane between the axon terminal and muscle surface. Additional features which confirmed the identity were the variety of types of axon terminal vesicle (round, flat, dense cored) found in sensory endings and the position of the endings along the intrafusal fibre. Occasionally the basement membrane was not present between the axon terminals and muscle membrane in motor endings of the 50df spindles. 8. There were 119 motor axons in this study of which 77 were unmyelinated. Of the 59 total spindle poles, 36 were from tenuissimus muscles and 23 from peroneal muscles. Four tandem spindles were present in this material. There were 55 bagl fibre poles, 66 bag2 fibre poles, 240 typical chain poles and 20 long chain poles of either large or thin diameter. In total there were 586 motor endings. 9. The length of the primary and secondary sensory endings, capsule and intrafusal fibres increased with age. 10. Motor axons. It was not possible to categorise the motor axons as gamma or beta. The numbers of motor axons supplying spindle poles was found to be 2-3 for each age group. Numbers of motor axons supplying individual intrafusal fibre poles were, 1-2 for bag fibres and 0-1 for chain fibres, both typical and long. These figures are in the same range as the numbers of ? axons which supply individual fibres of adult spindles. Polyneuronal innervation of the type where many axons supply a single motor endplate therefore does not occur during the period studied, but does in rat spindles, even after birth (Kucera, Walro and Reichler 1988). The distribution of motor axons to the intrafusal fibres, even as late in development as 28dpn, was less selective than in adult spindles. Myelination of motor axons was 92% complete by 28dpn. The selectivity did not depend on myelination prior to 21 dpn but myelinated axons had a smaller proportion of inappropriate innervation by 28dpn. All bag2 fibres, 90% of bagl fibres and 76% of typical chain fibres had some motor innervation by 28dpn. These values were very similar to adult innervation values. The innervation of long chain fibres was more variable in the young but they tended to be innervated when present, whereas only 25% are innervated in the adult. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Margaret H Gladden
Keywords: Physiology, Neurosciences
Date of Award: 1994
Depositing User: Enlighten Team
Unique ID: glathesis:1994-75667
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Dec 2019 09:15
Last Modified: 19 Dec 2019 09:15

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