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M Hollyday

Publications and source records attributed to M Hollyday.

30 records · Page 2Linked to original sources
PubMed1983

Neural pathway constraints in the motor innervation of the chick hindlimb following dorsoventral rotations of distal limb segments.

Several studies have demonstrated that motor axons can discriminate between dorsally and ventrally derived muscles. In this paper we present evidence that (1) the pathway axons take in the limb constrain their access to either dorsally or ventrally derived muscles, and therefore (2) the axon's ability to discriminate between dorsal and ventral is expressed already at the level of pathway selection into the limb. Surgically manipulated hindlimbs were produced consisting of a normal host thigh connected to a dorsoventrally rotated calf or to rotated and duplicated donor limb segments. The limb rotations were done distal to the level at which axons select a dorsally or ventrally destined pathway through the limb, such that at the level of the rotation, axons in each nerve were confronted with the opposite-from-normal set of muscles. In this situation, the relative influence of pathway availability versus dorsal/ventral muscle recognition could be assessed. The innervation of rotated limb segments was, in all cases, opposite from normal. Motoneurons which normally innervate dorsal muscles innervated ventrally derived muscles that had been rotated into a dorsal position. Likewise, normally ventrally destined axons served dorsal muscles in the rotated segments. Thus, motor axons did not alter their distal path to reach their normal set of muscles. While these results do not rule out intrinsic dorsal/ventral differences between muscles, they do demonstrate that muscle surface recognition is not necessary to account for dorsal/ventral discrimination in the innervation of normal, supernumerary, or duplicated limbs, nor is it sufficient to account for the innervation of rotated limb segments. These results also indicate that pathway guidance cues are an important influence on innervation patterns.

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PubMed1981

Rules of motor innervation in chick embryos with supernumerary limbs.

The positions of motoneurons supplying individual muscles in chick embryos with grafted supernumerary limbs have been identified using retrograde transport of horseradish peroxidase. For a given muscle, motor pool location varied depending on the embryonic origin of the muscle, the position of the limb along the rostrocaudal axis of the body, and the limb's orientation with respect to the body wall. Limb muscles derived from the ventral part of the embryonic premuscle mass were always innervated by medially located motoneurons. Muscles derived from the dorsal portion were consistently innervated by motoneurons in either intermediate or far lateral positions. These relationships were variant with changes in limb position and orientation. These findings indicate that motor axons can recognize and selectively innervate muscles derived from either dorsal or ventral muscle mass. In addition, the spinal nerves innervating each limb were identified. The type of limb plexus (e.g., crural, sciatic, or wing) and the peripheral branching patterns of the nerves within the limbs were also studied and found to be controlled by the limbs. The rostrocaudal variation in motor pool position similarly depended on the position and orientation of the limb. This rostrocaudal variation in motor pool position can be explained by the limb's ability to determine axonal outgrowth pathways and hence to constrain the possible target choices of outgrowing axons. The process of limb innervation involves interactions between motoneuron axons having intrinsic differences or specificities, and the character of the local environment of the limb into which they grow.

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PubMed1980

Organization of motor pools in the chick lumbar lateral motor column.

Motor pool positions for individual leg muscles were mapped in hatched chicks using intramuscular injections of HRP. Several leg muscles were also mapped in stage 38 (12-day) embryos. The results indicate that an adult map is formed by stage 38. The adult motor pool map can be viewed as being composed of two maps, one for muscles derived from the embryonic ventral muscle mass, the other for muscles derived from the dorsal mass. Each of these maps is a continuous, although distorted, representation of muscle precursor position on the original sheets of dorsal and ventral muscle mass. The developmental implications of these findings are discussed.

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PubMed1977

An autoradiographic study of the formation of the lateral motor column in the chick embryo.

An autoradiographic determination of the time of origin of the lateral motor columns (LMC) of the chick embryo has been made. The first motor neurons of the brachial LMC are born at stage 15; the earliest birthdates of lumbar LMC neurons are at stage 17. At least 95% of the motor neurons of both brachial and lumbar columns are produced by stage 23 (4 days). The remaining 5% of the motor neurons are produced during the next two days. A clear rostrocaudal gradient of motor neuron production is seen beoth between the brachial and lumbar LMCs and within the LMCs themselves. The LMCs are assembled in a mediolateral sequence: the early-born motor neurons settle medially, the later-born motor neurons settle more laterally. Observations were made of other large early-born neurons which remain permanently in the dorsal gray of the spinal cord.

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PubMed1977

Localization of motor neuron pools supplying identified muscles in normal and supernumerary legs of chick embryo.

Neuromuscular specificity has been investigated in chick embryos with a grafted supernumerary leg. The nerves of the lumbar plexus are divided between the two legs so that rostral nerves innervate the grafted leg and the caudal nerves supply the host's original leg. The basic topographic organization of the histologically definable motor neuron clusters of the lateral motor columns remains unchanged by the addition of a supernumerary leg. Intramuscular injections of identified leg muscles have been used to map the intraspinal location of specific motor pools in stage 38 (12-day) embryos. In the normal embryo, the gastrocnemius muscle is innervated by neurons in a central dorsal cluster of motor neurons in segments 26-29. In six experimental cases, the motor neurons supplying the gastrocnemius muscle of a rostrally placed grafted leg were consistently located in a specific medial cluster of neurons in segments 23-25. Motor neurons in this location never normally innervate a gastrocnemius muscle, even in the very young embryos during the period of naturally occurring cell death. This observation of a systematic mismatch between a particular motor cluster and an abnormally innervated muscle indicates the operation of a selective developmental process. A hierarchy of selective chemoaffinities may best explain our experimental results.

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PubMed1976

Reduction of the naturally occurring motor neuron loss by enlargement of the periphery.

Motor hyperplasia following the enlargement of the periphery by implantation of a supernumerary leg is not due to "remote control" of proliferation, as shown by motor neuron counts in 6-day chick embryos. We have tested the alternative hypothesis that we are dealing with reduction of the naturally occurring cell death. In normal development, the lumbar lateral motor column (l.m.c.) undergoes motor neuron degeneration resulting in a cell loss of at least 40%, which occurs between six and one-half and nine and one-half days. Following transplantation of supernumerary legs, cases selected for vigorous motility showed a numerical difference between experimental and contralateral (control) sides amounting to +11.0% to +27.5%. The transplants were innervated by varying combinations of thoracic and rostral lumbar nerves. We interpret our data in terms of survival of motor neurons which normally would have failed in a competition at the periphery but which were sustained by the enlarged peripheral fields. Our data do not permit a decision between the two alternatives: competition for synaptic sites or for a trophic agent. The surviving motor neurons are not limited to the rostral segments of the motor column but in most instances distributed along its entire rostro-caudal extent, implying a redistribution of all l.m.c. axons. The term "hyperplasia" is no longer appropriate for the phenomenon under consideration and should be replaced by the term "hypothanasia.""

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PubMed1975

Area specific reflexes from normal and supernumerary hindlimbs of Xenopus laevis.

Two area specific reflexes elicited by natural stimulation of different regions of the hindlimbs of Xenopus laevis have been identified. Light or intense mechanical stimulation of the foot evokes reflex activity in the ipsilateral knee flexor nerve; moderate pressure applied to the calf evokes reflex activity predominantly in the ipsilateral knee extensor nerve. The reflex responses have been recorded electrophysiologically to overcome the limitations of behavioral observations for determining the presence of activity in particular muscles. Normal area specific reflexes are elicited in the normal ipsilateral hindlimb by stimulation of grafted supernumerary hindlimbs innervated either by hindlimb (lumbar) or by non-limb (thoracic) spinal cord segments. The area specific reflexes can be elicited only if the limb is grafted to a host younger than stage 54-55 of Nieuwkoop and Faber ('56), the stage at which reflex movements are first observed behaviorally. Abnormal reflex responses are evoked by stimulation of supernumerary limbs innervated by either thoracic or lumbar segments when the limb buds are grafted to older larvae. Supernumerary forelimbs grafted at early stages and innervated by either thoracic or lumbar spinal cord segments generally fail to elicit area specific reflex responses in the normal hindlimb. Single-unit recordings of afferent fibers supplying the normal and supernumerary hindlimbs show that each limb receives a separate nerve supply. No evidence for branched afferent fibers has been found. The implications of these results for theories of neuronal specification are discussed, particularly the hypothesis that peripheral tissues are able to specify the central actions of afferent fibers that innervate them.

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