The formation of appropriate central and peripheral connexions by foreign sensory neurones of the bullfrog

The making of a proprioceptor: a tale of two identities

Proprioception, the sense of body position in space, has a critical role in the control of posture and movement. Aside from skin and joint receptors, the main sources of proprioceptive information in tetrapods are mechanoreceptive end organs in skeletal muscle: muscle spindles (MSs) and Golgi tendon organs (GTOs). The sensory neurons that innervate these receptors are divided into subtypes that detect discrete aspects of sensory information from muscles with different biomechanical functions. Despite the importance of proprioceptive neurons in motor control, the developmental mechanisms that control the acquisition of their distinct functional properties and positional identity are not yet clear. In this review, we discuss recent findings on the development of mouse proprioceptor subtypes and challenges in defining them at the molecular and functional level.

Fish Scales Dictate the Pattern of Adult Skin Innervation and Vascularization

As animals mature from embryonic to adult stages, the skin grows and acquires specialized appendages, like hairs, feathers, and scales. How cutaneous blood vessels and sensory axons adapt to these dramatic changes is poorly understood. By characterizing skin maturation in zebrafish, we discovered that sensory axons are delivered to the adult epidermis in organized nerves patterned by features in bony scales. These nerves associate with blood vessels and osteoblasts above scales. Osteoblasts create paths in scales that independently guide nerves and blood vessels during both development and regeneration. By preventing scale regeneration and examining mutants lacking scales, we found that scales recruit, organize, and polarize axons and blood vessels to evenly distribute them in the skin. These studies uncover mechanisms for achieving comprehensive innervation and vascularization of the adult skin and suggest that scales coordinate a metamorphosis-like transformation of the skin with sensory axon and vascular remodeling.

Anatomical coupling of sensory and motor nerve trajectory via axon tracking

It is a long-standing question how developing motor and sensory neuron projections cooperatively form a common principal grid of peripheral nerve pathways relaying behavioral outputs and somatosensory inputs. Here, we explored this issue through targeted cell lineage and gene manipulation in mouse, combined with in vitro live axon imaging. In the absence of motor projections, dorsal (epaxial) and ventral (hypaxial) sensory projections form in a randomized manner, while removal of EphA3/4 receptor tyrosine kinases expressed by epaxial motor axons triggers selective failure to form epaxial sensory projections. EphA3/4 act non-cell-autonomously by inducing sensory axons to track along preformed epaxial motor projections. This involves cognate ephrin-A proteins on sensory axons but is independent from EphA3/4 signaling in motor axons proper. Assembly of peripheral nerve pathways thus involves motor axon subtype-specific signals that couple sensory projections to discrete motor pathways.

Self-avoidance and tiling: Mechanisms of dendrite and axon spacing

The spatial pattern of branches within axonal or dendritic arbors and the relative arrangement of neighboring arbors with respect to one another impact a neuron's potential connectivity. Although arbors can adopt diverse branching patterns to suit their functions, evenly spread branches that avoid clumping or overlap are a common feature of many axonal and dendritic arbors. The degree of overlap between neighboring arbors innervating a surface is also characteristic within particular neuron types. The arbors of some populations of neurons innervate a target with a comprehensive and nonoverlapping "tiled" arrangement, whereas those of others show substantial territory overlap. This review focuses on cellular and molecular studies that have provided insight into the regulation of spatial arrangements of neurite branches within and between arbors. These studies have revealed principles that govern arbor arrangements in dendrites and axons in both vertebrates and invertebrates. Diverse molecular mechanisms controlling the spatial patterning of sister branches and neighboring arbors have begun to be elucidated.

Repulsive interactions shape the morphologies and functional arrangement of zebrafish peripheral sensory arbors

BACKGROUND

Trigeminal sensory neurons detect thermal and mechanical stimuli in the skin through their elaborately arborized peripheral axons. We investigated the developmental mechanisms that determine the size and shape of individual trigeminal arbors in zebrafish and analyzed how these interactions affect the functional organization of the peripheral sensory system.

RESULTS

Time-lapse imaging indicated that direct repulsion between growing axons restricts arbor territories. Removal of one trigeminal ganglion allowed axons of the contralateral ganglion to cross the midline, and removal of both resulted in the expansion of spinal cord sensory neuron arbors. Generation of embryos with single, isolated sensory neurons resulted in axon arbors that possessed a vast capacity for growth and expanded to encompass the entire head. Embryos in which arbors were allowed to aberrantly cross the midline were unable to respond in a spatially appropriate way to mechanical stimuli.

CONCLUSIONS

Direct repulsive interactions between developing trigeminal and spinal cord sensory axon arbors determine sensory neuron organization and control the shapes and sizes of individual arbors. This spatial organization is crucial for sensing the location of objects in the environment. Thus, a combination of undirected growth and mutual repulsion results in the formation of a functionally organized system of peripheral sensory arbors.

Target-independent specification of proprioceptive sensory neurons

Previous studies in the chick embryo have shown that sensory neurons fail to innervate muscle in the absence of motor neurons. Instead, motor neuron deletion causes more sensory axons to project to the skin. We used this experimental paradigm to determine when sensory neurons are specified to become proprioceptive afferents. Experimental embryos were treated with either saline or exogenous neurotrophin-3 (NT-3) to promote the survival of proprioceptive afferents. In saline-treated embryos, motor neuron deletion caused an increase in sensory neuron apoptosis on the deleted side, an effect reversed by NT3. Motor neuron deletion also eliminated the sartorious muscle nerve, as previously reported. In NT3-treated embryos, this altered nerve pattern was accompanied by the enlargement of the adjacent cutaneous nerve. These embryos were further analyzed by using immunohistochemistry for trkC (a receptor for NT3) retrograde and transganglionic labeling. Our results show that, following motor neuron deletion, more trkC+ afferents project in cutaneous nerves on the deleted side of NT3-treated embryos. Transganglionic labeling demonstrated that at least some of these neurons made spinal projections that are typical of proprioceptive afferents. These results therefore indicate that the proprioceptive phenotype is specified prior to target innervation and that these neurons can retain their identity despite projecting to inappropriate (cutaneous) targets.

Excess target-derived neurotrophin-3 alters the segmental innervation of the skin

It is thought that dermatomes are established during development as a result of competition between afferents of neighbouring segments. Mice that overexpress neurotrophins in the skin provide an interesting model to test this hypothesis, as they possess increased numbers of sensory neurons, and display hyperinnervation of the skin. When dermatomal boundaries were mapped in adult mice, it was found that those in nerve growth factor and brain-derived neurotrophic factor overexpressers were indistinguishable from wild-type animals but that overlap between adjacent segments was greatly reduced in neurotrophin-3 (NT-3) overexpressers. However, dermatomes in heterozygous NT-3 knockout mice displayed no more overlap than wild-types. In order to quantify differences across strains, innervation territories of thoracic dorsal cutaneous nerves were mapped and measured in adult mice. Overlap between adjacent dorsal cutaneous nerves was normal in nerve growth factor overexpressing mice, but much reduced in NT-3 overexpressers. However, this restriction was not reflected in the central projection of the dorsal cutaneous nerve, creating a mismatch between peripheral and central projections. Dorsal cutaneous nerve territories were also mapped in neonatal mice aged postnatal day 7-8. In neonates, nerve territories of NT-3 overexpressers overlapped less than wild-types, but in neonates of both strains the amount of overlap was much greater than in the adult. These results indicate that substantial separation of dermatomes occurs postnatally, and that excess NT-3 enhances this process, resulting in more restricted dermatomes. It may exert its effects either by enhancing competition, or by direct effects on the stability and formation of sensory endings in the skin.

Dynamic expression of neurotrophin receptors during sensory neuron genesis and differentiation

To identify potential functions for neurotrophins during sensory neuron genesis and differentiation, we determined the temporal and spatial protein expression patterns of neurotrophin receptors throughout the process of sensory neurogenesis in the dorsal root ganglia (DRG). We show that neurotrophin receptors are expressed early, being first detected on subsets of migrating neural crest cells, and that trkC is among the earliest markers of neural lineage specification. In the immature DRG, we find that both trkC and p75(NTR) are expressed on subsets of dividing progenitor cells in vivo. Furthermore, our data directly reveal distinct patterns of trk receptor expression by individual sensory neurons from the time of their inception with all early arising cells initially being trkC(+), some subsets of whom also coexpress either trkA or trkB or both. As sensory neurons innervate their targets and establish their mature identities, the spectrum of trk receptors expressed by individual neurons is altered. The stereotyped trk receptor expression profiles identified here may potentially correspond to distinct lineages of sensory neurons. These data, in conjunction with other studies, argue for multiple functions for neurotrophins during the process of sensory neuron differentiation, including effects on both neural crest and DRG mitotically active progenitor cells, in addition to possibly influencing the establishment of sensory neuron identity.

Neurotrophin-3 promotes the survival of a limited subpopulation of cutaneous sensory neurons

In the chick embryo, exogenous neurotrophin-3 (NT3) is sufficient to promote the differentiation of proprioceptive afferents even in the absence of limb muscle targets. To determine if NT3 can promote the differentiation of this phenotype in afferents with cutaneous targets, we analyzed the effects of chronic NT3 on cutaneous and muscle sensory neurons that express trkC, a receptor for NT3. In normal embryos, retrograde labeling and immunohistochemistry showed that about 75% of large-diameter muscle afferents express trkC, whereas only about 7% of large-diameter cutaneous afferents express this protein. After chronic treatment with NT3 during the cell death period, both populations of trkC(+) neurons were increased approximately twofold. Because this treatment is known to block cell death in sensory neurons, these results indicate that NT3 can promote the survival of both proprioceptive afferents and cutaneous afferents. To examine the phenotype of the cutaneous afferents rescued by NT3, we analyzed their projections and connections using transganglionic labeling and electrophysiological recording. The results indicate that exogenous NT3 neither altered the pattern of spinal projections nor caused cutaneous afferents to form monosynaptic connections with motor neurons. These results demonstrate that selective cell death does not contribute to the modality-specific pattern of spinal innervation and suggest that proprioceptive afferents may innervate muscle selectively.

Development of specific connectivity between premotor neurons and motoneurons in the brain stem and spinal cord

Astounding progress has been made during the past decade in understanding the general principles governing the development of the nervous system. An area of prime physiological interest that is being elucidated is how the neural circuitry that governs movement is established. The concerted application of molecular biological, anatomical, and electrophysiological techniques to this problem is yielding gratifying insight into how motoneuron, interneuron, and sensory neuron identities are determined, how these different neuron types establish specific axonal projections, and how they recognize and synapse upon each other in patterns that enable the nervous system to exercise precise control over skeletal musculature. This review is an attempt to convey to the physiologist some of the exciting discoveries that have been made, within a context that is intended to link molecular mechanism to behavioral realization. The focus is restricted to the development of monosynaptic connections onto skeletal motoneurons. Principal topics include the inductive mechanisms that pattern the placement and differentiation of motoneurons, Ia sensory afferents, and premotor interneurons; the molecular guidance mechanisms that pattern the projection of premotor axons in the brain stem and spinal cord; and the precision with which initial synaptic connections onto motoneurons are established, with emphasis on the relative roles played by cellular recognition versus electrical activity. It is hoped that this review will provide a guide to understanding both the existing literature and the advances that await this rapidly developing topic.

Peripheral specification of Ia synaptic input to motoneurons innervating foreign target muscles

Muscle sensory neurons, called Ia afferents, make monosynaptic connections with functionally related sets of motoneurons in the spinal cord. Previous work has suggested that peripheral target muscles play a major role in determining the central connections of Ia afferents with motoneurons. Here, we ask whether motoneurons can also be influenced by their target muscles in terms of the monosynaptic input they receive from Ia afferents, by transplanting thoracic motoneurons into the lumbosacral spinal cord so that they innervate foreign muscles. Three or four segments of thoracic neural tube from stage 14-15 chicken embryos were transplanted to the lumbosacral region of stage 16-17 embryos, and electrophysiological recordings were made from transplanted motoneurons after the embryos had reached stage 38-40. Transplanted thoracic motoneurons innervated limb muscles and received monosynaptic inputs from Ia afferents. These connections were not random: Most of the connections were formed between Ia afferents and motoneurons projecting to the same muscle (homonymous connections). Few aberrant connections were found although the anatomical distribution of afferents in the transplant indicated that they had ample opportunity to contact inappropriate motoneurons. We conclude that although peripheral target cues are not sufficient to respecify an already committed motoneuron (turn a thoracic motoneuron into a lumbosacral motoneuron), they do provide sufficient information for Ia afferent input to be functionally correct.

Extent of nociceptive dermatomes in adult rats is not primarily maintained by axonal competition

Nociceptive innervation territories of individual peripheral and spinal nerves in the skin of the rat hind paw were investigated. In addition, the hypothesis that competitive interactions among the axons from adjacent dorsal root ganglia (DRG) play an important role in maintenance of dermatomal extent in adult animals was tested. The area of innervation territories of individual spinal and peripheral nerves was determined by nociceptive pinch test of the skin after extirpation of adjacent DRGs or transection of adjacent peripheral nerves, respectively. Positions of nociceptive dermatomes and innervation territories of peripheral nerves were similar to the territories innervated by the C-fibers described earlier by dye extravasation technique. In contrast, our results convincingly demonstrated substantial overlap of nociceptive (probably A delta) fibers from adjacent dermatomes in which the autonomous innervation areas were only about one-half of the maximal areas. Nociceptive territories of peripheral nerves overlapped, too. Accordingly, we could find no autonomous innervation area of the sural nerve. Two weeks after extirpation of adjacent DRGs, the area of each of the isolated dermatomes L3, L4, and L5 increased only by about 10%, and it did not change detectably during the next 6 months. The results of our study (a) support the view that innervation fields supplied by the nociceptive (probably A delta) fibers are greater and display more overlap than those supplied by the C-fibers of the same nerve and (b) suggest that axonal competition for innervation territory is not decisive for maintenance of dermatomal borders in the adult rat.

Neurotrophin-3 promotes the differentiation of muscle spindle afferents in the absence of peripheral targets

The neurons of the dorsal root ganglia (DRG) that supply muscle spindles require target-derived factors for survival. One necessary factor for these neurons is neurotrophin-3 (NT3). To determine whether NT3 can promote the survival of these neurons in the absence of other target-derived factors, we analyzed the effects of exogenous NT3 after early limb bud deletion in the chick. In control embryos, limb bud deletion eliminated approximately 90% of the trkC-positive (trkC+) neurons in lumbar DRG on the deleted side. In addition, the deletion led to a dramatic loss of collateral sensory projections to motoneurons. Exogenous NT3 restored a normal population of trkC+ neurons in lumbar DRG on the deleted side and increased the number of trkC+ neurons in DRG with normal targets (contralateral lumbar and thoracic). The effect was highly selective; NT3 increased the number of trkC+ neurons without significantly changing the number of either trkA+ or trkB+ neurons. The effect of NT3 was attributable to the rescue of DRG neurons from cell death, because exogenous NT3 reduced the number of pyknotic nuclei without significantly altering proliferation. Analysis of spinal projections showed further that many of the trkC+ neurons rescued by NT3 projected to the ventral spinal cord. These neurons thus had central projections characteristic of muscle spindle afferents. Together, our results indicate that NT3 signaling is both necessary and sufficient for the development of the proprioceptive phenotype, even in the absence of other signals from limb muscle.

Neurotrophin-3 promotes the differentiation of muscle spindle afferents in the absence of peripheral targets

The neurons of the dorsal root ganglia (DRG) that supply muscle spindles require target-derived factors for survival. One necessary factor for these neurons is neurotrophin-3 (NT3). To determine whether NT3 can promote the survival of these neurons in the absence of other target-derived factors, we analyzed the effects of exogenous NT3 after early limb bud deletion in the chick. In control embryos, limb bud deletion eliminated approximately 90% of the trkC-positive (trkC+) neurons in lumbar DRG on the deleted side. In addition, the deletion led to a dramatic loss of collateral sensory projections to motoneurons. Exogenous NT3 restored a normal population of trkC+ neurons in lumbar DRG on the deleted side and increased the number of trkC+ neurons in DRG with normal targets (contralateral lumbar and thoracic). The effect was highly selective; NT3 increased the number of trkC+ neurons without significantly changing the number of either trkA+ or trkB+ neurons. The effect of NT3 was attributable to the rescue of DRG neurons from cell death, because exogenous NT3 reduced the number of pyknotic nuclei without significantly altering proliferation. Analysis of spinal projections showed further that many of the trkC+ neurons rescued by NT3 projected to the ventral spinal cord. These neurons thus had central projections characteristic of muscle spindle afferents. Together, our results indicate that NT3 signaling is both necessary and sufficient for the development of the proprioceptive phenotype, even in the absence of other signals from limb muscle.

The generation of neuronal heterogeneity in a rat sensory ganglion

Adult sensory neurons differ chemically, morphologically, and functionally, but the factors that generate their diversity remain unclear. For example, neuropeptides are generally found in small neurons, whereas abundant neurofilament is common in large neurons. Neurons containing the neuropeptides calcitonin gene-related peptide (CGRP) or substance P were quantified using immunohistochemistry in rat lumbar dorsal root ganglion (DRG) at times before and after sensory neurons contact central and peripheral targets in vivo. No neurons in the newly formed DRG expressed neuropeptide or neuropeptide mRNA, but neuropeptides were detectable about the time that axons connect with peripheral targets. To determine the requirement for target in neuropeptide regulation, embryonic DRG neurons were isolated at times before central and peripheral connections had formed, placed in culture, and immunocytochemically assayed for CGRP and substance P. Cultured neurons expressed neuropeptides with a time course and in proportions similar to those in vivo. Thus, some neurons in the embryonic DRG seem to be intrinsically specified to later express CGRP and substance P. The percentage of CGRP-immunoreactive neurons was not changed by cell density, non-neuronal cells, neurotrophins in addition to nerve growth factor (NGF), or antibody inactivation of neurotrophin-3 in the presence of NGF. To test the role of extrinsic cues on CGRP expression, DRG neurons were co-cultured with potential target tissues. Co-culture with a rat epidermal or smooth muscle cell line increased the proportion of CGRP-containing neurons, whereas primary skeletal muscle and 3T3 cells had no effects. Thus, multiple appropriate sensory neuron phenotypes arise in a regulated fashion in cultured neurons isolated before target connections have formed, and some candidate target tissues can modulate that intrinsic expression pattern.

The generation of neuronal heterogeneity in a rat sensory ganglion

Adult sensory neurons differ chemically, morphologically, and functionally, but the factors that generate their diversity remain unclear. For example, neuropeptides are generally found in small neurons, whereas abundant neurofilament is common in large neurons. Neurons containing the neuropeptides calcitonin gene-related peptide (CGRP) or substance P were quantified using immunohistochemistry in rat lumbar dorsal root ganglion (DRG) at times before and after sensory neurons contact central and peripheral targets in vivo. No neurons in the newly formed DRG expressed neuropeptide or neuropeptide mRNA, but neuropeptides were detectable about the time that axons connect with peripheral targets. To determine the requirement for target in neuropeptide regulation, embryonic DRG neurons were isolated at times before central and peripheral connections had formed, placed in culture, and immunocytochemically assayed for CGRP and substance P. Cultured neurons expressed neuropeptides with a time course and in proportions similar to those in vivo. Thus, some neurons in the embryonic DRG seem to be intrinsically specified to later express CGRP and substance P. The percentage of CGRP-immunoreactive neurons was not changed by cell density, non-neuronal cells, neurotrophins in addition to nerve growth factor (NGF), or antibody inactivation of neurotrophin-3 in the presence of NGF. To test the role of extrinsic cues on CGRP expression, DRG neurons were co-cultured with potential target tissues. Co-culture with a rat epidermal or smooth muscle cell line increased the proportion of CGRP-containing neurons, whereas primary skeletal muscle and 3T3 cells had no effects. Thus, multiple appropriate sensory neuron phenotypes arise in a regulated fashion in cultured neurons isolated before target connections have formed, and some candidate target tissues can modulate that intrinsic expression pattern.

Evidence that intersegmental competition influences the segmental distribution of the central terminals of muscle sensory afferents in the chicken embryo

The possibility that muscle sensory afferents that enter the spinal cord at different segmental levels compete for terminal space in the lateral motor column was tested by unilaterally ablating neural crest cells destined to generate dorsal root ganglia (DRGs). Newly migrating neural crest was surgically removed from several lumbosacral segments on one side of the spinal cord on day 3 of embryogenesis (d3). The resultant experimental preparations developed without DRGs on the operated side at the segmental levels where the ablation was performed. At d18 the central projection from the intact DRG immediately caudal to the operated segments was anatomically mapped with a lipophilic axonal tracer. The central projection from the same DRG on the opposite, unoperated side was mapped as a control. The density of terminals along the longitudinal axis of the lateral motor column was quantified in serial sections with an image analysis program. On both the operated and control sides, the majority of terminals were found within the segment of entry and the rostral and caudal neighboring spinal segments. However, the rostral neighboring segment received a larger proportion of terminals on the operated side compared to the control side. Thus, the absence of primary afferents entering the spinal cord at specific segments leads to a shift of the afferent projection to the lateral motor column from the neighboring segments towards the deafferented segments.

A critical period for the influence of peripheral targets on the central projections of developing sensory neurons

During development, the projections that sensory neurons make within the spinal cord are influenced by the specific targets they contact in the periphery. If sensory ganglia normally supplying principally cutaneous targets are forced to grow into limb muscles, in early stage tadpoles, many sensory neurons within these ganglia innervate limb muscles and subsequently develop spinal projections appropriate for muscle spindle afferents. If the same procedure is performed with adult frogs, however, these novel projections do not form. In this study, we have determined the developmental stages at which this sensitivity to peripheral targets exists. Axons from sensory neurons in thoracic (largely cutaneous) dorsal root ganglia were re-routed into the front leg at various stages through metamorphosis, and the central spinal projections of these re-routed fibers were assessed with HRP labeling. We found that thoracic sensory axons could be made to project to limb muscles throughout development, but that the central projections of these neurons were only appropriate for spindle afferents if the fibers were re-routed before stage XVIII, shortly before metamorphic climax. Because sensory neurons can regenerate specifically into the appropriate spinal laminae even in adult frogs, these results suggest that changes in either the DRG or the arm musculature occur by stage XVII so that DRG neurons cannot respond to novel peripheral targets.

Evidence for prenatal competition among the central arbors of trigeminal primary afferent neurons: single axon analysis

Previous studies from this laboratory have demonstrated that prenatal damage to vibrissae follicles results in significant increases in the brainstem representations of the remaining vibrissae as demonstrated by staining for the mitochondrial enzyme cytochrome oxidase (CO). Because CO is primarily a postsynaptic marker, these results do not directly address the question of whether there were changes in the projections of primary afferent fibers. To address this issue, we made intra-axonal recordings from individual vibrissa-related primary afferents in rats that sustained damage to vibrissae follicles on embryonic day 17, and then injected horseradish peroxidase (HRP) into these axons to visualize their terminal arbors in the brainstem at the level of trigeminal subnucleus interpolaris (SpI). All vibrissae-related primary afferents responded to deflection of one and only one vibrissa, and the terminal arbors of axons (N = 47) recovered from animals that sustained fetal peripheral lesions were significantly larger than those (N = 23) from normal rats. Fibers from fetally damaged animals had increased total fiber lengths and numbers of branch points. These results indicate that reduced competition among primary afferent axons results in increases in the terminal arbors that remain. These increases occur without any significant alteration in their peripheral receptive fields.

Development of specific muscle and cutaneous sensory projections in cultured segments of spinal cord

Development of sensory projections was studied in cultured spinal segments with attached dorsal root ganglia. In spinal segments from stage 30 (E6.5) and older chicken embryos, prelabeled muscle and cutaneous afferents established appropriate projections. Cutaneous afferents terminated solely within the dorsolateral laminae, whereas some muscle afferents (presumably Ia afferents) projected ventrally towards motoneurons. Development of appropriate projections suggests that sufficient cues are preserved in spinal segments to support the formation of modality-specific sensory projections. Further, because these projections developed in the absence of muscle or skin, these results show that the continued presence of peripheral targets is not required for the formation of specific central projections after stage 29 (E6.0). Development of the dorsal horn in cultured spinal segments was assessed using the dorsal midline as a marker. In ovo, this midline structure appears at stage 29. Lack of midline formation in stage 28 and 29 cultured spinal segments suggests that the development of the dorsal horn is arrested in this preparation. This is consistent with earlier reports suggesting that dorsal horn development may be dependent on factors outside the spinal cord. Because dorsal horn development is blocked in cultured spinal segments, this preparation makes it possible to study the consequences of premature ingrowth of sensory axons into the spinal cord. In chicken embryos sensory afferents reach the spinal cord at stage 25 (E4.5) but do not arborize within the gray matter until stage 30. During this period dorsal horn cells are still being generated. In spinal segments, only those segments that have developed a midline at the time of culture support the formation of midline at the time of culture support the formation of specific sensory projections.(ABSTRACT TRUNCATED AT 250 WORDS)

Normal and abnormal pathfinding of facial nerve fibers in the chick embryo

Development of the facial nerve was studied in normal chicken embryos and after surgical disruption of ingrowing sensory facial nerve fibers at 38-72 h of incubation. Disruption of facial nerve fibers by otocyst removal often induced a rostral deviation of the facial nerve and ganglion to the level of the trigeminal ganglion. Cell bodies of the geniculate ganglion trailed their deviating neurites and occupied an abnormal rostral position adjacent to the trigeminal ganglion. Deviating facial nerve fibers were labeled with the carbocyanine fluorescent tracer DiI in fixed tissue. Labeled fibers penetrated the cranium adjacent to the trigeminal ganglion, but they did not follow the trigeminal nerve fibers into the brain stem. Rather, after entering the cranium, they projected caudally to their usual site of entrance and proceeded towards their normal targets. This rostral deviation of the facial nerve was observed only after surgery at 48-72 h of incubation, but not in cases with early otocyst removal (38-48 h). A rostral deviation of the facial nerve was seen in cases with partial otocyst removal when the vestibular nerve was absent. The facial nerve followed its normal course when the vestibular nerve persisted. We conclude that disruption of the developing facial pathway altered the routes of navigating axons, but did not prevent pathfinding and innervation of the normal targets. Pathfinding abilities may not be restricted to pioneering axons of the facial nerve; later-developing facial nerve fibers also appeared to have positional information. Our findings are consistent with the hypothesis that navigating axons may respond to multiple guidance cues during development. These cues appear to differ as a function of position of the navigating axon.

Times of origin of brachial sensory neurons are not correlated with neuronal phenotype

The times of origin (birthdays) of sensory and motor neurons that innervate the triceps brachii muscles of the bullfrog (Rana catesbeiana) were determined to learn whether neurons innervating a specific target are generated at a particular developmental time. 3H-thymidine (3H-TdR) was made available continuously throughout a specific developmental period. All neurons that innervated the triceps muscle in juvenile frogs (identified by filling cells retrogradely with HRP) were generated prior to metamorphosis. Triceps motoneurons were all postmitotic by early limb bud stage V. Triceps sensory neurons were generated over a protracted period of larval development, from stage V through early pre-metamorphic stage XV. Most large triceps sensory neurons were generated before the majority of the small cells. However, there was considerable overlap in the times of origin of the two populations; both large and small cells were generated at all stages of sensory neurogenesis. There was thus no strict relationship between sensory soma size and birthdate. Late-generated sensory neurons tended to be located in clusters within ganglia, whereas HRP-filled triceps neurons were not. These 3H-labeled clusters may represent clones of neurons which would indicate that late stage neuroblasts give rise to neurons that supply different peripheral targets. The time course of triceps neuronal generation paralleled that of all other brachial sensory neurons implying that the time of last cell division does not in itself determine either the target a neuron will innervate or the sensory modality to which it will respond.

Specification of synaptic connections between sensory and motor neurons in the developing spinal cord

Experimental studies of mechanisms underlying the specification of synaptic connections in the monosynaptic stretch reflex of frogs and chicks are described. Sensory neurons innervating the triceps brachii muscles of bullfrogs are born throughout the period of sensory neurogenesis and do not appear to be related clonally. Instead, the peripheral targets of these sensory neurons play a major role in determining their central connections with motoneurons. Developing thoracic sensory neurons made to project to novel targets in the forelimb project into the brachial spinal cord, which they normally never do. Moreover, these foreign sensory neurons make monosynaptic excitatory connections with the now functionally appropriate brachial motoneurons. Normal patterns of neuronal activity are not necessary for the formation of specific central connections. Neuromuscular blockade of developing chick embryos with curare during the period of synaptogenesis still results in the formation of correct sensory-motor connections. Competitive interactions among the afferent fibers also do not seem to be important in this process. When the number of sensory neurons projecting to the forelimb is drastically reduced during development, each afferent still makes central connections of the same strength and specificity as normal. These results are discussed with reference to the development of retinal ganglion cells and their projections to the brain. Although many aspects of the two systems are similar, patterned neural activity appears to play a much more important role in the development of the visual pathway than in the spinal reflex pathway described here.

Changes in spinal cord reflexes after cross-anastomosis of cutaneous and muscle nerves in the adult rat

Evidence exists that the specification of afferent nerves and their central connections in the embryo may depend in part on influences from the peripheral target innervated. We have now investigated whether such peripheral determination persists in the adult rat using the unmyelinated afferent system of C fibres, which differ chemically in the adult depending on their target. We have previously shown that if the cutaneous sural nerve and the muscle gastrocnemius nerve are cross-anastomosed so that they grow to each other's target, the A fibres establish functional endings and the C fibres change their chemistry to that which is appropriate for the new target. Here we report that in normal adult rats, a short train of stimuli to the cutaneous sural nerve produced a brief facilitation of the flexion reflex, lasting on average only 5 min, whereas similar stimulation of the gastrocnemius-muscle nerve enhanced this reflex for an average of 54 min. In cross-anastomosed animals, stimulation of the gastrocnemius nerve (innervating skin) induced a brief potentiation of the flexion reflex, lasting on average only 3 min. By contrast, stimulation of sural nerve (innervating muscle) produced a potentiation of this reflex lasting 57 min. Thus the ability of adult afferent nerves to potentiate the flexion reflex depends on the target with which they make contact. We propose that tissue-specific factors influence some of the central actions of primary afferent neurons in the adult.

Lack of fiber type selectivity during reinnervation of neonatal rabbit soleus muscle

Fast and slow contracting fibers in neonatal mammalian skeletal muscle are each innervated in a highly specific manner by motor neurons of the corresponding type, even at an age when polyinnervation is widespread. Chemospecific recognition is a possible mechanism by which this pattern of innervation could be established. We have investigated this possibility by studying the degree of specificity during reinnervation of rabbit soleus muscle following nerve crush on Postnatal Day 1 or 4. We assayed fiber type composition by measuring the twitch rise times of motor units within 2 days of the onset of functional reinnervation (5-6 days after nerve crush). In contrast to the broad, bimodal distribution of single motor unit twitch rise times seen in normal muscles, motor units in reinnervated muscles yielded a narrower, unimodal distribution of rise times. Rise times of reinnervated units were intermediate to those of normal fast and slow units, suggesting that reinnervated units were composed of a mixture of fast and slow contracting fibers. An alternative possibility, that specific reinnervation was masked by contractile dedifferentiation of muscle fibers, was examined by maintaining a transmission blockade induced by botulinum toxin poisoning for an equivalent interval. Twitch rise times of treated motor units exhibited the distinctly bimodal distribution characteristic of normal muscles, suggesting that muscle fibers can retain contractile diversity during a transient period of denervation. We carried out computer simulations to estimate the amount of rise time diversity induced by varying degrees of specificity during reinnervation. Based on this analysis, we conclude that there is little if any selective reinnervation of muscle fiber types at the ages studied.

Development of central projections of lumbosacral sensory neurons in the chick

The development of central projections of sensory neurons in lumbosacral dorsal root ganglia (DRGs) was examined by using horseradish peroxidase labeling techniques in chick embryos from stage 23 (E4) to stage 39 (E13). Our results show that primary afferents reach the spinal cord by stage 23. Afferent axons extend in the primordium of the dorsal funiculus for several segments rostral and caudal to their segment of entry for over 24 hours before invading the gray matter at stage 28 (E6). Sensory fibers grow into the vicinity of motoneuron dendrites by stage 32 (E7.5), about the time that reflexes and apparent monosynaptic EPSPs can first be elicited. Dense projections into the dorsal laminae of the spinal cord, presumably representing cutaneous afferents, appear somewhat later, at about stage 39 (E13), when the segmental projection pattern begins to resemble the mature pattern.

Plasticity of primary afferent acid phosphatase expression following rerouting of afferents from muscle to skin in the adult rat

We have examined the possibility that reinnervation of a new peripheral target by primary afferent neurones can alter the histochemical properties of those afferents in the adult rat. The hindlimb sural and gastrocnemius nerves largely supply skin and muscle, respectively. In adult animals these nerves were cut and rejoined to either their own distal stumps (self-anastomosis) or that of the other nerve (cross-anastomosis) and allowed to regenerate for 12-16 weeks to reinnervate an appropriate or inappropriate target. Fluoride-resistant acid phosphatase (FRAP) is a chemical marker found in many unmyelinated afferents. We have determined the FRAP expression in normal and regrown nerves and examined its distribution in the dorsal horn of animals with self- and cross-anastomosed nerves. While normal and self-anastomosed sural nerves stained heavily for FRAP, gastrocnemius nerves showed either no staining or only the occasional fibre. Cross-anastomosed gastrocnemius nerves, now innervating the skin, showed a significant increase in staining, in some cases approaching the levels normally seen in sural nerves. Conversely, cross-anastomosed sural nerves (innervating muscle) showed decreased FRAP staining. In the normal dorsal horn the terminals of FRAP containing afferents form a thin band extending throughout the mediolateral extent of lamina II (Devor and Claman: Brain Res. 190:17-28, '80). One week after axotomy of the sural nerve, FRAP is depleted from its terminals and a gap appears in the normal FRAP staining pattern in the lumbar enlargement of the spinal cord. The new expression of FRAP in cross-anastomosed nerves was also seen in their terminals in the dorsal horn.(ABSTRACT TRUNCATED AT 250 WORDS)

Specificity of sensory projections to the spinal cord during development in bullfrogs

Sensory neurons in dorsal root ganglia of frogs project to areas of the spinal cord they do not normally innervate following removal of adjacent ganglia at tadpole stages (Frank and Westerfield, J. Physiol. (Lond.) 324:495-505, '82b). A possible explanation of this phenomenon is that sensory neurons project to wider areas of the spinal cord in tadpoles than in adult frogs and that partial deafferentation causes the retention of these widespread projections. Therefore, the specificity of sensory projections to the spinal cord in tadpoles was assessed by staining individual dorsal roots with horseradish peroxidase. Thoracic sensory neurons project to thoracic segments of the spinal cord and to the brainstem in tadpoles, like thoracic sensory neurons in adult frogs. They rarely arborize in the brachial region even at stages when no other sensory fibers arborize at this level. Furthermore, their projections are restricted to the dorsal horn at all stages. Conversely, hypoglossal sensory neurons, which project into the intermediate gray matter in the adult, also project to this area in tadpoles. The finding that sensory neurons in tadpoles only project to areas of the spinal cord that they innervate in the adult suggests that the novel projections observed following partial deafferentation of the spinal cord are actually induced by the operation. An additional finding was that forelimb afferents, which project to an area extending from the obex to midthoracic levels in adult frogs, arborize at rostral spinal levels and at thoracic levels several stages before they form projections to the region around their own dorsal root. These differences in the stages at which projections to different levels of the spinal cord develop suggest that local properties of the spinal cord may control the timing of sensory fiber arborization.

Activation patterns of embryonic chick lumbosacral motoneurones following large spinal cord reversals

1. Embryonic chick motoneurones were caused to innervate inappropriate hindlimb muscles by rotating the presumptive lumbosacral region of the neural tube in stage 15-16 embryos which is prior to the outgrowth of motoneurone axons. 2. The activation patterns of motoneurones in control and spinal cord reversal embryos were analysed from electromyographic (e.m.g.) recordings of stage 36 limb muscles during evoked movement sequences in an isolated spinal cord-limb preparation. Histograms representing the frequency of activation were constructed for each muscle. The muscle's pattern of activation was classified as flexor-like or extensor-like and compared to the activation patterns of control muscles. 3. A series of control operations was performed in which the prospective lumbosacral region of the neural tube was removed and replaced in its original orientation. Muscles in these embryos were innervated by their normal motoneurone pools and they were activated normally, indicating that the neural tube operation per se does not alter the activation pattern of motoneurones. Furthermore, some muscles (twelve out of sixty-one) in spinal cord reversal embryos had normal activation patterns and appeared to be innervated by their original motoneurones. Based on these results and the result of a previous study (Landmesser & O'Donovan, 1984 b), it is concluded that motoneurones in reversed spinal cords are activated in a manner appropriate for their original identity. 4. The majority of muscles (thirty-three out of sixty-one) in large spinal cord reversal embryos were activated during an appropriate phase of the kicking cycle. Of the remaining muscles, 16% were activated inappropriately (i.e. extensor muscles were activated as flexors, and vice versa), and 30% had a novel 'mixed' flexor- and extensor-like activation pattern. However, the activation pattern of most muscles differed markedly from that of any other control muscles regardless of whether the muscle was activated appropriately or inappropriately as a flexor or extensor. The abnormal activation patterns are a likely consequence of the diffuse distribution of inappropriate motoneurones projecting to foreign muscles in embryos with large spinal cord reversals. Since it is likely that motoneurones are still activated according to their original identity, the activation patterns of individual foreign motor units projecting to a muscle sum in e.m.g. recordings to produce a novel abnormal activation pattern.(ABSTRACT TRUNCATED AT 400 WORDS)

The development of sensory projection patterns in embryonic chick hindlimb under experimental conditions

In the chick, sensory neurons grow to their segmentally appropriate target sites in the hindlimb from the outset during normal development. To elucidate the underlying mechanisms, we performed various manipulations of the neural tube, including the neural crest, or of the hindlimb, before axonal outgrowth and assessed the resulting sensory projections using retrograde and anterograde HRP labeling and electrophysiological techniques. Previous experiments had shown that motoneurons are specified to project to their appropriate target muscles prior to axon outgrowth and that they respond to cues in the limb in order to grow to those targets (C. Lance-Jones and L. Landmesser, 1980, J. Physiol. (London) 302, 559-602; C. Lance-Jones and L. Landmesser, 1981, Proc. R. Soc. London, B 214, 19-52). When several segments of neural tube and neural crest were deleted, sensory neurons in the remaining segments still projected along their correct pathways, as did motoneurons. In situations in which motoneurons grew to their correct targets from altered positions with respect to the limb (e.g., small neural tube reversals), sensory neurons also tended to project along the segmentally appropriate pathways both to skin and to muscle. In situations in which motoneurons were displaced greater distances from their normal point of entry into the limb and made wrong connections (e.g., large neural tube reversals, anterior-posterior limb reversals), sensory neurons also projected incorrectly. The patterns of sensory projections to muscles were, in each situation, generally similar to the motoneuron projections. These results are consistent with the possibility that sensory neurons, like motoneurons, are specified with respect to their peripheral connectivity. Alternatively, the results suggest that motoneurons may play a role in the process of pathway selection by sensory neurons.

Skin sensory innervation patterns in embryonic chick hindlimb following dorsal root ganglion reversals

During embryonic development skin sensory neurons in lumbosacral dorsal root ganglia (DRGs) establish their dermatomes and axonal projections in a precise, orderly fashion in the chick. To investigate mechanisms responsible for this specific outgrowth, the rostrocaudal order of DRGs T7-LS3 was reversed by rotating the corresponding segments of neural crest, either alone or together with the underlying neural tube in St.15-16 embryos. The resulting skin sensory innervation patterns, mapped physiologically or anatomically at St.29-40, differed between the two experimental groups. Following neural tube rotations DRGs tended to establish innervation patterns that were consonant with their original position in the embryo. Axons from these rotated DRGs generally projected into the appropriate pathways and innervated the appropriate region of skin. Neural crest rotations left the ventral neural tube (including the motor neuron precursors) largely intact. In this case rotated DRGs tended to establish innervation patterns in accordance with their new position in the embryo, almost as if no rotation had been made. These results cannot be explained solely by the inherent specificity of sensory neurons. Instead, the results are largely consistent with the suggestion (Honig et al., 1986; Landmesser and Honig, 1986) that motor axons can direct the outgrowth of sensory axons and thereby influence the establishment of sensory innervation patterns. Other mechanisms that may also affect the development of sensory innervation patterns are discussed.

Normal electrical activity is not required for the formation of specific sensory-motor synapses

These studies sought to determine the role of stretch-evoked electrical activity in muscle sensory axons on the formation of specific monosynaptic connections made by these neurons onto spinal motoneurons. Normal activity was disrupted by tenotomy during the developmental stages that the central connections are made. The pattern of connections was normal, suggesting that normal activity is not required in this system for the formation of specific sensory-motor connections.

Morphogenesis of an identified leech neuron: segmental specification of axonal outgrowth

We have investigated the development of segmental diversity in an identified leech neuron, the Retzius cell. Retzius cells in the genital segments differ from those in other segments in lacking central axons and contacting different peripheral targets: the genitalia. These differences are not apparent during initial axon outgrowth, when all Retzius cells follow the same morphogenetic pattern. Rather, they first appear about the time the peripheral axons of the genital segment Retzius cells contact the genital primordia. This suggests that the pattern of central and peripheral axonal outgrowth may be modified by an interaction with peripheral targets.

The organization of motoneurons in the turtle lumbar spinal cord

The distribution of motoneurons in the lumbosacral spinal cord of the turtle Pseudemys scripta elegans was studied by using the technique of retrograde transport of horseradish peroxidase. A total of 19 different hindlimb muscles were injected with varying amounts of horseradish peroxidase. The resulting distribution of labeled motoneurons was studied in both longitudinal and transverse sections of spinal cord. Motoneurons innervating a particular hindlimb muscle are clustered in longitudinally arranged motorpools. Motorpools of different muscles can show considerable overlap in both the rostrocaudal and transverse planes. The distribution of the various motorpools demonstrates a somatotopic organization of motoneurons within the lumbar spinal cord. Motoneurons innervating more distally positioned muscles are generally found in the more caudal segments, while motoneurons supplying proximal muscles are distributed throughout almost the whole lumbosacral intumescence. Motoneurons innervating anterodorsally positioned muscles are found in the ventrolateral part of area IX in the ventral horn, while more dorsomedially positioned motoneurons innervate the posteroventral muscles. These features are consistent with observations in other tetrapods, although the somatotopic representation of motoneurons is more evident in higher vertebrates such as chicken and cat. The observed motorpool distribution is discussed in relation to the presumed ontogeny of the spinal cord and hindlimb muscles and also in relation to the functions of the investigated muscles.

The effects of neural crest deletions on the development of sensory innervation patterns in embryonic chick hind limb

Selected lumbosacral dorsal root ganglia (d.r.g.s) were eliminated from chick embryos by removing the parent neural crest, and the dermatomes and axonal projection patterns established in the hind limb by the remaining intact d.r.g.s were studied physiologically and/or anatomically. Dermatomes of intact d.r.g.s expanded into denervated skin regions, partially but never completely replacing the lost innervation; some skin regions consistently remained without apparent innervation. Dermatome expansion was detected in young embryos, soon after skin innervation was established, suggesting that skin sensory axons in operated embryos ignored their usual dermatome borders. The axonal pathways (nerve trunks) normally supplied by axons from the deleted d.r.g.s were missing, and axons from intact d.r.g.s were only rarely found in aberrant pathways. In contrast, the relative distribution of axons from intact d.r.g.s within their usual complement of pathways was altered, with axonal projections shifting toward, but not into the deleted pathways. Shifts in axonal projection patterns were observed in embryos prior to the onset of sensory cell death. Thus, d.r.g. deletions appeared to affect the initial growth of axons into the limb. Together these results suggest that during normal development competitive interactions among axons from neighbouring d.r.g.s play an important role in establishing the borders between dermatomes and in determining the distribution of axons within projection pathways in the limb.

The activation patterns of embryonic chick motoneurones projecting to inappropriate muscles

Chick lumbosacral motoneurones were caused to innervate foreign muscles by surgically rotating or shifting the limb bud about the anterior-posterior axis in stage 17-18 embryos. The activation pattern of such wrongly projecting motoneurones was assessed at stages 35-38 by recording electromyographic activity from muscles in an isolated spinal cord/hind limb preparation. Muscle activity was classed as flexor- or extensor-like according to the characteristics of the patterned sequence of bursts elicited by a single shock to the thoracic cord. Wrongly projecting motoneurones did not have their activation pattern altered to one appropriate for the muscle innervated; therefore in some cases a particular muscle was activated with a pattern similar to its original one, and in other cases in an opposite manner. Mixed flexor-extensor-like activation of a single muscle was, however, rare. The identity of motoneurones projecting to a muscle was determined by their cord location following retrograde labelling with horseradish peroxidase. This allowed us to conclude that motoneurones could develop their normal pattern of activation even when projecting to foreign muscles. It is concluded that the cord circuits (presumably composed of local interneurones responsible for the activation of motoneurones in the isolated cord preparation are not altered by retrograde influences from the muscle. Wrongly projecting motoneurones, which were maintained throughout the normal cell death period, were activated during spontaneous embryonic movements, and in many cases were found to have a behaviourally inappropriate activation pattern. These observations are discussed in relation to proposed mechanisms by which developmental errors in connectivity are corrected.

Hyperplasia in the spinal sensory system of the frog. II. Central and peripheral connectivity patterns

Central and peripheral connectivity patterns of hyperplastic dorsal root ganglia (DRGs) in Rana pipiens are examined in order to determine the relative roles of peripheral and central contacts in the production of DRG hyperplasias. The hyperplasias are produced in the intact hindlimb DRGs after the removal in tadpoles and young postmetamorphic frogs of neighboring DRGs (Davis and Constantine-Paton, '83). The peripheral target zones of the hyperplastic DRGs, determined by physiological recordings of sensory receptive fields, are found to undergo a significant degree of expansion relative to controls. Peripheral expansion is most pronounced in caudalmost DRG 10, and this effect occurs in experimental animals operated during larval and postmetamorphic stages. Further, anatomical labelling of peripheral sensory fibers coursing to the hindlimb reveals that the hyperplastic DRG 10 actually contains additional fibers projecting to the denervated regions. The central projection of the hyperplastic DRG 10 does not show corresponding increases in longitudinal arborization after the application of horseradish peroxidase to the appropriate dorsal roots. These observations are made on some of the same experimental animals in which peripheral fields are shown to have vastly expanded. We conclude that the peripheral processes of the hyperplastic DRGs are less rigidly specified than the central terminations, and that it is the periphery which plays the primary role in controlling the cell numbers increases. A second aim of this investigation is to identify whether sexually dimorphic connectivity patterns in normal frogs explain the production of DRG 10 hyperplasias exclusively in male experimental animals (Davis and Constantine-Paton, '83). We apply the same techniques used in our connectivity studies of hyperplastic DRGs to the investigation of connectivity patterns of DRG 10s in normal males and females. No sex-dependent differences in peripheral and central connectivity are found. Thus, since normal male and female frogs possess an equivalent amount of target space for DRG 10, the unique production of hyperplasias in male experimental animals cannot be explained solely on the basis of connectivity. We speculate on what other factors may be involved.

Development of sensory-motor synapses in the spinal cord of the frog

The development and specificity of monosynaptic sensory-motor synapses were studied in the brachial spinal cord of bullfrog tadpoles. Intracellular and extracellular recordings were made from motoneurones innervating several different muscles of the forelimb. Excitatory synaptic potentials (e.p.s.p.s) were elicited by stimulation of various peripheral muscle nerves. Sensory and motor axons in the triceps brachii muscle nerves were electrically excitable at stage XIII, the earliest stage studied. Their conduction velocities were 0.2-0.4 m/s. These velocities increased during subsequent development so that by stage XXII they were approximately 5 m/s. Before stage XVII, synaptic potentials evoked in motoneurones by stimulation of the triceps sensory fibres had a long central latency and fatigued easily. These potentials were probably mediated polysynaptically. At stage XVII, the first short-latency triceps synaptic potentials appeared. They had central latencies of less than 3 ms and represented the direct, monosynaptic input from muscle sensory cells on to motoneurones. During subsequent development the percentage of triceps motoneurones innervated by triceps sensory fibres increased, while the number of long-latency polysynaptic inputs decreased. Both the electrical and chemical components, characteristic of these monosynaptic e.p.s.p.s in adult frogs, were prominent from the time the e.p.s.p.s first appeared. The pattern of innervation of brachial motoneurones by triceps sensory afferents was specific from the beginning. Triceps sensory fibres innervated most triceps motoneurones but very few subscapular or pectoralis motoneurones, just as in adult frogs. At no time were there appreciable numbers of 'aberrant' connexions. The developmental time course of several different classes of sensory-motor connexions was similar. Thus the synaptic specificity of this system cannot be explained by a differential timing of synaptogenesis.

The effect of lesions in the neural crest on the formation of synaptic connexions in the embryonic chick spinal cord

1. The pattern of synaptic activity in lateral gastrocnemius (l.g.) motoneurones in the lumbar spinal cord of chick embryos (Stage 44-45, 19-21 d of incubation) has been examined using intracellular recording. In the motoneurones of normal chick embryos, stimulation of different peripheral, sciatic nerve branches gave rise to characteristic synaptic responses. Stimulation of the lateral gastrocnemius nerve caused a monosynaptic e.p.s.p. which was graded by the intensity of nerve stimulation. Stimulation of synergistic muscle afferents also caused a brief latency e.p.s.p., followed by longer latency excitatory and inhibitory synaptic potentials. Stimulation of antagonistic muscle afferents or cutaneous afferents gave rise to longer latency inhibitory and excitatory synaptic potentials respectively.2. The synaptic activity of l.g. motoneurones was also recorded in embryos in which short segments of the lumbar neural crest had been destroyed by microcautery at 3 d of incubation (Stage 18). The embryos developed without sensory ganglia and dorsal roots in the corresponding region.3. At 19-21 d of incubation, the amplitude of the l.g. e.p.s.p. of l.g. motoneurones in deafferented segments was on the average only a half to a third of the amplitude seen in motoneurones of intact spinal segments. However, both the l.g. and synergist e.p.s.p.s were larger than those seen in acutely deafferented segments of normal embryos.4. In spite of the weak monosynaptic input from l.g. and synergistic afferents, the pattern of synaptic activity evoked by antagonistic muscle afferent or cutaneous afferent stimulation was not different from normal. This was even the case for gastrocnemius motoneurones in which no early e.p.s.p. could be evoked by stimulating the l.g. or synergistic muscle nerves.5. No muscle spindles could be seen in sections of l.g. muscles from embryos with extensive lesions of the lumbosacral neural crest. Incomplete lesions of l.g. segments reduced the number of spindles in the muscle.6. These results suggest that when motoneurones are deprived of part of their normal synaptic input before the formation of peripheral connexions, the identity of the motoneurones (in terms of the origin of their synaptic input) is preserved. Missing synaptic inputs are either replaced by appropriate afferent fibres, if they are available, or not at all. The chick sensory ganglion cells with monosynaptic connexions to motoneurones appear to be unable to compensate significantly for peripheral or central defects in the innervation of the hind limb. They behave as if their developmental possibilities were quite rigidly determined at an early embryonic stage.

Synaptic organization of sensory and motor neurones innervating triceps brachii muscles in the bullfrog

1. The anatomy and physiology of sensory-motor pathways were studied in the brachial spinal cord of adult bullfrogs to characterize the properties and specificity of these connexions.2. Motoneurones innervating a given forelimb muscle are located in discrete and reproducible regions of the lateral motor column. Yet only a fraction of the motoneurones in a particular region innervates any one muscle.3. The central projections of sensory afferent axons from the triceps muscles extend throughout the rostro-caudal length of the brachial spinal cord. Within this region these projections terminate in an area containing many motoneuronal dendrites.4. Within the triceps motor pool sensory neurones from the triceps muscles produce monosynaptic potentials only in triceps motoneurones even though these motoneurones are mingled with motoneurones innervating other muscles.5. Motoneurones innervating each of the three heads of the triceps muscles, medial, internal and external, receive monosynaptic input from their own, homonymous muscle head. Sensory fibres from the medial head also innervate 98% of the heteronymous motoneurones projecting to the internal or external heads, and nearly 90% of the medial triceps motoneurones are innervated by sensory axons from the other two heads.6. Similarly, other brachial motoneurones receive monosynaptic input from sensory axons in their own muscle nerves. However, most of the synaptic potentials evoked in triceps motoneurones by stimulation of muscle nerves other than triceps are of longer latency and probably involve polysynaptic pathways.7. Thus, the pattern of synaptic connexions between muscle sensory afferents and motoneurones in the frog's spinal cord is specific. Furthermore, comparison with homologous pathways in the cat's spinal cord suggests that the strength and pattern of these connexions are similar.