Central projections of the lateral line nerves in the shovelnose sturgeon

Primary projections of the anterior ( ALLN ) and posterior ( PLLN ) lateral line nerves were traced in the shovelnose sturgeon by means of horseradish peroxidase (HRP) histochemistry and silver degeneration. The trunk of the ALLN divides into dorsal and ventral roots as it enters the medulla. Fibers of the dorsal root form ascending and descending branches that terminate within the ipsilateral dorsal octavolateralis nucleus and the dorsal granular component of the lateral eminentia granularis. Fibers of the ventral root of the ALLN , as well as fibers of the PLLN , enter the medulla ventral to the dorsal root of the ALLN where some of the fibers terminate among the dendrites of the magnocellular octaval nucleus. The bulk of the fibers form ascending and descending branches that terminate within the ipsilateral medial octavolateralis nucleus. A portion of the ascending fibers continue more rostrally and terminate in the ipsilateral eminentia granularis and bilaterally in the cerebellar corpus. Some fibers of the descending rami of both the ALLN and PLLN extend beyond the caudal limit of the medial octavolateralis nucleus to terminate in the caudal octavolateralis nucleus. The HRP cases also revealed retrogradely filled large neurons whose axons course peripherally in the lateral line nerve and are likely efferent to the lateral line organs.     

The primary projections of the lateral-line nerves of the Florida gar, Lepisosteus platyrhincus

Gars, like most other ray-finned fishes, possess lines of superficial and canal neuromasts that form highly ordered spatial arrays on the head and trunk. These neuromasts are innervated by three pairs of cranial nerves: anterior, middle and posterior lateral-line nerves. Application of horseradish peroxidase to the roots of these nerves indicates that the afferent fibers of each nerve terminate throughout the rostrocaudal extent of the ipsilateral octavolateralis column, composed of medial and caudal octavolateralis nuclei, as well as rostrally in the eminentia granularis of the cerebellum. The afferents of each nerve terminate within different regions of these nuclei, thus preserving a rostrocaudal somatotopy of the peripheral receptors. The individual rami of these nerves innervate peripheral receptors in a dorsoventral sequence; however, application of horseradish peroxidase to these rami indicates an almost total overlap of the terminal fields of adjacent rami and, thus, little or no preservation of dorsoventral somatotopy of the peripheral receptors. Similarly, the terminal fields of fibers that innervate canal and superficial neuromasts overlap extensively, and there is no evidence for separate maps of canal and superficial neuromasts. Furthermore, the position of the terminal fields of superficial neuromasts is not consistent with a hypothesis that these receptors may have evolved into electroreceptors in some teleosts. However, these experiments do indicate that superficial neuromasts, like canal neuromasts, are innervated by efferent fibers of rostral and caudal efferent nuclei, but there is no evidence of a diencephalic efferent nucleus as occurs in teleosts.     

Primary projections of the lateral line nerves in adult lampreys

Primary lateralis projections in silver lampreys, Ichthyomyzon unicuspis, and young adult sea lampreys, Petromyzon marinus, were examined utilizing silver impregnation of degenerating fibers and transganglionic transport of horseradish peroxidase. Anterior lateral line nerve afferents terminate throughout the neuropil of the electroreceptive dorsal nucleus and in the lateral neuropil of the mechanoreceptive medial nucleus. The superficial ophthalmic, buccal and recurrent rami of the anterior lateral line nerve all project to the ipsilateral dorsal nucleus; the recurrent ramus, supplying electroreceptors on the trunk, possesses the largest terminal field. No somatotopy was apparent in the projections to the dorsal nucleus. Primary afferents in the posterior lateral line nerve project to the medial neuropil of the ipsilateral medial nucleus, continue rostrally into the cerebellum and cross to the central neuropil of the contralateral medial nucleus. Lateralis projections in lampreys resemble those in nonteleost bony fishes and elasmobranchs; however, the contralateral projection of the posterior lateral line nerve is presently recognized only in petromyzontids.     

The development and postnatal organization of primary afferent projections to the rat thoracic spinal cord

Primary afferent projections to the thoracic spinal cord in fetal and postnatal rats were labelled by applying horseradish peroxidase (HRP) to the central stumps of cut peripheral nerves. Diaminobenzidine (DAB) and tetramethyl benzidine (TMB) histochemical processing procedures were used to reveal the HRP reaction product. In postnatal rats, individual muscle nerves were labelled to reveal the organization of muscle afferent projections to the motor nuclei. The terminals of muscle afferents were distributed widely across the dendritic arbors of motoneurons supplying the same muscles. No spatial segregation of the terminations of different populations of muscle afferents was discernable. Afferents supplying different regions of the skin were labelled by applying HRP to the dorsal and ventral primary rami of the spinal nerves. Afferents in the dorsal rami projected to lateral portions of both the ipsilateral and contralateral dorsal horns while afferents in the ventral rami projected to the medial portions of both dorsal horns. The projections of the dorsal rami were shifted caudally relative to those of the ventral rami. This relationship reflects the fact that the regions of skin innervated by the dorsal rami are displaced caudally relative to those innervated by the corresponding ventral rami. In fetuses, dorsal rami were labelled alone or in combination with ventral rami. These experiments disclosed the time course of development of the projections to different laminae of the spinal gray matter and revealed that afferents in the two primary rami project to appropriate regions in the ipsilateral and contralateral dorsal horns from the very outset.     

Segregation of electroreceptive and mechanoreceptive lateral line afferents in the hindbrain of chondrostean fishes

The anterior lateral line nerve (ALLN) in the chondrostean fishes (sturgeon and paddlefishes) consists of both fibers innervating ampullary electroreceptors and fibers innervating the mechanoreceptive neuromasts of the cephalic lateral line system. The fibers of the posterior lateral line nerve (PLLN) innervate only mechanoreceptive neuromasts on the body trunk. The ALLN enters the medulla via dorsal and ventral roots; the dorsal root projects to the dorsal octavolateralis nucleus (DON), whereas the ventral root and the PLLN project principally to the medial octavolateralis nucleus (MON). Previous studies in elasmobranchs have demonstrated that fibers of the dorsal root of the ALLN convey electrosensory information, and fibers of the ventral root are concerned with mechanoreceptive information. Electrophysiological and neuroanatomical methods are employed in this study in order to determine if there exists a similar segregation of electroreceptive and mechanoreceptive lateral line afferents within the chondrostean medulla. In specimens of shovelnose, Scaphirhynchus platorynchus, and Atlantic sturgeon, Acipenser oxyrhynchus, and paddlefish, Polyodon spathula, evoked potentials recorded from the hindbrain and elicited by electric fields reached maximum amplitude within the DON and decreased in amplitude through the cerebellar crest. Evoked potentials elicited by stimulation of the posterior lateral line nerve achieved maximum amplitude within the MON. Single and multiple unit recordings revealed that units within the DON responded only to electric field stimulation, whereas units recorded in the MON responded only to mechanical stimulation. Horseradish peroxidase implanted beneath isolated patches of ampullae in Polyodon revealed fibers innervating electroreceptors projecting to the DON via the dorsal root of the ALLN. These results demonstrate a segregation of electroreceptive and mechanoreceptive lateral line afferent fibers in the chondrostean hindbrain, similar to that seen in elasmobranchs. This supports the contention that the electrosensory systems of elasmobranchs and chondrosteans are homologous, and are derived from the common ancestor of elasmobranch and actinopterygian fishes.     

The peripheral distribution and central projections of the sensory rami of the facial nerve in goldfish, Carassius auratus

Taste buds in goldfish and other cyprinids are found not only within the oropharyngeal cavity but also scattered over the external body surface. The external taste buds are innervated by branches of the facial nerve that terminate centrally in an enlargement of the medulla termed the facial lobe. The peripheral distribution and areas of innervation of the rami of the facial sensory nerve were determined by using a modification of the Sihler technique and by examination of a Bodian-stained head series. The central projections of individual rami of the facial sensory nerve were traced by means of the horseradish peroxidase (HRP) technique. Fibers of the facial sensory nerve distribute over the head and trunk via nine rami. The supraorbital ramus distributes fibers to taste buds above the eye. The palatine, maxillary, and mandibular rami innervate taste buds of the rostral palate, upper lip, and lower lip, respectively. The three rami of the hyomandibular trunk innervate taste buds on the operculum, branchiostegal rays, and in the lower cheek region. A facial recurrent ramus was also found that distributes fibers to taste buds on the trunk and pectoral fin via two rami, the lateral recurrent ramus and pectoral recurrent ramus. The facial sensory rami map somatotopically on the facial lobe. Overall, the projections follow an anteroposterior orientation with the long axis of the body tilted slightly ventrally. The lips and rostral palate make up a disproportionately large portion of the map, taking up nearly the entire ventral extent of the lobe. The trunk and pectoral fin regions map broadly across the dorsal portion of the lobe. Further, projections to the nucleus of the descending trigeminal tract were observed with labeling of the supraorbital, maxillary, and mandibular rami, and the rami of the hyomandibular trunk. Projections to the facial motor nucleus were also observed with labeling of maxillary and mandibular rami, perhaps indicating a monosynaptic reflex are. These projections have not been reported in previous studies on the teleostean facial taste system.     

Muscle Afferents Innervating Skin Form Somatotopically Appropriate Connections in the Adult Rat Dorsal Horn

We have studied the somatotopic reorganization in dorsal horn neurons after a disruption in the normal spatial arrangement of primary sensory axons in adult rats. Muscle afferents were redirected to skin by cutting and cross-anastomosing the hindlimb gastrocnemius nerve (GN) and sural nerve (SN). It has previously been shown that after 10 – 12 weeks GN afferents innervate the hairy skin of the lateral ankle and calf (previously innervated by SN afferents) and become potentially capable of relaying information on the location and intensity of stimuli applied to the skin. We determined the receptive field and response properties of dorsal horn neurons in the lumbar spinal cord, in regions where the lower hindlimb is normally represented. In control animals (with intact or self-anastomosed sural nerves) very few neurons (<8%) received any synaptic input from the GN as assessed by electrical stimulation of the nerve. In contrast, when this nerve innervated skin, many cells responded to GN stimulation, and these nearly all had receptive field components in the former SN territory. Moreover, in animals with cross-anastomosed nerves, cells without GN inputs all had receptive fields outside the former SN skin territory. We have shown that in all likelihood GN afferents substituted for SN afferents in subserving the low and high threshold receptive fields of dorsal horn neurons. Furthermore, for many neurons, receptive fields were formed from inappropriately regrown GN afferents and adjacent intact cutaneous afferents (in the tibial or common peroneal nerves). Therefore, when GN afferents innervate skin in adult animals, they alter their central connectivity in an appropriate manner for their new peripheral terminations, so that an orderly somatotopic representation of the hind limb skin is maintained. We suggest that this plasticity of dorsal horn somatotopy is driven in part by activity-dependent mechanisms.     

Medullary and mesencephalic pathways and connections of lateral line neurons of the spiny dogfish Squalus acanthias

The neuronal connections of the electrosensory dorsal and the mechanosensory medial octavolateralis nuclei of the spiny dogfish Squalus acanthias were studied by horseradish peroxidase, autoradiographic and axonal degeneration methods. Efferents from each nucleus, in addition to extensive commissural components, give rise to ipsilateral and contralateral lemnisci that ascend to midbrain levels and terminate among the cells of the lateral mesencephalic nucleus (LMN). Within LMN, electrosensory and mechanosensory neurons distribute dorsolateral and ventromedial in position, respectively. Ascending fibers of both modalities also terminate within the central zone of the optic tectum. The LMN of spiny dogfish sharks that possess a primitive pattern of midbrain organization is homologous to parts of the lateral mesencephalic nuclear complex of batoids that possess a more derived pattern of midbrain organization. Other fiber connections of the dorsal and medial octavolateralis nuclei appear to differ from each other, indicating that electrosensory and mechanosensory lateral line information is carried over separate pathways at least to midbrain levels of the brain stem. For example, nucleus B, a feedback center, occupies a position in the descending lateral line pathways of sharks and skates similar to nucleus praeeminentialis of many electrosensory teleosts. The dorsal octavolateralis nucleus of sharks and skates receives afferents from nucleus B but there is no evidence that nucleus B directly feeds back to the medial octavolateralis nucleus of the spiny dogfish. Moreover, unlike the dorsal nucleus, the medial nucleus of Squalus is reciprocally linked with the octaval system.     

Organization of the six motor nuclei innervating the ocular muscles in lamprey

The topography of motoneurons supplying each of the six ocular muscles of the lamprey, Lampetra fluviatilis, was studied by selective application of HRP to the cut nerves of identified muscles. In addition, the distributions of motoneuron populations to both eyes were studied simultaneously with fluorescein and rhodamine coupled dextran-amines (FDA and RDA) applied to cut ocular muscle nerves of either side. The motoneuron pool of the caudal oblique muscle is represented bilaterally in the trochlear (N IV) motor nucleus. The dorsal rectus muscle is innervated from a contralateral group of oculomotor (N III) motoneurons and the remaining four muscles exclusively from the ipsilateral side (N III and N VI). The inferior and posterior rectus muscles are both innervated by the abducens nerve. In contrast to all jawed vertebrates, only three eye muscles (the dorsal rectus, rostral rectus, and rostral oblique) are innervated by the oculomotor nerve in lampreys (N III). Lampreys have a motor nucleus similar to the accessory abducens nucleus previously described only in tetrapods. They lack the muscle homologous to the nasal rectus muscle of elasmobranchs and the medial rectus muscle of osteognathostomes. The distribution of the dendrites of different groups of motoneurons was studied and is considered in relation to inputs from tectum and the different cranial nerves.     

Receptive fields of single cells from the face zone of the cat rostral dorsal accessory olive

Natural stimulation was used to map the receptive fields of single cells recorded from the rostal medial portion of the dorsal accessory olive (rDAO) and the subjacent principal olive (PO) of the barbiturate anesthetized cat. Previous reports indicated a somatotopic mapping of the entire contralateral body within the rDAO which included a small face zone and a larger zone with a very precise map for the limbs. While concentrating on the face zone of the rDAO we confirmed the previously reported somatotopy (face: rostral and medial; forelimb: caudal and medial; hindlimb: caudal and lateral; and trunk: rostal and lateral) and found a somatotopy within, and adjacent to, the face zone. At the border between rDAO regions representing forelimb and face, cells with forelimb fields were found to lie dorsally to cells with facial fields. Within the rDAO face region, cells with large facial fields lie dorsally to cells with small facial fields. In both cases, the more ventral cells lie in the ventral lamella of the PO, which suggests a functional as well as physical continuity between rDAO and the ventral lamella of the PO. We therefore conclude that the face zone in the rDAO and the face zone in the PO form one continuous and complete map of the face with an orderly progression of receptive fields. Furthermore, we have found that stimulation of the red nucleus can inhibit rDAO cells with facial receptive fields just as it does cells with receptive fields from the rest of the body.     

On neuronal homology: a comparison of similar axons in Musca, Sarcophaga, and Drosophila (Diptera: Schizophora)

Transganglionic transport of horseradish peroxidase (HRP) was used to study the organization of the thoracic spinal nerve projection to the dorsal horn in rats. Labeling was found in the superficial dorsal horn 16-20 hours after application of HRP to the cut ends of various spinal nerve rami. Labeling was restricted to the outer part of the substantia gelatinosa at these stages. Longer survivals (25-48 hours) gave rise to labeling of the deep part of substantia gelatinosa and deeper parts of the dorsal horn as well. The dorsal ramus projected to the lateral third of the horn from half a segment rostral to half a segment caudal to the entry segment. The ventral ramus projected to the medial two-thirds of the horn from 1 1/2 segments rostral to half a segment caudal to the entry segment. The two branches of the ventral ramus that were examined projected to separate medial and lateral compartments for the entire ventral ramus. There was a distinct lateromedial shift of the projection found from rostral through caudal levels within the projection compartment for each nerve. The results indicate that the dorsal horn projection of thoracic spinal nerve branches is organized in longitudinal compartments which are arranged in a strictly somatotopic fashion.     

Distribution and innervation of taste buds in the axolotl

Adult axolotls have approximately 1,400 taste buds in the epithelium of the pharyngeal roof and floor and the medial surfaces of the visceral bars. These receptors are most dense on the lingual surfaces of the upper and lower jaws, slightly less dense throughout lateral portions of the pharyngeal roof and floor, and more sparse within medial portions of the pharyngeal roof and floor, except for a median oval patch of receptors located rostrally between the vomerine tooth fields. Each taste bud is a pear-shaped organ, situated at the center of a raised hillock and averaging 80 and 87 microm in height and width, respectively. Each comprises 50 to 80 cells, which can be classified as basal, dark fusiform, or light fusiform, based on differences in their morphology. The distal ends of the apical processes of the fusiform cells reach the surface of each hillock, forming a single taste pore with an average diameter of 15 microm. Each apical process terminates in one of three ways: as short, evenly spaced microvilli; as long clustered microvilli; or as large, stereocilia-like microvilli. The pharyngeal epithelium and associated taste buds in axolotls are innervated solely by rami of the facial, glossopharyngeal and vagal nerves. Approximately, the rostral one half of the pharyngeal roof is innervated by the palatine rami of the facial nerve, whereas the caudal one half of the pharyngeal roof is innervated by the pharyngeal rami of the glossopharyngeal and vagal nerves. The lingual surface of the lower jaw is innervated by the pretrematic (mandibular) ramus of the facial nerve. The dorsal two-thirds of the visceral arches, and the ventral one-third of the visceral arches and the pharyngeal floor, are innervated by both the pretrematic and post-trematic rami of the glossopharyngeal and vagal nerves, respectively.     

Peripheral organization and central projections of the electrosensory nerves in gymnotiform fish

The electrosensory system of weakly electric gymnotiform fish is described from the receptor distribution on the body surface to the termination of the primary afferentsin the posterior lateral line lobe (PLLL). There are two types of electroreceptor(ampullary and tuberous) and a single type of lateral line mechanoreceptor (neuromast). Receptor counts in Apteronotus albifronsshow that (1) neuromasts are distributed as in other teleosts; (2) ampullary receptors number 151 on one side of the head and 208 on one side of the body; (3) tuberous receptors were estimated to number 3,000-3,500 on one side of the head and 3,500-5,000 on one side of the body. The distribution of each receptor type is described. Each receptor is innervated by a single primary afferent. Electro-sensory afferents have myelinated cell bodies in the ganglion of the anterior lateral line nerve (ALLN). The distribution of these ganglion cell diameters is strongly bimodal in Apteronotus and Eigenmannia: The smaller-diameter cells may be those which innervate ampullary electroreceptors, the larger-diameter tuberous electroreceptors. Transganglionic HRP transport techniques were used to determine the first-order connections of the anterior lateral line nerve in six species of gymnotiform fish. Small branches of the ALLN were labeled so as to determine the somatotopic organization in the PLLL. The PLLL is divided into four segments from medial to lateral, termed medial, centromedial, centrolateral, and lateral segments (Heiligenberg and Dye, '81). Representations of the head are found rostrally in each zone, and the trunk is mapped caudally in each zone. Thus there are four body maps in the PLLL. The medial segment receives ampullary input (Heiligenberg and Dye, '82) and maps the dorsoventral body axis mediolaterally, as does the tuberous centrolateral segment. The tuberous centromedial and lateral segments map the dorsoventral axis lateromedially. Thus the medial and centromedial segments meet belly to belly, the centromedial and centrolateral segments meet back to back, and the centrolateral and lateral segments meet belly to belly. Adjacent electrosensory maps within the PLLL are therefore always mirror images.     

Bilateral and multimodal sensory interactions of single cells in the pigeon's midbrain

Standard microelectrode recording techniques were employed to monitor single unit activity in the pigeon's nucleus intercollicularis and medial substantia grisea et fibrosa periventricularis in response to visual, tactile and auditory stimuli. Approximately 40% of the units were driven exclusively by visual stimuli, 8% by tactile stimuli, 47% by both visual and tactile stimuli and a very small percentage by auditory stimuli. Visual receptive fields were generally excitatory in the contralateral eye and suppressive in the ipsilateral eye. Most units were movement selective and some demonstrated direction sensitivity, summation and habituation. Units were generally insensitive to stimulus shape or contrast reversal. Somatosensory receptive fields were located on both sides of the body and were either excitatory or suppressive or both. Ipsilateral visual and somatosensory bimodal inputs were most often of the same sign while ipsilateral visual and contralateral somatosensory bimodal inputs tended to be of opposite sign. Visual and somatosensory receptive field locations of bimodal units tended to be in register.     

Topographical representation of the peripheral nerve branches of the facial nucleus of the opossum: a study utilizing horseradish peroxidase

The main facial nucleus of a marsupial, the North American opossum (Didelphis marsupialis virginiana), was subdivided into 6 portions by localizing HRP-positive neurons after injecting all muscles supplied by each major peripheral motor branch of the facial nerve. In the medial lobe of this dumbbell-shaped nucleus the caudal auricular nerve, rostral auricular ramus and cervical ramus were represented dorsemedially, dorsolaterally and ventrally, respectively. The lateral lobe contained zygomatic ramus cells dorsemedially, marginal mandibular ramus cells ventromedially and buccal rami cells laterally. The cells supplying the caudal digastric muscle were in the accessory facial nucleus. Thus, even though the facial nucleus of the opossum lacks distinct ramal subdivisions in Nissl preparations, such are evident after HRP labeling.     

Organization of somatosensory input to the deep collicular laminae in hamster

Retrograde transport of horseradish peroxidase (HRP) was used to delineate the sources of somatosensory input to the hamster's superior colliculus. Cells in the ipsilateral somatosensory cortex and contralateral dorsal horn of the spinal cord, dorsal column nuclei, lateral cervical nucleus, internal basilar nucleus, nucleus of the spinal trigeminal tract and deep layers of the superior colliculus were labeled following HRP injections centered in the deep tectal laminae.The response characteristics of somatosensory corticotectal, spinotectal and intertectal neurons were investigated with extracellular single unit recording methods and, with the exception of the fact that the receptive fields of corticotectal and spinotectal neurons were consistently smaller than those of cells recorded in the colliculus, the response characteristics of these neurons were quite similar to those of somatosensory neurons in the deep layers of the tectum. Lesions of the somatosensory cortex or dorsal half of the spinal cord were also combined with single unit recording in the colliculus to determine whether or not such damage altered the incidence and/or response characteristics of deep layer somatosensory cells. These lesions had no appreciable effect upon the functional organization of the deep tectal laminae. The implications of these results with regard to the convergence of visual and somatosensory information in the tectum are discussed.     

Peripheral configuration and central projections of the lateral line system in Astronotus ocellatus (Cichlidae): a nonelectroreceptive teleost

The lateral line system of Astronotus ocellatus comprises one trunk canal, one tail canal, and three head canals. The sensory receptors on the head are innervated by rami of the dorsal anterior, ventral anterior, and posterior lateral line nerves, and those along the trunk and tail by rami of the posterior lateral line nerve. The peripheral configuration of lateral line canals and nerves was examined in whole mount preparations, the central connections of restricted groups of endorgans studied using HRP and degeneration methods, and the neuronal morphology and cytoarchitecture of the lateralis region investigated with Nissl, silver, and Golgi methods. Neurons of the lateralis cell column are diffusely arrayed and of variable morphology. They are oriented primarily in the transverse plane and, with the exception of a dorsal lamina of large multipolar cells, are not organized into zones. Lateralis fibers bifurcate on entering the brainstem, course in lateral tracts, and give off medially directed collaterals to terminate in the ipsilateral nucleus medialis and nucleus caudalis. In addition, fibers terminate in the eminentia granularis of the cerebellum, but only fibers supplying endorgans in the head canals penetrate the granule cell zone of the cerebellar corpus. Fibers supplying sense organs in adjacent canals overlap in their central endings, whereas fibers of distantly separated receptors do not overlap. The rami supplying trunk and tail canal organs do not project as far rostrally in the central neuropil as do the other rami. Endings of posterior lateral line fibers lie dorsal to those of the anterior lateral line nerves, and some lateralis fibers terminate within the confines of the magnocellular, descending, and posterior nuclei of the octavus column. Although there is spatial order to the lateralis projections, there is no clear somatotopic organization in the lateralis region.     

Altered responses to cutaneous stimuli in the second somatosensory cortex following lesions of the postcentral gyrus in infant and juvenile macaques

Infant macaques recover tactile abilities better than older animals after somatosensory cortical lesions. To investigate the neural basis of this phenomenon, we ablated the hand representation in primary somatosensory cortex (SI) of infant and juvenile Macaca mulatta and recorded in ipsilateral second somatosensory cortex (SII) a year later. We also made tracer injections to verify the lesion boundaries and to study the connections of SII after the lesion of SI. Similar to the report of Pons et al. (Science 237:417-420, '87), we found that substantial portions of the SII hand area were unresponsive to cutaneous stimulation of the hand in both age groups. Particularly, there were no cutaneous receptive fields restricted to the digits. Some responses were elicited in each animal by mechanical stimulation of the hand, including a proportion related to cutaneous receptive fields. This proportion was higher in the infants than in the juveniles, which may explain the greater capacity of the infants for recovery of tactile function after SI lesions. The residual somatic drive in the SII hand area of the juveniles was attributable to sparing of parts of areas 3a and 3b. However, in the infants, this explanation was not tenable since the responses noted in SII occurred even after total ablations of the postcentral gyrus. The pattern of corticocortical connections revealed by injections of HRP into the medial margin of the SI lesion and of Fast Blue into SII in one infant confirmed the absence of SI inputs to the region of SII where responses were recorded from the hand. Representations of body parts other than the hand were normally responsive, and their location was consistent with normal somatotopy in SII.     

Altered responses of nociceptive cat lamina I spinal dorsal horn neurons after chronic sciatic neuroma formation

The activity of lumbar spinal dorsal horn lamina I neurons with afferent drive from the sciatic nerve was studied in intact cats and in cats with acute sciactic nerve transection or chronic sciatic nerve transection with neuroma formation. The majority (51 of 75) of neurons recorded in lamina I ipsilateral to a neuroma had no receptive field and could only be identified by their responses to electrical stimulation of the sciatic nerve. The remainder could be activated by the sciatic nerve, but their responses to mechanical stimulation were irregular in comparison to the stable responses of cells recorded in control animals and to the responses of cells contralateral to chronic nerve lesions. Animals with acute nerve transections demonstrated as loss sciatic nerve-innvervated cells with receptive fields except for those cells located on the lateral edge of the dorsal horn, which had normal, proximal receptive fields and response characteristics. In addition, the characteristic somatotopy of lamina I cells was not observed in some cats with chronic neuromata. The mediolateral distribution of cell types indicated that some cells had altered receptive fields following chronic nerve transection. The data presented for lamina I neurons agrees with the observation of spinal cord plasticity first presented for cat dorsal horn cells. Since there is no evidence for a redistribution of intact afferent fibers following chronic nerve transection in adult mammals, the mechanism of altered somatotopy may involved alterations in synaptic efficacy at existing synapses.     

Retinal projections in adult and newborn grey squirrels

Retinal projections in newborn squirrels were compared to those in adults by using horseradish peroxidase (HRP) as a highly sensitive anterograde tracer. In both newborn and adult squirrels, the HRP reaction product was found in the dorsal lateral geniculate nucleus, the superior colliculus, the pretectal nuclei, and the nuclei of the accessory optic tract. Thus, newborn squirrels have retinal input to most or all structures normally innervated in the adult. However, the pattern of terminations differed in the newborn from that in the adult, and this was especially apparent in the dorsal lateral geniculate nucleus and the superior colliculus. In the dorsal lateral geniculate nucleus, the regions of ipsilateral and contralateral retinal inputs were clearly less segregated than in adults, although the adult laminar pattern of retinal terminations was partially apparent, even though there was yet no cytoarchitectural evidence of the adult lamination pattern. In the superior colliculus, a marked difference was seen in the pattern of ipsilateral retinal terminations. In the adult, ipsilateral retinotectal input was restricted to a narrow, dense, patchy, mediolateral band in stratum opticum in the rostral colliculus. In the newborn, the ipsilateral retinotectal input was less dense, free of patches, spread in thickness to include much of the stratum opticum and the superficial grey, and spread in extent to include all but the caudal pole of the colliculus. These observations are consistent with the prevailing view that visual connections are initially widespread and become restricted during the course of development.