Basilar Pontine Nuclei Research Articles

Afferents to the cerebellar flocculus in cat with special reference to pathways conveying vestibular, visual (optokinetic) and oculomotor signals

Horseradish peroxidase (HRP) was injected into the cerebellar flocculus of 20 cats to determine: (a) the proportions of afferents from the various brain stem nuclei; (b) possible projections from the basilar pontine nuclei; and (c) sources of saccadic eye movement signals recorded from flocculus Purkinje cells. Results confirm earlier findings that the flocculus receives large numbers of mossy fibre afferents from the vestibular and perihypoglossal nuclei, bilaterally, and climbing fibres from the contralateral inferior olive (dorsal cap, ventrolateral outgrowth, medial accessory olive, ventral bend of principal olive). In addition, large numbers of HRP-labeled neurons have been identified within: (i) the basilar pontine nuclei, bilaterally, where they are distributed in columns in the dorsolateral, lateral, ventral medial and dorsomedial nuclei; (ii) the nucleus reticularis tegmenti pontis; (iii) several of the cranial motor nuclei, VI, VII, X (retrofacial n.), XI (n. ambiguus), and XII; (iv) the raphe magnus, pontis and obscurus; (v) the lateral reticular nucleus, pars subtrigeminalis. Finally, new information is presented which shows that large numbers of flocculus projecting neurons are located within the medial longitudinal fasciculus at two locations; one just rostral to the hypoglossal nucleus and another group extends 2-3 mm rostral to the abducens nucleus. These groups are bilateral, and have been termed, respectively, the caudal and intermediate interstitial nucleus of the medial longitudinal fasciculus. Both groups correspond in location to physiologically identified neurons in cat which fire in relation to saccadic eye movements. Their projection to the flocculus, in part, explains the saccadic discharge of Purkinje cells in the flocculus.

GABAergic neural elements in the rat basilar pons: Electron microscopic immunochemistry

Previous light microscopic immunoperoxidase studies of glutamic acid decarboxylase (GAD)-immunoreactive neural elements in the rat basilar pontine nuclei revealed immunocytochemical reaction product in neuronal somata and axon terminals. In the present study, pre-embedding immunoperoxidase labeling of GAD or gamma-aminobutyric acid (GABA) and postembedding immunogold labeling of GABA allowed the ultrastructural visualization of these neural elements in the basilar pontine nuclei of colchicine-treated animals. At the electron microscopic level, immunolabeled neuronal somata exhibited smoothly contoured nuclei, whereas some dendrites also contained reaction product after immunocytochemical treatment and were postsynaptic to both immunoreactive and nonimmunoreactive axon terminals. Synaptic boutons immunoreactive for GAD or GABA exhibited cross-sectional areas that ranged from 0.1 to 3.8 microns 2 and generally appeared round or elongated in most sections. The majority (95%) of immunolabeled boutons contained pleomorphic synaptic vesicles and formed symmetric synapses at their postsynaptic loci; however, boutons exhibiting round vesicles and boutons forming asymmetric synapses (5%) were also immunopositive. Small (less than 1.5 microns 2) GAD- or GABA-labeled axon terminals formed synaptic contact mainly with small dendritic profiles, dendritic spines, and neuronal somata, whereas large labeled boutons (greater than 1.5 microns 2) formed synapses with all sizes of dendritic profiles. Occasionally, a single immunolabeled bouton formed synaptic contact with two separate postsynaptic dendrites. It is suggested that the immunolabeled neuronal somata and dendrites observed in the rat basilar pontine nuclei represent a population of pontine local circuit neurons; however, it is known that GABAergic cell groups extrinsic to the pontine gray provide afferent projections to the basilar pons, and therefore at least some immunoreactive axon terminals present in the pontine nuclei are derived from these extrinsic sources. The ultrastructural observation of GABAergic neural elements in the rat basilar pontine nuclei confirms previous light microscopic findings and provides an anatomical substrate through which GABAergic neurons, whether arising from an intrinsic or extrinsic source, might exert an inhibitory influence on target cells within the pontine nuclei.

Evidence for the presence of presynaptic dendrites and GABA-immunogold labeled synaptic boutons in the monkey basilar pontine nuclei

Two general categories of GABA-immunoreactive (GABA-Ir) boutons are present in the monkey basilar pontine nuclei (BPN). One type characterized by a pale or lucent appearance is involved both in glomerular arrangements that include serial, triadic synapses, and in non-glomerular synapses. The second type of GABA-Ir bouton exhibits a wide variety in size and shape and contains a greater complement of synaptic vesicles than the first type, giving it a darker appearance in comparison to the pale GABA-Ir boutons. Such boutons participate only in non-glomerular synapses. It is suggested that the pale GABA-Ir boutons arise from the intrinsic population of GABA neurons, while the darker appearing boutons might take origin from one of the GABA-ergic afferent systems that reach the BPN.

Convergence of cortical and cerebellar projections on single basilar pontine neurons: A light and electron microscopic study in the rat

A protocol that involved a combination of two orthogradely transported tracer substances, wheat agglutinin-horseradish peroxidase and Phaseolus vulgaris leucoagglutinin injected at separate locations in the same animal was utilized to investigate the possible congruence of axonal projection fields formed by the cerebral cortical and cerebellar afferents to the basilar pontine nuclei. When large placements of tracer material were made in the cerebellar nuclei to label the cerebellopontine projections and a second tracer was injected in one of several cerebral cortical areas to visualize certain corticopontine projections, it was noted that axon terminal zones of the cortical and cerebeller systems occupied greater or lesser amounts of the same basilar pontine territory depending on the location of the cerebral cortical injection. Cerebellopontine terminal fields exhibited their greatest congruency with projections from the motor cortex containing the representation for facial musculature and with projections from the forelimb sensorimotor cortex. A lesser degree of overlap was observed when cerebellar projection zones were visualized in combination with basilar pontine projections from sensory face cortex, hindlimb sensorimotor cortex, visual cortex and auditory cortex. In addition, it was apparent that portions of the cerebellopontine and corticopontine terminal fields did not overlap at all. A related series of electron microscopic experiments was undertaken to establish that within the zones of overlapping cerebellar and cortical projections, there was in fact a convergence of the two afferent systems on single basilar pontine neurons. Boutons of the corticopontine system were labeled by the orthograde transport of wheat germ agglutinin-horseradish peroxidase injected into the sensorimotor cortex while cerebellopontine terminals were marked for electron microscopic identification in the same animal by transecting the brachium conjunctivum and allowing sufficient time for boutons in the pontine nuclei to exhibit degeneration. Although the number of definitive examples of convergence was small, nonetheless it was possible to observe single basilar pontine neuron dendrites receiving synaptic contacts from both the cortical and cerebellar afferents systems. Taken together these obervations indicate that some basilar pontine neurons receive a dual or convergent input from the cerebral cortex and cerebellar nuclei. It is difficult to estimate the prevalence of such convergence since cortical and cerebellar inputs typically contact distal and proximal pontine neuron dendrites, respectively, thus limiting the chances that both types of boutons can be observed in contact with a single basilar pontine neuron dendrite. Since the basilar pons contains both projection and local circuit neurons, and it has been shown that the cerebellopontine system includes excitatory and inhibitory components, it must remain for future electrophysiological studies to clarify the functional significance of this convergent circuitry.

Survey of noncortical afferent projections to the basilar pontine nuclei: a retrograde tracing study in the rat.

The retrograde transport of the conjugate wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was used in the rat to identify the cell bodies of origin for all subcortical projections to the basilar pontine nuclei (BPN). A parapharyngeal surgical approach was used to allow the injection micropipette to enter the BPN from the ventral aspect of the brainstem and thus avoid any potential for false-positive labeling due to transection and injury-filling of axonal systems located dorsal to the basilar pontine gray. A surprisingly large number of BPN afferent cell groups were identified in the present study. Included were labeled somata in the lumbar spinal cord and a large variety of nuclei in the medulla, pons, and midbrain, as well as labeled cells in diencephalic and telencephalic nuclei such as the zona incerta, ventral lateral geniculate, hypothalamus, amygdala, nucleus basalis of Meynert, and the horizontal nucleus of the diagonal band of Broca. Quite a number of cell groups known to project directly to the cerebellum also exhibited labeled somata in the present study. To explore the possibility that such neurons were labeled because their axons were transected and injury-filled as they coursed through the BPN injection site to enter the cerebellum via the brachium pontis, a series of rats received complete, bilateral lesions of the brachium pontis followed 30-60 minutes later with multiple, diffuse injections of WGA-HRP (12-16 placements per animal) throughout the cerebellar cortex. In another series of animals, the massive cerebellar WGA-HRP injections were not preceded by brachium pontis lesions. In the latter cases, each of the cell groups in question that were known to project directly to the cerebellum exhibited labeled somata. However, when the cerebellar HRP injections were preceded by brachium pontis lesions, each of the cell groups in question continued to exhibit labeled somata in numbers comparable to that observed in the nonlesion cases. This implies that such neurons project to the BPN and the cerebellar cortex and that the axons of these particular neurons do not project to the cerebellum via the brachium pontis.

Measuring cholinergic sensitivity: II. Arecoline effects on metabolic activity in pontine regions of rat brain

Brain imaging studies may help to localize areas particularly sensitive to cholinergic agonists. A study in rats using the 2-deoxyglucose method for studying changes in brain metabolic activity was performed. Among regions in the pons/medulla, arecoline, 0.05 mg/kg ip, increased glucose utilization in the dorsal raphe, median raphe, and basilar pontine nuclei without producing behavioral or cardiovascular effects. These brain areas, along with the hippocampus, may be pertinent to studies of muscarinic supersensitivity in humans.

Collateral branches of cerebellopontine axons reach the thalamus, superior colliculus, or inferior olive: A double-fluorescence and combined fluorescence-horseradish peroxidase study in the rat

Retrograde double-labeling methods that used two different fluorescent dyes or a fluorescent dye in combination with wheat germ agglutinin-horseradish peroxidase were used in the rat to study the collateralization of cerebellopontine fibers to the thalamus, the superior colliculus, or the inferior olive. In cases with combined basilar pontine nuclei and thalamus injections, double-labeled neurons were located in the rostral part of the lateral cerebellar nucleus as well as within the interpositus anterior and interpositus posterior nuclei. These cells are medium to large in size and multipolar-shaped. A much smaller number of double-labeled cells was observed in the combined basilar pontine nuclei and superior colliculus injections. In these cases most of the double-labeled cells were intermediate- to large-sized and either bipolar- or multipolar-shaped. Such neurons were distributed throughout the rostrocaudal extent of the lateral cerebellar nucleus, with only a few double-labeled cells located in the interpositus anterior and posterior nuclei. Finally, in the cases with combined basilar pontine nuclei and inferior olive injections, double-labeled cells were located in interpositus anterior and posterior nuclei and the medial portion of the lateral cerebellar nucleus. The double-labeled cells were relatively small in size and most were spindle-shaped. No double-labeled cells were observed in the medial cerebellar nucleus in any of the three injection combinations. Based upon the observation of double-labeled neurons in the deep cerebellar nuclei in each of the three injection combinations involving the basilar pontine nuclei, we conclude that cerebellar projections to the basilar pons arise in part as collaterals of axons that project to the thalamus, superior colliculus, or the inferior olive.

A review of recent observations concerning the synaptic organization of the basilar pontine nuclei.

Ultrastructural studies are described that have identified in the basilar pontine nuclei (BPN), the synaptic boutons formed by the corticopontine, cerebellopontine, tectopontine, and dorsal column nuclei-pontine afferent projection systems. In addition, immunocytochemical studies visualized neuronal somata, dendrites, and synaptic boutons that contain immunoreactivity for GABA or the synthesizing enzyme glutamic acid decarboxylase (GAD). Based upon differences in the mode of degeneration and postsynaptic locus of degenerative synaptic boutons in the BPN, it is suggested that two types of cortical neurons and three classes of deep cerebellar nuclear cells project to the BPN. For similar reasons, it appears that two types of neurons in the dorsal column nuclei project to the BPN while only one type of afferent synaptic bouton takes origin from the superior colliculus. Furthermore it appears that the population of BPN neurons projecting to the paramedian lobule receives convergent inputs from the cutaneous periphery and the corresponding region of sensorimotor cortex. Studies employing GAD immunohistochemistry indicate that GABA-ergic neurons and axon terminals are present in the BPN and thus support the suggestion that a local inhibitory interneuron is present within the BPN. Taken together these observations suggest that basilar pontine neurons might play a more active role in the integration of various types of information destined for the cerebellar cortex than has previously been recognized.

Parasagittal organization of mossy fiber collaterals in the cerebellum of the mouse.

We have observed that WGA-HRP injections in lobule VIII of the mouse result in the labeling of mossy fiber terminals in the anterior lobe (lobules I-V), which are distributed in five distinct parasagittal bands. Injections in the anterior lobe label mossy fiber terminations in lobules VIII and IX. We interpret these results as indicating that an extensive system of mossy fiber collaterals exists between the anterior lobe and lobule VIII (less so to IX), which terminates as discrete parasagittal bands in the anterior lobe. Intermediate bands are thus occupied by fibers that do not send collaterals to the posterior vermis (VII-IX). In an attempt to identify the source(s) of this collateral system we have used double retrograde tracing techniques. Following injections of one tracer in the anterior lobe and another in lobule VIII we observe large numbers of double retrogradely labeled neurons in the lateral reticular nucleus, the basilar pontine nuclei, and the spinal cord. Thus, these mossy fiber sources are the most likely origins for the banded collateral system. Our studies do not allow us to distinguish whether one, or more than one, of these regions contribute to the system.

A double retrograde fluorescent tracing analysis of dorsal column nuclear projections to the basilar pontine nuclei, thalamus, and superior colliculus in the rat.

Injections of the fluorescent dyes Nuclear yellow and True blue were used to determine that the dorsal column nuclei project in collateral fashion to the basilar pontine nuclei (BPN) and the ventral posterolateral nucleus of the thalamus or the BPN and the superior colliculus. Results indicated that relatively few dorsal column nuclear cells project to both the basilar pons and the superior colliculus. In contrast, many dorsal column nuclear cells that project to the BPN also give rise to collateral projections to the thalamus. Thus it is suggested that the latter dorsal column-BPN connections might at least represent in part the anatomical substrate for the electrophysiological demonstration that cerebellar granule cells can be activated at relatively short latency by peripheral tactile receptor stimulation.

Origin and ultrastructural identification of dorsal column nuclear synaptic terminals in the basilar pontine gray of rats

The ultrastructural characteristics of HRP-WGA-labeled or degenerating axon terminals arising from neurons in the dorsal column nuclei (DCN) were identified within the contralateral basilar pontine nuclei (BPN) following unilateral HRP-WGA injections or ablations of the DCN. The cells of origin of these projections were also identified through the application of the retrograde tracer HRP-WGA. Two groups of degenerating DCN-pontine terminals were identified. Both formed asymmetrical synaptic contacts with dendritic shafts and/or dendritic appendages of pontine neurons. One group of degenerating terminals contained small, round synaptic vesicles, while the other exhibited a mixture of dense core and pleomorphic vesicles. The former group, which clearly represented the majority of degenerating terminals observed, was interpreted to progress from an early filamentous form of degeneration to a later electron-dense variety and to originate from dorsally located DCN cells distributed primarily at the level of and just caudal to the area postrema. Other DCN-labeled neurons were more ventrally located and were postulated to give rise to those degenerative boutons that contained a mixture of dense core and pleomorphic-shaped vesicles. The present study also identified the cells of origin of two additional projections to the basilar pons: one from cells in the external cuneate nucleus and another from neurons of the medullary reticular formation.

Synapse formation and other ultrastructural features of postnatal development in the basilar pontine nuclei of the rat.

The postnatal development of the basilar pontine nuclei (BPN) was studied in the rat from birth through postnatal day 21. Although 3 maturational phases could be discerned within this time span, the events taking place within each phase overlap to some degree and the postnatal maturation of this region is best characterized as a continuum comprised of several simultaneously occurring processes. The early postnatal period (birth to postnatal day 5) is best described as a period of relative immaturity. Although generally similar to the adult BPN cytoarchitectonically, cell density in the early neonate is substantially higher than in the adult and this obscures to some degree the various subdivisions of the pontine gray. Neuronal somata are quite small in cross-sectional area relative to the adult and the neuropil is similarly immature and exhibits wide expanses of extracellular space and few synaptic boutons. These features are in marked contrast to the second developmental period which extends from postnatal days 6 through 13 and is characterized by an intense wave of synaptic proliferation. It is during this period that each of the various types of synaptic boutons that have been described in the adult make their appearance. Curiously, during this same time period, as well as in the early postnatal period, there also appears to be an ongoing process of spontaneous axon terminal elimination. The third developmental period begins on postnatal day 14 and extends through day 21 at which time the cytoarchitectonics, cytology and synaptic organization of the BPN are adult-like. During this period, no additional types of synaptic boutons appear, although the number of boutons continues to increase in a fairly uniform manner across all the various categories of synaptic boutons.

Certain basilar pontine afferent systems are GABA-ergic: Combined HRP and immunocytochemical studies in the rat

Injection of the tracer substance wheat germ agglutinin-horseradish peroxidase (WGA-HRP) directly into the basilar pontine nuclei using a ventral surgical approach resulted in the labeling of somata in many areas both rostral and caudal to the basilar pons. Certain of the sections that had been reacted for HRP were also incubated in antiserum prepared against glutamic acid decarboxylase (GAD) and processed according to routine peroxidase anti-peroxidase immunocytochemical procedures. Neuronal somata exhibiting both HRP and GAD reaction products were considered to represent GABA-ergic neurons that provide axonal projections to the basilar pontine nuclei. Such double-labeled neurons were observed within the zona incerta, anterior pretectal nucleus, lateral cerebellar nucleus, perirubral area, and the pontine and medullary reticular formation.

Classical conditioning of the nictitating membrane response of the rabbit. III. Connections of cerebellar lobule HVI.

We report the connections of cerebellar cortical lobule HVI in the rabbit. We have studied the anterograde and retrograde transport of wheatgerm-agglutinated horseradish peroxidase (WGA-HRP) following its injection into HVI to reveal efferent and afferent connections. All of the cases showed strong anterograde transport to the anterior interpositus nucleus (AIP) - indicating that this is the major efferent target of HVI. Retrogradely labelled cells were found in the inferior olivary, spinal trigeminal, lateral reticular, inferior vestibular and pontine nuclei. Within the olive, the medial part of the rostral dorsal accessory olive (DAO) and the adjacent medial part of the principal olive (PO) were consistently labelled in all cases. This area is known to receive somatosensory information from the face and neck. There was no projection to the hemispheral part of lobule VI from visual parts of the olive within the dorsal cap and medial parts of the medial accessory olive. Likely sources of visual and auditory information to HVI are the dorsolateral basilar pontine nuclei and nucleus reticularis tegmenti pontis, which were densely labelled in all cases. These anatomical findings are consistent with the suggestion that, during NMR conditioning, information related to the periorbital shock unconditional stimulus (US) may be provided by climbing fibres to HVI and light and white noise conditional stimulus (CS) information may be supplied by pontine mossy fibres.

GAD-immunoreactive neural elements in the basilar pontine nuclei and nucleus reticularis tegmenti pontis of the rat. I. Light microscopic studies.

Immunocytochemical procedures employing the unlabeled antibody enzyme (PAP) method were used to visualize those neuronal elements in the basilar pontine nuclei (BPN) and nucleus reticularis tegmenti pontis (NRTP) of the rat which contain glutamic acid decarboxylase (GAD), a key enzyme involved in the synthesis of the neurotransmitter gamma-aminobutyric acid (GABA). Neuronal somata, axons, and axon terminals in the BPN and NRTP exhibited GAD-immunoreactivity. Both thick and thin varieties of labeled axons and terminals were distributed in varying densities throughout the BPN and NRTP. The greatest accumulation of labeled terminals was noted in the ventrolateral and lateral border regions of the BPN while a slightly less dense aggregation was observed along the ventral, ventromedial and midline regions of the pontine gray. Labeled fibers and terminals were also observed in the dorsal and ventral peduncular regions as well as central portions of the lateral, ventral, and medial pontine areas. Axonal and terminal labeling was present throughout the NRTP but no focal increases in density similar to those in the BPN were apparent. No obvious GAD-labeled fiber bundles could be observed to enter the BPN or NRTP. However, small fascicles of labeled axons were seen to course ventrally around the dorsolateral aspect of the cerebral peduncle to reach lateral pontine areas while other labeled axons descended through clefts in the mid-portion of the cerebral peduncle or passed through the medial lemniscus and around the medial portion of the cerebral peduncle to enter the pontine gray.

Projections to the basilar pontine nuclei from face sensory and motor regions of the cerebral cortex in the rat.

Orthograde axonal transport tracing methods were used to describe the projections to the basilar pontine nuclei (BPN) which arise within the face representation of motor or somatosensory cerebral cortex. Injections centered in motor face (MF) cortex resulted in the labeling of several corticopontine terminal fields which exhibit a rostrocaudal columnar arrangement within the ipsilateral BPN. The location of such terminal zones is consistent with the somatotopic pattern of termination previously described for limb sensorimotor cortices. In contrast, the projections from somatosensory face (SF) cortical regions largely terminate in BPN areas separate from those receiving either limb sensorimotor or MF inputs. Both MF and SF cortices also give rise to projections to the contralateral BPN; those from SF cortex are less extensive than those of MF origin. In addition to their relationship with limb sensorimotor corticopontine terminations, the MF projections to the BPN also seem to partially overlap the projection zones of the cerebellopontine system, particularly the regions projected upon by the lateral cerebellar nucleus. The SF projections, on the other hand, appear to terminate in BPN areas that also receive input from either the dorsal column nuclei or the spinal trigeminal complex. There is only minimal potential overlap between MF and SF projections in the BPN. With regard to the pontocerebellar system, the projections from MF cortex terminate among BPN neurons which project to the cerebellar hemispheres, particularly lobus simplex, crus I and crus II. The SF projections also overlap BPN neurons which project to the lateral hemispheres in addition to the paraflocculus and vermal lobules VII and IXa,b. Taken together these observations suggest that subsets of BPN neurons might exist such that some receive convergent inputs from systems whose function can generally be regarded as motor (sensorimotor cortex, cerebellopontine) while another population of BPN neurons might integrate signals from systems which transmit somatosensory information (dorsal column nuclei, spinal trigeminal).

Afferent projections to the cerebellar flocculus in the pigmented rat demonstrated by retrograde transport of horseradish peroxidase.

The horseradish peroxidase (HRP) retrograde transport method was used to identify brainstem afferents to the cerebellar flocculus in the pigmented rat. Injections of the enzyme were made through recording microelectrodes, making it possible to localize the injection site by physiological criteria. Clearly, the largest number of afferents arise from the bilateral vestibular and perihypoglossal nuclei and from the contralateral dorsal cap (of Kooy) of the inferior olive. Additionally, a substantial number arise bilaterally from: (1) the nucleus reticularis tegmenti pontis (NRTP); (2) several of the cranial motor nuclei including the abducens, retrofacial and facial nuclei and the nucleus ambiguus; (3) the rostral part of the lateral reticular nucleus (subtrigeminal nucleus); (4) the raphe pontis and raphe magnus and (5) neurons intercalated among the medial longitudinal fasciculus (MLF) just rostral to the hypoglossal nucleus and another group rostral to the abducens nucleus. The basilar pontine nuclei contained a large number of lightly labeled neurons in all flocculus injections which were discretely located within the dorsolateral, lateral and medial divisions. These areas were labeled bilaterally but with a slight contralateral preponderance. Injection into the flocculus, but involving the adjacent ventral paraflocculus, produced a heavier labeling of pontine neurons with a slightly different distribution. Therefore, we tentatively conclude that the flocculus receives input from these pontine visual centers (dorsolateral, lateral and medial nuclei), perhaps through collateral projections from neurons projecting to the paraflocculus. The present study demonstrates strong similarities between the rat and other species studied (e.g., rabbit, cat, monkey) in terms of the brainstem nuclei projecting to the flocculus. Most noticeable in quantitative terms are the pathways known to mediate vestibular (vestibular and perihypoglossal nuclei) and visual (optokinetic) information (e.g., NRTP). Additionally, we can provide morphological evidence that the midline and paramedian pontine tegmentum, identified in the cat and monkey as containing saccade-related neurons, send large numbers of projections to the rat flocculus. Given these similarities, the rat may be a suitable animal model in which to study the pathways underlying visual-vestibular interaction and saccadic mechanisms in the flocculus.

The cerebellopontine system in the rat. I. Autoradiographic studies.

This study utilized light microscopic autoradiographic procedures to describe the projections from the three major subdivisions of the deep cerebellar nuclei (DCN) to the basilar pontine nuclei (BPN). Although the vast majority of cerebellopontine axons reached the BPN via the descending limb of the brachium conjunctivum (BC) after crossing the midline within the midbrain, a relatively small number of ipsilaterally projecting fibers was also observed. Fascicles of cerebellopontine axons left the main bundle of descending limb fibers throughout much of the rostrocaudal length of the BPN and passed around and through the medial lemniscus and cerebral peduncle to enter the pontine gray. The lateral cerebellar nucleus gave rise to the largest number of cerebellopontine fibers, whose terminal fields exhibited both diffuse and patchlike labeling patterns within each of the major subdivisions of the BPN including medial, ventral, lateral, and dorsal areas. Projections from the interpositus complex exclusive of its posterior division were fewer and less widely distributed than those from the lateral nucleus. Interpositopontine fibers terminated primarily in the caudal one-half of the BPN in medial, ventral, and lateral regions and overlapped somewhat with projections from the lateral cerebellar nucleus. Pontine projections emanating from the medial cerebellar nucleus were the fewest and most restricted in distribution relative to the other two cerebellar efferent systems. Such fibers formed a patchlike network of terminal fields which extended throughout much of the rostrocaudal length of the BPN in medial and dorsomedial regions. A relatively small but considerable number of ipsilateral cerebellopontine fibers terminated in pontine regions, which often mirrored the typical contralateral projection fields. Although it proved difficult to determine the precise origin of the ipsilateral fiber systems, it appeared that each of the three major DCN subdivisions made some contribution. Also it was apparent that considerable overlap existed between cerebellopontine projection zones and those of other pontine afferents including sensorimotor, visual, and auditory cortices, the superior colliculus, and the mammillary nuclei of the hypothalamus. Moreover, cerebellopontine terminal fields were congruent in some instances with discrete clusters of BPN neurons which serve as the source of pontocerebellar fiber systems, reaching portions of the lateral cerebellar hemispheres, posterior vermis, and the paraflocculus.

A transient component of the developing corticospinal tract arises in visual cortex

Following injections of tritiated leucine restricted to occipital (visual) regions of cerebral cortex during the first postnatal week (days 3–6), labeled corticofugal axons were followed caudally through the pons, medullary pyramid, pyramidal decussation and into the cervical spinal cord. However, when similar injections were made into visual cortex during the second postnatal week, labeled corticofugal axons could not be traced beyond mid-pontine levels where abundant axon terminal labeling was evident in the basilar pontine nuclei. Since axonal labeling did not subsequently appear at levels caudal to the pons during later stages of development or in the adult, it is suggested that the contribution to the corticospinal system made by visual cortical axons in the first postnatal week is a transient connection which is eventually lost either by a process of selective axon collateral elimination or cell death.

Electron microscopic identification of superior colliculo-pontine axon terminals

Synaptic boutons emanating from axons of superior colliculus origin were identified by electron microscopy in the neuropil of the basilar pontine nuclei. Such boutons were relatively small (0.6–2.0 μm) and exhibited electron-dense degeneration within a 1–2 day period following electrolytic lesions which involved much of the superior colliculus. Degenerating boutons were observed in synaptic contact with dendritic shafts and spines as well as neuronal somata. The reactive boutons were rapidly engulfed by phagocytic elements and were no longer visible in the neuropil after 6 days of survival.