猫小脑第Ⅶ小叶皮层—核团投射的电生理学研究

在三碘季铵酚制动的去大脑猫上,记录了小脑后叶的第Ⅶ小叶皮层浦肯野细胞(PC)对分别刺激顶核、间位核和齿状核的逆行场电位和逆行单位反应,以确定小脑皮层PC对这三个核团投射的空间分布。在鉴定了PC对其靶核团的投射后,用特制的模拟自然屈腕运动的刺激装置来推动猫同侧前肢的掌背,造成腕关节一次轻微的屈曲,观察该PG对这一刺激的单位反应。实验资料用电子计算机处理,作出平均诱发电位和刺激后时间直方图。 本文以电生理学方法揭示,猫小脑后叶第Ⅶ小叶皮层-核团投射存在较明确的纵区分布模式,纵区之间的分界线走向有一定的弯曲,与前叶略有不同。小脑后叶皮层从中线到两侧2.8mm为顶核区(FZ);其外侧为间位核区(IZ),最大宽度约为3.5mm;齿状核区(DZ)约始于5.0mm处。这三个不同纵区的PC对外周自然屈腕刺激都有反应,但反应细胞的百分数不同,FZ有59％的PC对外周刺激有反应,IZ为84％,DZ为20％。这些结果表明后叶第Ⅶ小叶具有类似于前叶的功能分布,IZ的PC对外周刺激有更大的调制作用,提示该皮层-核团投射的纵区结构有其特定的功能意义。     

The rat olivocerebellar system visualized in detail with anterograde PHA-L tracing technique, and sprouting of climbing fibers demonstrated after subtotal olivary lesions

The rat olivocerebellar climbing fiber system has been investigated at the light and electron microscopic level with anterograde Phaseolus vulgaris leucoagglutinin (PHA-L) tracing. From PHA-L Injections in different parts of the inferior olive labelled axons could be traced to the contralateral cerebellum. Arriving in the deep cerebellar white matter, the olivocerebellar axons ran around and through the cerebellar nuclei. Plexuses of labelled terminal fibers appeared in the cerebellar nuclei, and the density of this innervation was estimated to 1-4 million varicosities per mm3. Ultrastructurally, these boutons engaged in asymmetric synapses with small dendrites. Bundles of labelled fibers continued into the folial white matter, and terminated as climbing fibers in sagittal zones of the cerebellar cortex. Both the cortical and nuclear terminations of the olivocerebellar system are strictly topographically organized. The plasticity of climbing fibers was studied after partial lesions of the inferior olive induced by 3-acetylpyridine. One to 6 months after the lesion, surviving climbing fibers demonstrated extensive sprouting. The newly formed axons originated from parent climbing fiber plexuses, grew in the direction of parallel fibers, and formed terminal plexuses around several neighbouring Purkinje cells. As normal climbing fiber terminals, these terminals formed asymmetric synapses with spines of proximal Purkinje cell dendrites, and evidence by Benedetti et al. (1983) shows that the regenerated innervation is electrophysiologically functional. It is suggested that denervated Purkinje cells release a trophic substance, which stimulate surviving climbing fibers to sprouting, axonal growth and synapse formation.     

Three-dimensional architecture of identified cerebral neurosecretory cells in an insect

The organization of identified neurosecretory cell groups in the larval brain of the tobacco hornworm, Manduca sexta, was investigated immunocytologically. Computer-assisted three-dimensional reconstruction was used to examine the architecture of the neurosecretory cell groups. The group III lateral neurosecretory cells (L-NSC-III) which produce the prothoracicotropic hormone are located dorsolaterally in the protocerebrum and extend axons medially that decussate to the contralateral lobe prior to exiting the brain through the nervi corporis cardiaci I + II. The group IIa2 medial neurosecretory cells (M-NSC IIa2) are located anteriorly in the medial dorsal protocerebrum. The axons of these cells also exit the brain via the contralateral nervi corporis cardiaci I + II. However, their axons traverse a different pathway through the brain from that of the L-NSC III axons. Each of the cell groups possesses elaborate dendrites with terminal varicosities. The dendrites can be classified into specific fields based upon their location and projection pattern within the brain. The dendrites for these two neurosecretory cell groups overlap in specific regions of the protocerebral neuropil. After the axons of these neurosecretory cells exit the brain through the retrocerebral nerve, they innervate the corpus allatum where they arborize to form neurohemal terminals in strikingly different patterns. The L-NSC III penetrate throughout the glandular structure and the M-NSC IIa2 terminals are restricted to the external sheath. A third group of cerebral neurosecretory cells, the ventromedial neurons (VM) which stain with the monoclonal antibody to prothoracicotropic hormone in Manduca, are located anteriorly in the medial region of the brain. The axons of these cells do not exit the brain to the retrocerebral complex, but rather pass through the circumesophageal connectives and ventral nerve cord. These neurons appear to be the same VM neurons that produce eclosion hormone. One dendritic field of the L-NSC III terminates in close apposition to the VM neurons. The distinct morphologies of these neurosecretory cell groups in relation to other cell groups and the distribution of neuropeptides within the neurons suggest that insect neurosecretory cells, like their vertebrate counterparts, may have multiple regulatory roles.     

Cerebello-vestibular connections of the anterior vermis. A retrograde tracer study in different mammals including primates

Purkinje cells were retrogradely labelled from large injections of wheatgerm-coupled horseradish peroxidase in the vestibular nuclei, including Deiters' nucleus. The labelled Purkinje cells were located in two parallel strips in the anterior vermis; the medial strip is located within the A-zone, the lateral strip corresponds to the B-zone. In the ventral part of the anterior lobe the two strips fuse into a single band, in the dorsal part of the anterior lobe they are separated by a wedge-shaped area, corresponding to the X-zone. The B-zone proceeds in the simple lobule, where it deviates laterally and where it terminates at the centre of the ansoparamedian lobule. Identical zonal patterns were observed in cat, rabbit, rat and monkey. The demarcation of the anterior vermis by the lateral border of the B-zone, and the differences in the projection of the A and B-zone are briefly discussed.     

Clostridium botulinum C2 toxin: low pH-induced pore formation is required for translocation of the enzyme component C2I into the cytosol of host cells

The binary Clostridium botulinum C2 toxin consists of two individual proteins, the transport component C2II (80 kDa) and the enzyme component C2I, which ADP-ribosylates G-actin in the cytosol of cells. Trypsin-activated C2II (C2IIa) forms heptamers that bind to the cell receptor and mediate translocation of C2I from acidic endosomes into the cytosol of target cells. Here, we report that translocation of C2I across cell membranes is accompanied by pore formation of C2IIa. We used a radioactive rubidium release assay to detect C2IIa pores in the membranes of Chinese hamster ovary cells. Pore formation by C2IIa was dependent on the cellular C2 toxin receptor and an acidic pulse. Pores were formed when C2IIa was bound to cells at neutral pH and when cells were subsequently shifted to acidic medium (pH < 5.5), but no pores were detected when C2IIa was added to cells directly in acidic medium. Most likely, acidification induces a change from "pre-pore" to "pore" conformation of C2IIa, and formation of the pore conformation before membrane binding precludes insertion into membranes. When C2I was present during binding of C2IIa to cells prior to the acidification step, C2IIa-mediated rubidium release was decreased, suggesting that C2I interacted with the lumen of the C2IIa pore. A decrease of rubidium efflux was also detected when C2I was added to C2IIa-treated cells after the acidification step, suggesting that C2I interacted with C2IIa in its pore conformation. Moreover, C2I also interacted with C2IIa channels in artificial lipid membranes and blocked them partially. C2I was only translocated across the cell membrane when C2IIa plus C2I were bound to cells at neutral pH and subsequently shifted to acidic pH. When cell-bound C2IIa was exposed to acidic pH prior to C2I addition, only residual intoxication of cells was observed at high toxin concentrations, and binding of C2I to C2IIa was slightly decreased. Overall, C2IIa pores were essential but not sufficient for translocation of C2I. Intoxication of target cells with C2 toxin requires a strictly coordinated pH-dependent sequence of binding, pore formation by C2IIa, and translocation of C2I.     

Tissue Distributions of Dhurrin and of Enzymes Involved in Its Metabolism in Leaves of Sorghum bicolor

Three zones of carboxypeptidase inhibitory activity were observed when heat-stable extracts of potato tubers (cv. Russet Burbank) were chromatographed on carboxymethyl cellulose. The isoinhibitors found in these zones were denoted I, II, and III based upon their order of elution from this column. The predominant form (II) had previously been suggested to be a mixture of two polypeptides (IIa and IIb) differing in that IIa possessed an additional residue of glutamine (Hass et al. 1975 Biochemistry 14: 1334). These closely related isoinhibitors (IIa and IIb) were separated by equilibrium ion exchange chromatography and characterized. Isoinhibitor I was shown to be identical to II except for two replacements, Ser-30 → Ala and Arg-32 → Gly. These replacements had no significant effect on apparent K_i values toward either carboxypeptidase A or B. Isoinhibitor III, which was identical to II except that it lacked the amino terminal pyrrolidone carboxylic acid and following glutamine residue, was also functionally indistinguishable from II in inhibition studies. It was concluded that at least two and possibly as many as five genes code for the various isoinhibitor species which are present in potato tubers.     

The host cell chaperone Hsp90 is essential for translocation of the binary Clostridium botulinum C2 toxin into the cytosol

Clostridium botulinum C2 toxin is the prototype of the binary actin-ADP-ribosylating toxins and consists of the binding component C2II and the enzyme component C2I. The activated binding component C2IIa forms heptamers, which bind to carbohydrates on the cell surface and interact with the enzyme component C2I. This toxin complex is taken up by receptor-mediated endocytosis. In acidic endosomes, heptameric C2IIa forms pores and mediates the translocation of C2I into the cytosol. We report that the heat shock protein (Hsp) 90-specific inhibitors, geldanamycin or radicicol, block intoxication of Vero cells, rat astrocytes, and HeLa cells by C2 toxin. ADP-ribosylation of actin in the cytosol of toxin-treated cells revealed that less active C2I was translocated into the cytosol after treatment with Hsp90 inhibitors. Under control conditions, C2I was localized in the cytosol of toxin-treated rat astrocytes, whereas geldanamycin blocked the cytosolic distribution of C2I. At low extracellular pH (pH 4.5), which allows the direct translocation of C2I via C2IIa heptamers across the cell membrane into the cytosol, Hsp90 inhibitors retarded intoxication by C2I. Geldanamycin did not affect toxin binding, endocytosis, and pore formation by C2IIa. The ADP-ribosyltransferase activity of C2I was not affected by Hsp90 inhibitors in vitro. The cytotoxic actions of the actin-ADP-ribosylating Clostridium perfringens iota toxin and the Rho-ADP-ribosylating C2-C3 fusion toxin was similarly blocked by Hsp90 inhibitors. In contrast, radicicol and geldanamycin had no effect on anthrax lethal toxin-induced cytotoxicity of J774-A1 macrophage-like cells or on cytotoxic effects of the glucosylating Clostridium difficile toxin B in Vero cells. The data indicate that Hsp90 is essential for the membrane translocation of ADP-ribosylating toxins delivered by C2II.     

Cellular uptake of Clostridium botulinum C2 toxin requires oligomerization and acidification

The actin-ADP-ribosylating binary Clostridium botulinum C2 toxin consists of two individual proteins, the binding/translocation component C2II and the enzyme component C2I. To elicit its cytotoxic action, C2II binds to a receptor on the cell surface and mediates cell entry of C2I via receptor-mediated endocytosis. Here we report that binding of C2II to the surface of target cells requires cleavage of C2II by trypsin. Trypsin cleavage causes oligomerization of the activated C2II (C2IIa) to give SDS-stable heptameric structures, which exhibit a characteristic annular or horseshoe shape and form channels in lipid bilayer membranes. Cytosolic delivery of the enzyme component C2I is blocked by bafilomycin but not by brefeldin A or nocodazole, indicating uptake from an endosomal compartment and requirement of endosomal acidification for cell entry. In the presence of C2IIa and C2I, short term acidification of the extracellular medium (pH 5.4) allows C2I to enter the cytosol directly. Our data indicate that entry of C2 toxin into cells involves (i) activation of C2II by trypsin-cleavage, (ii) oligomerization of cleaved C2IIa to heptamers, (iii) binding of the C2IIa oligomers to the carbohydrate receptor on the cell surface and assembly with C2I, (iv) receptor-mediated endocytosis of both C2 components into endosomes, and finally (v) translocation and release of C2I into the cytosol after acidification of the endosomal compartment.     

The binary Clostridium botulinum C2 toxin as a protein delivery system: identification of the minimal protein region necessary for interaction of toxin components.

The binary Clostridium botulinum C2 toxin is composed of the enzyme component C2I and the binding component C2II, which are individual and non-linked proteins. Activated C2IIa mediates cell binding and translocation of C2I into the cytoplasm. C2I ADP-ribosylates G-actin at Arg-177 to depolymerize actin filaments. A fusion toxin containing the N-terminal domain of C2I (residues 1-225) transports C3 ADP-ribosyltransferase from Clostridium limosum into cells (Barth, H., Hofmann, F., Olenik, C., Just, I., and Aktories, K. (1998) Infect. Immun. 66, 1364-1369). We characterized the adaptor function of C2I and its interaction with C2IIa. The fusion toxin GST-C2I(1-225)-C3 was efficiently transported by C2IIa, indicating that C2IIa translocates proteins into the cytosol even when the C2I(1-225) adaptor was positioned in the middle of a fusion protein. Amino acid residues 1-87 of C2I were sufficient for interaction with C2IIa and for translocation of C2I fusion toxins into HeLa cells. Residues 1-87 were the minimal part of C2I to bind to C2IIa on the cell surface, as detected by fluorescence-activated cytometry. An excess of C2I(1-87) (but not of further truncated C2I fragments) competed with Alexa488-labeled C2I for binding to C2IIa. Also, the fragment C2I(30-431) and the fusion toxin C2I(30-225)-C3 competed with C2I-Alexa488 for binding to C2IIa. C2I(30-225)-C3 did not induce cytotoxic effects on cells when applied together with C2IIa, indicating that amino acid residues 1-29 are involved in translocation of C2I but are not absolutely essential for binding to C2IIa.     

Central neural pathways for angiotensin-induced thirst.

Evidence is reviewed implicating the preoptic region in angiotensin-induced thirst. The most responsive area according to results obtained with behavioral, electrophysiological, and autoradiographic mapping techniques is at the caudal border of the medial preoptic region and rostral border of the anterior hypothalamus. The neural pathway from this preoptic site for angiotensin-induced thirst extends along the medial forebrain bundle through the midlateral hypothalamus to the paramedial midbrain tegmentum and to an area ventrolateral to the central gray. Lesions of this pathway in the midlateral hypothalamus and rostral midbrain significantly attenuated drinking induced by microinjections of angiotensin II into the preoptic area but did not disrupt water intake induced by microinjections of angiotensin II into the subfornical organ or cerebral ventricles. Although the efferent pathways from angiotensin-receptive sites in the subfornical organ and cerebral ventricles are unknown, it appears from these observations that the medial forebrain bundle is not involved. Lesions of the medial forebrain bundle-lateral hypothalamus also do not disrupt drinking induced by microinjections of hypertonic saline into the preoptic region although lesions placed 1 mm further lateral do. Since fat lateral hypothalamic lesions are without effect on drinking induced by centrally administered angiotensin II, this suggests that intracellular and extracellular thirst signals are subserved by separate neural pathways in the hypothalamus.     

Synapsin IIa Bundles Actin Filaments

Abstract: Synapsins are neuron-specific phosphoproteins associated with small synaptic vesicles in the presynaptic nerve terminal. Synapsin I, which has been demonstrated to bundle F-actin in vitro, has been postulated to regulate neurotransmitter release by cross-linking synaptic vesicles to the actin cytoskeleton. To investigate the possible interaction of synapsin II with actin filaments, we expressed synapsin II in Spodoptera frugiperda and High Five insect cells using a recombinant baculovirus. Purified recombinant synapsin IIa was incubated with F-actin, and bundle formation was evaluated by light scattering and electron microscopy. Synapsin IIa was found to bundle actin filaments. Dose-response curves indicated that synapsin IIa was more potent than synapsin I in bundling actin filaments. These data suggest that synapsin IIa may cross-link synaptic vesicles and actin filaments in the nerve terminal.     

Effects of unilateral lesion of the inferior olive on L-[3H] aspartate receptors binding in synaptic membranes of cat cerebellar cortex

In adult cats, local injection of kainic acid (KA) in the inferior olive (IO) of one side, from which the crossed olivocerebellar projection originates, produced asymmetric postural and motor deficits, attributed to selective damage of the olivary neurons. Since aspartate is one of the putative transmitters of the olivocerebellar fibers, experiments were performed to find out whether 6-8 days after injection of KA within the IO of one side produced changes in aspartate receptors binding in different zones of the cerebellar cortex. In particular, binding in the contralateral zones of the cerebellar cortex was referred to proteins contained in membrane suspensions and compared with the control values obtained in the same experiments from the ipsilateral zones. Binding of L-[3H] aspartate decreased on the average to 53.4% of the control value in the medial zone and to 86.1% of the control value in the intermediate and lateral zones of the cerebellar cortex. This reduction varied in different experiments according to the side of the injection, in agreement with the well known pattern of regional distribution of the olivocerebellar projection within the cerebellar cortex. These findings favour aspartate as the putative neurotransmitter of the climbing fibers. The demonstration that binding of aspartate decreased in the cerebellar cortex of one side, 6-8 days after injection of KA in the corresponding IO, indicates that plastic events occur at this level following destruction of the olivocerebellar pathway. In particular, the reduced binding can be attributed either to a decrease in number of the postsynaptic receptor sites for aspartate or to a decreased affinity of this amino acid for the corresponding receptors. These findings, however, do not exclude that an hypersensitivity by denervation may occur at the level of individual Purkinje cells when they are deprived of the climbing fibers input. In order to answer this question further experiments are required to find out how the binding for aspartate is modified at increasing time intervals after the olivary lesion.     

Prospective Neural Areas and their Morphogenetic Movements during Neural Plate Formation in the Xenopus Embryo. II. Disposition of Transplanted Ectoderm Pieces of X. borealis Animal Cap in Prospective Neural Areas of Albino X. laevis gastrulae.

Small pieces of the animal cap of X. borealis gastrulae were transplanted into various regions of the noninvoluting marginal zone of albino X. laevis gastrulae, and the distribution of the donor cells was analyzed by quinacrine fluorescence staining.
The present study indicated that the prospective central nervous system (CNS) lies as a belt-shaped area in the noninvoluting marginal zone of early gastrulae. This belt-shaped prospective neural area extends as far as 0.7 mm (115° to the vegetal pole) above the blastopore in the dorsal midline and 1.3 mm lateral (130° to the dorsal midline) to the dorsal midline. The ectoderm of the dorsal region extends in the animal-vegetal direction and forms the ventral side of the CNS. The dorsalateral and lateral regions converge toward the dorsal midline and extended in the animal-vegetal direction. The former constitutes the lateral side of the anterior CNS, and the latter the dorso-lateral side of the posterior CNS.
The outer layer of ectoderm which was transplanted onto the inner layer of the host gastrula differentiated into neural tissues.
The prospective areas of the CNS and their morphogenetic movement during Xenopus embryogenesis are also discussed with regard to neural induction.     

Analysis of the efferent projections of the lateral geniculate nucleus with special reference to the innervation of the subcommissural organ and related areas

In order to define central neurons projecting to the subcommissural organ (SCO) and to related areas in the postero-medial diencephalon, Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected into the lateral geniculate nucleus of the rat. PHA-L-labelled neurons send axonal processes medially through the posterior thalamic nuclei and the posterior commissure to the other hemisphere. Branches of fibres originating from this projection form a plexus of nerve terminals in the underlying precommissural nucleus and in the nucleus of the posterior commissure. A small number of PHA-L-immunoreactive nerve fibres penetrate from the precommissural nucleus into the lateral part of the SCO. A few labelled fibres penetrate directly from the posterior commissure into the medial part of the caudal SCO. Most of the PHA-L-immunoreactive fibres occur in the hypendymal layer, although a few terminate near the ependymal cells of the organ. Many labelled fibres are found in the ventricular ependyma adjacent to the SCO, some fibres lying close to the ventricular lumen. These results were obtained only if the tracer was delivered into the intergeniculate leaflet of the lateral geniculate nucleus (IGL). The IGL innervates both the suprachiasmatic nucleus and the pineal organ; the connections between the IGL and the midline structures, including the SCO, suggest that these areas are influenced by the circadian system.     

Stage-dependent border cell and carbon flow from roots to rhizosphere

Rising CO(2) levels in the atmosphere have drawn attention to the important role of soil in sequestering carbon. This project goal was to quantify soil carbon deposition associated with border cell release and exudation from root growth zones. Carbon was measured with a Carlo Erba C/N analyzer in soil from the rhizosphere of mature grasses and, in separate experiments, in soil collected around root growth zones. Root border cells in "rhizosphere soil" (silica sand) were counted using a compound microscope after soil sonication and extraction with surfactant. For sand-grown Bromus carinatus, Zea mays, and Cucumis sativus, young seedlings (with roots shorter than 2 cm) released thousands of border cells, while older root tips released only hundreds. For a variety of native annual and perennial grasses and invasive annual grasses (Nassella pulchra, B. carinatus, B. diandrus, B. hordeaceus, Vulpia microstachys, Aegilops triuncialis, Lolium multiflorum, Zea mays), the rhizosphere of mature root systems contained between 18 and 32 μg C g(-1) sand more than that of the unplanted controls. Spatial analysis of the rhizosphere around the cucumber growth zone confirmed C enrichment there. The root tip provided C to the rhizosphere: 4.6 μg C in front of the growing tip, with the largest deposition, 20.4 μg C, to the rhizosphere surrounding the apical 3 mm (root cap/meristem). These numbers from laboratory studies represent the maximum C that might be released during flooding in soils. Scaling up from the organ scale to the field requires a growth analysis to quantify root tip distributions in space and time.     

Cellular uptake of Clostridium botulinum C2 toxin: membrane translocation of a fusion toxin requires unfolding of its dihydrofolate reductase domain

The Clostridium botulinum C2 toxin is the prototype of the family of binary actin-ADP-ribosylating toxins. C2 toxin is composed of two separated nonlinked proteins. The enzyme component C2I ADP-ribosylates actin in the cytosol of target cells. The binding/translocation component C2II mediates cell binding of the enzyme component and its translocation from acidic endosomes into the cytosol. After proteolytic activation, C2II forms heptameric pores in endosomal membranes, and most likely, C2I translocates through these pores into the cytosol. For this step, the cellular heat shock protein Hsp90 is essential. We analyzed the effect of methotrexate on the cellular uptake of a fusion toxin in which the enzyme dihydrofolate reductase (DHFR) was fused to the C-terminus of C2I. Here, we report that unfolding of C2I-DHFR is required for cellular uptake of the toxin via the C2IIa component. The C2I-DHFR fusion toxin catalyzed ADP-ribosylation of actin in vitro and was able to intoxicate cultured cells when applied together with C2IIa. Binding of the folate analogue methotrexate favors a stable three-dimensional structure of the dihydrofolate reductase domain. Pretreatment of C2I-DHFR with methotrexate prevented cleavage of C2I-DHFR by trypsin. In the presence of methotrexate, intoxication of cells with C2I-DHFR/C2II was inhibited. The presence of methotrexate diminished the translocation of the C2I-DHFR fusion toxin from endosomal compartments into the cytosol and the direct C2IIa-mediated translocation of C2I-DHFR across cell membranes. Methotrexate had no influence on the intoxication of cells with C2I/C2IIa and did not alter the C2IIa-mediated binding of C2I-DHFR to cells. The data indicate that methotrexate prevented unfolding of the C2I-DHFR fusion toxin, and thereby the translocation of methotrexate-bound C2I-DHFR from endosomes into the cytosol of target cells is inhibited.     

Separate populations of neurons in the oculomotor nucleus project to the cerebellum and the abducent nucleus. A retrograde fluorescent double-labelling study in the cat

Retrograde transport of fluorescent substances was used in order to investigate possible branching of axons from neurons in the oculomotor nucleus in the cat. Rhodamine-B-isothiocyanate (RITC) was injected into the cerebellar hemisphere, while Fluoro-Gold was implanted into the abducent nucleus. Neurons single-labelled with either of the dyes were found in the oculomotor nucleus in all cases, but no double-labelled neurons were found. The labelled cells were smaller than motoneurons and located in partly overlapping areas along the dorsal border of the oculomotor nucleus, with the RITC labelled cerebellar projecting cells concentrated medially and the Fluoro-Gold labelled neurons projecting to the abducent nucleus concentrated laterally. The RITC labelled cells were found throughout the rostrocaudal extent of the nucleus, while the Fluoro-Gold labelled cells were mainly found caudally. The present findings demonstrate that oculomotor neurons projecting to the feline cerebellum and abducent nucleus represent separate cell populations.     

Morphology of electrophysiologically identified baroreceptor afferents and second order neurones in the brainstem of the cat

Baroreceptor afferent fibres and second order baroreceptor neurones were identified by their discharge pattern and were intracellularly injected with horseradish peroxidase. Three afferent fibres and three second order neurones were reconstructed by camera lucida drawings from serial sections of the brainstem. The afferent fibres were classified as A delta-fibres and had terminal arborizations with synaptic boutons in the dorsomedial region of the nuclei of the solitary tract (TS). The afferent fibres had additional collaterals with a medial projection to the commissural nucleus and in a direction lateral to the TS. The terminals of these collaterals could not be demonstrated. The second order neurones were located in the same dorsomedial region as the synaptic boutons of the afferent fibres. Neurones were small and spindle-shaped with two primary dendrites: one dendrite projected cranially along the medial border of the TS, and the second one projected caudally and medially into the commissural nucleus. The unmyalinated axons of these neurones could be traced over a distance of 1 mm. In only one neurone could an axon collateral be detected. The axons projected dorsally around the TS in a ventrolateral direction beyond the boundaries of the nuclei of the TS. The axon collateral projected in the medial direction into the commissural nucleus. In no case were axon terminals demonstrated.