Anatomy of the rat medial geniculate body: II. Dendritic morphology

The medial geniculate body (MGB) of the rat was studied with Golgi methods to determine the distribution of neurons identified by dendritic morphology. These findings were compared with major divisions and constituent nuclei established by somatic and fiber architectonics, and by connections with temporal neocortex (Clerici et al.: Society of Neuroscience Abstracts 12:1272, 1986; 13:325, 1987; Anatomical Record 218:23, 1987; Winer and Larue: Journal of Comparative Neurology 257:282-315, 1987; Clerici and Coleman: Journal of Comparative Neurology 297:14-31, 1990). It was found that an elaboration of the prototypical scheme proposed by Morest (Journal of Anatomy 98:611-630, 1964) for partitioning the mammalian MGB is valid for characterizing the rat MGB. Two predominant categories of principal neuron dendritic patterning were identified: a bushy cell having tufted dendritic fields and a stellate cell with a radiate dendritic domain. Tufted neurons have large caliber dendritic trunks that divide profusely into daughter branches close to the soma with intertwining higher order branches that maintain a relatively restricted dendritic field. Stellate neurons typically emit primary dendrites in all directions that then divide dichotomously at wide angles at subsequent orders of branching to produce a somewhat spheroidal dendritic field. In the present study, the rat MGB is found to be a tripartite structure composed of ventral (MGv), dorsal (MGd), and medial (MGm) divisions, each uniquely characterized by constituent dendritic morphology. The paramount neuronal class of the MGv is the tufted principal cell. In the ventral and ovoid nuclei of the MGv the neuronal orientation of highly oriented bitufted cells is in register with afferent brachial axons. In the ventral nucleus, this arrangement approximates vertical with a dorsomedial tilt most prominent rostrally; in the ovoid nucleus, tufted cells adhere to the double spiraled course of afferent axons. The transition zone between ventral and ovoid nuclei contains tufted neurons that align with radially oriented fibers issuing from the junction of the ovoid and midgeniculate bundles. Bitufted neurons of the marginal zone parallel fibers at the lateral margin of the geniculate. Within the MGd the dorsal and caudodorsal nuclei are characterized by stellate cells with extensive dendritic arbors and busy neurons with dendritic branches less tufted than those observed in the MGv. The deep dorsal nucleus contains bitufted neurons that polarize with the long axis of the midgeniculate bundle and intermingle with stellate neurons. The suprageniculate nucleus includes neurons with large somata and long, sparsely branched and dorsoventrally oriented dendrites orthagonal to corticothalamic axons, as well as smaller neurons and classical stellate cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Disruption of auditory but not visual learning by destruction of intrinsic neurons in the rat medial geniculate body

The contribution of intrinsic neurons in the medial geniculate body (MG) of the rat to the classical conditioning of emotional responses to acoustic stimuli was examined. Injection of ibotenic acid, which destroys cell bodies but not axons of passage, into the MG disrupted the conditioned changes in mean arterial pressure and emotional behavior elicited by a tone previously associated with footshock. Unconditioned responses elicited by the tone and by the footshock were unaffected. Moreover, the same animals readily learned to associate a visual stimulus with footshock. Thus, destruction of intrinsic neurons in the MG selectively disrupts the conditioning of emotional responses to acoustic stimuli. The MG appears to be the afferent link in a modality specific emotional learning pathway.     

Anatomy of the auditory thalamocortical system in the mongolian gerbil: Nuclear origins and cortical field‐, layer‐, and frequency‐specificities

Knowledge of the anatomical organization of the auditory thalamocortical (TC) system is fundamental for the understanding of auditory information processing in the brain. In the Mongolian gerbil (Meriones unguiculatus), a valuable model species in auditory research, the detailed anatomy of this system has not yet been worked out in detail. Here, we investigated the projections from the three subnuclei of the medial geniculate body (MGB), namely, its ventral (MGv), dorsal (MGd), and medial (MGm) divisions, as well as from several of their subdivisions (MGv: pars lateralis [LV], pars ovoidea [OV], rostral pole [RP]; MGd: deep dorsal nucleus [DD]), to the auditory cortex (AC) by stereotaxic pressure injections and electrophysiologically guided iontophoretic injections of the anterograde tract tracer biocytin. Our data reveal highly specific features of the TC connections regarding their nuclear origin in the subdivisions of the MGB and their termination patterns in the auditory cortical fields and layers. In addition to tonotopically organized projections, primarily of the LV, OV, and DD to the AC, a large number of axons diverge across the tonotopic gradient. These originate mainly from the RP, MGd (proper), and MGm. In particular, neurons of the MGm project in a columnar fashion to several auditory fields, forming small‐ and medium‐sized boutons, and also hitherto unknown giant terminals. The distinctive layer‐specific distribution of axonal endings within the AC indicates that each of the TC connectivity systems has a specific function in auditory cortical processing. J. Comp. Neurol. 522:2397–2430, 2014. © 2014 Wiley Periodicals, Inc.     

The role of the medial geniculate region in differential Pavlovian conditioning of bradycardia in rabbits

The present study examined the role of the medial geniculate region (MGN) in differential Pavlovian conditioning of bradycardia and corneo-retinal potential (CRP) to acoustic stimuli in rabbits. Injections of horseradish peroxidase into the amygdala central nucleus, an area that mediates the bradycardia-conditioned response (CR), produced cell body and fiber labeling at the ventral and medial borders of the MGN. Then, bilateral electrolytic lesions were made at the medial border of the MGN or in control sites dorsal and/or rostral to the MGN. Ten days after surgery, lesioned and unoperated control animals were subjected to 7 days of differential Pavlovian conditioning. In the control lesion and unoperated groups, the CS+ consistently elicited larger bradycardia responses than the CS-. However, animals with bilateral lesions in the medial MGN did not demonstrate differential bradycardia CRs. Bradycardia response magnitude in MGN lesion animals was not affected. Evidence of CRP differential conditioning was present in each group. The present findings suggest that a region just medial to the MGN is involved in bradycardia differential conditioning in rabbits. The fact that bradycardia responses were still present after medial MGN lesions suggests that other auditory regions may also be involved in the mediation of the bradycardia CR.     

Different distributions of large and small retinal ganglion cells in the cat after HRP injections of single layers of the lateral geniculate body and the superior colliculus

Retinal ganglion cells were labeled with HRP after injecting layers of GL or single strata within the stratum griseum superficiale (SGS). Only small cells were labeled after injecting small cell C layers and upper SGS. Only large cells were labeled after injecting lower SGS. Small and large cells were labeled after injecting medial interlaminar nucleus (MIN) and layers A and A₁.     

Thalamocortical projections to rat auditory cortex from the ventral and dorsal divisions of the medial geniculate nucleus

The ventral and dorsal medial geniculate (MGV and MGD) constitute the major auditory thalamic subdivisions providing thalamocortical inputs to layer IV and lower layer III of auditory cortex. No quantitative evaluation of this projection is available. Using biotinylated dextran amine (BDA)/biocytin injections, we describe the cortical projection patterns of MGV and MGD cells. In primary auditory cortex the bulk of MGV axon terminals are in layer IV/lower layer III with minor projections to supragranular layers and intermediate levels in infragranular layers. MGD axons project to cortical regions designated posterodorsal (PD) and ventral (VA) showing laminar terminal distributions that are quantitatively similar to the MGV-to-primary cortex terminal distribution. At the electron microscopic level MGV and MGD terminals are non-γ-aminobutyric acid (GABA)ergic with MGD terminals in PD and VA slightly but significantly larger than MGV terminals in primary cortex. MGV/MGD terminals synapse primarily onto non-GABAergic spines/dendrites. A small number synapse on GABAergic structures, contacting large dendrites or cell bodies primarily in the major thalamocortical recipient layers. For MGV projections to primary cortex or MGD projections to PD or VA, the non-GABAergic postsynaptic structures at each site were the same size regardless of whether they were in supragranular, granular, or infragranular layers. However, the population of MGD terminal-recipient structures in VA were significantly larger than the MGD terminal-recipient structures in PD or the MGV terminal-recipient structures in primary cortex. Thus, if terminal and postsynaptic structure size indicate strength of excitation then MGD to VA inputs are strongest, MGD to PD intermediate, and MGV to primary cortex the weakest.     

Binaural response patterns in subdivisions of the medial geniculate body

Auditory evoked potentials (AEPs) to binaural click stimulation were examined in the ventral (MGv) and caudomedial (MGcm) subdivisions of the medial geniculate body (MG) in guinea pigs. Binaural stimulation caused a decrease in amplitude for the response component recorded from the MGv, but an increase in amplitude for the AEP component recorded from the MGcm. Findings suggest that the evoked responses recorded from MGv and MGcm are functionally distinct. The inhibitory binaural response (BR) pattern seen in MGv was similar to that of the middle latency response (MLR) component recorded over the temporal cortex, while the additive BR pattern typical of the MGcm was similar to that of the surface midline MLR component. Furthermore, these data imply that the binaural response patterns seen in the primary and non-primary auditory cortex may be processed and encoded at the thalamic level. It is concluded that the distinct BR patterns noted for the two MG subdivisions reflect the predominant type of binaurally responsive neurons within the respective pathways.     

Anatomical studies on the nucleus reticularis tegmenti pontis in the pigmented rat. I. Cytoarchitecture, topography, and cerebral cortical afferents

The nucleus reticularis tegmenti pontis (NRTP) is a precerebellar reticular nucleus that has been found to be related to cerebropontocerebellar pathways and, more recently, to eye movements. The present study investigates the cytoarchitecture, the topography, and the cerebral cortical projections to the NRTP in the pigmented rat. The cytoarchitecture and topography of the NRTP was determined by examination of Nissl-stained material sectioned in the transverse and sagittal planes. Two cytoarchitectonically distinct portions of the NRTP are apparent; a central subdivision (NRTPc) composed of large multipolar, small spherical, and fusiform neurons, and a pericentral subdivision (NRTPp) composed of loosely packed small fusiform and spherical neurons. The NRTPc is located dorsal to the medial lemniscus and pyramidal tracts over the caudal two-thirds of the pons. It extends caudodorsally to the region just rostral and ventral to the abducens nucleus. The NRTPp is adjacent to the lateral margins of the NRTPc, rostrally, and lies ventral to the caudal portions of the NRTPc. Large injections of horseradish peroxidase (HRP) were made into the cerebellum in order to determine the degree to which each subdivision of the NRTP contributes to the cerebellar projection. A high percentage of NRTPc neurons and a lower percentage of NRTPp neurons were labeled. These differences in labeling density and neuronal morphology noted above confirm the appropriateness of subdividing the NRTP into central and pericentral subdivisions. The cerebral cortical afferents to the NRTP were examined by placing small iontophoretic injections of HRP into the NRTPc and NRTPp. A systematic examination of all cortical areas revealed that the HRP-labeled neurons are entirely localized within pyramidal layer V of three major cortical areas: the ipsilateral prefrontal cortex (Brodmann areas 8, 8a, 11, and 32); the ipsilateral motor and somatosensory cortices (Brodmann areas 2, 4, 6, and 10), and the bilateral cingular cortex (Brodmann areas 24a, 24b, 29c, and 29d). By far, the heaviest cortical labeling with HRP injections into the medial NRTPc is within the cingular cortex that may, in the rat, be homologous to the frontal eye field of the cat and monkey. In contrast, injections involving the lateral NRTPc or the NRTPp produced labeling within wide regions of the cortex with the greatest number in the somatomotor cortex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Topographic and cytoarchitectonic organization of thalamic neurons related to their targets in low-, middle-, and high-frequency representations in cat auditory cortex

We studied the topographic organization of thalamic projections upon different ranges of cortical frequency representation. Thalamic neurons were labeled by injecting horseradish peroxidase (HRP) or tritiated bovine serum albumin into auditory cortex. Injections in individual brains were confined to the same range of frequency representation, and distributed through three or four tonotopic cortical fields in order to label as much of the thalamic projection upon a limited range of frequency representation as practicable. Low, middle, and high ranges of the frequency representation were injected in different brains. The spatial organizations of arrays of labeled neurons are described, and each array is divided into a ventral division and lateral posterior complex (lateral part of the posterior thalamic group), both composed mainly of small cells; and a medial division, composed mainly of medium and large cells. The ventral and medial divisions (located laterally and medially within the medial geniculate body (MGB), respectively), both contact the lateral posterior complex which is located rostrally. The HRP cytoarchitecture of the three divisions is described, and the portions of the ventral division corresponding with the physiologically and cytoarchitectonically defined ventral nucleus are identified. Relatively few labeled neurons were found within other thalamic areas. The topographic organizations of the ventral division (and its tonotopic subdivision, the ventral nucleus), the lateral posterior complex (also tonotopically organized), and the medial division are described. There are planar and concentric components of the topographic organization in the ventral nucleus. Within the planar component, the low-frequency area is located laterally and the high-frequency area is located rostromedially. Within the concentric component, the low-frequency area is located centrally and the high-frequency area is located peripherally. Low-, middle-, and high-frequency areas course without interruption through the planar and concentric components. In the lateral posterior complex, the low-frequency area is located rostrally, and the high-frequency area is located caudally adjoining the high-frequency area in the ventral nucleus. The topographic organizations of the ventral nucleus and lateral posterior complex are consistent with tonotopic maps of these regions. The medium- and large-cell portion of the medial division is also topographically organized, although there may be more overlap among low-, middle-, and high-frequency arrays than in the ventral nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)     

The medial geniculate body of the tree shrew, Tupaia glis. I. Cytoarchitecture and midbrain connections.

In this study of the medial geniculate body in the tree shrew eight subdivisions are identified on the basis of differences recognized in Nissl-stained material. Experiments using the methods of anterograde and retrograde axonal transport and anterograde degeneration show that each subdivision has a unique pattern of connections with the midbrain. The ventral division of the medial geniculate body contains at least two subdivisions, the ventral nucleus and the caudomarginal nucleus. The ventral nucleus is characterized by densely-packed cells and receives topographically organized projections from the central nucleus of the inferior colliculus. The caudomarginal nucleus, on the other hand, receives its major midbrain projections from the medial nucleus in the inferior colliculus. In the dorsal division four subdivisions are distinguished. The suprageniculate nucleus contains large, loosely-packed cells and receives projections from the deep layers of the superior colliculus and from the midbrain tegmentum. The dorsal nucleus receives projections from the midbrain tegmentum. The deep dorsal and anterodorsal nuclei have neurons which resemble those in the dorsal nucleus. Both receive projections from the roof nucleus of the inferior colliculus but the deep dorsal nucleus receives an additional projection from the parabrachial tegmentum. The medial division has a rostral and a caudal subdivision. The ascending projections to the rostral nucleus are from the lateral zone in the inferior colliculus and from the spinal cord. The caudal nucleus contains cells with large somas and receives projections from most of the midbrain areas which project to the other subdivisions of the medial geniculate body.     

Single unit activity in the auditory cortex and the medial geniculate body of the rhesus monkey: Behavioral modulation

Three rhesus monkeys were trained to respond to a given auditory signal, the nature of which could be predicted from a preceding visual stimulus. The activity of 28 units in the auditory cortex and 53 units in the Medial Geniculate Body (MGB) of the monkey was recorded during task-performance conditions, as well as in the non-performance conditions. The activity of about one third of the cortical and MGB units was independent of the behaving status of the animal. In other units, the response to an auditory signal delivered during task-performance conditions as compared to the response recorded during non-performance periods was either augmented or attenuated. Furthermore, it was found that the spontaneous activity of most of the MGB and cortical units was continuously affected by either an excitatory or an inhibitory input, activated by the behavioral state. The temporal characteristics of behavioral modulation were studied by computing an amplification curve for all MGB units characterized by a ‘through stimulus excitation’ type of response. Analysis of these curves together with the behavioral effect on the spontaneous activity allows the suggestion of possible mechanisms by which the behavioral state of the monkey modulates the activity in the thalamocortical segment of the auditory system.     

Ascending components of the medial forebrain bundle from the lower brain stem in the rat, with special reference to raphe and catecholamine cell groups. A study by the HRP method

The afferent connections of the medial forebrain bundle (MFB) arising from the lower brain stem have been investigated by means of horseradish peroxidase (HRP) with sensitive substrate. The injection was made iontophoretically into MFB at various levels.After injection of HRP into MFB, a significant number of HRP-labeled neurons were observed in the following structures of the lower brain stem: (1) raphe nuclear group, (2) locus coeruleus, (3) n. laterodorsalis tegmenti, (4) parabrachial area, (5) A₁, A₂, A₄, A₅ and A₇ areas where noradrenaline-containing neurons were disseminated, (6) A₈, A₉ and A₁₀ areas which contain dopamine neurons, (7) surrounding area of the fasciculus longitudinalis medialis at the level of the n. prepositus hypoglossi, (8) n. prepositus hypoglossi and (9) mesencephalic gray matter. As a rule, the ascending projections are ipsilateral and course in the medial part of MFB.Regarding the raphe nuclei, we have demonstrated that the caudal raphe nuclei, such as n. raphe magnus and obscurus (but not n. raphe pallidus), also send their axons to the hypothalamus. Particularly, the axons of n. raphe magnus ascend in MFB to reach the level of the preoptic or anterior septal area. Furthermore, in accordance with previous reports, HRP-labeled cells were also identified in the n. raphe dorsalis, centralis superior and pontis, respectively. It should be further noted that labeled cells appeared in the n. linearis caudalis.In addition, the present study indicates a number of non-aminergic cell groups as sources of ascending MFB fibers.On the whole, the present study further clarified the organization of the components of the MFB ascending from the lower brain stem, and provided some additional anatomical substrates for the physiology of the control of the forebrain by the lower brain stem neurons.     

Functional organization of the medial geniculate body''s subdivisions of the awake squirrel monkey

The response properties of 138 cells in the medial geniculate body (MGB), of the awake squirrel monkey (Saimiri sciureus), to 7 species-specific vocalizations were studied. Cells were divided into 4 subgroups: 26 in the ventral, 24 in the medial and 46 in the lateral subdivision. Forty-two cells located on the borders between the subdivisions represent the fourth group. No significant differences were found between the subdivisions with respect to their selectivity, nor did cells in any subdivision respond preferentially to any particular vocalization. On the other hand, the response patterns of the ventral and the lateral subdivisions showed significant differences (P < 0.001, X²-test) from those of the medial subdivision. Most of the cells in the medial subdivision (87.5%) responded with similar response pattern to the 7 vocalizations (mainly ‘on’ or ‘sustain’), while most of the cells in the ventral and the lateral subdivisions (61.5% and 69.6% respectively) responded with complex, time-locked and different patterns to the various vocalizations. Cells that exhibited a response characterized as an intermediate between the two types were accumulated mainly on or close to the borders between the medial and the other subdivisions of the MGB. The possible role of each response patterns is discussed with respect to the projection of the subdivisions to the cortex.     

Long term potentiation in the magnocellular medial geniculate nucleus of the anesthetized cat

Long term potentiation (LTP) has been suggested as a mechanism in learning. The magnocellular division of the medial geniculate nucleus (MGm) is known to develop discharge plasticity rapidly during behavioral learning. The ability of the MGm also to develop long term potentiation was studied in cats under barbiturate anesthesia. Monosynaptic responses elicited in the MGm by periodic (0.2 Hz) stimulation of the brachium of the inferior colliculus (BIC) developed significant increases in amplitude and decreases in latency, which were maintained for at least 1 h, following brief high frequency stimulation of the BIC. Antidromic] responses recorded in the inferior colliculus were unchanged. These findings provide a link between learning-induced physiological pla plasticity and LTP, and demonstrate that the auditory system can develop long term potentiation.     

Audiogenic kindling increases neuronal responses to acoustic stimuli in neurons of the medial geniculate body of the genetically epilepsy-prone rat

Frequent repetition of audiogenic seizure (AGS) (‘AGS kindling’) in the severe substrain of genetically epilepsy-prone rats (GEPR-9s) results in the appearance of cortical epileptiform electrographic activity, increases of seizure duration and additional convulsive behaviors. These findings suggest that the initial AGS network, which is located primarily in the brainstem, has undergone expansion to the forebrain. The medial geniculate body (MGB) is a thalamic structure that is the first major auditory nucleus efferent to the AGS-initiating site in the inferior colliculus. The MGB is not required for AGS induction, but it has been implicated in the expanded AGS network in GEPR-9s based on focal, pharmacological blockade experiments. The present study examined changes in acoustically evoked MGB neuronal responses in awake and behaving GEPR-9s and in anesthetized GEPR-9s after 14 repetitive AGS-inducing stimuli given daily. An elevated number of action potentials was observed in the MGB neuronal responses after AGS kindling in GEPR-9s. This increase of MGB neuronal responses was associated with a loss of habituation and lasted for at least 28 days after the 14th AGS. An increase in the incidence of sustained acoustic responses in MGB neurons was observed after repetitive AGS in GEPR-9s. Increases in the peak latency and threshold of MGB neuronal responses were also observed after AGS kindling. MGB neurons exhibited a rapid tonic firing during tonic seizures in behaving GEPR-9s, suggesting that the MGB may be implicated in the propagation of seizure activity. However, MGB neuronal firing was silent during post-tonic clonus, a behavior seen in GEPR-9s only after AGS repetition, suggesting that MGB does not play a direct role in the generation of this convulsive behavior. Thus, changes in neuronal firing in nuclei efferent to the MGB, in the expanded neuronal network for repetitive AGS, may be responsible for the generation of post-tonic clonus in GEPR-9s.     

Connections of cat auditory cortex: I. Thalamocortical system

Despite the functional importance of the medial geniculate body (MGB) in normal hearing, many aspects of its projections to auditory cortex are unknown. We analyzed the MGB projections to 13 auditory areas in the cat using two retrograde tracers to investigate thalamocortical nuclear origins, topography, convergence, and divergence. MGB divisions and auditory cortex areas were defined independently of the connectional results using architectonic, histochemical, and immunocytochemical criteria. Each auditory cortex area received a unique pattern of input from several MGB nuclei, and these patterns of input identify four groups of cortical areas distinguished by their putative functional affiliations: tonotopic, nontonotopic, multisensory, and limbic. Each family of areas received projections from a functionally related set of MGB nuclei; some nuclei project to only a few areas (e.g., the MGB ventral division to tonotopic areas), and others project to all areas (e.g., the medial division input to every auditory cortical area and to other regions). Projections to tonotopic areas had fewer nuclear origins than those to multisensory or limbic-affiliated fields. All projections were organized topographically, even those from nontonotopic nuclei. The few divergent neurons (mean: 2%) are consistent with a model of multiple segregated streams ascending to auditory cortex. The expanded cortical representation of MGB auditory, multisensory, and limbic affiliated streams appears to be a primary facet of forebrain auditory function. The emergence of several auditory cortex representations of characteristic frequency may be a functional multiplication of the more limited maps in the MGB. This expansion suggests emergent cortical roles consistent with the divergence of thalamocortical connections.     

The organization of the rat lateral geniculate body by immunohistochemical analysis of neuroactive substances

The organization of neuroactive substances in the rat lateral geniculate body (LGB) was studied with available immunohistochemical stainings. In the dorsal lateral geniculate nucleus (DLG), there existed only γ-aminobutyric acid (GABA)-like immunoreactive neurons. Immunoreactive fiber plexuses for substance P (SP), cholecystokinin-8 (CCK) and vasoactive intestinal polypeptide (VIP) were present in the lateral margin of the DLG, just beneath the optic tract. There were immunoreactive neurons and fibers for GABA, SP, leucine-enkephalin (ENK) and neuropeptide Y (NPY) in the intergeniculate leaflet (IGL). ENK-, NPY- and SP-like immunoreactive neurons in the IGL were mainly medium-sized, and bipolar or spindle-shaped kwith a few dendrites oriented dorsoventrally. In the IGL, use of double-labeled immunofluorescence demonstrated that a few neurons exhibited both ENK- and SP-like immunoreactivities, and a few neurons had both GABA- and ENK-like immunoreactivities. Although the morphologyh of ENK-like immunoreactive neurons resembled to NPY-like immunoreactive neurons, both neurons were clearly different neurons. Many GABA-, ENK- and SP-like immunoreactive neurons and fibers were found in the ventral lateral geniculate nucleus (VLG). These immunoreactive neurons were mainly medium-sized, and bipolar in shape, while a few immunoreactive neurons were multipolar shape. Neurons containing ENK and fibers containing SP mainly existed in the lateral half of the parvocellular part and in the medial half of magnocellular part of the VLG. In this region, abotu one-thrid of teh GABA-like immunoreactive neurons contained ENK-like immunoreactivity. Many SP neurons mainly existed in the medial half of the parvocellular part of the VLG. CCK- and VIP-like immunoreactive fibers were present in the lateral half of the magnocellular part of the VLG. Immunoreactive fibers for calcitonin gene-related peptide, corticotropin-releasing factor, neurotensin and tyrosine hydroxylase were disseminated throughout the LGB. The subdivisions of the LGB were discussed, based upon the distribution of neuroactive substances.     

Anatomy of the auditory thalamocortical system of the guinea pig

We investigated the projection from the medial geniculate body (MG) to the tonotopic fields (the anterior field A, the dorsocaudal field DC, the small field S) and to the nontonotopic ventrocaudal belt in the auditory cortex of the guinea pig. The auditory fields were first delimited in electrophysiological experiments with microelectrode mapping techniques. Then, small quantities of horseradish peroxidase (HRP) and/or fluorescent retrograde tracers were injected into the sites of interest, and the thalamus was checked for labeled cells. The anterior field A receives its main thalamic input from the ventral nucleus of the MG (MGv). The projection is topographically organized. Roughly, the caudal part of the MGv innervates the rostral part of field A and vice versa. After injection of tracer into low or medium best-frequency sites in A, we also found a topographic gradient along the isofrequency contours: the dorsal (ventral) part of a cortical isofrequency strip receives afferents from the rostral (caudal) portions of the corresponding thalamic isofrequency band. However, it is not so obvious whether such a gradient exists also in the high-frequency part of the projection. A second, weaker projection to field A originates in a magnocellular nucleus that is situated caudomedially in the MG and was therefore named the caudomedial nucleus. The dorsocaudal field DC receives input from the same nuclei as the anterior field, but the location of the labeled cells in the MGv is different. This was demonstrated by injection of different tracers into sites with like best frequencies in fields A and DC, respectively. After injection of HRP into the 1-2-kHz isofrequency strip in field A and injection of Nuclear Yellow (NY) into the 1-2-kHz site in field DC, the labeled cells in the MGv form one continuous array that runs from caudal to rostral over the whole extent of the MGv. The anterior part of this array consists of NY-labeled cells; i.e., it projects to field DC. The caudal part is formed by HRP-labeled cells; i.e., it innervates field A. These findings indicate that there is only one continuous tonotopic map in the MGv. This map is split when projected onto the cortex so that two adjacent tonotopic fields (A and DC) result. The cortical maps are rotated relative to the thalamic map in that rostral portions of the MGv project to caudal parts of the tonotopic cortex and vice versa.(ABSTRACT TRUNCATED AT 400 WORDS)     

Human fetal hippocampal development: I. Cytoarchitecture,myeloarchitecture, and neuronal morphologic features

To characterize better the process of anatomic development of the human hippocampus, we studied the cytoarchitecture, myeloarchitecture, and neuronal morphology in human fetal and postnatal hippocampi. Twenty cases were studied in which the ages ranged from 9 weeks gestation through 62 years. Fixed, paraffin-embedded, hippocampal sections were stained with cresyl violet for Nissl substance and immunolabeled for myelin basic protein. The hippocampal region at 9 weeks contains 4 layers: a ventricular zone, an intermediate zone, a homogeneous-appearing hippocampal plate comprised of bipolar-shaped neurons, and a wide marginal zone. At 15–19 weeks, individual subfields can be distinguished. A distal-to-proximal gradient of cytoarchitectural and neuronal morphologic maturity is seen, with the subiculum appearing more developed than the ammonic subfields and the dentate gyrus appearing least mature. Within each subfield, an “inside-out” gradient of maturity is also evident. By 32–34 weeks gestational age, neurons in CA2 and CA3 have undergone rapid enlargement and morphologic maturation, surpassing CA1, which still contains some immature neurons. The dentate gyrus is the latest area to develop, only assuming a mature cytoarchitecture after 34 weeks. The essential cytoarchitectural appearance of the hippocampal subfields is stable after birth, although there is progressive neuronal enlargement and a decrease in neuronal density throughout childhood into adulthood. Myelination is first evident near term, with strong myelin basic protein immunoreactivity present in the angular bundle, alveus, and fimbria and relatively scant immunoreactivity in the nascent perforant pathway. Myelination in the hippocampus increases in childhood until adolescence, after which the pattern remains unchanged. These studies delineate normal neuroanatomic development and can be used to understand better the mechanisms underlying human neurodevelopmental and neurodegenerative disorders of the hippocampal formation. © 1996 Wiley-Liss, Inc.