The Functional Architecture of the Medial Geniculate Body and the Primary Auditory Cortex

Differential Na(+)-K(+)-ATPase activity in rat lemniscal and non-lemniscal auditory thalami

1. Using whole-cell recording and confocal immunofluorescent microscopy, we have investigated the differential electrogenic activity, subunit expression and subcellular distribution of the Na(+)-K(+)-ATPase in the lemniscal (ventral) and non-lemniscal (dorsal) pathways of the rat medial geniculate body (MGB) in vitro. 2. Bath application of Na(+)-K(+)-ATPase inhibitors strophanthidin or dihydro-ouabain produced a transient, dose-dependent inward current or membrane depolarization which were significantly larger in dorsal MGB neurones than in ventral cells (45.9 +/- 6.45 vs. 24.3 +/- 4.1 pA; P < 0.05). Electrophysiological and morphometric measurements showed that the dorsal MGB neurones had a significantly lower input conductance and a smaller somata than their ventral counterparts. The level of the resting membrane potential also differed by about 6 mV between the two cell populations, with the dorsal cells being more hyperpolarized (-74.2 +/- 0.6 vs. -67.7 +/- 1.3 mV; P < 0.001). 3. Incubation of enzymatically dissociated MGB neurones with fluorescent monoclonal antibodies against alpha 1-alpha 3 isoforms of Na(+)-K(+)-ATPase showed that both dorsal and ventral cells expressed primarily alpha 3 subunits. Confocal laser scanning revealed, however, that the mean pixel density of alpha 3 fluorescent antibodies in the plasma membrane domain, but not in the cytoplasmic compartment, was about 40% higher in dorsal neurones than in the ventral cells (29.7 +/- 4.7 vs. 16.9 +/- 2.3 grey shadow per pixel; P < 0.05). 4. The above results suggest that the electrogenic activity of the Na(+)-K(+)-ATPase is differentially regulated between lemniscal and non-lemniscal auditory thalami through a mechanism that probably involves differential pump densities in the cell membrane.

Corticofugal modulation of frequency processing in bat auditory system

Auditory signals are transmitted from the inner ear through the brainstem to the higher auditory regions of the brain. Neurons throughout the auditory system are tuned to stimulus frequency, and in many auditory regions are arranged in topographical maps with respect to their preferred frequency. These properties are assumed to arise from the interactions of convergent and divergent projections ascending from lower to higher auditory areas; such a view, however, ignores the possible role of descending projections from cortical to subcortical regions. In the bat auditory system, such corticofugal connections modulate neuronal activity to improve the processing of echo-delay information, a specialized feature. Here we show that corticofugal projections are also involved in the most common type of auditory processing, frequency tuning. When cortical neurons tuned to a specific frequency are inactivated, the auditory responses of subcortical neurons tuned to the same frequency are reduced. Moreover, the responses of other subcortical neurons tuned to different frequencies are increased, and their preferred frequencies are shifted towards that of the inactivated cortical neurons. Thus the corticofugal system mediates a positive feedback which, in combination with widespread lateral inhibition, sharpens and adjusts the tuning of neurons at earlier stages in the auditory processing pathway.

Complementary expression of parvalbumin and calbindin D-28k delineates subdivisions of the rabbit medial geniculate body

The complementary pattern of immunohistochemical staining for the calcium-binding proteins parvalbumin (PV) and calbindin D-28k (CB) was used to delineate four major subdivisions of the rabbit medial geniculate body (MGB). PV immunoreactivity predominates in the ventral and medial divisions, whereas CB-immunoreactive cells characterize the dorsal and internal divisions. The ventral nucleus is strongly PV+ due to dense neuropil labeling and moderately labeled somata. The medial nucleus contains both medium-sized and large PV+ somata, as well as thick PV+ axons and terminals. The wedge-shaped internal nucleus composed of densely labeled CB+ cells, separates the dorsal and ventral nuclei rostrally, and expands caudally to encapsulate the posterior MGV. Large multipolar CB+ neurons with radiate dendrites characterize the dorsal nucleus. The differential expression of PV and CB also distinguishes the deep dorsal and superficial dorsal subnuclei in the dorsal division and a ventrolateral component in the ventral division. A comparison with studies of MGB connectivity in a variety of species suggests that PV immunoreactivity is highest in subdivisions that receive a substantial input from the central nucleus of the inferior colliculus and that project to primary auditory cortex. In contrast, CB immunoreactivity characterizes nuclei that receive input primarily from other sources, such as the paracentral nuclei of the inferior colliculus, the lateral tegmentum, and the spinal cord, and that project to secondary auditory areas. The ability of calcium-binding protein immunohistochemistry to delineate neuronal compartments across indistinct cytoarchitectonic borders makes it a powerful tool for guiding future connectional and physiological studies of the MGB.

Auditory cortex of the rufous horseshoe bat: 1. Physiological response properties to acoustic stimuli and vocalizations and the topographical distribution of neurons

The extent and functional subdivisions of the auditory cortex in the echolocating horseshoe bat, Rhinolophus rouxi, were neurophysiologically investigated and compared to neuroarchitectural boundaries and projection fields from connectional investigations. The primary auditory field shows clear tonotopic organization with best frequencies increasing in the caudorostral direction. The frequencies near the bat's resting frequency are largely over-represented, occupying six to 12 times more neural space per kHz than in the lower frequency range. Adjacent to the rostral high-frequency portion of the primary cortical field, a second tonotopically organized field extends dorsally with decreasing best frequencies. Because of the reversed tonotopic gradient and the consistent responses of the neurons, the field is comparable to the anterior auditory field in other mammals. A third tonotopic trend for medium and low best frequencies is found dorsal to the caudal primary field. This area is considered to correspond to the dorsoposterior field in other mammals. Cortical neurons had different response properties and often preferences for distinct stimulus types. Narrowly tuned neurons (Q10dB > 20) were found in the rostral portion of the primary field, the anterior auditory field and in the posterior dorsal field. Neurons with double-peaked tuning curves were absent in the primary area, but occurred throughout the dorsal fields. Vocalization elicited most effectively neurons in the anterior auditory field. Exclusive response to pure tones was found in neurons of the rostral dorsal field. Neurons preferring sinusoidal frequency modulations were located in the primary field and the anterior and posterior dorsal fields adjacent to the primary area. Linear frequency modulations optimally activated only neurons of the dorsal part of the dorsal field. Noise-selective neurons were found in the dorsal fields bordering the primary area and the extreme caudal edge of the primary field. The data provide a survey of the functional organization of the horseshoe bat's auditory cortex in real coordinates with the support of cytoarchitectural boundaries and connectional data.

A quantitative analysis of parvalbumin neurons in rabbit auditory neocortex

Parvalbumin (PV) is a calcium-binding protein present in GABAergic cells in the cerebral cortex and in thalamic relay neurons. In the present study, parvalbumin immunocytochemistry (PVi) and stereological methods were used to obtain estimates of cortical volume, total neuron number, laminar density, and the percentage of PV-immunoreactive neurons in auditory neocortex. PVi clearly delineated the primary auditory cortex (AI), which was characterized by two PV+ bands: dense terminal-like labeling within lamina III/IV and PV+ somata in lamina VIa. Stereological analysis of Nissl-stained sections revealed that the total number of neurons in rabbit AI was 1.48 x 10(6) with a mean neuronal density of 55 x 10(3)/mm3. Based on a mean cortical thickness of 1.92 mm, there are approximately 106,000 neurons in a 1 mm2 column of auditory cortex. PVi yields an extraordinary Golgi-like staining of nonpyramidal cells in all cortical layers. PV+ nonpyramidal cells constitute approximately 7.0% of the neurons in AI. There were significant differences in the morphology and density of PV+ neurons across layers. Although only 5% of cells in lamina I were PV+, three nonpyramidal cell types were present. Lamina II had the highest numerical density within AI but the lowest percentage of PV+ neurons (3.3%). Lamina II, however, contained the greatest diversity of PV+ nonpyramidal cell types, which included small multipolar cells, bipolar cells, and, less frequently, large cells of the bitufted, bipolar, and stellate varieties. Lamina IV had one of the highest numerical densities (67.6 x 10(3) neurons/mm3) and contributed nearly 27% of the total neuron number in AI. The numerical density of PV+ nonpyramidal cells was also greatest within lamina IV (7.1 x 10(3)/mm3) where they formed 10.4% of the neuronal population. PV+ nonpyramidal cells in lamina IV and lamina III were predominantly large basket-type cells with bitufted dendritic domains and tangentially oriented local axonal plexuses. The terminal-like label within lamina III/IV derived in part from the basket-cell axons, which formed pericellular arrays around unstained somata. Cell-sparse lamina V contained the largest PV+ nonpyramidal cells in AI. These cells, which formed 11% of the neuron population in lamina V, were notable for their tangentially oriented dendritic fields and local axonal arbors. PVi partitioned lamina VI into VIa and VIb. Large multipolar nonpyramidal cells were distributed throughout lamina VI and made up approximately 6% of the total population. Lamina VIa contained a band of lightly labeled PV+ pyramidal neurons that formed 15% of the neuronal population.(ABSTRACT TRUNCATED AT 400 WORDS)

The neurons of the medial geniculate body in the mustached bat (Pteronotus parnellii)

The neurons in the medial geniculate body were studied in Golgi preparations from adult mustached bats (Pteronotus parnellii). Their somatic and dendritic configurations were compared with those of cells in other, nonecholocating mammals. A second goal was to use the thalamic nuclear subdivisions derived from Golgi material to integrate the findings in parallel studies of cytoarchitecture, immunocytochemistry, and tectothalamic connections. Three primary divisions are defined. The ventral division is large and has a stereotyped neuronal organization. Medium-sized perikarya (about 10 microns in diameter) represent tufted neurons; the fibrodendritic plexus forms laminae in the lateral part along which midbrain axons terminate. A smaller, possibly intrinsic, neuron with thin, sparse dendrites is rarely impregnated. Neurons in the larger, medial part, which represents frequencies of 60 kHz and higher, have more spherical dendritic fields; their branching pattern remains tufted, and the laminar organization was less evident. The dorsal division is about equal in size, and it has many nuclei and a corresponding neuronal diversity. These neurons are medium-sized except in the suprageniculate nucleus, where many cells are larger. Four dorsal division nuclei are recognized. Each has neurons with radiate or weakly tufted dendritic arbors. Superficial dorsal nucleus neurons are oriented from medial to lateral, imparting a slightly laminated appearance to the neuropil. A few smaller, stellate neurons with modest dendritic domains are present. Suprageniculate nucleus neurons have radiating dendritic fields that project spherically; they have fewer branches than dorsal nucleus neurons. The posterior limitans nucleus is dorsomedial to the suprageniculate nucleus; it has small neurons with long, sparsely branched dendrites. The rostral pole nucleus, included in the dorsal division on cytoarchitectonic grounds, had too few neurons impregnated to reveal its neuronal architecture. The medial division, the smallest of the main parts, is one nucleus with at least six types of cells, including the magnocellular, bushy tufted, disc-shaped, medium-sized multipolar, elongated, and small stellate neurons. There is no laminar arrangement. Many of the neurons resemble those in rodent, marsupial, carnivore, and primate auditory thalamic nuclei. Despite such morphological correspondences, functional differences, such as the evolution of combination sensitivity, suggest that structurally comparable auditory thalamic neurons may subserve diverse physiological representations.

Cytoarchitecture of the medial geniculate body in the mustached bat (Pteronotus parnellii)

The cytoarchitectonic organization of the medial geniculate body and adjoining thalamic nuclei was analyzed in the mustached bat (Pteronotus parnellii). These subdivisions provide a reference for structural, physiological, connectional, and neurochemical work. Most nuclei recognized in other mammals exist in the mustached bat, although the relative volume of the three divisions was species specific. The ventral division contains medium-sized neurons and a few smaller cells and is well developed. Neurons in the lateral part lie in regularly aligned rows corresponding to the laminae in Golgi material; in the medial part, these laminae are obscured by fibers. The dorsal division has at least four nuclei, each with a unique cytoarchitecture and myeloarchitectonic organization. The suprageniculate nucleus is prominent and has many large radiate neurons. Cells in the superficial dorsal nucleus have weekly laminated dendrites, while dorsal nucleus neurons have spherical dendritic fields. There is a wide range of neuropil patterns within the dorsal division. The suprageniculate nucleus has thick myelinated axons, while the fibers in the superficial and dorsal nuclei are much thinner. The rostral pole nucleus becomes prominent in the anterior one-half of the auditory thalamus; its architectonic affiliation is equivocal, and connectional and immunocytochemical studies suggest that it may belong to the dorsal division. The medial division is one nucleus with many types of neurons, and it has coarse axons without laminar orientation. It is the smallest of the divisions and is present throughout the medial geniculate complex, except at the caudal tip and at the rostral pole. Many features of medial geniculate body organization evident in other mammals are recognized in the mustached bat. These include a prominent ventral division, some of whose neurons have a laminar organization, and a comparatively small medial division that is devoid of fibrodendritic laminae. Other features, such as the presence of a large rostral pole nucleus, whose homologue in other species is uncertain, or the sparse number or small cells that may participate in local circuits, set it apart from carnivores and primates and suggest that there are species specific patterns of medial geniculate body organization.

Projections of physiologically defined subdivisions of the inferior colliculus in the mustached bat: targets in the medial geniculate body and extrathalamic nuclei

This study examined the output of the central nucleus of the inferior colliculus to the medial geniculate body and other parts of the nervous system in the mustached bat (Pteronotus parnellii). Small deposits of anterograde tracers (horseradish peroxidase, [3H]leucine, Phaseolus vulgaris leucoagglutinin, wheat germ agglutinin conjugated to horseradish peroxidase, or biocytin) were made at physiologically defined sites in the central nucleus representing major components of the bat's echolocation signal. The topography, frequency specificity, and axonal morphology of these outputs were studied. The medial geniculate body was a major target of inferior collicular neurons, with three distinct input patterns. The projection to the ventral division was tonotopically organized, but had a relatively sparse contribution from neurons representing frequency modulated components of the biosonar pulse. The second input was to the rostral medial geniculate body, in which projections from inferior collicular neurons representing constant frequency sonar components were separated from those representing frequency modulated components. A third input was to the suprageniculate nucleus, which received strong, topographically arranged projections. Inputs to the dorsal nucleus and medial division were also observed. Extrathalamic regions receiving input included the pontine gray, external nucleus of the inferior colliculus, pericollicular tegmentum, nucleus of the brachium of the inferior colliculus, and pretectum. These central nucleus projections differed in organization and the structure of axon terminals, suggesting different physiological influences on their target nuclei. These results demonstrate that the central nucleus has divergent projections to various sensory and premotor nuclei, besides its well-established projection to the medial geniculate body.

Laminar distribution and neuronal targets of GABAergic axon terminals in cat primary auditory cortex (AI)

The form, density, and neuronal targets of presumptive axon terminals (puncta) that were immunoreactive for gamma-aminobutyric acid (GABA) or its synthesizing enzyme, glutamic acid decarboxylase (GAD), were studied in cat primary auditory cortex (AI) in the light microscope. High-resolution, plastic-embedded material and frozen sections were used. The chief results were: 1) There was a three-tiered numerical distribution of puncta, with the highest density in layer Ia, an intermediate number in layers Ib-IVb, and the lowest concentration in layers V and VI, respectively. 2) Each layer had a particular arrangement: layer I puncta were fine and granular (less than 1 micron in diameter), endings in layers II-IV were coarser and more globular (larger than 1 micron), and layer V and VI puncta were mixed in size and predominantly small. 3) The form and density of puncta in every layer were distinctive. 4) Immunonegative neurons received, in general, many more axosomatic puncta than immunopositive cells, with the exception of the large multipolar (presumptive basket) cells, which invariably had many puncta in layers II-VI. 5) The number of puncta on the perikarya of GABAergic neurons was sometimes related to the number of puncta in the layer, and in other instances it was independent of the layer. Thus, while layer V had a proportion of GABAergic neurons similar to layer IV, it had only a fraction of the number of puncta; perhaps the intrinsic projections of supragranular GABAergic cells are directed toward layer IV, as those of infragranular GABAergic neurons may be. Since puncta are believed to be the light microscopic correlate of synaptic terminals, they can suggest how inhibitory circuits are organized. Even within an area, the laminar puncta patterns may reflect different inhibitory arrangements. Thus, in layer I the fine, granular endings could contact preferentially the distal dendrites of pyramidal cells in deeper layers. The remoteness of such terminals from the spike initiation zone contrasts with the many puncta on all pyramidal cell perikarya and the large globular endings on basket cell somata. Basket cells might receive feed-forward disinhibition, pyramidal cells feed-forward inhibition, and GABAergic non-basket cells would be the target of only sparse inhibitory axosomatic input. Such arrangements imply that the actions of GABA on AI neurons are neither singular nor simple and that the architectonic locus, laminar position, and morphological identity of a particular neuron must be integrated for a more refined view of its role in cortical circuitry.

Morphology and spatial distribution of GABAergic neurons in cat primary auditory cortex (AI)

This is a survey of the distribution, form, and proportion of neurons immunoreactive for gamma-aminobutyric acid (GABA) or glutamic acid decarboxylase (GAD) in cat primary auditory cortex (AI). The cells were studied in adult animals and were classified with respect to their somatic size, shape, and laminar location, and with regard to the origins and branching pattern of their dendrites. These attributes were used to relate each of the GAD-positive neuronal types to their counterparts in Golgi preparations. Each layer had a particular set of GABAergic cell types that is unique to it. There were 10 different GABAergic cell types in AI. Some were specific to one layer, such as the horizontal cells in layer I or the extraverted multipolar cells in layer II, while other types, such as the small and medium-sized multipolar cells, were found in every layer. The number and proportion of GABAergic cells were determined by using postembedding immunocytochemistry. The proportion of GABAergic neurons was 24.6%. This was slightly higher than the values reported elsewhere in the neocortex. The laminar differences in density and proportion of GABAergic and non-GABAergic neurons were also comparable (though somewhat higher) to those found in other cortical areas: thus, 94% of layer I cells were GABAergic, while the values in other layers ranged from 27% (layer V) to 16% (layer VI). Layer VI had the most heterogeneous population of GABAergic neurons. The proportion of these cells across different regions within AI was studied. Since some receptive field properties such as sharpness of tuning and aurality are distributed non-uniformly across AI, these might be reflected by regional differences across the cerebral cortex. There were significantly more GABAergic somata in layers III and IV in the central part of AI, along the dorsoventral axis, where physiological studies report that the neurons are tuned most sharply (Schreiner and Mendelson [1990] J. Neurophysiol. 64:1442-1459). Thus, there may be a structural basis for certain aspects of local inhibitory neuronal organization.

Response properties of single units in areas of rat auditory thalamus that project to the amygdala. I. Acoustic discharge patterns and frequency receptive fields

Projections from the auditory thalamus to the amygdala have been implicated in the processing of the emotional significance of auditory stimuli. In order to further our understanding of the contribution of thalamoamygdala projections to auditory emotional processing, acoustic response properties of single neurons were examined in the auditory thalamus of chloral hydrate-anesthetized rats. The emphasis was on the medial division of the medial geniculate body (MGm), the suprageniculate nucleus (SG), and the posterior intralaminar nucleus (PIN), thalamic areas that receive inputs from the inferior colliculus and project to the lateral nucleus of the amygdala (AL). For comparison, recordings were also made from the specific thalamocortical relay nucleus, the ventral division of the medial geniculate body (MGv). Responses latencies were not statistically different in MGv, MGm, PIN, and SG, but were longer in the posterior thalamic region (PO). Overall, frequency tuning functions were narrower in MGv than in the other areas but many cells in MGm were as narrowly tuned as cells in MGv. There was some organization of MGv, with low frequencies represented dorsally and high frequencies ventrally. A similar but considerably weaker organization was observed in MGm. While the full range of frequencies tested (1-30 kHz) was represented in MGv, cells in MGm, PIN, and SG tended to respond best to higher frequencies (16-30 kHz). Thresholds were higher in PIN than in MGv (other areas did not differ from MGv). Nevertheless, across the various areas, the breadth of tuning was inversely related to threshold, such that more narrowly tuned cells tended to have lower thresholds. Many of the response properties observed in MGm, PIN, and SG correspond with properties found in AL neurons and thus add support to the notion that auditory responses in AL reflect thalamoamygdala transmission.

Response properties of single units in areas of rat auditory thalamus that project to the amygdala. II. Cells receiving convergent auditory and somatosensory inputs and cells antidromically activated by amygdala stimulation

The purpose of this study was to further our understanding of the contribution of auditory thalamoamygdala projections to conditioned emotional memories formed when auditory and noxious somatosensory stimuli are associated. Single unit activity was recorded in the acoustic thalamus of chloral hydrate-anesthetized rats in response to auditory (white noise, clicks, tones) and somatosensory (foot-shock) stimulation. The thalamic areas focused on were the medial division of the medial geniculate body (MGm), the suprageniculate nucleus (SG), and the posterior intralaminar nucleus (PIN), thalamic areas that receive inputs from both the inferior colliculus and the spinal cord and that project to the lateral nucleus of the amygdala (AL). For comparison, recordings were also made from the specific thalamocortical relay nucleus, the ventral division of the medial geniculate body (MGv), which receives projections from the inferior colliculus but not from the spinal cord. Auditory but not somatosensory responses were recorded from MGv, while both auditory and somatosensory responses were frequently found in MGm, PIN, and SG. In these areas, convergent auditory and somatosensory responses were more frequently found rostrally than caudally. Within a thalamic subregion, the acoustic response properties of the convergence cells were not different from the response properties of unimodal auditory cells. Some cells that responded to somatosensory but not auditory stimuli showed a potentiated response when tested with simultaneous presentation of auditory and somatosensory stimuli. In some studies, thalamic cells that project to the amygdala were antidromically activated by stimulation of the AL. Consistent with anatomical tracing results, antidromically activated cells were found in MGm, PIN, and SG, but not in MGv. Antidromically activated cells were more likely to respond to auditory stimuli than to somatosensory stimuli, but unimodal somatosensory and convergence cells were also found. These findings, which provide the first characterization of acoustic response properties of multimodal cells in the auditory thalamus and of cells in the auditory thalamus that project to amygdala, suggest insights into the emotional functions of the thalamoamygdala pathway.

Morphology of GABAergic neurons in the inferior colliculus of the cat

The goal of the present study was to provide a comprehensive and quantitative description of neurons immunoreactive for gamma-aminobutyric acid (GABA) in the inferior colliculus (IC) of the cat. Neurons were investigated with two different antisera and two different incubation methods. Free-floating frozen or vibratome-cut sections were incubated either with an antiserum to glutamic acid decarboxylase (GAD) or to GABA conjugated to protein with glutaraldehyde. Additional 1.5-microns-thick sections were incubated with the GABA antiserum after embedding and removal of the plastic. Quantitative data were obtained from much of this material. Despite the use of these different antisera and reaction methods, the results obtained were remarkably similar. The results show that GAD- or GABA-positive neurons represent a significant population of cells in the central nucleus of the IC, up to 20% of the neurons. Most of these neurons have large or medium-sized perikarya. In contrast, immunonegative neurons are medium-sized or small. Many GABA-positive neurons had proximal dendrites or somata oriented in parallel to the fibrodendritic laminae of the central nucleus and are presumed to be disc-shaped neurons. Other have an orthogonal orientation and are presumed to be stellate cells. Large GABA-positive neurons form two groups, those with many axosomatic endings and those with few. Collectively, these observations suggest that there are several types of GABAergic neuron in the central nucleus and, by extension, that these may participate in many types of inhibitory circuits.

Patterns of GABAergic immunoreactivity define subdivisions of the mustached bat's medial geniculate body

The anatomy and the spatial distribution of neurons and axonal endings (puncta) immunoreactive for glutamic acid decarboxylase (GAD) or gamma-aminobutyric acid (GABA) were studied in the medial geniculate body of the mustached bat (Pteronotus parnellii). The principal findings are that: 1) most GABAergic neurons are present in the dorsal and ventral divisions with few, if any, in the medial division; 2) only a small fraction, about 1% or less, of auditory thalamic neurons are immunopositive; 3) the density of immunoreactive puncta is independent on the number of GABAergic neurons in the thalamic divisions, with the ventral division having the largest number/unit area, the medial division about 75% of this value, and the dorsal division only about 50%; and 4) the form of the puncta was unique to each division, those in the ventral division being medium-sized and comparatively simple, those in the medial division predominantly large, coarse, and complex, while dorsal division ending were finer and more delicate. These patterns recapitulate, with some significant exceptions, those found in the rat and cat. The puncta could originate from several sources; while many may arise from intrinsic GABAergic Golgi type II local circuit neurons, these cells may not be the only or even the principal source. Thus, the dorsal division contains comparatively many immunopositive cells though fewer puncta than might be expected if the bulk of these were to arise from auditory thalamic interneurons. This suggests that other, extrinsic sources, such as the thalamic reticular nucleus, may be the source of such endings. A second point is that the form and density of the puncta is regionally specific within the medial geniculate complex. These local patterns might have a significant and regionally specific role in controlling the differential excitability of auditory thalamic neurons. The distribution of presumptive synaptic endings also has implications for the number and arrangement of glomeruli or synaptic nests. Thus, these circuit elements, which are common to the thalamic nuclei in other species, might play an important role in local synaptic circuits between different types of cells. If so, then the structural variations embodied in these patterns could subserve functional arrangements that differ among species. Such patterns might reflect concomitant physiological differences in the organization of local circuits within the microchiropteran medial geniculate body.