Specific patterns of neuron arrangement and of synaptic articulation in the medial geniculate body

The organization and physiology of the auditory thalamus and its role in processing acoustic features important for speech perception

The auditory thalamus, or medial geniculate body (MGB), is the primary sensory input to auditory cortex. Therefore, it plays a critical role in the complex auditory processing necessary for robust speech perception. This review will describe the functional organization of the thalamus as it relates to processing acoustic features important for speech perception, focusing on thalamic nuclei that relate to auditory representations of language sounds. The MGB can be divided into three main subdivisions, the ventral, dorsal, and medial subdivisions, each with different connectivity, auditory response properties, neuronal properties, and synaptic properties. Together, the MGB subdivisions actively and dynamically shape complex auditory processing and form ongoing communication loops with auditory cortex and subcortical structures.

Auditory thalamic organization: Cellular slabs, dendritic arbors and tectothalamic axons underlying the frequency map

A model of auditory thalamic organization is presented incorporating cellular laminae, oriented dendritic arbors and tectothalamic axons as a basis for the tonotopic map at this level of the central auditory system. The heart of this model is the laminar organization of neuronal somata in the ventral division of the medial geniculate body (MGV) of the rabbit, visible in routine Nissl stains. Microelectrode studies have demonstrated a step-wise ascending progression of best frequencies perpendicular to the cell layers. The dendritic arbors of MGV neurons are aligned parallel to the cellular laminae and dendritic tree width along the frequency axis corresponds closely with the frequency steps seen in microelectrode studies. In the laminated subdivision, tectothalamic axons terminate in the form of bands closely aligned with the laminae and dendritic arbors of thalamic relay neurons. The bands of tectothalamic axons extend in the anterior-posterior (A-P) plane forming a dorsal-ventral series of stacked frequency slabs. In the pars ovoidea region, the homologous spiraling of somata, dendritic fields and tectothalamic axons appear to represent a low-frequency area in this species. At least two types of tectothalamic terminals were found within the bands: large boutons frequently arranged in a glomerular pattern and smaller boutons arising from fine caliber axons. We propose that the rabbit is an ideal model to investigate the structural-functional basis of functional maps in the mammalian auditory forebrain.

Comparison of the fine structure of cortical and collicular terminals in the rat medial geniculate body

Neurons throughout the rat medial geniculate body, including the dorsal and ventral divisions, display a variety of responses to auditory stimuli. To investigate possible structural determinants of this variability, measurements of axon terminal profile area and postsynaptic dendrite diameter were made on inferior colliculus and corticothalamic terminal profiles in the medial geniculate body identified by anterograde tracer labeling following injections into the inferior colliculus or cortex. Over 90% of the synapses observed were axodendritic, with few axosomatic synapses. Small (<0.5 microm(2)) and large (>1.0 microm(2)) collicular profiles were found throughout the medial geniculate, but were smaller on average in the dorsal division (0.49+/-0.49 microm(2)) than in the ventral division (0.70+/-0.64 microm(2)). Almost all corticothalamic profiles were small and ended on small-caliber dendrites (0.57+/-0.25 microm diameter) throughout the medial geniculate. A few very large (>2.0 microm(2)) corticothalamic profiles were found in the dorsal division and in the marginal zone of the medial geniculate. GABA immunostaining demonstrated the presence of GABAergic profiles arising from cells in the inferior colliculus. These profiles were compared with GABAergic profiles not labeled with anterograde tracer, which were presumed to be unlabeled inferior colliculus profiles or thalamic reticular nucleus profiles. The distributions of dendritic diameters postsynaptic to collicular, cortical and unlabeled GABAergic profiles were compared with dendritic diameters of intracellularly labeled medial geniculate neurons from rat brain slices. Our results demonstrate a corticothalamic projection to medial geniculate body that is similar to other sensory corticothalamic projections. However, the heterogeneous distributions of excitatory inferior collicular terminal sizes and postsynaptic dendritic diameters, along with the presence of a GABAergic inferior collicular projection to dendrites in the medial geniculate body, suggest a colliculogeniculate projection that is more complex than the ascending projections to other sensory thalamic nuclei. These findings may be useful in understanding some of the differences in the response characteristics of medial geniculate neurons in vivo.

Anatomic, intrinsic, and synaptic properties of dorsal and ventral division neurons in rat medial geniculate body

Anatomic, intrinsic, and synaptic properties of dorsal and ventral division neurons in rat medial geniculate body. Presently little is known about what basic synaptic and cellular mechanisms are employed by thalamocortical neurons in the two main divisions of the auditory thalamus to elicit their distinct responses to sound. Using intracellular recording and labeling methods, we characterized anatomic features, membrane properties, and synaptic inputs of thalamocortical neurons in the dorsal (MGD) and ventral (MGV) divisions in brain slices of rat medial geniculate body. Quantitative analysis of dendritic morphology demonstrated that tufted neurons in both divisions had shorter dendrites, smaller dendritic tree areas, more profuse branching, and a greater dendritic polarization compared with stellate neurons, which were only found in MGD. Tufted neuron dendritic polarization was not as strong or consistent as earlier Golgi studies suggested. MGV and MGD cells had similar intrinsic properties except for an increased prevalence of a depolarizing sag potential in MGV neurons. The sag was the only intrinsic property correlated with cell morphology, seen only in tufted neurons in either division. Many MGV and MGD neurons received excitatory and inhibitory inferior colliculus (IC) inputs (designated IN/EX or EX/IN depending on excitation/inhibition sequence). However, a significant number only received excitatory inputs (EX/O) and a few only inhibitory (IN/O). Both MGV and MGD cells displayed similar proportions of response combinations, but suprathreshold EX/O responses only were observed in tufted neurons. Excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) had multiple distinguishable amplitude levels implying convergence. Excitatory inputs activated alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors the relative contributions of which were variable. For IN/EX cells with suprathreshold inputs, first-spike timing was independent of membrane potential unlike that of EX/O cells. Stimulation of corticothalamic (CT) and thalamic reticular nucleus (TRN) axons evoked a GABAA IPSP, EPSP, GABAB IPSP sequence in most neurons with both morphologies in both divisions. TRN IPSPs and CT EPSPs were graded in amplitude, again suggesting convergence. CT inputs activated AMPA and NMDA receptors. The NMDA component of both IC and CT inputs had an unusual voltage dependence with a detectable DL-2-amino-5-phosphonovaleric acid-sensitive component even below -70 mV. First-spike latencies of CT evoked action potentials were sensitive to membrane potential regardless of whether the TRN IPSP was present. Overall, our in vitro data indicate that reported regional differences in the in vivo responses of MGV and MGD cells to auditory stimuli are not well correlated with major differences in intrinsic membrane features or synaptic responses between cell types.

The organization of corticothalamic projections: reciprocity versus parity

All neocortical areas receive inputs from and project back to the thalamus. It is often said that the corticothalamic projections are organized in a way that reciprocates the spatial distribution of thalamocortical pathways. The present review examines to what extent this rule of reciprocity is actually supported by the most recent neuroanatomical data, particularly those relating to the central organization of the vibrissal sensory system in the rat. A critical survey of previous studies is made and new results are presented concerning the fine-grained organization of corticothalamic projections in this sensory system. Together, prior results and the present set of new data confirm the existence of both, reciprocal and nonreciprocal patterns of corticothalamic connectivity. This conclusion leads us to propose that the spatial organization of corticothalamic connections complies with a more fundamental rule, the rule of parity, from which reciprocity follows as a general, but not obligatory consequence. The rule of parity states that the distribution of corticothalamic projections across and within the thalamic nuclei is determined by the branching patterns of the different classes of prethalamic afferents. The anatomical, developmental and physiological consequences of this rule are discussed. The rule of parity suggests that, according to the behavioral context, both prethalamic and corticothalamic pathways may function in a feedback mode.

Experimentally induced retinal projections to the ferret auditory thalamus: development of clustered eye-specific patterns in a novel target

We have examined the relative role of afferents and targets in pattern formation using a novel preparation, in which retinal projections in ferrets are induced to innervate the medial geniculate nucleus (MGN). We find that retinal projections to the MGN are arranged in scattered clusters. Clusters arising from the ipsilateral eye are frequently adjacent to, but spatially segregated from, clusters arising from the contralateral eye. Both clustering and eye-specific segregation in the MGN arise as a refinement of initially diffuse and overlapped projections. The shape, size, and orientation of retinal terminal clusters in the MGN closely match those of relay cell dendrites arrayed within fibrodendritic laminae in the MGN. We conclude that specific aspects of a projection system are regulated by afferents and others by targets. Clustering of retinal projections within the MGN and eye-specific segregation involve progressive remodeling of retinal axon arbors, over a time period that closely parallels pattern formation by retinal afferents within their normal target, the lateral geniculate nucleus (LGN). Thus, afferent-driven mechanisms are implicated in these events. However, the termination zones are aligned within the normal cellular organization of the MGN, which does not differentiate into eye-specific cell layers similar to the LGN. Thus, target-driven mechanisms are implicated in lamina formation and cellular differentiation.

Morphology and spatial distribution of corticothalamic terminals originating from the cat auditory cortex

In this paper we studied the morphology and spatial distribution of corticothalamic axons and terminals originating from the auditory cortical fields of the cat. The anterograde tracer biocytin was injected at electrophysiologically characterized loci in the primary (AI) (N = 2), anterior (AAF) (N = 1), posterior (PAF) (N = 1) and secondary (AII) (N = 2) auditory fields. In all cases, two different types of labeled terminals were found in the auditory thalamus: small spherical endings (1-2 microns) and giant, finger-like endings (5-10 microns). After biocytin injections in AI and AAF, the majority of anterogradely labeled axons terminated in the rostral half of the pars lateralis (LV) of the ventral division of the medial geniculate body (vMGB). In LV, the corticothalamic axons ramified profusely, giving rise to dense terminal fields forming well delineated curved stripes, with small spherical endings. Additional terminal fields formed by small endings were observed in the medial division of the medial geniculate body (mMGB). Giant endings were observed in a small area in the dorsal nucleus (D) of the dorsal division of the medial geniculate body (dMGB), near its border with the vMGB. PAF projections were located in the caudal half of vMGB and in mMGB, where only small terminals were found. Giant endings were seen in the superficial part of dMGB emerging from labeled corticothalamic axons oriented in parallel to the dorsal surface of the MGB. Projections from AII gave rise to a main terminal field of small endings in D; a second terminal field consisting of giant endings intermingled with small endings was found in the deep dorsal nucleus (DD) of dMGB. We conclude that small terminals serve the feedback projection to the thalamic nucleus from which the injected cortical field receives its main input, whereas giant terminals cross the borders between the parallel ascending auditory pathways.

The descending auditory pathway and acousticomotor systems: connections with the inferior colliculus

In this review the following major points are emphasized. First, the descending auditory system includes 3 separate, but parallel pathways connecting the AC, MGB and IC. Each pathway makes a strong set of connections with a distinctive area from each of 3 auditory centers. The three sets of connections are mutually exclusive, such that the pathways describe 3 separate corticocolliculo-geniculate systems. Thus, multiple feedback loops between the AC and the IC are formed which create a great capacity for parallel processing of auditory information. Second, the IC projects to the SOC and, in particular, to the source of olivocochlear efferent neurons. The connections of the IC with the AC rostrally, and with the olivocochlear neurons caudally, imply a descending trisynaptic pathway from the cortex to the cochlea whose travel time could better that of the ascending pathway and thus provide an efficient feedback mechanism. It is probable that the IC influences cochlear signal processing. The reciprocal connectivity between any two of either the IC, SOC or the CN, again, affords to the auditory system remarkable parallel processing capabilities. Finally, the descending auditory, and 'extra-auditory' connections of the IC bestow a functional separateness to the 3 nuclei of the IC, a view that is best illustrated by description of the ICX as an acousticomotor nucleus, having connections with the SC, cerebellum and somatosensory and vocalization systems. More sophisticated questions about the descending auditory system will incorporate these present observations and test functional implications to which they allude.

Identification and structure of neurons in the medial geniculate body projecting to primary auditory cortex (AI) in the cat

The neuronal types in the ventral nucleus of the cat medial geniculate body projecting to the primary auditory cortex (AI) were investigated using the retrograde transport of horseradish peroxidase. These cells were compared with the morphology of neurons as revealed in Golgi and Nissl preparations, plastic-embedded tissue, and electron microscopic material. After large injections, more than 90% of the neurons in the ventral nucleus, the principal nucleus of the lemniscal auditory pathway, were labeled, and the population of labeled cells included both large and small neuronal somata. Since the ventral nucleus contains only two varieties of cells--large neurons with bushy dendrites and an unbranched axon, and smaller cells with thin dendrites and a locally projecting axon--it is concluded that at least some of the small cells, previously believed to be interneurons, may function both as local circuit and as projection neurons. These findings were confirmed in toluidine blue-stained, 1-2 micron thick sections, and in the electron microscope, where small cells with sparse cytoplasm and a deeply invaginated nuclear envelope often contained intracellular horseradish peroxidase granules, as well as the larger neurons. Besides the small, labeled neurons in the ventral nucleus, many labeled cells were seen in the interstitial nucleus of the brachium of the inferior colliculus. This hitherto poorly characterized group of cells is embedded among the fibers of the brachium of the inferior colliculus. Many of the morphologically distinct varieties of cells in the medial division of the medial geniculate body, including small neurons, were labeled. Thus, in addition to the route embodied by the large bushy neurons which project to primary auditory cortex, at least one other pathway--represented by certain of the small cells in the ventral nucleus, reaches the primary auditory cortex.

Fine structure of the superficial layers of the viper optic tectum. A Golgi and electron-microscopic study

The superficial layers of the viper optic tectum, which receive fibers from he retina, were studied using both light and electron microscopes. The optic fibers layer, or stratum opticum, is composed of 200 to 250 tight fascicles containing thin fibers, nearly all of which are myelinated. The main optic terminal layers, the stratum griseum et fibrosum superficiale, the greatest part of the cellular population is composed of small vertically oriented neurons and horizontal nerve cells, many of which are probably local circuit neurons. The neuropil of the stratum griseum et fibrosum superficiale is made up of small nerve elements, including three types of profiles containing synaptic vesicles; 1) boutons with pleiomorphic synaptic vesicles (P), representing over 47% of the total population of profiles containing synaptic vesicles and comprising three subgroups (P1, P2, and P3); 2) boutons with spheroidal synaptic vesicles (S), forming more than 29% of the total populations of profiles containing synaptic vesicles and comprising two categories, S1 and S2 (S2, the more numerous, represents the optic boutons, which make up 22% of the total populations of profiles containing synaptic vesicles); and 3) dendrites with pleiomorphic vesicles, accounting for approximately 23% of the total populations of profiles containing synaptic vesicles. A study of synaptic patterns revealed a large number of serial synapses and a lesser number of triplets or triadic synapses. The presynaptic components are boutons containing spheroidal (S1, S2) or pleiomorphic (P1, P2, P3) synaptic vesicles. The intermediate profile was always a dendrite with synaptic vesicles which frequently belonged to the small neurons of the stratum griseum et fibrosum superficiale. Comparison of the present results with other recent data shows that the synaptic circuitry in the optic tectum of Vipera aspis closely resembles the pattern observed in the optic tectum of other vertebrates, ranging form fish to mammals. However, quantitative differences exist, especially with regard to the proportion of dendrites containing synaptic vesicles. Their number seems to be higher in sauropsidians than in mammals, particularly in primates.

Effects of changes in cortical arousal and of auditory cortex cooling on neuronal activity in the medial geniculate body

The activity of cells in the medial geniculate body (MGB) of adult cats was recorded during different states of cortical arousal with and without cooling of the auditory cortex. In the absence of auditory cortex cooling, the overall mean unit spontaneous discharge rate was 49% higher during desynchroized Electrocorticogram (ECoG) periods (high cortical arousal). Responses to sound were somewhat more prominent vis-à-vis the spontaneous activity during periods of high arousal. Changes in spontaneous discharge rate associated with arousal shifts were significantly reduced during auditory cortex cooling. When the ECoG changed from desynchronized to synchronized activity, MGB cells showed a change in discharge pattern, typically characterized by an increase in both high-rate bursts and long-interval pauses. These changes were duplicated for most cells by cooling of the auditory cortex. Corticofugal fiber discharge thus has an effect on MGB neuronal activity which is dependent on the level of cortical arousal. This effect is most likely a result of direct corticogeniculate activity, though indirect auditory cortex - brainstem - MGB routes may also be involved.

Presynaptic dendrites and serial synapses in the intermediate and deep layers of the cat superior coliculus

Presynaptic dendrites (PSDs) which participate in the serial synapses have frequently been found in the intermediate and deep layers of the cat superior colliculus. The PSDs are presynaptic to small dendritic shafts or spines with symmetrical membrane thikening, and postsynaptic to axon terminals with asymmetrical synaptic contact. Two types of the axon terminals are observed, both of which contain pleomorphic vesicles.

Anatomic evidence suggestive of dendrodendritic synapses in the opossum basilar pons

In Golgi-prepared material, opossum basilar pontine intrinsic neurons measured less than 22 microns and gave origin to only two or three primary dendrites which gradually tapered in diameter, branched infrequently and exhibited few spines or protrusions. Characteristically, such neurons gave rise to several axon-like processes which might take origin from proximal or distal dendrites as well as the soma. Morphologically similar neurons have been observed in several other regions throughout the CNS where they have been shown to be a source of presynaptic dendritic elements. That opossum basilar pontine intrinsic neurons might also give rise to presynaptic dendrites was supported by the following electron microscopic observations. Profiles containing a small cluster of pleomorphic vesicles, occasional ribosomes and numerous microtubules formed synaptic active sites in which the membrane densities were intermediate between Gray's type I and type II. Such presumed presynaptic dendritic profiles were observed to participate in serial synaptic arrangements in which they were always postsynaptic to a round vesicle bouton and presynaptic to another dendritic element. The above features are compatible with electron microscopic descriptions of presynaptic dendrites in other CNS regions and thus suggest that basilar pontine intrinsic neurons may represent a source of presynaptic dendritic elements.

Serial and triadic synapses in the cerebellar nuclei of the cat

A quantitative electron microscopic investigation of the nucleus interpositus in cat cerebellum reveals that about 1.5% of all observed synapses are established between synaptic vescile-bearing profiles. It is shown by serial sections that 70% of these synaptic complexes are triadic arrangements and 30% are serial synapses. Further analysis discloses that the first presynaptic element in the triadic and serial synapses may be one of four different axonal types: (A) Purkinje-cell axons; (B) and (C) afferent fibers containing large round vesicles and originating from the brain stem (probably mossy and climbing fiber collaterals); and (D) axon terminals containing small round vesicles. Indirect evidence suggests that type D profiles are the recurrent axon collaterals of the projective neurons. The second, postsynaptic and presynaptic, vesicle-bearing process in these complexes is either a class D terminal, or a somewhat more "dendrite-like" profile (Class E) containing flattened vesicles, and identified as belonging to processes of local Golgi type II interneurons.