Electron microscope study of sources in the cat primary auditory cortex (AI) projecting to the medial geniculate body

An electron microscope study of retrogradely labeled pyramidal neurons in layer VI of the primary auditory cortex (AI) after injecting horseradish peroxidase (HP) into the medial geniculate body was carried out in cats. Not less than 57.8±1.9% on average of the perimeter of perikaryon profiles of corticogeniculate neurons labeled with HP were found to be covered with astroglia processes. Between three and eight synapses occupying an average of 10.8±1.0% of the perimeter length were found on the perikaryon profiles of these neurons. Nearly all synapses (a total of 98.7%) at the soma of corticogeniculate neurons had symmetrical active zones, being made up of axonal terminals with flattened synaptic vesicles. Anterogradely HP-labeled axonal terminals of geniculocortical fibers were also found in the neuropil of layer VI in area AI, in addition to retrogradely labeled neurons. They contained large round synaptic vesicles and formed asymmetrical synapses. The potential role of axosomatic synapses in the shaping of corticogeniculate neuronal activity is discussed.A. A. Bogomolets Institute, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 2, pp. 171–178, March–April, 1990.     

Ultrastructural characteristics of callosal neurons in deep layers of cat primary auditory cortex (AI)

We have carried out an electron microscopic investigation of retrogradely HRP-labeled nonpyramidal neurons in layers V and VI of the primary auditory cortex (AI), which are sources of transcallosal projections. We have established that on average 15.8±1.7% of the perikaryon surface of these cells is occupied by axo-somatic synapses. We detected in ultrathin sections from two to nine synapses on the profiles of the perikaryon of callosal neurons. All of these axo-somatic synapses are formed by axon terminals containing small flat synaptic vesicles and are characterized by symmetrical contacts. The length of the cross section of the contacts was on average 1.6±0.1 µm. The axon terminals of callosal fibers, antegradely labeled by the enzyme, form in the deep layers of the cortex asymmetrical synapses on the spines and stems of the dendrites. A possible functional significance of the axo-somatic synapses in the production of the impulse activity of callosal neurons in the deep layers of the AI region, is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 5, pp. 549–556, May, 1991.     

Ultrastructural parameters of GABA-ergic and non-GABA-ergic synaptic contacts on neurons of the catsubstantia nigra

Nigrothalamic neurons were identified in the reticular part of thesubstantia nigra using labelling by the retrograde axonal transport of horseradish peroxidase. Nine parameters of the synaptic contacts (n=195) were analyzed, including the size and shape of terminals and size and type of synaptic vesicies. Sixty-six percent of axon terminals studied formed symmetric contacts and contained large polymorphic vesicles (group I). Two-thirds of these synapses were located on the distal dendrites, while one-third was distributed on the perikarya and proximal dendrites. Synapses of group II (29% of all synapses analyzed) exhibited asymmetric contacts and contained round agranular vesicles. Among these synapses, 79% were located on the distal dendrites, 19% were located on the proximal dendrites, and only 2% were located on the neuronal perikarya. Axon terminals of group III (5% of total population) exhibited symmetric contact and contained small polymorphic vesicles. High-resolution immunogold EM histochemistry indicated that 63% of the group-I axon terminals were GABA-positive. The majority of synapses on the labelled nigrothalamic neurons (21 contacts of 25) belonged to group I. Among these 21 synapses, 19 were axo-somatic and mostly GABA-positive. Within group II, 30% of synapses showed slightly expressed GABA-positivity.Neirofiziologiya/Neurophysiology, Vol. 27, No. 2, pp. 147–157, March–April, 1995.     

Ultrastructure and morphometric parameters of glutamatergic synapses on nigrothalamic and unidentified neurons of the reticular zone of theSubstantia nigra in cats

Nigrothalamic neurons were identified into thesubstantia nigra by their retrograde labelling with horseradish peroxidase. Axon terminals that contain glutamate (the excitatory transmitter) were revealed immunocytochemically with an immunogold electron microscopic technique. Ultrastructural parameters (the large and small diameters of axon terminals, area of their profiles, coefficient of form of profiles, large and small diameters of synaptic vesicles) were analyzed in all 240 synapses under study. Synaptic contacts localized on both nigrothalamic and unidentified neurons belonged to three morphologically specific groups. Synapses of the groups I and III, according to classification by Rinvik and Grofova, were characterized by a symmetric type of synaptic contact and contained polymorphic synaptic vesicles. Contacts in group-II synapses were asymmetric, and respective terminals contained round vesicles. Among the studied synapses, 65.8% were classified as group-I contacts, 25.0% belonged to group II, and 9.2% belonged to group III. Glutamate-positive axon terminals formed predominantly group-II synapses; such connections constituted 70% of this group's synapses. Sixty percent of glutamate-positive synapses were localized on the distal dendrites and 23% on the proximal dendrites, while 17% of such synapses were distributed on the somata of nigral neurons. Such a pattern of distribution of glutamate-positive synapses was observed on both nigrothalamic and unidentified nigral neurons. About 7% of glutamate-positive synapses were formed by very large axon terminals containing round synaptic vesicles; yet, the contacts of these terminals were of a symmetric type. Twenty percent of group-I synapses, i.e., synapses considered inhibitory connections, were found to manifest a weak immune reaction to glutamate.Neirofiziologiya/Neurophysiology, Vol. 28, No. 6, pp. 285–295, November–December, 1996.     

Synaptic apparatus of the feline primary auditory cortex (area Al)

Electron microscope research was conducted on the synaptic apparatus of the feline primary auditory cortex (Al). A total of 2096 profiles of axonal terminals (AT) were found over a total area of 8230 µm² of ultrathin slices at different layers of this cortical layer — an average of 255 profiles per 1000 µm² of the surface area on these slices. The AT profiles occupied about 8.9% of the surface of these cross-sections. It was found that 52% of the AT containing synaptic vesicles formed asymmetrical or symmetrical synaptic contacts (83.9% and 16.1% respectively) and that AT had no contacts which could be considered synaptic junctions on 48% of slices. It was also observed that 45.3% of the AT forming contacts synapsed on spines, 48.5% on dendrites, and 6.2% on neuronal somata. Finally, 95.4% and 4.6% of axo-spinal synapses contained rounded and flattened vesicles respectively; equivalent figures for axodendritic synapses were 79.4% and 20.6% respectively and 19.8 and 80.2% for axosomatic synapses. Calculations revealed an average of 322.8 × 10⁶ AT over 1 mm³ of cat auditory cortex. Organizational aspects of synaptic apparatus at different layers of area A1 were ascertained.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 4, pp. 533–543, July–August, 1990.     

Synaptic organization of the cat parietal association cortex (area 5b)

An electron microscopy study was made of synaptic organization in the cat association cortex, area 5b. A total of 1635 axonal terminals were discovered over 6215 µm² (240 electronic imagings of slices of different association cortex layers); i.e., an average of 263±16 terminals per 1000 µm² expanse. It was found that 75.5% of axon terminals contained synaptic vesicles and formed either one- or two-sided contact with postsynaptic structures; 24.5% of axonal terminals contained synaptic vesicles but formed no distinct synaptic contacts with nearby neurons; 84.9% of terminals contained round-shaped or slightly oval synaptic vesicles; 7.8% had both rounded and elongated shapes, and vesicles were very elongated in the remaining 7.3%. Of the axonal terminals having synaptic contacts, axo(dendritic)-spinal terminals accounted for 46.6%, and axodendritic and axosomatic endings amounted to 50.0% and 3.4% respectively (in all 77% of axosomatic terminals contained elongated vesicles and maintained symmetrical contact, while 23% had round-shaped vesicles and formed asymmetrical contact). Calculations show that for each 1 mm³ an average of 258 million axonal terminals are found forming synaptic contacts in the cat association cortex as well as 84 million terminals containing synaptic vesicles but not forming contact.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 2, pp. 174–185, March–April, 1989.     

Morphological characteristics of callosal neurons of the cat primary auditory cortex (AI)

Cortical stratification of neurons forming callosal projections to the primary cortical area (AI) was investigated in cats using horseradish peroxidase axonal transport techniques. The population of area AI callosal neurons was found to be composed of several groups of cells. The group comprising around 60% of all callosal neurons of this area consists of large layer III pyramidal neurons. Callosal neurons belonging to this layer have a mean perikaryon profile area of 261.8±8.2 µm²; they account for 22% of all cells found in the layer. The second group, comprising 27% of all area AI callosal neurons, was largely made up of large layer V and VI cells; these could not be classed as pyramidal neurons due to the shape of their somata and the geometry of their dendritic arborization. Perikaryon profile in these nonpyramidal neurons occupied an area of 250.3±8.4 µ². No callosal neurons were observed in layer I. These account for 6 and 7% of total numbers of callosal neurons of area AI in layers II and IV. Callosal neurons were found to form projections to all layers of area AI in the contralateral hemisphere. Highest density of callosal fiber endings was observed in layers II and III.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 2, pp. 249–256, March–April, 1990.     

A GABAergic inhibitory microcircuit controlling cholinergic outflow to the airways.

GABA is the main inhibitory neurotransmitter that participates in the regulation of cholinergic outflow to the airways. We have tested the hypothesis that a monosynaptic GABAergic circuit modulates the output of airway-related vagal preganglionic neurons (AVPNs) in the rostral nucleus ambiguus by using a dual-labeling electron microscopic method combining immunocytochemistry for glutamic acid decarboxylase (GAD) with retrograde tracing from the trachea. We also determined the effects of blockade of GABAA receptors on airway smooth muscle tone. The results showed that retrogradely labeled AVPNs received a significant GAD-immunoreactive (GAD-IR) terminal input. Out of a pooled total of 3,161 synaptic contacts with retrogradely labeled somatic and dendritic profiles, 20.2% were GAD-IR. GAD-IR terminals formed significantly more axosomatic synapses than axodendritic synapses (P < 0.02). A dense population of GABAergic synaptic contacts on AVPNs provides a morphological basis for potent physiological effects of GABA on the excitability of AVPNs. GAD-IR terminals formed exclusively symmetric synaptic specializations. GAD-IR terminals were significantly larger (P < 0.05) in both length and width than unlabeled terminals synapsing on AVPNs. Therefore, the structural characteristics of certain nerve terminals may be closely correlated with their function. Pharmacological blockade of GABAA receptors within the rostral nucleus ambiguus increased activity of putative AVPNs and airway smooth muscle tone. We conclude that a tonically active monosynaptic GABAergic circuit utilizing symmetric synapses regulates the discharge of AVPNs.     

Callosal connections in the cat parietal cortex during postnatal ontogenesis

By means of the method based on the retrograde axonal transport of horseradish peroxidase topography, quantitative and qualitative composition of homotopic neurons in the cat cerebral parietal associative cortex performing callosal connections have been studied. When comparing the data of the experiment with those previously obtained on distribution of the axonal terminals in the comissural neurons, certain places are revealed where concentration of the homotopic callosal connections of the parietal cortex field 7 take place. A morphological characteristic of the longaxonal pyramidal and stellate neurons forming these connections is presented.     

A survey of the cytology and synaptic organization of the insular trigeminal-cuneatus lateralis nucleus in the rat, including an identification of spinal afferent inputs

The cytology and synaptic organization of the insular trigeminal-cuneatus lateralis (iV-Cul) nucleus was examined in the rat. In addition, the ultrastructural morphology and synaptic connectivity of anterogradely labeled spinal afferent axons terminating in iV-Cul were examined following injection of horseradish peroxidase (HRP) into the cervical spinal cord. The uniformity of the ultrastructural features of iV-Cul neurons supports the presence of a homogeneous neuronal population. The most prominent feature of the iV-Cul neuropil is the presence of numerous interdigitating astrocytic processes, which extensively isolate neuronal somata and processes. iV-Cul contains a heterogeneous population of axonal endings that can be separated into three categories, depending upon whether they contain predominantly spherical-shaped agranular synaptic vesicles (R endings), predominantly pleomorphic-shaped agranular synaptic vesicles (P endings), or a heterogeneous population of dense-core vesicles (DC endings). The R endings represent the majority of axonal endings in iV-Cul and establish asymmetrical axodendritic and axospinous synaptic contacts, primarily along the distal portions of the dendritic tree. P endings establish symmetrical axosomatic, axodendritic, and axospinous synaptic contacts and exhibit a more generalized distribution along the somadendritic tree. DC terminals establish asymmetrical axodendritic synaptic contacts with distal dendritic processes and are the least frequently observed endings in the iV-Cul neuropil. Numerous synaptic glomeruli, exhibiting a single large central R bouton that establishes multiple axodendritic or axospinous synapses, characterize the iV-Cul neuropil. Axoaxonic synapses are conspicuously absent from the iV-Cul neuropil and glomeruli. The anterograde HRP labeling of spinal afferent axons that terminate in iV-Cul indicates that the terminals along these fibers are of the R type and that they are engaged predominantly in synaptic glomeruli. The results of this study indicate that the synaptic organization of iV-Cul is distinctly different from that of neighboring somatosensory nuclei, and supports the recent suggestion that this nucleus should be considered a separate precerebellar spinal relay nucleus in the lateral medulla.     

A Survey of the Cytology and Synaptic Organization of the Insular Trigeminal—Cuneatus Lateralis Nucleus in the Rat,Including an Identification of Spinal Afferent Inputs

The cytology and synaptic organization of the insular trigeminal—cuneatus lateralis (iV-Cul) nucleus was examined in the rat. In addition, the ultrastructural morphology and synaptic connectivity of anterogradely labeled spinal afferent axons terminating in iV-Cul were examined following injection of horseradish peroxidase (HRP) into the cervical spinal cord. The uniformity of the ultrastructural features of iV-Cul neurons supports the presence of a homogeneous neuronal population. The most prominent feature of the iV-Cul neuropil is the presence of numerous interdigitating astrocytic processes, which extensively isolate neuronal somata and processes. iV-Cul contains a heterogeneous population of axonal endings that can be separated into three categories, depending upon whether they contain predominantly spherical-shaped agranular synaptic vesicles (R endings), predominantly pleomorphic-shaped agranular synaptic vesicles (P endings), or a heterogeneous population of dense-core vesicles (DC endings). The R endings represent the majority of axonal endings in iV-Cul and establish asymmetrical axodendritic and axospinous synaptic contacts, primarily along the distal portions of the dendritic tree. P endings establish symmetrical axosomatic, axodendritic, and axospinous synaptic contacts and exhibit a more generalized distribution along the somadendritic tree. DC terminals establish asymmetrical axodendritic synaptic contacts with distal dendritic processes and are the least frequently observed endings in the iV-Cul neuropil. Numerous synaptic glomeruli, exhibiting a single large central R bouton that establishes multiple axodendritic or axospinous synapses, characterize the iV-Cul neuropil. Axoaxonic synapses are conspicuously absent from the iV-Cul neuropil and glomeruli. The anterograde HRP labeling of spinal afferent axons that terminate in iV-Cul indicates that the terminals along these fibers are of the R type and that they are engaged predominantly in synaptic glomeruli. The results of this study indicate that the synaptic organization of iV-Cul is distinctly different from that of neighboring somatosensory nuclei, and supports the recent suggestion that this nucleus should be considered a separate precerebellar spinal relay nucleus in the lateral medulla.     

Distribution and ultrastructure of thalamic afferents in the cat auditory cortex

Ultrastructural features of thalamic afferent fibers were studied in the cat auditory cortex in the early stages (on the 4th day) of experimental degeneration produced by destruction of the medial geniculate body. A coordinate grid was used in conjunction with an electron micro-scope to study the topography of the degenerating elements over wide areas of sections, so that the density of degeneration could be determined quantitatively in different layers of the cortex. Degenerating axons were found in all layers. Most of the large (5–7 µ) degenerating axons are located in layer VI; their diameters were smaller in the upper layers of the cortex. Degenerative changes affecting synaptic terminals of thalamo-cortical afferents were of the "dark" type. Fibers of the geniculo-cortical tract were shown to terminate mainly in cortical layer IV. A few degenerating synapses were found in the molecular layer. Terminals with sperical synaptic vesicles are found mainly on the spines of dendrites where they form "asymmetrical" contacts. A few degenerating axo-somatic synapses were observed on stellate neurons in layer IV. The results are discussed in connection with electrophysiological investigations of the cat auditory cortex during stimulation of specific afferent fibers.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 6, pp. 612–620, November–December, 1972.     

Nerve cells and synaptic connections in the intestinal nerve of the snail,Helix pomatia L.

Summary The ultrastructure of nerve cells and the finestructural organization of synaptic contacts have been investigated in the intestinal nerve in the snail Helix pomatia. Three types of nerve cells, occurring singly or in groups, can be distinguished on the basis of the ultrastructure of their perikaryon and content of granules. The peripheral output of these nerve cells has been verified by retrograde CoCl₂ and NiCl₂ staining. Both axosomatic and axo-axonic specialized synaptic contacts occur in the intestinal nerve. Presynaptic elements of these synaptic contacts contain 100–120 nm granular vesicles or 120–200 nm neurosecretory-like granules. Following intracellular horseradish peroxidase (HRP) labelling of identified central neurons responsible for peripheral regulatory processes, several labelled axons running toward the periphery can be followed throughout the branches of the intestinal nerve. These labelled axon processes (either primary axon or small collaterals) form specialized synaptic contacts, inside the intestinal nerve, and are always in a postsynaptic position. The occurrence of peripheral axo-somatic and axo-axonic synapses provides a morphological basis for integrative processes taking place in the intestinal nerve (peripheral nervous system) of Helix pomatia.     

Morphology of the cat auditory cortex

The ultrastructural features of the primary auditory cortex of the cats and the character of the endings of geniculo-cortical afferent fibers in the early stages of experimental degeneration evoked by destruction of the medial geniculate body were studied. In all layers of the cortex asymmetrical synapses with round synaptic vesicles on dendritic spines and on thin dendritic branches of pyramidal and nonpyramidal neurons are predominant. Symmetrical synapses with flattened or polymorphic vesicles are distributed chiefly on the bodies of the neurons and their large dendrites. Because there are few symmetrical synapses which could be regarded as inhibitory it is postulated that inhibitory influences may also be transmitted through asymmetrical synapses with round vesicles. Other types of contacts between the bodies of neurons, dendrites, and glial processes also were found in the auditory cortex. Degenerating terminals of geniculo-cortical fibers were shown to terminate chiefly in layer IV of the cortex on pyramidal and nonpyramidal neurons. Degeneration was of the dark type in asymmetrical synapses with round vesicles. The results are dicussed in connection with electrophysiological investigations of the auditory cortex.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 5, No. 5, pp. 519–524, September–October, 1973.     

Studies of cardiac ganglia in pre- and postnatal rabbits

Summary This investigation was undertaken to describe the ultrastructure of cardiac ganglia in rabbits from day 18 of gestation to day 35 postpartum. Special attention was directed to the types of synaptic contacts made with the principal neurons and with the small granule-containing cells. The cardiac ganglia in all animals consisted mainly of parasympathetic postganglionic neurons, supporting cells, and small granule-containing (small intensely fluorescent) cells. The neurons received afferent synaptic terminals of two types. One type contained mainly small clear vesicles typical of most cholinergic terminals. The second type contained mainly small dense-core vesicles (these were most prominent after treatment of the animal with 5-hydroxydopamine), and were considered to be adrenergic terminals. These adrenergic terminals are probably part of an inhibitory system in the ganglia. The small granule-containing cells received typical afferent synaptic terminals of the cholinergic type, and also formed specialized contacts with certain axonal terminals. These latter specializations are considered to be reciprocal synapses which probably have a role in modulating ganglionic transmission.Supported by the Kentucky Heart Association and the Heart Association of Louisville and Jefferson County     

Ultrastructural organization of the chemically sensitive cat ventrolateral medulla

Differences in the location of putative inhibitory (F-type) synapses were revealed during research into the ultrastructural organization of the chemically sensitive cat ventrolateral medulla (VLM). These synapses are made up of axonal terminals filled with flattened synaptic vesicles with the long axis measuring 60–80 nm. They are mainly located in the caudal portion of the test area, while S-type synapses with spherical electron-transparent synaptic vesicals, with a mean diameter of 50 nm, are distributed fairly evenly within the confines of the test area. It is postulated that neuronal structure of the chemically sensitive cat VLM have a different functional significance in the exerting of central neurogenous control over circulatory function.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 3, pp. 300–305, May–June, 1989.     

Structure and origin of axonal terminals innervating neurons of the vestibular ganglia

Structural organization of the frog axonal terminals, realizing the synaptic contact with neurons of the vestibular ganglion have been studied electron microscopically. Two kinds of axonal terminals are revealed. They differ in quantitative and qualitative composition of their synaptic vesicles convergated simultaneously in various combinations to the ganglious neurons. Using the axonal retrograde transport of horseradish peroxidase, the nature of these axonal terminals is analysed. Their source is demonstrated to be neurons of the reticular formation and ventral vestibular nucleus of the medulla oblongata, as well as Purkinje's cells of the cerebellum.     

Synaptic changes in the feline association cortex (area 5b) following lesion of the thalamic posterolateral nucleus

Numbers of intact and degenerating axonal terminals (AT) were determined in area 5b of the cat association cortex two, four, and 30 days after electrolytic destruction of the posterolateral thalamic nucleus (PL). The proportion of degenerating AT in area 5b two and four days after PL lesion was found to be 7.4 and 7.3%, respectively. Intact AT in the same field accounted for 83.6% of the total in intact animals four days after PL lesion. A 16.4% reduction occurred, consisting of AT with round vesicles and asymmetrical synaptic contacts (i.e., primarily excitatory AT); 40% of the damaged AT formed synapses on dendrites and 60% on spines. It follows that neurons of cortical association area 5b receive direct excitatory and mainly axonospinal afferents from the thalamic PL. Degenerating AT accounted for 4.6% 30 days after PL lesion, as compared with a decline of 31.2% and a rise of 7.0% in axospinal and axodendritic AT, respectively. No change occurred in axosomatic AT. The total number of AT had declined by 10.0%. Aspects of reinnervation of area 5b neurons following degeneration of the neuronal synapses induced by PL lesion are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 5, pp. 661–667, September–October, 1989.     

Afferent projections to the mesencephalic locomotor region of the cat brain

Afferent projections to the functionally identified mesencephalic locomotor region were investigated in cats using the horseradish peroxidase retrograde axonal transport technique. Sources of afferent projections to this region were discovered in different structures of the fore-, mid-, and hindbrain. Numbers of horseradish peroxidase-labeled neurons were calculated in different brain structures after injecting this enzyme into the mesencephalic locomotor region. Apart from the endopeduncular nucleus, different hypothalamic structures, and the substantia nigra, labeled neurons were discovered in the central tegmental region, the central gray, raphe and vestibular nuclei, the solitary tract nucleus, and the brain stem reticular formation. Neurons accumulating horseradish peroxidase were also discovered in nuclei where ascending sensory tracts originate. This fact serves to bring out the structural inhomogeneity of the midbrain locomotor region; electrical stimulation of this area is an effect which may be attributed to excitation of neurons found within it and activation of accompanying fiber tracts.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev Translated from Neirofiziologiya, Vol. 18, No. 6, pp. 763–773, November–December, 1986.     

Morphological characteristics of the neuronal population in the feline auditory cortex projecting into the medial geniculate body

The location and morphological profile of auditory cortex neurons projecting to the medial geniculate body were investigated in adult cats using horseradish peroxidase retrograde axonal transport techniques. Sources of descending projections to the medial geniculate body from auditory cortex areas I and II were found to be neurons belonging to deep-lying layers (layer VI and layer V to a lesser extent). By far the majority of corticogeniculate neurons in the auditory cortex were pyramidal cells. In layer VI of the primary auditory area (A1), the number of corticogeniculate neurons reaches 60% of all cells belonging to that layer. The average area (M±m) of the profile of perikarya of corticogeniculate neurons in layer VI, area Al equaled 139.3±2.5 µm² and 219.5±7.0 µm² in layer V neurons; average size of long diameter: 15.0±0.19 and 18.3±0.4 µm respectively. The lower regions of layers III and IV in area Al were found to be the termination point of the greater mass of anterogradely-labeled geniculocortical fibers (terminals of relay neuron axons belonging to the medial geniculate body).A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 4, July–August, pp. 513–521, 1989.