Location of motoneurons in the oculomotor nucleus and the course of their axons in the oculomotor nerve

Subdivisions of the oculomotor nucleus, and the course of axons in the brainstem and more peripherally in the oculomotor nerve of the cat, were studied by directly applying horseradish peroxidase solution to the transected nerve-branch stump in the orbit. The medial rectus subdivision consisted of two subgroups, and intermingling between subdivisions was found. About 20% of the motoneurons controlling the medial rectus muscle were scattered in the medial longitudinal fasciculus or a more ventrolateral area. A few motoneurons controlling the inferior rectus or inferior oblique muscle were also located in the medial longitudinal fasciculus. Axons to the superior branch that supplied the superior rectus and levator muscle coursed in the dorsolateral half of the oculomotor nerve. In contrast, those to the medial rectus, inferior rectus, and inferior oblique muscles were scattered diffusely in the oculomotor nerve.     

Extra- and intracellular HRP analysis of the organization of extraocular motoneurons and internuclear neurons in the guinea pig and rabbit

The distribution of extraocular motoneurons and abducens and oculomotor internuclear neurons was determined in guinea pigs by injecting horseradish peroxidase (HRP) into individual extraocular muscles, the abducens nucleus, the oculomotor nucleus, and the cerebellum. Motoneurons in the oculomotor nucleus innervated the ipsilateral inferior rectus, inferior oblique, medial rectus, and the contralateral superior rectus and levator palpebrae muscles. Most motoneurons of the trochlear nucleus projected to the contralateral superior oblique muscle although a small number innervated the ipsilateral superior oblique. The abducens and accessory abducens nuclei innervated the ipsilateral rectus and retractor bulbi muscles, respectively. The somata of abducens internuclear neurons formed a cap around the lateral and ventral aspects of the abducens nucleus. The axons of these internuclear neurons terminated in the medial rectus subdivision of the contralateral oculomotor nucleus. At least two classes of guinea pig oculomotor internuclear interneurons exist. One group, located primarily ventral to the oculomotor nucleus, innervated the abducens nucleus and surrounding regions. The second group, lying mainly in the dorsal midline area of the oculomotor nucleus, projected to the cerebellum. Intracellular staining with HRP demonstrated similar soma-dendritic organization for oculomotor and trochlear motoneurons of both guinea pigs and rabbits. Dendrites of oculomotor motoneurons radiated symmetrically from the soma to cover approximately one-third of the entire nucleus, and each motoneuron sent at least one dendrite into the central gray overlying the oculomotor nucleus. In both species, a small percentage of oculomotor motoneurons possessed axon collaterals that terminated both within and outside of the nucleus. The dendrites of trochlear motoneurons extended into the medial longitudinal fasciculus and the reticular formation lateral to the nucleus. Our data on the topography of motoneurons and internuclear neurons in the guinea pig and soma-dendritic organization of motoneurons in the guinea pig and rabbit show that these species share common organizational and morphological features. In addition, comparison of these data with those from other mammals reveals that dendritic complexity (number of dendrites per motoneuron) of extraocular motoneurons exhibits a systematic increase with animal size.     

GABAergic and glycinergic presympathetic neurons of rat medulla oblongata identified by retrograde transport of pseudorabies virus and in situ hybridization

Electron microscopy suggests that up to half the synaptic input to sympathetic preganglionic neurons (SPGNs) is GABAergic or glycinergic. A proportion of this input is suspected to originate from neurons located within the medulla oblongata. The present study provides definitive evidence for the existence of these supraspinal presympathetic (PS) neurons with inhibitory phenotypes. PS neurons were identified by retrograde trans-synaptic migration of pseudorabies virus (PRV) injected into the adrenal gland. GABAergic or glycinergic cell bodies were identified by the presence of glutamate decarboxylase (GAD)-67 mRNA or glycine transporter (GlyT)-2 mRNA detected with in situ hybridization (ISH). Neither GABAergic nor glycinergic PS neurons were tyrosine hydroxylase (TH)-immunoreactive (ir). GABAergic PS neurons were located within the ventral gigantocellular nucleus, gigantocellular nucleus alpha, and medial reticular formation, mostly medial to the TH-ir PS neurons. About 30% of GABAergic PS neurons were serotonergic cells located in the raphe pallidus (RPa) and parapyramidal region (PPyr). Glycinergic PS neurons had the same general distribution as the GABAergic cells, except that no glycinergic neurons were located in the RPa or PPyr and none were serotonergic. PRV immunohistochemistry combined with ISH for both GlyT2 and GAD-67 mRNAs showed that at least 63% of midline medulla GABAergic PS neurons were also glycinergic and 76% of glycinergic PS neurons were GABAergic. In conclusion, the rostral ventromedial medulla contains large numbers of GABAergic and glycinergic neurons that innervate adrenal gland SPGNs. Over half of these PS neurons may release both transmitters. The physiological role of this medullary inhibitory input remains to be explored.     

Internuclear neurons in the ocular motor system of frogs.

Medial and lateral rectus motoneurons of frogs were localized after retrograde labeling with horseradish peroxidase (HRP) injected in the medial rectus muscle or applied on the cut end of the abducens nerve. Coordinates of these cell columns were used as target areas for the injection of small amounts of HRP (20-60 nl) and [3H]leucine (25-40 nl) and as search areas for retrogradely and anterogradely labeled internuclear neurons (INT) in in vivo and in vitro experiments. HRP injection in the medial rectus subdivision of the oculomotor nucleus (n = 6) resulted in retrograde labeling of cell bodies in the contralateral principal abducens nucleus. On the average about 16 cells per animal were found. Somatic diameters were about 13.5 +/- 2.8 microns (n = 32). The number and the size of these abducens internuclear neurons (AbINT) are smaller than those of lateral rectus motoneurons (n = 75; diameter: 19 +/- 3.2 microns). A crossed projection of AbINT to medial rectus motoneurons in the contralateral oculomotor nucleus is further supported by autoradiographic results. Following injection of [3H]leucine into the abducens nucleus, a high density of silver grains was visible within the contralateral oculomotor nucleus, mainly in the caudal part of the oculomotor nucleus, where medial rectus motoneurons are located. Injection of [3H]leucine in vivo (n = 4) and in vitro (n = 3) resulted in a similar high density of silver grains within the contralateral oculomotor nucleus, but the background level of silver grains was significantly higher after in vitro (264 +/- 38/2,500 microns2) than after in vivo injections (195 +/- 17/2,500 microns2). HRP injection in the principal abducens nucleus (n = 9) resulted in retrograde labeling of cell bodies in the medial rectus subdivisions of the bilateral oculomotor nuclei. Ipsilateral projections predominated, with about 10 (+/- 8) labeled cells over contralateral projections (about 3 +/- 2). Average diameters of these oculomotor internuclear neurons (OcINT) were again smaller (10.8 +/- 2 microns; n = 18) than those of medial rectus motoneurons (14.4 +/- 3 microns; n = 52). In addition, retrogradely labeled cells were consistently encountered in the bilateral vestibular nuclei, the cerebellar nuclei, the dorsal brainstem caudal to the abducens nuclei, and ipsilaterally in the pretectum. Most of the vestibular neurons were located in the rostral part of the vestibular nuclear complex. These neurons might constitute part of the three-neuronal arc of the vestibulo-ocular reflex in the frog. Labeled cells in the pretectum were restricted to the ipsilateral posterior thalamic nucleus (P).(ABSTRACT TRUNCATED AT 400 WORDS)     

Patterns of extraocular innervation by the oculomotor complex in the chick

The horseradish peroxidase retrograde tracer technique was used to map the projection pattern of the oculomotor nuclear complex to the extraocular muscles in the chick embryo. The following projection pattern was found: The dorsolateral oculomotor subnucleus innervates the ipsilateral inferior rectus muscle, the dorsomedial subnucleus innervates the ipsilateral medial rectus muscle, a lateral division of the ventromedial subnucleus innervates the ipsilateral inferior oblique muscle, and a medial division of the ventromedial subnucleus innervates the contralateral superior rectus muscle. The so-called central nucleus also innervates the contralateral superior rectus muscle. This pattern was extremely discrete, with virtually no overlapping representations. These results provide the first evidence for a functional medial-lateral subdivision of the ventromedial subnucleus. This pattern relates to the unusual development of this subnucleus and suggests that only part of the primordium for this cell group migrates across the midline during its ontogeny, rather than all of it, as was previously believed. The subnuclear organization of the avian oculomotor complex is also considered in comparison to such functional organization in other species.     

Location and quantitative analysis of the motoneurons innervating the extraocular muscles of the guinea-pig, using horseradish peroxidase (HRP) and double or triple labelling with fluorescent substances

The location of the motoneurons innervating the extraocular muscles of the guinea-pig was investigated using horseradish peroxidase (HRP) and the fluorescent substances fast blue, propidium iodide and nuclear yellow as retrograde tracers. The innervation of the inferior rectus, medial rectus and inferior oblique muscles is exclusively ipsilateral, and these neurons form three well-defined and mutually separate subnuclei in the oculomotor nucleus. The subgroup innervating the medial rectus lies exclusively along the medial face of the oculomotor nucleus, with no aberrant neurons in the medial longitudinal fasciculus, as have been found in other mammals. The superior rectus and levator palpebrae are innervated almost entirely by contralateral motoneurons located both in the oculomotor nucleus and in a variety of extranuclear positions (in the periaqueductal grey, among the fibres of medial longitudinal fasciculus and ventral to this bundle). There is no anteroposterior separation between the oculomotor and trochlear nuclei, since superior rectus and levator palpebrae neurons are found flanking the latter laterally all along its anterior half. In the caudal two-thirds of the oculomotor nucleus the motoneurons innervating the superior rectus and levator palpebrae are partially intermingled with those corresponding to the ipsilaterally-innervated muscles, particularly those of the inferior rectus.     

Comparison of vestibular and abducens internuclear projections to the medial rectus subdivision of the oculomotor nucleus in the monkey

Comparisons were made of projections from the vestibular nuclei (VN) and abducens internuclear neurons (AIN) to cell group A of the medial rectus subdivision (MRS) of the oculomotor nuclear complex. Cell group A, the major component of the MRS, receives projections only from the ipsilateral VN and the contralateral AIN. Neither ipsilateral vestibular projections to cell group A, arising from the medial vestibular nucleus, nor projections from MVN to the opposite abducens nucleus, match the massive projection of AIN to the MRS.     

Afferents to the abducens nucleus in the monkey and cat

The abducens nucleus is a central coordinating element in the generation of conjugate horizontal eye movements. As such, it should receive and combine information relevant to visual fixation, saccadic eye movements, and smooth eye movements evoked by vestibular and visual stimuli. To reveal possible sources of these signals, we retrogradely labeled the afferents to the abducens nucleus by electrophoretically injecting horseradish peroxidase into an abducens nucleus in four monkeys and two cats. The histologic material was processed by the tetramethyl benzidine (TMB) method of Mesulam. In both species the largest source of afferents to the abducens nucleus was bilateral projections from the ventrolateral vestibular nucleus and the rostral pole of the medial vestibular nucleus. Scattered neurons were also labeled in the middle and caudal levels of the medial vestibular nucleus. Large numbers of neurons were labeled in the ventral margin of the nucleus prepositus hypoglossi in the cat and in the common margin of the nucleus prepositus and the medial vestibular nucleus in the monkey, a region we call the marginal zone. Substantial numbers of retrogradely labeled neurons were found in the dorsomedial pontine reticular formation both caudal and rostral to the abducens nuclei. In the monkey, large numbers of labeled neurons were present in the contralateral medial rectus subdivision of the oculomotor complex, while smaller numbers occurred in the ipsilateral medial rectus subdivision and elsewhere in the oculomotor complex. In the cat, large numbers of retrogradely labeled cells were present in a small periaqueductal gray nucleus immediately dorsal to the caudal pole of the oculomotor complex, and a few labeled neurons were also dispersed through the caudal part of the oculomotor complex. Occasional labeled neurons were present in the contralateral superior colliculus in both species. The size and distribution of the labeled neurons within the intermediate gray differed dramatically in the two species. In the cat, the retrogradely labeled neurons were very large and occurred predominantly in the central region of the colliculus, while in the monkey, they were small to intermediate in size and were distributed more uniformly within the middle gray. Among the afferent populations present in the monkey, but not in the cat, was a group of scattered neurons in the ipsilateral rostral interstitial nucleus of the medial longitudinal fasciculus and a denser, bilateral population in the interstitial nucleus of Cajal.(ABSTRACT TRUNCATED AT 400 WORDS)     

Localization of motoneurons controlling the extraocular muscles of the rat

The localization of extraocular motoneurons in the rat was investigated by injecting horseradish peroxidase and [¹²⁵I]wheat germ agglutinin¹⁷ as retrogade tracer substances into individual eye muscles. The organization of subnuclei was found to be most similar to the rabbit. The subgroups representing the medial rectus and inferior rectus muscles are located in the rostral two thirds of the ipsilateral oculomotor nucleus (nIII) with some medial rectus motoneurons scattered laterally along the edge of the medial longitudinal fasciculus. The motor pool controlling the inferior oblique muscle is located in the middle third of the ipsilateral nIII. The motoneurons of the superior rectus muscles are in the caudal two-thirds of contralateral nIII while the levator palpebrae muscle has a bilateral innervation in the oculomotor nucleus. The motoneurons of the superior oblique are located in the contralateral trochlear nucleus although a few labeled neurons were scattered laterally in amongst the fibers of the medial longitudinal fasciculus. The cell bodies of lateral rectus motoneurons regional separation between the latter and internuclear neurons was found after injecting HRP into the oculomotor nucleus.     

Localization of retractor bulbi motoneurons in the rabbit

Motoneurons innervating the rabbit retractor bulbi muscle have been identified by retrograde transport of horseradish peroxidase (HRP). Following injection of HRP into single slips or all 4 slips of the retractor bulbi muscle, labeled motoneurons were consistently observed in the abducens (ABD) nucleus and in the accessory abducens (ACC) nucleus located ventral, lateral and rostral to the ABD. Axons from the ACC motoneurons could be seen to enter the VIth nerve. Injection of HRP into the lateral rectus muscle produced consistent labeling of motoneurons in the ABD nucleus overlapping the distribution of retractor bulbi motoneurons, but labeling was never observed in the ACC nucleus. The number of labeled ABD neurons after lateral rectus injections was far less (36%) than after injection into all 4 slips of the retractor bulbi muscle (72%). Injection of HRP into the superior oblique, superior rectus or medial rectus muscle produced labeling of motoneurons in the corresponding subdivisions of the oculomotor nucleus or trochlear nucleus but no labeled motoneurons were observed in either the ABD or ACC nuclei. Some highly inconsistent labeling of oculomotor nucleus was observed after retractor bulbi or lateral rectus muscle injections and this was judged to be due to intraorbital diffusion of the HRP. It was concluded that the retractor bulbi muscle is innervated by motoneurons located in both the ABD and ACC nuclei.     

Glycinergic/GABAergic synapses in the lateral superior olive are excitatory in neonatal C57Bl/6J mice.

The lateral superior olive (LSO), a nucleus involved in sound localization, receives tonotopically organized, inhibitory input from the medial nucleus of the trapezoid body (MNTB). To better understand the development of this glycinergic/GABAergic pathway, we used Gramicidin-perforated patch clamp recordings to characterize MNTB-evoked postsynaptic potentials in LSO neurons of neonatal C57Bl/6J mice. We found that during the first postnatal week, MNTB-evoked responses change from being depolarizing to being hyperpolarizing. Most interestingly, depolarizing glycinergic/GABAergic synaptic potentials were able to trigger action potentials, demonstrating that the MNTB-LSO pathway can act as a true excitatory pathway. This transient excitatory action of immature MNTB-LSO synapses might play an important role in activity-dependent sharpening of the tonotopic organization of inhibitory connections in the LSO.     

Anatomical and physiological characteristics of vestibular neurons mediating the horizontal vestibulo-ocular reflex of the squirrel monkey

The anatomical characteristics of vestibular neurons, which are involved in controlling the horizontal vestibulo-ocular reflex, were studied by injecting horseradish peroxidase (HRP) into neurons whose response during spontaneous eye movements had been characterized in alert squirrel monkeys. Most of the vestibular neurons injected with HRP that had axons projecting to the abducens nucleus or the medial rectus subdivision of the oculomotor nucleus had discharge rates related to eye position and eye velocity. Three morphological types of cells were injected whose firing rates were related to horizontal eye movements. Two of the cell types were located in the ventral lateral vestibular nucleus and the ventral part of the medial vestibular nucleus (MV). These vestibular neurons could be activated at monosynaptic latencies following electrical stimulation of the vestibular nerve; increased their firing rate when the eye moved in the direction contralateral to the soma; had tonic firing rates that increased when the eye was held in contralateral positions; and had a pause in their firing rate during saccadic eye movements in the ipsilateral or vertical directions. Eleven of the above cells had axons that arborized exclusively on the contralateral side of the brainstem, terminating in the contralateral abducens nucleus, the dorsal paramedian pontine reticular formation, the prepositus nucleus, medial vestibular nucleus, dorsal medullary reticular formation, caudal interstitial nucleus of the medial longitudinal fasciculus, and raphé obscurus. Eight of the cells had axons that projected rostrally in the ascending tract of Deiters and arborized exclusively on the ipsilateral side of the brainstem, terminating in the ipsilateral medial rectus subdivision of the oculomotor nucleus and, in some cases, the dorsal paramedian pontine reticular formation or the caudal interstitial nucleus of the medial longitudinal fasciculus. Two MV neurons were injected that had discharge rates related to ipsilateral eye position, generated bursts of spikes during saccades in the ipsilateral direction, and paused during saccades in the contralateral direction. The axons of those cells arborized ipsilaterally, and terminated in the ipsilateral abducens nucleus, MV, prepositus nucleus, and the dorsal medullary reticular formation. The morphology of vestibular neurons that projected to the abducens nucleus whose discharge rate was not related to eye movements, or was related primarily to vertical eye movements, is also briefly presented.     

Glutamatergic afferent projections to the dorsal raphe nucleus of the rat

Based on WGA-apo-HRP-gold (WG) retrograde tracing, the present study revealed that different subdivisions of the dorsal raphe (DR) such as dorsomedial, ventromedial, lateral wing, and caudal regions receive unique, topographically organized afferent inputs, that are more restricted than previously reported. Phaseolus vulgaris leucoagglutinin anterograde tracing studies confirmed that the medial prefrontal cortex provides the major afferent input to each subdivision of the DR. Double-labeling studies combining WG tracing and glutamate immunostaining indicated that the medial prefrontal cortex, various hypothalamic nuclei including perifornical, lateral, and arcuate nuclei, and several medullary regions such as lateral and medial parabrachial nuclei, and laterodorsal tegmental nucleus provide the major glutamatergic input to each subregion of the DR. It should be noted that the degree of glutamatergic input from these afferent sites was specific for each DR subdivision. The present findings indicated that dorsomedial, ventromedial, lateral wing, and caudal subdivisions of the DR receive excitatory inputs from both cortical and subcortical sites which might be involved in regulation or modulation of a broad range of systems, including sensory and motor functions, arousal and sleep-wake cycle, biorhythmic, cognitive, and affective behaviors.     

Vestibulo-oculomotor connections in an elasmobranch fish,the atlantic stingray,Dasyatis sabina

In elasmobranch fishes, including the Atlantic stingray, the medial rectus muscle is innervated by the contralateral oculomotor nucleus. This is different from most vertebrates, in which the medial rectus is innervated by the ipsilateral oculomotor nucleus. This observation led to the prediction that the excitatory vestibulo-extraocular motoneuron projections connecting each semicircular canal to the appropriate muscle should use a contralateral projection from the vestibular nuclei to the motoneurons. This hypothesis was examined in the Atlantic stingray by injecting horseradish peroxidase unilaterally into the oculomotor nucleus. It was found that vestibulo-oculomotor projections arise from the ipsilateral anterior octaval nucleus and the contralateral descending octaval nucleus. The same pattern was observed when the trochlear nucleus was involved in the injection. There were no cells labeled in the region of the abducens nucleus, and no candidate for a nucleus prepositus hypoglossus was identified. The presence of compensatory eye movements, the directional sensitivity of the semicircular canals, the location of the motoneurons innervating each eye muscle, and our results indicate that the excitatory input to the extraocular motoneurons is derived from the contralateral descending octaval nucleus, and the inhibitory input is derived from the ipsilateral anterior octaval nucleus. The absence of both abducens internuclear interneurons and a nucleus prepositus hypoglossus suggests that eye movements, particularly those in the horizontal plane, are controlled differently in elasmobranchs than in other vertebrates examined to date. © 1994 Wiley-Liss, Inc.     

A survey of oral cavity afferents to the rat nucleus tractus solitarii

Visualization of myelinated fiber arrangements, cytoarchitecture, and projection fields of afferent fibers in tandem revealed input target selectivity in identified subdivisions of the nucleus tractus solitarii (NTS). The central fibers of the chorda tympani (CT), greater superficial petrosal nerve (GSP), and glossopharyngeal nerve (IX), three nerves that innervate taste buds in the oral cavity, prominently occupy the gustatory-sensitive rostrocentral subdivision. In addition, CT and IX innervate and overlap in the rostrolateral subdivision, which is primarily targeted by the lingual branch of the trigeminal nerve (LV). In the rostrocentral subdivision, compared with the CT terminal field, GSP appeared more rostral and medial, and IX was more dorsal and caudal. Whereas IX and LV filled the rostrolateral subdivision diffusely, CT projected only to the dorsal and medial portions. The intermediate lateral subdivision received input from IX and LV but not CT or GSP. In the caudal NTS, the ventrolateral subdivision received notable innervation from CT, GSP, and LV, but not IX. No caudal subnuclei medial to the solitary tract contained labeled afferent fibers. The data indicate selectivity of fiber populations within each nerve for functionally distinct subdivisions of the NTS, highlighting the possibility of equally distinct functions for CT in the rostrolateral NTS, and CT and GSP in the caudal NTS. Further, this provides a useful anatomical template to study the role of oral cavity afferents in the taste-responsive subdivision of the NTS as well as in subdivisions that regulate ingestion and other oromotor behaviors.     

Projections from the anteroventral cochlear nucleus to the lateral and medial superior olivary nuclei

The projections from the cochlear nucleus to the lateral and medial superior olivary nuclei were studied in the cat by use of retrograde transport of horseradish peroxidase to demonstrate the connections. The medial superior olivary nucleus receives input only from the anterior and posterodorsal subdivisions of the anterior division of the anteroventral cochlear nucleus (AA and APD, respectively; Brawer, Morest, and Kane: J. Comp. Neurol. 155: 251-300, 1974). These two subdivisions are populated almost exclusively by spherical bushy cells. Like the medial superior olivary nucleus, the lateral superior olivary nucleus receives inputs from AA and APD. In addition, the lateral superior olivary nucleus receives projections from the posterior subdivision (AP) of the anterior division and also from the posterior division of the anteroventral cochlear nucleus. The projections to the medial superior olivary nucleus are bilateral, whereas the projections to the lateral superior olivary nucleus are almost entirely ipsilateral. One implication of the results is that the medial superior olivary nucleus receives inputs from only one cell type--the spherical bushy cell--but that, at the least, two cell types project to the lateral superior olivary nucleus. Both the olivary nuclei receive input from most, if not all, of the dorsoventral extent of the anteroventral cochlear nucleus, implying that both receive input from neurons arrayed across the entire frequency representation of the anteroventral cochlear nucleus. All of the projections appear to be organized topographically such that frequency representation is preserved.     

The medial geniculate body of the tree shrew, Tupaia glis. II. Connections with the neocortex.

In this study the temporal cortex of the tree shrew was subdivided on the basis of cytoarchitectonic criteria, and the connections of each subdivision with the thalamus and midbrain were analyzed with retrograde and anterograde techniques. The results indicate that, with one exception, each subdivision of the medial geniculate body projects to a separate cortical area. The primary auditory cortex receives projections from the ventral nucleus. Surrounding the primary cortex are at least five additional cytoarchitectonically distinct areas which receive projections from the remaining medial geniculate subdivisions. The evidence suggests that there is very little overlap in the projections from each of these geniculate subdivisions. An exception is the projection of the caudal nucleus of the medial division. This subdivision apparently projects to most, if not all, of the cortical target of the medial geniculate body. Although the cortical projections of the caudal nucleus overlap those of the other medial geniculate subdivisions, the laminar distribution of its terminations in cortex is different. The caudal nucleus projects primarily to layer VI whereas the other subdivisions of the medial geniculate body project primarily to layer IV and the adjacent part of layer III. Anterograde techniques were also used to study the projections from the cortex back to the thalamus and to the midbrain. The projections to the thalamus precisely reciprocate the thalamocortical connections. The projections to the midbrain are to the same areas which the preceding study (Oliver and Hall, '78) showed give rise to ascending projections to the medial geniculate body. An exception is the central nucleus of the inferior colliculus which apparently does not receive a projection from the temporal cortex.     

Thalamotelencephalic organization in birds

Investigation of thalamo-telencephalic connections reveals correspondences between the avian and mammalian thalamic subdivisions (which may or may not mean true homologies). Based mainly on hodological comparisons, the avian thalamus possesses the principal anatomical and functional subdivisions characteristic for mammals. The current review is focused on a comparative analysis of intralaminar, midline and mediodorsal nuclei. There is evidence for matching subdivisions in the case of midline thalamic and mediodorsal nuclei within the avian dorsal thalamic zone, whereas such correspondence is evident, if less complete, in the case of the intralaminar nuclei. Thalamic connections are also relevant to the debated issue of the avian 'prefrontal' cortex. From the current study it is suggested that the prefrontal analogue regions of the bird may spread across the rostrocaudal extent of telencephalon, the rostral nidopallial/mesopallial region (formerly known as medial neostriatum/hyperstriatum) being one subdivision, receiving direct input from the paraventricular thalamic nucleus homologue of midline thalamic region (the medial juxtaventricular region of the nucleus dorsomedialis posterior). Hodological evidence from the current study and other reports argues for the possibility that the area corticoidea dorsolateralis might be hodologically comparable to the cingulate cortex, receiving input from a mediodorsal thalamic-relevant subdivision (lateral subdivision of nucleus dorsomedialis anterior, and medial aspect of nucleus dorsolateralis pars medialis), which also projects on the caudal nidopallium close to (but not coextensive with) the nidopallium caudolaterale, another potential analogue of avian prefrontal cortex. The rostral dorsolateral aspect of nucleus dorsomedialis anterior thalami and the dorsal aspect of nucleus dorsolateralis pars medialis are partially comparable to the mammalian intralaminar nuclei, sharing connections to non-limbic 'corticoid' areas (the Wulst), and the reticular thalamic nuclei.     

The localization of the motor neurons innervating the extraocular muscles in the oculomotor nuclei of the cat and rabbit,using horseradish peroxidase

The localization of the motor neurons innervating the extraocular muscles in the oculomotor nuclei of adult cats and rabbits was investigated by means of retrograde labelling with horseradish peroxidase (HRP). The groups consisting of the motor neurons innervating an individual muscle lay in the nucleus as elongated columns extending in a longitudinal direction. The position of each group in the transverse section varied according to the rostro-caudal level of the nucleus. In the cat and rabbit, entire contralateral innervation of the superior rectus and entire ipsilateral innervation of three muscles of the inferior rectus, medial rectus and inferior oblique were similarly observed. However, the arrangement of individual motor groups differed considerably in both animals except for the group innervating the inferior rectus which was generally found in the ventral position running through the rostral two-thirds of the oculomotor nucleus. In the case of cats, the central caudal nucleus bilaterally innervated the levator palpebrae superioris. The motor neurons innervating this muscle in the rabbit (which lacks the central caudal nucleus) formed a rostro-caudal club-shaped column close to the group innervating the superior rectus. The aberrant cellular mass in the adjoining medial longitudinal fasciculus which belongs to the medial rectus appears to play an important role in the eye movement, because it commonly appears in various animals.     

Architectonic subdivisions of the amygdalar complex of a primitive marsupial (Didelphis aurita)

The architecture of the amygdaloid complex of a marsupial, the opossum Didelphis aurita, was analyzed using classical stains like Nissl staining and myelin (Gallyas) staining, and enzyme histochemistry for acetylcholinesterase and NADPH-diaphorase. Most of the subdivisions of the amygdaloid complex described in eutherian mammals were identified in the opossum brain. NADPH-diaphorase revealed reactivity in the neuropil of nearly all amygdaloid subdivisions with different intensities, allowing the identification of the medial and lateral subdivisions of the cortical posterior nucleus and the lateral subdivision of the lateral nucleus. The lateral, central, basolateral and basomedial nuclei exhibited acetylcholinesterase positivity, which provided a useful chemoarchitectural criterion for the identification of the anterior basolateral nucleus. Myelin stain allowed the identification of the medial subdivision of the lateral nucleus, and resulted in intense staining of the medial subdivisions of the central nucleus. The medial, posterior, and cortical nuclei, as well as the amygdalopiriform area did not exhibit positivity for myelin staining. On the basis of cyto- and chemoarchitectural criteria, the present study highlights that the opossum amygdaloid complex shares similarities with that of other species, thus supporting the idea that the organization of the amygdala is part of a basic plan conserved through mammalian evolution.