The segmentation of the posterior cerebral artery: a microsurgical anatomic study.

There are still different descriptions of the segmentation of the posterior cerebral artery, although there is a radiological and anatomical consensus on the segmentation of the anterior and the middle cerebral artery. This study aims to define the most appropriate localization for origin and end points of the segments through reviewing the segmentation of the posterior cerebral artery. The segments and the cortical branches originating from those segments of the 40 posterior cerebral arteries of 20 cadaver brains were examined under operating microscope. In this research, the P1, P2, P3, P4, and P5 classification of the segmentation of the posterior cerebral artery is redefined. This redefinition was made to overcome the complexities of previous definitions. The P1 segment in this research takes its origin from the basilar tip and ends at the junction with the posterior communicating artery. The average diameter of this segment at the origin was 2.21 mm (0.9-3.3), and the average length was 6.8 mm (3-12). The P2 segment extends from the junction with the posterior communicating artery to the origin of the lateral temporal trunk. This point usually situates on one level of posterior of the cerebral peduncle. The average diameter of this segment at the origin was 2.32 mm (1.3-3.1), and the average length was 20.1 mm (11-26). The P3 segment extends from the origin of the lateral temporal trunk to the colliculus where both the posterior cerebral arteries are the nearest to each other (quadrigeminal point) and is located at the anterior-inferior of the splenium. The average diameter of this segment at the origin was 1.85 mm (1.2-2.7), and the average length was 16.39 mm (9-28). The P4 begins at the quadrigeminal point and ends at the top of the cuneus. The average diameter of this segment at the origin was 1.55 mm (1.1-2.2). While the P5 segment is named as the terminal branches of the major terminal branches of the posterior cerebral artery, no definite border was found between the P4 and the P5 segments. In this study, the segmentation of the posterior cerebral artery, developed by Krayenbühl and Yasargil, was redefined to be more appropriate for radiological and anatomical purposes.

Neurogenesis during optic tectum regeneration in Xenopus laevis.

The regenerative neurogenesis of the optic tectum of larval Xenopus laevis has been studied analyzing the proliferative and morphogenetic phases of the regeneration process after removal of one optic lobe. To this end, short-term and long-term pulses were carried out using the thymidine analog BrdU, selectively incorporated into cells during the S phase of the cell cycle. Results indicate that while in early larvae (stage 49/50, according to Nieuwkoop & Faber 1967) regeneration occurs mainly at the expense of the stem cells present in extensive proliferation zones ("matrix areas") of the midbrain, in late larvae (stage 55/56) regeneration occurs at the expense of stem cells present in very limited matrix areas of the brain and of quiescent cells, which re-enter the cell cycle following trauma. Moreover, in early larvae, morphogenesis of the optic tectum is carried out according to a precise spatio-temporal order from rostro-caudal to latero-medial. By contrast, in late larvae, the topographical order of the regenerative morphogenesis of the optic lobe is completely altered. As a consequence, the regenerated optic tectum in early larvae has an apparently normal structure, while the regenerated optic tectum in late larvae lacks stratification.

Fictive Scratching Patterns in Brain Cortex-Ablated, Midcollicular Decerebrate, and Spinal Cats.

Background: The spinal cord's central pattern generators (CPGs) have been explained by the symmetrical half-center hypothesis, the bursts generator, computational models, and more recently by connectome circuits. Asymmetrical models, at odds with the half-center paradigm, are composed of extensor and flexor CPG modules. Other models include not only flexor and extensor motoneurons but also motoneuron pools controlling biarticular muscles. It is unknown whether a preferred model can explain some particularities that fictive scratching (FS) in the cat presents. The first aim of this study was to investigate FS patterns considering the aiming and the rhythmic periods, and second, to examine the effects of serotonin (5HT) on and segmental inputs to FS. Methods: The experiments were carried out first in brain cortex-ablated cats (BCAC), then spinalized (SC), and for the midcollicular (MCC) preparation. Subjects were immobilized and the peripheral nerves were used to elicit the Monosynaptic reflex (MR), to modify the scratching patterns and for electroneurogram recordings. Results: In BCAC, FS was produced by pinna stimulation and, in some cases, by serotonin. The scratching aiming phase (AP) initiates with the activation of either flexor or extensor motoneurons. Serotonin application during the AP produced simultaneous extensor and flexor bursts. Furthermore, WAY 100635 (5HT1A antagonist) produced a brief burst in the tibialis anterior (TA) nerve, followed by a reduction in its electroneurogram (ENG), while the soleus ENG remained silent. In SC, rhythmic phase (RP) activity was recorded in the soleus motoneurons. Serotonin or WAY produced FS bouts. The electrical stimulation of Ia afferent fibers produced heteronymous MRes waxing and waning during the scratch cycle. In MCC, FS began with flexor activity. Electrical stimulation of either deep peroneus (DP) or superficial peroneus (SP) nerves increased the duration of the TA electroneurogram. Medial gastrocnemius (MG) stretching or MG nerve electrical stimulation produced a reduction in the TA electroneurogram and an initial MG extensor burst. MRes waxed and waned during the scratch cycle. Conclusion: Descending pathways and segmental afferent fibers, as well as 5-HT and WAY, can change the FS pattern. To our understanding, the half-center hypothesis is the most suitable for explaining the AP in MCC.

Role of the superior colliculus in the expression of acute and kindled audiogenic seizures in Wistar audiogenic rats.

PURPOSE: The role of the superior colliculus (SC) in seizure expression is controversial and appears to be dependent upon the epilepsy model. This study shows the effect of disconnection between SC deep layers and adjacent tissues in the expression of acute and kindling seizures. METHODS: Subcollicular transections, ablation of SC superficial and deep layers, and ablation of only the cerebral cortex were evaluated in the Wistar audiogenic rat (WAR) strain during acute and kindled audiogenic seizures. The audiogenic seizure kindling protocol started 4 days after surgeries, with two acoustic stimuli per day for 10 days. Acute audiogenic seizures were evaluated by a categorized seizure severity midbrain index (cSI) and kindled seizures by a severity limbic index (LI). RESULTS: All subcollicular transections reaching the deep layers of the SC abolished audiogenic seizures or significantly decreased cSI. In the unlesioned kindled group, a reciprocal relationship between limbic and brainstem pattern of seizures was seen. The increased number of stimuli provoked an audiogenic kindling phenomenon. Ablation of the entire SC (ablation group) or of the cerebral cortex only (ctx-operated group) hampered the acquisition of limbic behaviors. There was no difference in cSI and LI between the ctx-operated and ablation groups, but there was a difference between ctx-operated and the unlesioned kindled group. There was also no difference in cSI between SC deep layer transection and ablation groups. Results of histologic analyses were similar for acute and kindled audiogenic seizure groups. CONCLUSIONS: SC deep layers are involved in the expression of acute and kindled audiogenic seizure, and the cerebral cortex is essential for audiogenic kindling development.

Visuomotor behaviors in larval zebrafish after GFP-guided laser ablation of the optic tectum.

The optic tectum is the largest visual center in most vertebrates and the main target for retinal ganglion cells (RGCs) conveying visual information from the eye to the brain. The retinotectal projection has served as an important model in many areas of developmental neuroscience. However, knowledge of the function of the tectum is limited. We began to address this issue using laser ablations and subsequent behavioral testing in zebrafish. We used a transgenic zebrafish line that expresses green-fluorescent protein in RGCs projecting to the tectum. By aiming a laser beam at the labeled retinal fibers demarcating the tectal neuropil, the larval tectum could be selectively destroyed. We tested whether tectum-ablated zebrafish larvae, when presented with large-field movements in their surroundings, displayed optokinetic responses (OKR) or optomotor responses (OMR), two distinct visuomotor behaviors that compensate for self-motion. Neither OKR nor OMR were found to be dependent on intact retinotectal connections. Also, visual acuity remained unaffected. Tectum ablation, however, slowed down the OKR by reducing the frequency of saccades but left tracking velocity, gain, and saccade amplitude unaffected. Removal of the tectum had no effect on the processing of second-order motion, to which zebrafish show both OKR and OMR, suggesting that the tectum is not an integral part of the circuit that extracts higher-order cues in the motion pathway.

Parameters of transplant-mediated pupilloconstriction in rats with unilateral olivary pretectal lesions.

Embryonic retina, transplanted to the midbrain of neonatal rats, innervates the host brain and mediates a pupilloconstrictor response in the host eye. We hypothesise that there is a dynamic interaction between normal host and transplant photic input to the pupilloconstrictor centres. This study aims to characterise the nature of these interactions and determine the impact of experimental lesions on this reflex system. Pupillary diameter in normal rats is regulated by integration of intensity levels of the light delivered to the two eyes. The integration occurs at the primary input nucleus, the olivary pretectal nucleus, and at the output nucleus, the Edinger-Westphal nucleus. We have examined the pattern of integration of inputs delivered through the host eye and a retinal transplant placed over the midbrain at birth. Restriction of the site of integration to one olivary pretectal nucleus by ablating the contralateral nucleus causes a substantial enhancement of the transplant-mediated response and a major reduction in the host eye-mediated response. Although the pattern of change is quite similar to that seen between the two eyes of a normal animal after a similar lesion, the magnitude of improvement of the transplant response is much greater. The integration that occurs between transplant and host inputs is dynamic, and the efficacy of the transplant can be modulated by the competing host input. These results have implications for the use of neural transplants in degenerative diseases and might also offer insight into the limited functional recovery that occurs after neurological injury.

"Extra" optic fibers exclude normal fibers from tectal regions in goldfish.

A small group of selected optic fibers were surgically deflected from one tectum into the other, thus creating a novel additional projection originating from a small area of ipsilateral retina. The normal fibers to this "recipient" tectum were also severed so that both the deflected and the normal fibers regrew into this tectum at about the same time. The reinnervation pattern was analyzed by autoradiography and electrophysiologic mapping. Both techniques showed that the deflected fibers and the "normal" fibers failed to intermix. The deflected fibers typically formed several well-defined patches of innervation in roughly the appropriate region of denervated recipient tectum. The normal fibers filled in the remaining uninnervated tectal areas and were completely or nearly completely exculded from the patches occupied by the deflected fibers. This segregation was often quite sharp having an apparent average overlap less than 25-50 micron. The electrophysiology indicated that the projections of both deflected and normal fibers were retinotopically organized but that the mapping by the normal fibers was compressed. This compression, an apparent consequence of being squeezed onto a smaller than normal region of tectum, was similar to that previously observed following ablations of part of the tectum. The negligible surgical damage in the present experiment, however, excludes the kind of cytochemical reorganization previously suggested to produce compression. The findings also provide evidence for a competitive type of interaction between optic fibers.

Perception of apparent motion of colored stimuli after commissurotomy.

One subject with complete forebrain commissurotomy (L.B.), another with posterior callosotomy (D.K.), and eight normal controls were presented with successive pairs of red and/or green lights, on either side of the retinal midline and within the left and right visual fields. All of the subjects could discriminate the direction of apparent motion in all three locations, although L.B. was poorer on bilateral than unilateral presentations. Moreover, on bilateral presentations, L.B. was significantly poorer at identifying the color of the first light than the color of the second light. In contrast, D.K., like the control subjects, was equally good at identifying the color of either light. These and other results provide evidence that a subcortical shift in attention enables L.B. to discriminate the direction of apparent motion across the midline. On the other hand, a more robust mechanism involving the middle temporal area of the cortex must be responsible for tracking motion in D.K. and the control subjects. On the basis of these findings, it is suggested that the superior colliculus may contribute to direction sensitivity in the middle temporal area by mediating shifts in spatial attention.

The pretectum mediates rapid eye movement sleep regulation by light.

A variety of sensory stimuli (e.g., visual, auditory, and thermal) are known to induce rapid eye movement (REM) sleep in mammals. Studies have examined the induction of REM sleep in albino rats by light-to-dark transitions, a phenomenon referred to as REM sleep triggering. Recent research has demonstrated that aspiration lesions of the superior colliculus (SC) and pretectal area attenuated REM sleep triggering. To define more specifically the area or areas involved in mediating REM sleep responses to changes in illumination, fiber-sparing neurotoxic lesions were made to the pretectum (PT) or the SC. Lesions of the PT attenuated REM sleep triggering, whereas lesions of the SC did not. Thus, the role of the PT may be expanded to include the regulation of REM sleep in response to photic stimulation in albino rats. These findings provide a paradigm in which to study mechanisms of REM sleep generation and the effects of light on behavioral state.

Spatial discrimination in goldfish following bilateral tectal ablation.

Goldfish were classically conditioned to discriminate between right and left or nasal and temporal presentations of a spot-of-light conditioned stimulus (CS). After the conditioning, the fish were administered bilateral optic tectum ablation followed by weekly sessions of conditioning trials to test for retention or relearning of the discrimination response. As their behavioral photosensitivity is greatly decreased, the ablates were dark-adapted prior to each session and trials were administered in darkness. Right x left discrimination was retained postoperatively but the nasal x temporal discrimination was blocked. Sham-operated controls discriminated between the nasal and temporal CS when dark-adapted and tested in darkness. Subsequent transection of the optic nerves obliterated response to the CS, indicating that tectum ablates detect and respond to the CS retinally and not extraretinally. We conclude that memory of visual spatial learning is mediated by non-tectal brain structures and that the ablate can discriminate between right- and left-eye input but sees the CS too diffusely to distinguish its location within the monocular field.

Posttraining lesion of the superior colliculus interferes with feature-negative discrimination of fear-potentiated startle.

Though much is known about the neural circuits involved in the elicitation of fear, little is known about the neural circuits responsible for the reduction of fear. The present experiments investigated the contribution of the superior colliculus (SC) and the dorsal periacquaductal gray (dPAG) in the reduction of conditioned fear produced by an auditory feature trained in a feature-negative discrimination procedure. In this procedure, light plus foot shock training trials are interspersed with trials in which the light is preceded by a noise and this noise and light compound is not followed by foot shock. At the end of this feature-negative discrimination training, rats were given excitotoxic lesions of the SC or dPAG. Feature-negative discrimination of fear was assessed with the fear-potentiated startle effect in which conditioned fear is operationally defined as potentiated startle amplitude in the presence versus the absence of the light. Feature-negative discrimination of fear is evidenced by a reduction in fear-potentiated startle to the light when the noise feature accompanies the light. Lesions of the SC, but not the dPAG, interfered with feature-negative discrimination of fear-potentiated startle suggesting that the SC plays a role in feature-negative discrimination of fear. Both SC and dPAG lesions facilitated startle amplitude in the absence of the light suggesting that these structures may exert a tonic inhibition on the acoustic startle reflex. The SC receives polymodal sensory information and is known to project forebrain areas involved in the production of conditioned fear. Thus, the SC may be an important component of the feature-negative discrimination circuit.

Parameters of retinal graft-mediated responses are related to underlying target innervation.

Retinae taken from embryonic mice were transplanted to the brain of newborn rats. One eye was removed from the recipients at this time to promote innervation of host visual centers. After 4-6 weeks the optic nerve of the remaining eye was transected to eliminate all visual afferents of host origin and 4-6 days later the grafts were exposed surgically for testing. Nineteen animals with grafts located on the dorsal surface of the cerebellum form the basis of this study. Animals differing markedly in behavioral performance were selected for simultaneous immunohistochemical processing. Measurements of peak density and area of mouse-specific immunoreactivity associated with the olivary pretectal nuclei (OPN) were compared to pupillary response parameters using the Pearson correlation coefficient. This analysis was performed on the combined data from all groups and revealed a number of highly significant correlations between both area and density of labelling and pupillary response parameters. In all 7 co-processed groups, the best responders within each group showed the heaviest innervation of the olivary pretectal nucleus. These findings indicate therefore that the degree to which a transplanted retina innervates the appropriate host brain target is an important determinant of graft efficacy. The xenograft-mediated pupillary reflex provides a model which permits detailed analysis of the relationship between connectivity and behavior in a simple relay system.

The effects of lesions of the superior colliculus on locomotor orientation and the orienting reflex in the rat.

The effects of bilateral removal of the superior colliculus or visual cortex on visually guided locomotor movements in rats performing a brightness discrimination task were investigated directly with the use of cine film. Rats with collicular lesions showed patterns of locomotion comparable to or more efficient than those of normal animals when approaching one of 5 small doors located at one end of a large open area. In contrast, animals with large but incomplete lesions of visual cortex were distinctly impaired in their visual control of approach responses to the same stimuli. On the other hand, rats with collicular damage showed no orienting reflex or evidence of distraction in the same task when novel visual or auditory stimuli were presented. However, both normal and visual-decorticate rats showed various components of the orienting reflex and disturbance in task performance when the same novel stimuli were presented. These results suggest that although the superior colliculus does not appear to be essential to the visual control of locomotor orientation, this midbrain structure might participate in the mediation of shifts in visual fixation and attention. Visual cortex, while contributing to visuospatial guidance of locomotor movements, might not play a significant role in the control and integration of the orienting reflex.

Cardiovascular responses to microinjections of glutamate into the nucleus tractus solitarii of unanesthetized supracollicular decerebrate rats.

In anesthetized rats, microinjections of excitatory amino acids (EAAs) into the nucleus tractus solitarii (nTS), in a region located immediately rostral to the calamus scriptorius (CS), have been generally reported to elicit depressor and bradycardic responses. On the other hand, in conscious freely moving rats, similar microinjections have been reported to elicit pressor and bradycardic responses. These divergent results have been attributed to the effect of anesthetics. A reinvestigation of the effects of EAAs into the nTS in unanesthetized animals became necessary in order to resolve this controversy. The microinjection technique used in freely moving conscious rats suffers from several technical limitations; for example, microinjections cannot be delivered stereotaxically. In order to avoid these limitations, the present experiments were carried out in unanesthetized supracollicular decerebrate rats. A systematic mapping of nTS in these rats, using microinjections of the solutions of EAAs in artificial cerebrospinal (aCSF) fluid, confirmed that depressor and bradycardic responses are elicited from all the sites in the nTS extending from the CS to a level about 1 mm rostral to it. Pressor responses were elicited by microinjections of l-glutamate (l-Glu) only from the chemoreceptor projection site (a region of the commissural subnucleus, 0.1-0.5 mm caudal to the CS, 0-0.5 mm lateral to the midline and 0.4-0.5 mm deep from the medullary surface). The pressor responses elicited from the aforementioned site were accompanied with bradycardia; this response may be due to diffusion of l-Glu to the dorsal motor nucleus of vagus because the bradycardia disappeared when the depth of the microinjection was reduced to 0.3, instead of 0.5 mm, from the dorsal medullary surface. When urethane was administered intravenously in unanesthetized decerebrate rats, the responses to microinjections of l-Glu remained unchanged, i.e., depressor and bradycardic responses were elicited from all the sites in the nTS extending from the CS to a level about 1 mm rostral to it and pressor and tachycardic responses were elicited from the chemoreceptor projection site. These observations indicated that there is no anesthetic-induced qualitative alteration of the cardiovascular responses to microinjections of EAAs into the nTS.

Localization and source of gamma aminobutyric acid immunoreactivity in the isthmic nucleus of the frog Rana esculenta.

The distribution of gamma-aminobutyric acid (GABA)-containing neurons and nerve fibers was studied in the isthmic nucleus of the frog Rana esculenta using light and electron microscopical immunohistochemical techniques. Approximately 0.5% of isthmic cells showed GABA immunopositivity, and the majority of these cells was found in the anterior one-third of the nucleus. A meshwork of GABA-immunostained fine beaded axons filled the entire isthmic nucleus. The GABA-immunoreactive terminals formed pericellular basket-like structures around a few cells both in the medulla and the cortex of the isthmic nucleus. To determine the source of GABA-positive fibers in the isthmic nucleus lesion experiments were carried out. After unilateral tectal ablation no change was observed in GABA immunoreactivity. Hemisectioning the tegmentum close to the anterior border of the isthmic nucleus, transection of the caudal tectal commissure and decussatio veli, or electrical lesioning of the anterodorsal tegmental nucleus all resulted in a moderate decrease in the density of GABA-positive fibers. Our results suggest that the majority of GABA-positive fibers derives from local GABA-positive cells, but some GABAergic afferents seem to arise in the tegmentum.

Successful laser-assisted excision of a metastatic midbrain tumor.

Metastatic lesions to the midbrain are rare. They are found in 1 to 3% of autopsy series of solitary brain metastases. The consensus of opinion in the current literature is that they are inoperable lesions and should be treated by radiation therapy alone. This is the first case report of a completely excised metastatic adenocarcinoma to the midbrain. The patient's clinical course has been stable, and there is no computed tomographic evidence of recurrence at 18 months follow-up.

Transcollicular approach to intrinsic tectal lesions.

OBJECTIVE: We used a paramedian, infratentorial-supracerebellar, transcollicular approach to resect 11 intrinsic tectal lesions, including 8 tumors and 3 hematomas, in 11 patients. The route of access to the lesions was designed to minimize the anatomic and functional damage to the surrounding structures. METHODS: Access was through one superior colliculus in each of seven patients, through one inferior colliculus in each of two patients, and through the superior and inferior colliculi of one side in each of two patients. RESULTS: Of the eight tumors, three were totally resected, four were nearly totally resected, and one was partially resected. The preoperative ocular symptoms did not change in six of these eight patients and worsened in two, and the neurological deficits, except ocular symptoms, improved in two. All three hematomas were completely removed, along with abnormal blood vessels in the wall of the hematoma cavity; all three of these patients experienced neurological improvement. CONCLUSION: We conclude that the paramedian, infratentorial-supracerebellar, transcollicular approach permits safe removal of intrinsic tectal lesions. Resection of the superior or inferior colliculus or both on one side seems to be neurologically well tolerated.

Light-dark discrimination after sciatic nerve transplantation to the sectioned optic nerve in adult hamsters.

In adult golden hamsters (Mesocricetus auratus), the optic nerve was bilaterally sectioned with one side grafted with the sciatic nerve to make a bridge to the ipsilateral superior colliculus. Shuttle-box avoidance was tested using light as the conditioned stimulus. Three of the five transplanted animals revealed statistically significant increase in percentages of avoidance. A significant increase in the avoidance scores was also observed in 15 normal hamsters, but none of 13 blind hamsters showed such an increase. Intertrial responses, which represent spontaneous responses, did not show significant changes. We conclude that some of the transplanted animals can discriminate between light and dark, probably through their restored visual pathway.

Neuroendoscopic approach to tectal tumors: a consecutive series.

OBJECT: The authors report a consecutive series of 10 patients who presented with signs and symptoms caused by tectal tumors. Clinical findings, radiographic features, neuroendoscopic management strategies, and histological findings are reported and discussed. METHODS: Since January 1990, 11 neuroendoscopic procedures were performed in 10 patients who harbored tectal tumors. The patients were followed for an average of 5 years (range 2 months-11 years), and a retrospective study was conducted in which case notes, radiological findings, operative notes, and histopathological findings were assessed. Magnetic resonance (MR) imaging was performed, and the images were used to classify patients into three groups: those with hypertrophy of the tectum in whom isointensity appeared on T1-weighted images (Group 1); those with a tectal tumor occupying the cerebral aqueduct in whom decreased signal intensity appeared on T1-weighted images, as well as no enhancement after gadolinium administration (Group 2); and those with a tectal tumor in whom mixed signal intensity and conspicuous evidence of contrast enhancement appeared on T1-weighted images (Group 3). The results of histological examination were consistent with MR imaging features: in Group 1, glial tissue or gliosis; in Group 2, benign astrocytoma; and in Group 3, malignant astrocytoma. Cerebrospinal fluid diversion was the only surgical treatment that provided relief from obstructive hydrocephalus. One patient in Group 3 underwent radiotherapy and subsequent partial tumor removal under neuroendoscopic guidance. Thereafter, the tumor remained in decline. All patients had normal intellectual status after undergoing surgery in which a neuroendoscope was used. CONCLUSIONS: Neuroendoscopic procedures can provide histological diagnosis, define the tumor-midbrain interrelationship, and be highly effective in treating obstructive hydrocephalus and in removing tectal tumors. This procedure may receive clinical application as a new management strategy for tectal glioma.

Inhibition of labyrinthine nystagmus by visual fixation: effects of ablation of visual cortex and superior colliculi.

A study was conducted to destroy two specific areas of the cat's visual system in order to determine if these lesions would affect the visual inhibition of calorically-induced vestibular nystagmus. The occipital visual cortex was removed in eight cats and the superior colliculi were removed bilaterally in nine cats. Postoperative vestibular testing revealed no significant change in the electronystagmography tracings and response to visual fixation. These findings suggest that, in cats, the visual inhibition of labyrinthine nystagmus is not dependent upon the integrity of the visual cortex or superior colliculi. The hypothesis is brought forward that the visual inhibition of the vestibular nystagmus is merely a reflex of the brain stem to light stimulus, mediated via the cerebellum.