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Biomedical subjects

A Grantyn

Publications and source records attributed to A Grantyn.

At least 37 records · Page 2Linked to original sources
PubMed1982

Axonal patterns and sites of termination of cat superior colliculus neurons projecting in the tecto-bulbo-spinal tract.

Horseradish peroxidase was injected in the somata or axons of neurons located in the intermediate and deep layers of the superior colliculus. A group of 34 neurons with physiologically identified projection in the predorsal bundle (tectobulbo-spinal neurons, TBSNs) and two commissural tecto-tectal neurons were characterized with regard to soma-dendritic profiles, axon trajectories, collateral branching, and terminations. TBSNs belong to the class of large, multipolar, wide field neurons. They send axons through the deep white layer without generating local collaterals. Prior to decussation, all TBSNs bifurcate into an ascending branch which reaches the caudal diencephalon, and a main axon descending to the medulla or spinal cord. Regularly spaced collaterals supply a variety of structures at all rostro-caudal levels. In the midbrain, preterminal and terminal ramifications are present in the medial and lateral reticular tegmentum, in the central grey (including its supraoculo-motor zone), in the nuclei of Cajal and Dark-schewitsch and in the medial aspects of the prerubral area and the fields of Forel. Rhombencephalic targets of TBSNs include the medial pontine and bulbar reticular formation, the abducens nucleus, the nucleus reticularis tegmenti pontis and the nucleus prepositus hypoglossi. An increased density of terminal ramifications was found in several brain stem regions related to the control of eye and head movements. The widespread connections of each individual TBSN suggest that neurons of this type may provide a spatio-temporal pattern of facilitation which promotes rapid orientation of eyes, head and body towards the contralateral hemifield but does not specify the details of movement to be executed.

Animals↗
PubMed1980

Sources of direct excitatory and inhibitory inputs from the medial rhombencephalic tegmentum to lateral and medial rectus motoneurons in the cat.

The paramedian pontine and bulbar tegmentum was explored by microstimulation to outline the sites of origin of direct excitatory and inhibitory inputs to lateral rectus (LR) and medial rectus (MR) motoneurons (MNs). In order to avoid activation of fibers of passage and axon reflexes originating outside the stimulation sites, experiments were carried out 4--22 days after brain stem transections causing degeneration of vestibulo-ocular pathways. Additionally, in some experiments the paramedian tegmentum was isolated from the contralateral side by midline transections. Mapping of stimulus sites from which monosynaptic EPSPs and IPSPs were elicited brought out the following preoculomotor reticular regions: 1. LR-MNs received monosynaptic IPSPs from the contralateral reticular formation corresponding to Nucl. reticularis points caudalis (R.p.c.) and the rostral part of Nucl. reticularis gigantocellularis (R.gc.). 2. Monosynaptic inhibitory input to MR-MNs could only be demonstrated after degeneration of excitatory pathways ascending from the internuclear neurons of the VIth nucleus and from the ipsilateral vestibular nuclei. Monosynaptic IPSPs originated in the ipsilateral dorso-medial tegmentum through the entire extent of the Nucl. reticularis pontis oralis and rostral R.p.c. including the region of the ipsilateral VIth nucleus. 3. Monosynaptic excitation of LR-MNs was induced by stimulation of the ipsilateral R.p.c. and the rostral half of the paramedian bulbar tegmentum (R.gc.). 4. The sites from which monosynaptic EPSPs were evoked in MR-MNs were confined to the contralateral VIth nucleus and its immediate vicinity. No evidence could be obtained for direct excitatory inputs to MR-MNs from the ipsilateral paramedian tegmentum. It is concluded that the paramedian rhombencephalic reticular formation contains four pools of premotor neurons related to coordination of conjugate horizontal eye movements. Two of them are excitatory for LR- and MR-MNs with ipsilateral ON-directions, the other two mediate reciprocal inhibition of the antagonistic motor nuclei.

Animals↗
PubMed1979

Electroanatomy of tectal efferent connections related to eye movements in the horizontal plane.

1. Excitatory and inhibitory oligosynaptic pathways from the superior colliculus (CS) to ocular motoneurons engaged in horizontal eye movements were investigated in cats using acute and chronic brain stem transections in combination with intracellular recordings. 2. Isolation of the medial ponto-bulbar tegmentum from vestibular nuclei and adjacent lateral tegmental structures did not impair short-latency EPSPs and IPSPs induced by collicular stimulation in lateral rectus motoneurons (LR-MNs). On the contrary, responses were enhanced after chronic de-efferentation of vestibular nuclei. This suggests compensatory synaptic rearrangement in the tecto-reticulo-abducens pathways. 3. Midsagittal mesencephalic transections eliminated not only crossed excitatory but also ipsilateral inhibitory CS action on LR-MNs indicating that underlying pathways undergo decussation within the midbrain. 4. Midsagittal transections at different pontine and bulbar levels were performed to locate the second decussation of the inhibitory pathway. Ipsilateral IPSPs were eliminated only by deep lesions extending for about 1.5 mm rostral and caudal to the 6th nuclei. 5. Investigation of medial rectus motoneurons (MR-MNs) revealed two types of excitatory responses to CS-stimulation: (a) di- or trisynaptic EPSPs characterized by a fast rising phase and pronounced frequency potentiation; (b) slowly rising EPSPs displaying little or no frequency potentiation. 'Fast' EPSPs were abolished by all types of pontine lesions interrupting transmission through the contralateral 'abducens region' and may thus be relayed by internuclear neurons within or adjacent to the 6th nucleus. 'Slow' EPSPs persisted after transverse sections at midpontine and rostral pontine levels. 6. The trajectory of tectofugal inhibitory pathway to MR-MNs could not be followed due to a marked suppression of IPSPs under pentobarbital anesthesia. Persistence of IPSPs in LR-MNs under same conditions indicated that reciprocal inhibition of LR- and MR-MNs is mediated by different populations of inhibitory interneurons.

Animals↗
PubMed1977

[Excitatory effect of the superior colliculus on the motor neurons of extra-ocular muscles of cats].

The synaptic actions which might underline the caccadic eye movements induced by stimulation of the superior colliculus have been studied in the unanesthetized cat "encéphale isolé". Intracellular recordings of extraocular muscle medial rectus motoneurons have revealed a minimally disynaptic excitatory projection from the superior colliculus to the ipsilateral medial rectus motoneurons (latency 1.3 to 2.6 msec).

Action Potentials↗
PubMed1975

Postsynaptic potentials in cat abducens motoneurons evoked by stimulation of cortical eye fields.

Intracellular recordings were carried out on abducens motoneurons of encephale isole cats in order to analyse synaptic influences of cortical areas engaged in control of saccadic eye movements. It was found that, in addition to the "frontal eye field" (FEF), eye movements containing a contraversive component may be triggered by electrical stimulation of the 1st and the 2nd sensorimotor areas (SM). Correspondingly, sustained postsynaptic responses (EPSPs) and rhythmic firing of abducens motoneurons could be reliably induced by prolonged stimulus trains. In this respect, the efficiencies of FEF and SM were about the same. They appeared to be higher than the efficiency of excitatory pyramidal actions on spinal motoneurons as reported by others. EPSPs elicited from both regions by short stimuli were, on the major part, polysynaptic. Quite complex multineuronal chains appeared to be stronger engaged in the transmission of FEF effects. EPSPs of SM origin contained a disynaptic fraction which could not be reliably identified in FEF responses. Recipocal innervation of abducens nuclei on both sides was found to be reflected in the asymmetry of excitatory and inhibitory influences from two hemispheres: EPSPs predominated in responses to contralateral, IPSPs and mixed PSPs - to ipsilateral stimulation.

Animals↗