Brachium pontis lesions in cats partly reproduce the cerebellar dysfunction of voluntary reaching movements

Disrupting cortico-cerebellar communication impairs dexterity

To control reaching, the nervous system must generate large changes in muscle activation to drive the limb toward the target, and must also make smaller adjustments for precise and accurate behavior. Motor cortex controls the arm through projections to diverse targets across the central nervous system, but it has been challenging to identify the roles of cortical projections to specific targets. Here, we selectively disrupt cortico-cerebellar communication in the mouse by optogenetically stimulating the pontine nuclei in a cued reaching task. This perturbation did not typically block movement initiation, but degraded the precision, accuracy, duration, or success rate of the movement. Correspondingly, cerebellar and cortical activity during movement were largely preserved, but differences in hand velocity between control and stimulation conditions predicted from neural activity were correlated with observed velocity differences. These results suggest that while the total output of motor cortex drives reaching, the cortico-cerebellar loop makes small adjustments that contribute to the successful execution of this dexterous movement.

Preservation of pointing accuracy toward moving targets after extensive visual cortical ablations in cats

Impairments in reaching toward stationary and moving targets were studied in cats after restricted or extensive removal of visual cortical areas (areas 17, 18 and 19 and lateral suprasylvian visual areas). Regardless of the extent of the cortical lesion, cats were at first unable to localise and reach for a stationary target whereas they were soon able to detect and accurately point toward a mobile one. Moreover, the onset latency of such movements was dramatically increased. During post-operative re-training, the cats were unable to improve their accuracy scores when reaching towards stationary targets. In contrast, full compensation was observed for the accuracy of reaching movements directed toward moving targets. A partial recovery was observed for movement latency values that progressively decreased but left a permanent 30-40 ms impairment following extensive lesions. The role of extrageniculate messages and alternative routes involving other cortical areas in taking in charge the visuomotor activity is discussed.

Organization of the pontine nuclei

The pontine nuclei provide the cerebellar hemispheres with the majority of their mossy fiber afferents, and receive their main input from the cerebral cortex. Even though the vast majority of pontine neurons send their axons to the cerebellar cortex, and are contacted monosynaptically by (glutamatergic) corticopontine fibers, the information-processing taking place is not well understood. In addition to typical projection neurons, the pontine nuclei contain putative GABA-ergic interneurons and complex synaptic arrangements. The corticopontine projection is characterized by a precise but highly divergent terminal pattern. Large and functionally diverse parts of the cerebral cortex contribute; in the monkey the most notable exception is the almost total lack of projections from large parts of the prefrontal and temporal cortices. Within corticopontine projections from visual and somatosensory areas there is a de-emphasis of central vision and distal parts of the extremities as compared with other connections of these sensory areas. Subcorticopontine projections provide only a few percent of the total input to the pontine nuclei. Certain cell groups, such as the reticular formation, project in a diffuse manner whereas other nuclei, such as the mammillary nucleus, project to restricted pontine regions only, partially converging with functionally related corticopontine connections. The pontocerebellar projection is characterized by a highly convergent pattern, even though there is also marked divergence. Neurons projecting to a single cerebellar folium appear to be confined to a lamella-shaped volume in the pontine nuclei. The organization of the pontine nuclei suggests that they ensure that information from various, functionally diverse, parts of the cerebral cortex and subcortical nuclei are brought together and integrated in the cerebellar cortex.

Visual 'cortical-recipient' and 'tectal-recipient' pontine zones play distinct roles in cat visuomotor performance

In order to test the hypothesis that visual information reaching the cerebellum through the pontine nuclei is involved in the control of visually guided movements, the effects of bilateral kainic acid pontine lesions have been analysed in cats performing a reaching movement towards a spot of light that was either stationary or moving. In 4 cats, the lesion was restricted either to the ventromedian region (cortical-recipient zone) or to the dorsolateral nucleus (tectal-recipient zone) of the pons. A major and persistent impairment was seen when the cerebellum was deprived of the pontine information influenced by the colliculus. While cats displayed no impairment when reaching towards a stationary target, they exhibited a strong accuracy deficit associated with an increased reaction time when reaching towards a moving target. In contrast, lesioning the pontine zone influenced by the visual cortex induced a transient accuracy deficit with moving targets and a transient delay in movement onset whatever the mode of target presentation. These results emphasise the involvement of visual pontine regions in the guidance of movements; they also confirm previous results showing that tectal visual information plays a more important role than that originating in the visual cortex when movements are directed towards moving targets.

Integration in descending motor pathways controlling the forelimb in the cat. 16. Visually guided switching of target-reaching

A task has been developed to investigate the ability of cats to switch the direction of an ongoing target-reaching forelimb movement with the aid of a visual cue. The cats were standing in front of two horizontal tubes (internal diameter 30 mm; shoulder level) with food. The entrances of the tubes were closed with opaque trap doors but during illumination inside a tube its trap door was unlocked allowing the cat to retrieve food with the paw. When the cats had learnt to select the illuminated tube for insertion the next step was to switch the illumination to the other tube during ongoing target-reaching. Limb lifting was performed when the light was switched on in one of the tubes and time was measured from breaking electrical contact between the paw and the floor. After 25-75 ms, illumination was shifted to the other tube and the latency to the earliest change in movement trajectory was measured. The trajectory was recorded with the aid of cameras detecting the position of infrared light emitting diodes fixed to the dorsal part of the wrist. Every 3 ms the position was fed into a computer, and the movement trajectory (horizontal and sagittal planes) was displayed graphically. The velocities in the direction of cartesian coordinates x, y and z (protraction, adduction-abduction, lifting) were also computed. Single tube trials and switching trials from either tube were made in a random series. In order to switch, the cats used a combination of braking the protraction and a sideways movement. Initially there was often some retraction of the paw to avoid hitting the trap door of the first illuminated tube, but with more proficiency braking decreased and the movement path became smoothly curved. During braking of protraction there was also deceleration of lifting but not enough to maintain a constant movement path in the sagittal plane. In sessions with single tube trials, the movement paths in the horizontal plane were reasonably straight. In sessions with intermixed switching trials the single tube paths became segmented or curved, seemingly in order to facilitate switching. The mean switching latency in four cats ranged from 83 to 118 ms. In the fastest cat the switching latency ranged from 70-106 ms.(ABSTRACT TRUNCATED AT 400 WORDS)

Visual projections to the pontine nuclei in the rabbit: orthograde and retrograde tracing studies with WGA-HRP

Visual projections to the pontine nuclei in the rabbit were examined by means of both orthograde and retrograde tracing of WGA-HRP. The tecto-pontine projection was examined following microinjections of WGA-HRP in the right superior colliculus. The projection to the pontine nuclei is strictly ipsilateral and terminates at middle and caudal levels of the pons. The projection is absent in rostral pontine nuclei. The strongest projection is to the dorsal border of the dorsolateral pontine nuclei and is the only projection seen when the primary injection site is confined to superficial laminae. When the primary injection site also includes intermediate and deep laminae, patches of labelled terminals are also seen within dorsolateral, lateral, peduncular, paramedian, and ventral pontine nuclei as well as in the contralateral nucleus reticularis tegmenti pontis. The striate corticopontine projection was also examined with orthograde tracing of WGA-HRP. The striate corticopontine projection is ipsilateral. Most labelled terminals were seen in dorsolateral and lateral pontine nuclei throughout the rostral half of pons with some additional terminal labelling in paramedian and peduncular nuclei. Labelled terminals were also seen in ventral pontine nuclei throughout the middle and caudal levels of the pons. In a retrograde tracing study, visual projections to the pontine nuclei were examined following microinjections of WGA-HRP into the pontine nuclei. Labelled cells were seen ipsilaterally in superficial and deep laminae of the superior colliculus and in layer V of striate and surrounding occipital cortex. The pontine nuclei also receive ipsilateral projections from the ventral lateral geniculate, the nucleus of the optic tract, anterior and posterior pretectal nuclei, and the dorsal and medial terminal nuclei of the accessory optic system. These pathways are potential sources of visual input to the cerebellum.