Firing characteristics of neurones in the superior colliculus and the pontomedullary reticular formation during orienting in unrestrained cats

Load changes in limbs during orienting in non-restrained cats

We simultaneously investigated eye and head movements and postural adjustment during orienting by measuring load force exerted by four limbs in cats. When light is moved from the fixation point to the target position, the head first begins moving towards the target position, and the eye moves in the opposite direction due to the vestibulo-ocular reflex (VOR). Later, the eye moves quickly in the target direction by saccade, synchronous with the remaining rapid head orientation movement. Head movement is classified as either 'head rotation' or 'head translation'. During head rotation, the load force in ipsilateral limb to the target position decreased, and that in the contralateral limb increased. During head translation, on the contrary, load force in the ipsilateral limb increased and that in the contralateral limb decreased. This phenomenon was observed in fore- and hindlimbs. The latencies of head movement are very similar with those of the load force change in many trials, and in case in which the head movement has short latency, the amount of load force change is larger. In contrast, when head movement has long latency, the amount of load force change is smaller. In a previous study, we recorded two types of neurons from ponto-medullary reticular formation. The firing of these neurons was related with head movement. The cervical reticulospinal neuron (C-RSN) in ponto-medullary reticular formation got off collateral to both neck and forelimb motoneurons. These types were named phasic neuron (PN) and phasic sustained neuron (PSN). We discuss the relation between load changes and the two types of neurons and postural adjustment during orienting.

Spatial characteristics of neurons in the central mesencephalic reticular formation (cMRF) of head-unrestrained monkeys

Prior studies of the central portion of the mesencephalic reticular formation (cMRF) have shown that in head-restrained monkeys, neurons discharge prior to saccades. Here, we provide a systematic analysis of the patterns of activity in cMRF neurons during head unrestrained gaze shifts. Two types of cMRF neurons were found: presaccadic neurons began to discharge before the onset of gaze movements, while postsaccadic neurons began to discharge after gaze shift onset and typically after the end of the gaze shift. Presaccadic neuronal responses were well correlated with gaze movements, while the discharge of postsaccadic neurons was more closely associated with head movements. The activity of presaccadic neurons was organized into gaze movement fields, while the activity of postsaccadic neurons was better organized into movement fields associated with head displacement. We found that cMRF neurons displayed both open and closed movement field responses. Neurons with closed movement fields discharged before a specific set of gaze (presaccadic) or head (postsaccadic) movement amplitudes and directions and had a clear distal boundary. Neurons with open movement fields discharged for gaze or head movements of a specific direction and also for movement amplitudes up to the limit of measurement (70 degrees). A subset of open movement field neurons displayed an increased discharge with increased gaze shift amplitudes, similar to pontine burst neurons, and were called monotonically increasing open movement field neurons. In contrast, neurons with non-monotonically open movement fields demonstrated activity for all gaze shift amplitudes, but their activity reached a plateau or declined gradually for gaze shifts beyond specific amplitudes. We suggest that presaccadic neurons with open movement fields participate in a descending pathway providing gaze signals to medium-lead burst neurons in the paramedian pontine reticular formation, while presaccadic closed movement field neurons may participate in feedback to the superior colliculus. The previously unrecognized group of postsaccadic cMRF neurons may provide signals of head position or velocity to the thalamus, cerebellum, or spinal cord.

Neuroplasticity after unilateral visual cortex damage in the newborn cat

Anatomical, electrophysiological, and behavioral studies implicate extrastriate cortex as a major contributor to the sparing of visually guided behaviors following lesions of primary visual cortex incurred early in life. Here we report considerable sparing of the ability to detect and localize stimuli in the hemifield contralateral to unilateral early lesions of all contiguous visually-responsive primary and extrastriate cortical regions (occipital, visuoparietal, and visuotemporal cortices). In the adult cat this same lesion induces a dense blindness and cats are unable to orient to any visual stimulus introduced into the contralesional hemifield. In the absence of cortical circuits, the neural sparing identified following the neonatal lesion is based on the superior colliculus and it occurs despite massive retrograde transynaptic degeneration of large numbers of retinal ganglion cells.

Integration of multiple motor segments for the elaboration of locomotion: role of the fastigial nucleus of the cerebellum

This chapter provides a conceptual overview of the role and operation of higher structures of the central nervous system (CNS) in the control of posture and locomotion in the mammal, including the nonhuman primate and the human. Both quadrupedal and bipedal locomotion require the integrated neural control of multiple body segments against gravity. During development, and in selected instances in the adult, motor learning is required, particularly for merging anticipatory and reactive CNS processes, the latter being necessary after tripping and stumbling. We have recently found that the fastigial nucleus (FN) of the cerebellum in the cat plays a particularly important role in the control of locomotion, by virtue of its critical position in uniting the cerebro-cerebellar and the spino-cerebellar loops of neural activity that participate in the integrated control of multiple body segments. Further understanding of the CNS structures that achieve this integration has come from our recent study of an intact nonhuman primate, the Japanese monkey, Macaca fuscata, as it learns to elaborate bipedal locomotion rather than its normal quadrupedal fashion. Based on findings from these two animal species, we now present a model of the overall integrated control of posture and locomotion that features the combined operation of parallel and distributed neural circuitry throughout the CNS.

The neural control of orienting: role of multiple-branching reticulospinal neurons

This chapter emphasizes the functional significance of the multiple-branching patterns of descending axons implicated in the control of movement. The example provided concerns orienting head movements, which are controlled by pathways from the superior colliculus (SC). Such control is mediated via cervical reticulospinal neurons (C-RSNs), which take origin in the nucleus reticularis pontis caudalis and nucleus reticularis gigantocellularis, and give off multiple collaterals along the full length of their axonal trajectory. Their projection is not only to lamina IX neck motor nuclei in upper cervical segments, but also to laminae VII-VIII in lower cervical segments. Thus, SC commands for head orienting are transmitted to both neck motoneurons and lower cervical spinal circuitry, which latter network controls appropriate postural adjustments by the coordinated control of motoneurons supplying the four limbs.

Synaptic organization of the rat parafascicular nucleus, with special reference to its afferents from the superior colliculus and the pedunculopontine tegmental nucleus

The synaptic organization of afferents to the parafascicular nucleus (Pf) of the thalamus was studied in rats. In the Pf, three types of axon terminals were identified: the first type was a small terminal with round synaptic vesicles forming an asymmetric synapse, the second type was a large terminal with round synaptic vesicles forming an asymmetric synapse, and the third type was a terminal with pleomorphic vesicles forming a symmetric synapse. They were named SR, LR and P boutons, respectively. In order to determine the origin of these axon terminals, biotinylated dextran amine (BDA) was injected into the main afferent sources of the Pf, the superior colliculus (SC) and the pedunculopontine tegmental nucleus (PPN). Axon terminals from the SC were both SR and LR boutons which made synaptic contacts with somata and dendrites. PPN afferents were SR boutons, which made synaptic contacts with somata and smaller dendrites. Double-labeled electron microscopic studies, in which a retrograde tracer (wheat germ agglutinin conjugated to horseradish peroxidase: WGA-HRP) was injected into the striatum and an anterograde tracer (BDA) into the SC revealed that SC afferent terminals made synapses directly with Pf neurons that projected to the striatum. Another experiment was performed to find out whether two different afferents converged onto a single Pf neuron. To address this question, two different tracers were injected into the SC and PPN in a rat. Electron microscopically, both afferent terminals from the SC and PPN made synaptic contacts with the same dendrite. Our results prove that a single neuron of the rat Pf received convergent projections from two different sources.

Brainstem control of head movements during orienting; organization of the premotor circuits

When an object appears in the visual field, animals orient their head, eyes, and body toward it in a well-coordinated manner (orienting movement). The head movement is a major portion of the orienting movement. Interest in the neural control of head movements in the monkey and human have increased in the 1990's, however, fundamental knowledge about the neural circuits controlling the orienting head movement continues to be based on a large number of experimental studies performed in the cat. Thus, it is crucial now to summarize information that has been clarified in the cat for further advancement in understanding the neural control of head movements in different animal species. The superior colliculus (SC) has been identified as the primary brainstem center controlling the orienting. Its output signal is transmitted to neck motoneurons via two major separate pathways: one through the reticulospinal neurons (RSNs) in the pons and medulla and the other through neurons in Forel's field H (FFH) in the mesodiencephalic junction. The tecto-reticulo-spinal pathway controls orienting chiefly in the horizontal direction, while the tecto-FFH-spinal pathway controls orienting in the vertical direction. In each pathway, a subgroup of neurons functions as premotor neurons for both extraocular and neck motoneurons, while others are specified for each, which allows both coordinated and separate control of eye and head movements. Head movements almost always produce shifts in the center of gravity that might cause postural disturbances. The postural equilibrium may be maintained by transmitting the orienting command to the limb segments via descending axons of the reticulospinal and long propriospinal neurons. The SC and brainstem relay neurons receive descending inputs from higher order structures such as the cerebral cortex, cerebellum, and basal ganglia. These inputs may serve context-dependent control of orienting by modulating the activities of the primary brainstem pathways.