M. E. Anderson

An autoradiographic study of efferent connections of the globus pallidus in Macaca mulatta

SummaryRadioactive amino acids were injected into restricted regions of the globus pallidus of rhesus macaques to allow identification of the organization and courses of efferent pallidal projections. The previously identified projection of the internal pallidal segment (GPi) to ventral thalamic nuclei showed a topographic organization, with the predominant projection from ventral GPi being to medial and caudal ventralis anterior (VA) and lateralis (VL) and from dorsal GPi to lateral and rostral VA and VL. Pallidal efferent fibers also extended caudally and dorsally into pars caudalis of VL, but they spared the portion of pars oralis of VL shown by others to receive input from the cerebellum. In addition to centromedian labeling in all animals, the parafascicular nucleus was also labeled when isotope was injected into dorsal GPi. The medial route from GPi to the midbrain tegmentum was more substantial than has been shown before, and along this route there was an indication that some fibers terminated in the prerubral region. The projection to the pedunculopontine nucleus was extensive, and fibers continued caudally into the parabrachial nuclei.Pallidal projections to the thalamus seem to be topographically organized but spare thalamic regions that interact with area 4. Caudally directed efferent fibers follow multiple routes and extend more caudally than to the pedunculopontine nuclei.

A horseradish peroxidase study of afferent connections of the globus pallidus in Macaca mulatta

SummaryThe high tonic discharge rates of globus pallidus neurons in awake monkeys suggest that these neurons may receive some potent excitatory input. Because most current electrophysiological evidence suggests that the major described pallidal afferent systems from the neostriatum are primarily inhibitory, we used retrograde transport of horseradish peroxidase (HRP) to identify possible additional sources of pallidal afferent fibers. The appropriate location was determined before HRP injection by mapping the characteristic high frequency discharge of single pallidal units in awake animals. In animals with injections confined to the internal pallidal segment, retrograde label was seen in neurons of the pedunculopontine nucleus, dorsal raphe nucleus, substantia nigra, caudate, putamen, subthalamic nucleus, parafascicular nucleus, zona incerta, medial and lateral subthalamic tegmentum, parabrachial nuclei, and locus coeruleus. An injection involving the external pallidal segment and the putamen as well resulted in additional labeling of cells in centromedian nucleus, pulvinar, and the ventromedial thalamus.

Context-Dependent Modulation of Movement-Related Discharge in the Primate Globus Pallidus

A selective contribution of the basal ganglia (BG) to memory-contingent motor control has long been hypothesized. The importance of memory context remains an open question, however, for the BG skeletomotor circuit. To investigate this question, we studied the perimovement discharge of a carefully selected group of 74 “arm-related” pallidal cells in two rhesus monkeys. The animals performed three tasks designed to dissociate multiple independent aspects of memory-contingent reaching while controlling movement kinematics. The activity of most neurons (88%) was influenced strongly by the memory demands of a task (remembering “where” or “when” to move), but the population as a whole showed no systematic preference for memory- or sensory-contingent conditions. The effects of memory context were primarily additive with those of movement kinematics (particularly movement direction). Considered separately, decreases and increases in firing had very different context preferences: decreases were nearly always larger for sensory-triggered movements, whereas increases were enhanced most often under memory-contingent conditions (i.e., self-initiated or self-guided movements). A similar pattern of preferences was found for both pallidal segments. The distinct context-specific enhancements of decreases and increases could not be explained as simple sensory responses or as interactions with preparatory or anticipatory processes present before movement initiation. Rather, they appear related to movement execution under specific contexts. Our results lead to the conclusion that movement facilitatory decreases in internal pallidal (GPi) activity are primarily greater under sensory-triggered conditions. GPi increases and their suppressive effects, perhaps on competing activity in pallidal-recipient centers, have increased prevalence under memory-contingent conditions.

Segmental reflex inputs to motoneurons innervating dorsal neck musculature in the cat

SummaryThe responses to Stimulation of upper cervical muscle and cutaneous afferents were studied in motoneurons innervating splenius, complexus, and biventer cervicis dorsal neck muscles of cats. Motoneurons innervating complexus and biventer cervicis fibers, which are in the deeper, longitudinally oriented muscles, were monosynaptically excited by ipsilateral Group I afferents from each of these muscles, but they did not receive significant input from splenius Group I afferents. Likewise, splenius motoneurons were not monosynaptically excited by ipsilateral afferents from complexus and biventer cervicis. Stimulation of ipsilateral cutaneous afferents produced predominant excitation in splenius motoneurons, predominant inhibition in biventer cervicis motoneurons, and inhibition or mixed responses in complexus motoneurons.None of the neck motoneurons studied showed postsynaptic potentials following single or multiple shock stimulation of contralateral muscle nerves at stimulus intensities expected to excite exclusively Group I afferents. Higher intensity stimulation of contralateral muscle afferents, as well as fibers in the greater auricular nerves, produced predominant inhibition in all three neck motoneuron pools.Segmentally-excited afferents to neck motoneurons, like those from supraspinal systems, appear to evoke different patterns of synaptic responses in splenius motoneurons than they do in motoneurons innervating fibers in the deeper, longitudinally oriented complexus and biventer cervicis muscles.

An electrophysiological characterization of projections from the pedunculopontine area to entopeduncular nucleus and globus pallidus in the cat

SummaryStimulation of the pedunculopontine region (PPN), in which neurons are filled by horseradish peroxidase injected into the entopeduncular nucleus (ENTO) or globus pallidus (GP) of the cat, excites ENTO and GP neurons both orthodromically and antidromically. Stimulus threshold mapping experiments and intracellular records of EPSPs provide evidence that the orthodromic excitation may be produced monosynaptically by the axons of PPN neurons. Antidromic excitation of ENTO axons from stimulation in PPN or the thalamus may elicit IPSPs in ENTO neurons via the action of recurrent collaterals.An excitatory synaptic action of PPN neurons on pallidal cells could be a partial basis for the high discharge rate characteristic of these neurons in awake animals.

A quantitative analysis of pallidal discharge during targeted reaching movement in the monkey

SummaryNeurons in the globus pallidus have been studied during reaching movements of the arm made at varying speeds. The reaching task is similar to one used in earlier experiments, in which disruption of normal pallidal output caused changes in movement time. The pallidal cells studied were those that showed task-related changes in activity and a modification of discharge when the arm was manipulated outside of the task. Neuronal discharge was assessed to evaluate two possible models, one in which the timing of task-related discharge varied as a function of movement time and the other in which the amplitude (mean firing rate) of the change in discharge varied as movement time varied. The relation between neuronal discharge and movement time was examined quantitatively on a trial-by-trial basis using a statistical algorithm that identified each phase of the change in neuronal discharge on each trial. A nonparametric statistic was used to determine the correlation between movement time and the duration or latency of changes in neural firing or the mean discharge during each phase of the cells response. For fifty-five percent of the 40 neurons studied, the timing of the cells response (duration or latency) varied as a function of movement time. For only 10 cells (25%) was there a significant correlation between movement time and the mean firing rate during one or more phases of the cells response. Both timing and frequency modulation with movement time were limited to cells responsive to manipulation at the wrist or the shoulder.

An analysis of potentially converging inputs to the rostral ventral thalamic nuclei of the cat

SummaryPotentially convergent inputs to cerebellar-receiving and basal ganglia-receiving areas of the thalamus were identified using horseradish peroxidase (HRP) retrograde tracing techniques. HRP was deposited iontophoretically into the ventroanterior (VA), ventromedial (VM), and ventrolateral (VL) thalamic nuclei in the cat. The relative numbers of labeled neurons in the basal ganglia and the cerebellar nuclei were used to assess the extent to which the injection was in cerebellar-receiving or basal ganglia-receiving portions of thalamus. The rostral pole of VA showed reciprocal connections with prefrontal portions of the cerebral cortex. Only the basal ganglia and the hypothalamus provided non-thalamic input to modulate these cortico-thalamo-cortical loops. In VM, there were reciprocal connections with prefrontal, premotor, and insular areas of the cerebral cortex. The basal ganglia (especially the substantia nigra), and to a lesser extent, the posterior and ventral portions of the deep cerebellar nuclei, provided input to VM and may modulate these corticothalamo-cortical loops. The premotor cortical areas connected to VM include those associated with eye movements, and afferents from the superior colliculus, a region of documented importance in oculomotor control, also were labeled by injections into VM. The dorsolateral portion of the VA-VL complex primarily showed reciprocal connections with the medial premotor (area 6) cortex. Basal ganglia and cerebellar afferents both may modulate this cortico-thalamo-cortical loop, although they do not necessarily converge on the same thalamic neurons. The cerebellar input to dorsolateral VA-VL was from posterior and ventral portions of the cerebellar nuclei, and the major potential brainstem afferents to this region of thalamus were from the pretectum. Mid- and caudo-lateral portions of VL had reciprocal connections with primary motor cortex (area 4). The dorsal and anterior portions of the cerebellar nuclei had a dominant input to this corticothalamo-cortical loop. Potentially converging brainstem afferents to this portion of VL were from the pretectum, especially pretectal areas to which somatosensory afferents project.