Arm-movement-related neurons in the primate superior colliculus and underlying reticular formation: comparison of neuronal activity with EMGs of muscles of the shoulder, arm and trunk during reaching.

Neuronal activity was recorded from the superior colliculus (SC) and the underlying reticular formation in two monkeys during an arm reaching task. Of 744 neurons recorded, 389 (52%) clearly modulated their activity with arm movements. The temporal activity patterns of arm-movement-related neurons often had a time course similar to rectified electromyograms (EMGs) of particular muscles recorded from the shoulder, arm or trunk. These reach cells, as well as the muscles investigated, commonly exhibited mono- or biphasic (less frequently tri- or polyphasic) excitatory bursts of activity, which were related to the (pre-)movement period, the contact phase and/or the return movement. The vast majority of reach cells exhibited a consistent activity pattern from trial to trial as did most of the muscles of the shoulder, arm and trunk. Similarities between the activity patterns of the neurons and the muscles were sometimes very strong and were especially notable with the muscles of the shoulder girdle (e.g. trapezius descendens, supraspinatus, infraspinatus or the anterior and medial deltoids). This high degree of co-activation suggests a functional linkage, though not direct, between the collicular reach cells and these muscles. Neuronal activity onset was compared with that of 25 muscles of the arms, shoulders and trunk. The majority of cells (78.5%) started before movement onset with a mean lead time of 149+/-90 ms, and 36.5% were active even before the earliest EMG onset. The neurons exhibited the same high degree of correlation (r=0.97, Spearman rank) between activity onset and the beginning of the arm movement as did the muscles (r=0.98) involved in the task. The mean neuronal reach activity (background subtracted) ranged between 7 and 193 impulses/s (mean 40.5+/-24.2). The mean modulation index calculated [(reach activity background activity)/reach activity+background activity)] was 0.75+/-0.23 for neurons (n=358) and 0.87+/-0.14 for muscles (n=25). As the monkeys fixated the reach target constantly during an arm movement, neuronal activity which was modulated in this period was not related to eye movements. The three neck muscles investigated in the reach task exhibited no reach-related activity modulation comparable to that of either the reach cells or the muscles of the shoulder, arm and trunk. However, tonic neck muscle EMG was monotonically related to horizontal eye position. The clear skeletomotor discharge characteristics of arm-movement-related SC neurons revealed in this study agree with those already known from other sensorimotor regions (for example the primary motor, the premotor and parietal cortex, the basal ganglia or the cerebellum) and are consistent with the possible role of this population of reach cells in the control of arm movements.

Anatomical distribution of arm-movement-related neurons in the primate superior colliculus and underlying reticular formation in comparison with visual and saccadic cells.

We recorded from 389 "reach" neurons (two monkeys) in the superior colliculus (SC) and underlying reticular formation (RF) or adjacent periaqueductal grey, whose activity was related to visually guided arm movements. Reach neurons were present from approximately 0.7 mm down to a depth of 6 mm below the surface of the SC (mean 3.7+/-1.3, n=389). Although this mean distribution was different from that of cells with visual (mean depth 1.7+/-1.4 mm, n=283) or saccadic responses (mean depth 2.0+/-1.4 mm, n=232), there was a large amount of overlap. Fifty-five per cent of all reach cells (213/389) were assumed to be located inside the SC. The others were considered to be located in the underlying RF. The characteristics of visual responses and saccadic bursts (e.g. response latencies, discharge rates, burst durations) of arm-movement-related neurons were not different from those of typical visual or saccade cells in the SC. Although reach neurons could be recorded in a large area of the SC, they were found more often in the lateral than in the medial parts (chi-squared=19.3, P<0.001). Possible pathways by which arm-movement-related neuronal activity in and below the SC might gain access to spinal motor structures are discussed. The location of arm-movement-related neurons described in this study is in accordance with the known target areas of skeletomotor-related corticotectal projections and with the sites of origin of tectofugal pathways. It is concluded that this population of reach cells is in a position to relay and transmit limb movement information to the spinal motor system, where it may influence (or interact with) motor commands coming from other motor centres.

Population coding of arm-movement-related neurons in and below the superior colliculus of Macaca mulatta.

It has been shown for the motor cortex of primates, that an arm trajectory is coded as a population vector formed by many neurons with activities correlated with arm movements. Recently, neurons in the primate superior colliculus that also display activities related to arm movements have been described. In the present paper we show that a subpopulation of this type of neuron is able to code for limb movement by the population vector. However, the cosine function cannot describe these neurons adequately. Rather the Fisher distribution yields a much better description of arm-movement-related cells in the superior colliculus.

Phylogenetic position of the spirochetal genus Cristispira.

Comparative sequence analysis of 16S rRNA genes was used to determine the phylogenetic relationship of the genus Cristispira to other spirochetes. Since Cristispira organisms cannot presently be grown in vitro, 16S rRNA genes were amplified directly from bacterial DNA isolated from Cristispira cell-laden crystalline styles of the oyster Crassostrea virginica. The amplified products were then cloned into Escherichia coli plasmids. Sequence comparisons of the gene coding for 16S rRNA (rDNA) insert of one clone, designated CP1, indicated that it was spirochetal. The sequence of the 16S rDNA insert of another clone was mycoplasmal. The CP1 sequence possessed most of the individual base signatures that are unique to 16S rRNA (or rDNA) sequences of known spirochetes. CP1 branched deeply among other spirochetal genera within the family Spirochaetaceae, and accordingly, it represents a separate genus within this family. A fluorescently labeled DNA probe designed from the CP1 sequence was used for in situ hybridization experiments to verify that the sequence obtained was derived from the observed Cristispira cells.