Biomedical subjects

M C Tresch

Publications and source records attributed to M C Tresch.

12 recordsLinked to original sources

PubMed2001

Muscle synergies encoded within the spinal cord: evidence from focal intraspinal NMDA iontophoresis in the frog.

This paper relates to the problem of the existence of muscle synergies, that is whether the CNS command to muscles is simplified by controlling their activity in subgroups or synergies, rather than individually. We approach this problem with two methods that have been recently introduced: intraspinal N-methyl-D-aspartate (NMDA) microstimulation and a synergy-extracting algorithm. To search for a common set of synergies encoded for by the spinal cord whose combinations would account for a large range of electromyographic (EMG) patterns, we chose, rather than examining a large range of natural behaviors, to chemically microstimulate a large number of spinal cord interneuronal sites in different frogs. A possible advantage of this complementary method is that it is task-independent. Visual inspection suggested that the NMDA-elicited EMG patterns recorded from 12 leg muscles might indeed be constructed from smaller subgroups of muscles whose activity co-varied, suggestive of synergies. We used a modification of our extracting computational algorithm whereby a nonnegative least-squares method was employed to iteratively update both the synergies and their coefficients of activation in reconstructing the EMG patterns. Using this algorithm, a limited set of seven synergies was found whose linear combinations accounted for more than 91% of the variance in the pooled EMG data from 10 frogs, and more than 96% in individual frogs. The extracted synergies were similar among frogs. Further, preferred combinations of these synergies were clearly identified. This was found by extracting smaller sets of four, five, or six synergies and fitting each synergy from those sets as a combination from the set of seven synergies extracted from the same frog; the synergy combinations from each frog were then pooled together to examine their frequency of occurrence. Concordance with this method of identifying synergy combinations was found by examining how the synergies from the set of seven correlated pair-wise as they reconstructed the EMG data. These results support the existence of muscle synergies encoded within the spinal cord, which through preferred combinations, account for a large repertoire of spinal cord motor output. These findings are contrasted with previous approaches to the problem of synergies.

Animals↗

PubMed2000

Contributions of intrinsic motor neuron properties to the production of rhythmic motor output in the mammalian spinal cord.

Motor neurons are endowed with intrinsic and conditional membrane properties that may shape the final motor output. In the first half of this paper we present data on the contribution of I(h), a hyperpolarization-activated inward cation current, to phase-transition in motor neurons during rhythmic firing. Motor neurons were recorded intracellularly during locomotion induced with a mixture of N-methyl-D-aspartate (NMDA) and serotonin, after pharmacological blockade of I(h). I(h) was then replaced by using dynamic clamp, a computer program that allows artificial conductances to be inserted into real neurons. I(h) was simulated with biophysical parameters determined in voltage clamp experiments. The data showed that electronic replacement of the native I(h) caused a depolarization of the average membrane potential, a phase-advance of the locomotor drive potential, and increased motor neuron spiking. Introducing an artificial leak conductance could mimic all of these effects. The observed effects on phase-advance and firing, therefore, seem to be secondary to the tonic depolarization; i.e., I(h) acts as a tonic leak conductance during locomotion. In the second half of this paper we discuss recent data showing that the neonatal rat spinal cord can produce a stable motor rhythm in the absence of spike activity in premotor interneuronal networks. These coordinated motor neuron oscillations are dependent on NMDA-evoked pacemaker properties, which are synchronized across gap junctions. We discuss the functional relevance for such coordinated oscillations in immature and mature spinal motor systems.

Animals↗

PubMed2000

New perspectives on spinal motor systems.

The production and control of complex motor functions are usually attributed to central brain structures such as cortex, basal ganglia and cerebellum. In traditional schemes the spinal cord is assigned a subservient function during the production of movement, playing a predominantly passive role by relaying the commands dictated to it by supraspinal systems. This review challenges this idea by presenting evidence that the spinal motor system is an active participant in several aspects of the production of movement, contributing to functions normally ascribed to 'higher' brain regions.

Adaptation, Physiological↗

PubMed2000

Motor coordination without action potentials in the mammalian spinal cord.

Coordination of neuronal activity to produce movement is generally thought to depend on spike activity in premotor interneuronal networks. Here we show that even without such activity, the neonatal rat spinal cord could produce a stable motor rhythm mediated by the synchronization of motor neuron oscillations across gap junctions. These rhythms, however, were not coordinated between motor pools in different parts of the spinal cord. We further showed that neural coordination through gap junctions contributed to the fundamental organization and function of spinal motor systems. These results suggest that neural coordination across gap junctions is important in motor-pattern generation in the mammalian spinal cord.

2-Amino-5-phosphonovalerate↗

PubMed2000

Population reconstruction of the locomotor cycle from interneuron activity in the mammalian spinal cord.

Lesion studies have shown that neuronal networks in the ventromedial regions of the neonatal rat spinal cord are critical for the production of locomotion. We examined whether the locomotor cycle could be accurately predicted based on the activity recorded in a population of spinal interneurons located in these regions during pharmacologically induced locomotion. We used a Bayesian probabilistic reconstruction procedure to predict the most likely phase of locomotion given the observed activity in the neuronal population. The population reconstruction was able to predict the correct locomotor phase with high accuracy using a relatively small number of neurons. This result demonstrates that although the spike activity of individual spinal interneurons in the ventromedial region is weak and varies from cycle to cycle, the locomotor phase can be accurately predicted when information from the population is combined. This result is consistent with the proposed involvement of interneurons within these regions of the spinal cord in the production of locomotion.

Action Potentials↗

PubMed1999

Responses to spinal microstimulation in the chronically spinalized rat and their relationship to spinal systems activated by low threshold cutaneous stimulation.

We describe the responses evoked by microstimulation of interneuronal regions of the spinal cord in unanesthetized rats chronically spinalized at T10-T12. One to three weeks after spinalization, sites in the lumbar spinal cord were stimulated using trains of low current microstimulation. The isometric force produced by stimulation of a spinal site was measured at the ankle. Responses were reliably observed from stimulation of a region within the first 1250 microm from the dorsal surface of the spinal cord. These responses were clearly not due to direct motoneuronal activation and were maintained after chronic deafferentation. The force evoked by microstimulation and measured at the ankle varied smoothly across the workspace. Simultaneous stimulation of two sites in the spinal cord produced a response that was a simple linear summation of the responses evoked from each of the sites alone. Microstimulation generally produced a highly non-uniform distribution of response directions, biased toward responses which pulled the limb toward the body. Within these distributions there appeared to be two main types of responses. These different types of responses were preferentially evoked by microstimulation of different rostrocaudal regions of the spinal cord. This anatomical organization paralleled the spinal cutaneous somatotopy, as assessed by recording cutaneous receptive fields of neurons at sites to which the microstimulation was applied. This relationship was maintained after chronic deafferentation. The findings described here in the rat spinal cord in large part replicate those previously described in the frog.

Animals↗

PubMed1999

The construction of movement by the spinal cord.

We used a computational analysis to identify the basic elements with which the vertebrate spinal cord constructs one complex behavior. This analysis extracted a small set of muscle synergies from the range of muscle activations generated by cutaneous stimulation of the frog hindlimb. The flexible combination of these synergies was able to account for the large number of different motor patterns produced by different animals. These results therefore demonstrate one strategy used by the vertebrate nervous system to produce movement in a computationally simple manner.

Animals↗

PubMed1999

Coding of locomotor phase in populations of neurons in rostral and caudal segments of the neonatal rat lumbar spinal cord.

Several experiments have demonstrated that rostral segments of the vertebrate lumbar spinal cord produce a rhythmic motor output more readily and of better quality than caudal segments. Here we examine how this rostrocaudal gradient of rhythmogenic capability is reflected in the spike activity of neurons in the rostral (L(2)) and caudal (L(5)) lumbar spinal cord of the neonatal rat. The spike activity of interneurons in the ventromedial cord, a region necessary for the production of locomotion, was recorded intracellularly with patch electrodes and extracellularly with tetrodes during pharmacologically induced locomotion. Both L(2) and L(5) neurons tended to be active in phase with their homologous ventral root. L(5) neurons, however, had a wider distribution of their preferred phases of activity throughout the locomotor cycle than L(2) neurons. The strength of modulation of the activity of individual L(2) neurons was also larger than that of L(5) neurons. These differences resulted in a stronger rhythmic signal from the L(2) neuronal population than from the L(5) population. These results demonstrate that the rhythmogenic capability of each spinal segment was reflected in the activity of interneurons located in the same segment. In addition to paralleling the rostrocaudal gradient of rhythmogenic capability, these results further suggest a colocalization of motoneurons and their associated interneurons involved in the production of locomotion.

Action Potentials↗

PubMed1998

Spinal cord modular organization and rhythm generation: an NMDA iontophoretic study in the frog.

Previous work using electrical microstimulation has suggested the existence of modules subserving limb posture in the spinal cord. In this study, the question of modular organization was reinvestigated with the more selective method of chemical microstimulation. N-methyl--aspartate (NMDA) iontophoresis was applied to 229 sites of the lumbar spinal cord gray while monitoring the isometric force output of the ipsilateral hindlimb at the ankle. A force response was elicited from 69 sites. At 18 of these sites, tonic forces were generated and rhythmic forces at 44. In the case of tonic forces, their directions clustered along four orientations: lateral extension, rostral flexion, adduction, and caudal extension. For the entire set of forces (tonic and rhythmic), the same clusters of orientations were found with the addition of a cluster directed as a flexion toward the body. This distribution of force orientations was quite comparable to that obtained with electrical stimulation at the same sites. The map of tonic responses revealed a topographic organization; each type of force orientation was elicited from sites that grouped together in zones at distinct rostrocaudal and depth locations. In the case of rhythmic sequences of force orientations, some were distinctly more common, whereas others were rarely elicited by NMDA. Mapping of the most common rhythms showed that each was elicited from two or three regions of the cord. These regions were close in location to the tonic regions that produced those forces that represented components specific to that rhythm. There was an additional caudal region from which the different rhythms also could be elicited. Taken together, these results support the concept of a modular organization of the motor system in the frog's spinal cord and delineate the topography of these modules. They also suggest that these modules are used by the circuitry underlying rhythmic pattern generation by the spinal cord.

Animals↗

PubMed1997

Segmental and propriospinal projection systems of frog lumbar interneurons.

Spinal interneuronal networks have been implicated in the coordination of reflex behaviors and limb postures in the spinal frog. As a first step in defining these networks, retrograde transport of horseradish peroxidase (HRP) was used to examine the anatomical organization of interneuronal circuitry in the lumbar spinal cord of the frog. Following neuronal degeneration induced by spinal transection and section of the dorsal and ventral roots, HRP was placed at different locations in the spinal cord and the positions of labeled neuronal cell bodies plotted using a Eutectics Neuron Tracing System. We describe four spinal interneuronal systems, three with cell bodies located in the lumbar cord and one with descending projections to the lumbar cord. Interneurons with cell bodies located in the lumbar cord include: (1) Lumbar neurons projecting rostrally. Those projecting to thoracic segments tended to be located in the lateral and ventrolateral gray and in the lower two-thirds of the dorsal horn, with projections that were predominantly uncrossed. Those projecting to the brachial plexus and beyond were located in the dorsal part of the dorsal horn (uncrossed) and in the lateral, ventrolateral, and ventromedial gray (crossed). (2) Lumbar neurons with segmental projections within the lumbar cord. These neurons, which were by far the most numerous, had both uncrossed and crossed projections and were distributed throughout the dorsal, lateral, ventrolateral, and ventromedial gray matter. (3) Lumbar neurons projecting to the sacral cord. This population, which arose mainly from the dorsal horn and lateral or ventrolateral gray, was much smaller than in the other systems. Neuronal density of some of these populations of lumbar interneurons appeared to vary with rostrocaudal level. Finally, a population of neurons with cell bodies in the brachial and thoracic segments that projects to the lumbar cord is described. The most rostral of these neurons were multipolar cells with uncrossed projections, while those with crossed projections were confined almost exclusively to the ventral half of the cord. The distribution of spinal interneurons reported here will provide guidance for future studies of the role of interneuronal networks in the control of movements using the spinal frog as a model system.

Animals↗

PubMed1995

Priming of locomotor initiation by electrical stimulation in the hypothalamus and preoptic region in the anesthetized rat.

Electrical stimulation at a locomotor site can prime (i.e., shorten the latency to initiate) stepping elicited by subsequent stimulation of the same or a different site. We tested for the priming effect in representative sites along the medial forebrain bundle, and determined if its magnitude showed regional differences. Rats (n = 20) were anesthetized with Nembutal and held in a stereotaxic apparatus over a wheel. Stepping was detected by accelerometers attached to the hindlimbs. Priming and test trains of stimulation (0.5-ms cathodal pulses, 50 Hz, 25-75 microA, 7-9-s train duration) separated by 20 s were delivered every 90 s. When the priming and test stimulations were applied to the same site, the priming effects were similar along the entire extent of the medial forebrain bundle. When the priming and test sites were different, the priming effect depended on their relative positions. Anterior stimulation primed posterior sites at magnitude comparable to those produced by stimulating the same posterior site. Posterior stimulation primed anterior sites at a level half of that produced by stimulation of the same anterior site. This pattern was found for priming and test sites that were ipsilateral and contralateral. Priming is a general and robust phenomenon with properties that may be useful for studying locomotor initiation pathways.

Anesthesia↗

PubMed1993

Double dissociation of spatial and object visual memory: evidence from selective interference in intact human subjects.

A functional dissociation of the spatial and object visual systems was produced by selective interference in intact young adults. Subjects were instructed to remember the location of a dot in a spatial memory test, and the form of an object memory test. As predicted by current notions of dissociable visual systems in the primate, spatial memory was selectively impaired by a movement discrimination spatial task, whereas object memory was selectively impaired by a color discrimination object task.

Adolescent↗