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J M Bower

Publications and source records attributed to J M Bower.

64 records · Page 4Linked to original sources
PubMed1983

Congruence of spatial organization of tactile projections to granule cell and Purkinje cell layers of cerebellar hemispheres of the albino rat: vertical organization of cerebellar cortex.

1. We compared the spatial pattern of shortest latency somatosensory (tactile) projections to the Purkinje cell (PC) layer and to the underlying granule cell (GC) layer in tactile areas of rat cerebellar cortex. Micro-mapping methods were used to sample single units in the PC layer and multiple units in the GC layer of both anesthetized and unanesthetized rats. Mechanical and electrical stimulation of the body surface were employed. Responsiveness of PCs to cutaneous stimulation was assessed by constructing histograms of simple spike activity and statistically comparing poststimulus activity to nonstimulated base-line PC activity. 2. We found that PCs respond to tactile stimulation with increases (7-10 ms) followed by decreases (8-15 ms) in simple spike activity. Increases in simple spike activity followed activation of the underlying GC layer by 1-4 ms, while decreases in simple spike activity were found 2-5 ms after GC layer activation. 3. PCs were found to have both excitatory and inhibitory receptive fields (RFs). Excitatory RFs were restricted to small areas of a single body part and for each PC were very similar or identical to the RFs of neurons in the immediately subjacent GC layer. Inhibitory PC RFs were larger, often containing more than one body part and for each PC, were only partially similar to the RFs of subjacent GCs. PC inhibitory RFs also often included body surfaces projecting to the nearby but not to the underlying GC layer. 4. Stimulation of a single peripheral locus resulted in small, distinct regions of PC layer excitation and inhibition. Areas of PC excitation overlie activated regions of the GC layer, while inhibited PCs overlie both activated and nonactivated GC regions. 5. We found PCs to be organized in groups or patches with respect to the specific body region that was capable of activating them (upper lip, lower lip, etc.). Adjacent patches of PCs often represented widely separated body parts. This pattern of PC layer activating RF projections was congruent with the pattern of excitatory RF projections to the underlying GC layer. 6. These results indicate that there is a vertical organization in GC-PC excitatory relations, while GC-induced PC inhibition is slightly more widely distributed. 7. Our finding that the patchlike activation of PCs is congruent with that of the underlying GC layer contrasts with the classical concept that PCs are activated by parallel fibers in a "beamlike" fashion from a patch of GCs. Thus, a reevaluation of the role of parallel fibers seems to us to be in order. 8. In conclusion, our results support the view that short-latency afferent tactile projections to both the GC and PC layers of cerebellar cortex are highly organized spatially. This specificity of body surface projections must be incorporated into modern views of the functional organization of cerebellar cortex.

Animals↗
PubMed1982

Graphical methods for three-dimensional rotation of complex axonal arborizations.

Methods are described for 3-dimensional visualization and spatial manipulation of complex axonal arborizations. These methods allow 2-dimensional camera lucida drawings of neuron processes passing through serial sections to be redrawn with an altered viewing perspective without need for a computer. Two methods are described: a method for 90 degrees rotation, and a method for approximating smaller angle rotations. The method for small angle rotation was developed to visualize the spatial relationships between cortical laminae (curved as well as planar) and axon branching patterns.

Animals↗
PubMed1981

Principles of organization of a cerebro-cerebellar circuit. Micromapping the projections from cerebral (SI) to cerebellar (granule cell layer) tactile areas of rats.

We defined spatial patterns of organization of projections from somatosensory cerebral cortex (SI) to the somatosensory cerebellar cortex of anesthetized albino rats using microelectrode (stimulation and recording) micromapping methods and low-threshold cutaneous (tactile) stimulation. Two sampling strategies were used: (1) a single cerebral SI locus in layers V-VI was stimulated electrically, while a responding region of the cerebellar granule cell (GC) layer was systematically mapped with a recording electrode; (2) the SI stimulating electrode was used as the mapping electrode while the cerebellar GC electrode remain fixed. We found highly specific patterns of connections between somatotopically organized SI cortex and the somatotopically fractured tactile cerebellar cortex. Using threshold stimulating currents in SI, the projections from small populations of neural elements were found to be highly restricted, terminating within the confines of only those tactile cerebellar hemispheric locations having the same receptive fields (RFs). These SI-GC projections conform to the patchy mosaic pattern of organization previously shown for peripheral tactile projections. SI projections to GC patches were either contralateral or ipsilateral, depending on the laterality of the peripheral projections to that patch. Each SI focus projected to only a portion of a patch; projections from several adjacent SI loci overlapped serially within a patch. As with the peripherally evoked GC layer responses, SI-evoked GC responses were organized in a columnar fashion and were maximal at middle levels of the GC layer; SI-GC latencies were 5-8 ms. These data reveal that this tactile-related cerebro-cerebellar circuit exhibits precisely organized patterns of projection.

Animals↗
PubMedDate not supplied

A comparative survey of automated parameter-search methods for compartmental neural models.

One of the most difficult and time-consuming aspects of building compartmental models of single neurons is assigning values to free parameters to make models match experimental data. Automated parameter-search methods potentially represent a more rapid and less labor-intensive alternative to choosing parameters manually. Here we compare the performance of four different parameter-search methods on several single-neuron models. The methods compared are conjugate-gradient descent, genetic algorithms, simulated annealing, and stochastic search. Each method has been tested on five different neuronal models ranging from simple models with between 3 and 15 parameters to a realistic pyramidal cell model with 23 parameters. The results demonstrate that genetic algorithms and simulated annealing are generally the most effective methods. Simulated annealing was overwhelmingly the most effective method for simple models with small numbers of parameters, but the genetic algorithm method was equally effective for more complex models with larger numbers of parameters. The discussion considers possible explanations for these results and makes several specific recommendations for the use of parameter searches on neuronal models.

Action Potentials↗
PubMedDate not supplied

Transient versus asymptotic dynamics of CaM kinase II: possible roles of phosphatase.

Calmodulin-dependent protein kinase II (CaMKII) is known to play a key role during induction of long-term potentiation (LTP). Given the dependence of LTP on the frequency of synaptic activation, several previous modeling efforts have proposed that biochemical properties of CaMKII itself might be in part responsible for this dependence. Recently, De Koninck and Schulman (1998) have provided direct experimental evidence that the enzyme itself is sensitive to the frequency of Ca(2+) activation. Here we demonstrate the ability of a detailed biophysical model constructed solely on enzyme kinetics of purified proteins to generate the frequency sensitivity demonstrated by De Koninck and Schulman. Quantitative analysis of the model reveals that this frequency sensitivity is provided by a mechanism different from those previously postulated. This analysis leads to specific predictions concerning the effects of mutations on this process. We further employ the model to examine the asymptotic behavior of CaMKII-phosphatase system during longer simulated periods of stimulation. The analyses of the model suggest that the transient and asymptotic frequency sensitivity of this enzyme are dependent on different biochemical mechanisms. These results may be applicable to Ca(2+)/calmodulin signaling pathways in general.

Animals↗
PubMedDate not supplied

Modulatory effects of parallel fiber and molecular layer interneuron synaptic activity on purkinje cell responses to ascending segment input: a modeling study.

Based on anatomical, physiological, and model-based studies, it has been proposed that synapses associated with the ascending segment of granule cell axons provide the principle excitatory drive on Purkinje cells which is then modulated by the more numerous parallel fiber synapses. In this study we have evaluated this idea using a detailed compartmental model of a cerebellar Purkinje cell by providing identical ascending segment synaptic inputs during different levels of random parallel fiber and molecular interneuron input. Results suggest that background inputs from parallel fibers and molecular layer interneurons can have a substantial effect on the response of Purkinje cells to ascending segment inputs. Interestingly, these effects are not reflected in the average firing rate of the Purkinje cell and are thus entirely dendritic in effect. These results are considered in the context of the known segregated spatial distribution of the parallel fibers and ascending segment synapses and a new hypothesis concerning the functional organization of cerebellar cortical circuitry.

Animals↗
PubMedDate not supplied

Lateral cerebellar hemispheres actively support sensory acquisition and discrimination rather than motor control.

This study examined a new hypothesis proposing that the lateral cerebellum is not activated by motor control per se, as widely assumed, but is engaged during the acquisition and discrimination of tactile sensory information. This proposal derives from neurobiological studies of these regions of the rat cerebellum. Magnetic resonance imaging of the lateral cerebellar output nucleus (dentate) of humans during passive and active sensory tasks confirmed four a priori implications of this hypothesis. Dentate nuclei responded to cutaneous stimuli, even when there were no accompanying overt finger movements. Finger movements not associated with tactile sensory discrimination produced no dentate activation. Sensory discrimination with the fingers induced an increase in dentate activation, with or without finger movements. Finally, dentate activity was greatest when there was the most opportunity to modulate the acquisition of the sensory tactile data: when the discrimination involved the active repositioning of tactile sensory surface of the fingers. Furthermore, activity in cerebellar cortex was strongly correlated with observed dentate activity. This distinct four-way pattern of effects strongly challenges other cerebellar theories. However, contrary to appearances, neither our hypothesis nor findings conflict with behavioral effects of cerebellar damage, neurophysiological data on animals performing motor tasks, or cerebellar contribution to nonmotor, perceptual, and cognitive tasks.

Adult↗