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PubMedDate not supplied

Cerebello-thalamic synapses and motor adaptation.

The cerebellum's influence on voluntary movement is mediated, in large part, through the cerebello-thalamo-cortical (CTC) pathway. Of particular relevance here are those neurons in the cerebellar nuclei that project, via thalamus, to pyramidal tract neurons in primary motor cortex. Several lines of evidence implicate cerebello-thalamic (CT) synaptic plasticity as a neural substrate underlying movement adaptation in adult animals. CT synapses exhibit a number of structural characteristics suggestive of a capacity for both formation of new synapses, and alterations in efficacy of transmission across existing synapses. Long-term potentiation can be evoked across CT synapses in vitro by high frequency stimulation, albeit in young animals. Evidence regarding the contribution of CT synaptic plasticity to two different types of movement adaptation in adult animals is conflicting. Adaptation involving a strengthening and re-coordination of voluntary movement is associated with an increase in density of CT synaptic boutons and an increase in number of synaptic vesicles available for immediate neurotransmitter release within each bouton. On the other hand, adaptation involving associative conditioning of a reduced sensorimotor neural circuit is associated with plasticity at thalamo-cortical but not CT synapses. These conflicting findings may reflect differences in the extent of synaptic re-organization that occurs at thalamic versus cortical levels, differences in the neural circuitry mediating each behavior, and/or differences in the spatio-temporal convergence of activity in the thalamus during the adaptive processes. It is concluded that CT synaptic plasticity can underlie movement adaptation if the adaptation requires reorganization of the cerebellum's influence on cerebral cortex.

Adaptation, Physiological↗
PubMed2003

Adaptive significance of circadian clocks.

Circadian clocks are ubiquitous and are found in organisms ranging from bacteria to mammals. This ubiquity of occurrence implies adaptive significance, but to date there has been no rigorous empirical evidence to support this. It is believed that an organism possessing circadian clocks gains fitness advantage in two ways: (i) by synchronizing its behavioral and physiological processes to cyclic environmental factors (extrinsic adaptive value); (ii) by coordinating its internal metabolic processes (intrinsic adaptive value). There is preliminary circumstantial evidence to support both. Several studies using organisms living in constant environments have shown that these organisms possess functional circadian clocks, suggesting that circadian clocks may have some intrinsic adaptive value. Studies to assess the adaptive value of circadian clocks in periodic environments suggest that organisms may have a fitness advantage in those periodic environments, which closely match their own intrinsic periodicity. Furthermore, evidence from organisms living in the wild, selection studies, and studies on latitudinal clines suggest that circadian clocks may have an extrinsic adaptive value as well. In this paper, I have presented several hypotheses for the emergence of circadian clocks and have reviewed some major empirical studies suggesting adaptive significance of circadian clocks.

Adaptation, Physiological↗
PubMed1977

Electrical and adaptive properties of rod photoreceptors in Bufo marinus. I. Effects of altered extracellular Ca2+ levels.

The effects of altering extracellular Ca(2+) levels on the electrical and adaptive properties of toad rods have been examined. The retina was continually superfused in control (1.6 mM Ca(2+)) or test ringer's solutions, and rod electrical activity was recorded intracellularly. Low-calcium ringer's (10(-9)M Ca(2+)) superfused for up to 6 min caused a substantial depolarization of the resting membrane potential, an increase in light-evoked response amplitudes, and a change in the waveform of the light-evoked responses. High Ca(2+) ringer's (3.2 mM) hyperpolarized the cell membrane and decreased response amplitudes. However, under conditions of either low or high Ca(2+) superfusion for up to 6 min, in both dark-adapted and partially light-adapted states, receptor sensitivity was virtually unaffected; i.e., the V-log I curve for the receptor potential was always located on the intensity scale at a position predicted by the prevailing light level, not by Ca(2+) concentration. Thus, we speculate that cytosol Ca(2+) concentration is capable of regulating membrane potential levels and light-evoked response amplitudes, but not the major component of rod sensitivity. Low Ca(2+) ringer's also shortened the period of receptor response saturation after a bright but nonbleaching light flash, hence accelerating the onset of both membrane potential and sensitivity recovery during dark adaptation. Exposure of the retina to low Ca(2+) (10(-9)M) ringer's for long periods (7-15 min) caused dark-adapted rods to lose responsiveness. Response amplitudes gradually decreased, and the rods became desensitized. These severe conditions of low Ca(2+) caused changes in the dark-adapted rod that mimic those observed in rods during light adaptation. We suggest that loss of receptor sensitivity during prolonged exposure to low Ca(2+) ringer's results from a decrease of intracellular (intradisk) stores of Ca(2+); i.e., less Ca(2+) is thereby released per quantum catch.

Adaptation, Ocular↗
PubMed1984

Wavelength dependence of dark adaptation in Phycomyces phototropism.

The wavelength dependence of phototropic dark adaptation in Phycomyces was studied between 347 and 545 nm. Dark adaptation kinetics were measured for wavelengths of 383, 409, 477, and 507 nm in the intensity range from 6.2 X 10(-2) to 2 X 10(-7) W X m-2. At these wavelengths, dark adaptation follows a biexponential decay as found previously with broadband blue light (Russo, V. E. A., and P. Galland, 1980, Struct. Bonding., 41:71; Lipson, E. D., and S. M. Block, 1983, J. Gen. Physiol., 81:845). We have found that the time constants of the fast and slow components depend critically on the wavelength. At 507 nm, dark adaptation kinetics were found to be monophasic. The phototropic latency after a step down by a factor of 500 was measured for 19 different wavelengths. Maximal latencies were found at 383, 477, and 530 nm; minimal latencies were found at 409 and 507 nm. With irradiation programs that employ different wavelengths before and after the step down, the dark adaptation kinetics depend critically on the sequence in which the two wavelengths are given. We have found too that not only do the adaptation kinetics vary with wavelength, but so also do the phototropic bending rate and the phototropic latencies in experiments without intensity change. The results imply that more than one photoreceptor is mediating phototropism in Phycomyces and that sensory adaptation is regulated by these photoreceptors.

Dark Adaptation↗
PubMed1990

Microvillar components of light adaptation in blowflies.

The process of light adaptation in blowfly photoreceptors was analyzed using intracellular recording techniques and double and triple flash stimuli. Adapting flashes of increasing intensity caused a progressive reduction in the excitability of the photoreceptors, which became temporarily suppressed when 3 x 10(6) quanta were absorbed by the cell. This suppression was confirmed by subsequently applying an intense test flash that photoactivated a considerable fraction of the 10(8) visual pigment molecules in the cell. The period of temporary desensitization is referred to as the refractory period. The stimulus intensity to render the receptor cell refractory was found to be independent of the extracellular calcium concentration over a range of 10(-4) and 10(-2) M. During the refractory period (30-40 ms after the adapting flash) the cell appears to be "protected" against further light adaptation since light absorption during this period did not affect the recovery of the cell's excitability. Calculations showed that the number of quantum absorptions necessary to induce receptor refractoriness is just sufficient to photoactivate every microvillus of the rhabdomere. This coincidence led to the hypothesis that the refractoriness of the receptor cells is due to the refractoriness of the individual microvilli. The sensitivity of the receptor cells after relatively weak adapting flashes was reduced considerably more than could be accounted for by the microvilli becoming refractory. A quantitative analysis of these results suggests that a photoactivated microvillus induces a local adaptation over a relatively small area of the rhabdomere around it, which includes several tens of microvilli. After light adaptation with an intense flash, photoactivation of every microvillus by the absorption of a few quanta produced only a small receptor response whereas photoactivation of every rhodopsin molecule in every microvillus produced the maximum response. The excitatory efficiency of the microvilli therefore increases with the number of quanta that are absorbed simultaneously.

Adaptation, Ocular↗
PubMed1990

Cellular mechanisms that underlie bleaching and background adaptation.

Experiments were performed on rod photoreceptors isolated from the eye of the larval tiger salamander to determine if the same or different mechanisms underlie the desensitization produced by dim background light (background adaptation) and that which persists in the steady state in darkness after a significant fraction of the photopigment is bleached (bleaching adaptation). We have examined adaptational effects after light that bleached between approximately 50% and 95% of the photopigment under conditions which preclude pigment regeneration. The steady-state desensitization, far greater than that predicted by quantum-catch loss, is relieved upon regeneration of the visual pigment with 11-cis retinal. We measured the spread of desensitization along the long axis of the rod after a local bright bleach at one end by comparing responses to dim local test flashes elicited in different regions of the outer segment, before and after bleaching. The space constant for this spread was less than 2.5 microns. We have previously measured the space constant for the longitudinal spread of desensitization during a local dim background in Ambystoma rods to be 7 microns. This is similar to a space constant of 6 microns measured under similar conditions in Bufo rods by Lamb et al. (1981. J. Physiol. 319:463-496). If calcium carries the signal for background desensitization, this difference in space constant for background and bleaching adaptation precludes it as the messenger for the steady component of bleaching adaptation. Experiments with isobutylmethyl xanthine (IBMX) also indicate that Ca2+ as well as c-GMP are unlikely regulators of bleaching desensitization, since elevation of cytosolic levels of both of these internal messengers by IBMX has little effect on sensitivity in bleach-adapted cells. All of our findings are consistent with the notion that bleaching adaptation is not mediated by a freely diffusible cytoplasmic messenger.

1-Methyl-3-isobutylxanthine↗
PubMed2005

Electrophysiological correlates of visual adaptation to faces and body parts in humans.

The existence of facial aftereffects suggests that shape-selective mechanisms at the higher stages of visual object coding -- similarly to the early processing of low-level visual features -- are adaptively recalibrated. Our goal was to uncover the ERP correlates of shape-selective adaptation and to test whether it is also involved in the visual processing of human body parts. We found that prolonged adaptation to female hands -- similarly to adaptation to female faces -- biased the judgements about the subsequently presented hand test stimuli: they were perceived more masculine than in the control conditions. We also showed that these hand aftereffects are size and orientation invariant. However, no aftereffects were found when the adaptor and test stimuli belonged to different categories (i.e. face adaptor and hand test, or vice versa), suggesting that the underlying adaptation mechanisms are category-specific. In accordance with the behavioral results, both adaptation to faces and hands resulted in a strong and category-specific modulation -- reduced amplitude and increased latency -- of the N170 component of ERP responses. Our findings suggest that shape-selective adaptation is a general mechanism of visual object processing and its neural effects are primarily reflected in the N170 component of the ERP responses.

Adaptation, Physiological↗
PubMed2005

Effects of human cerebellar thalamus disruption on adaptive control of reaching.

Lesion or degeneration of the cerebellum can profoundly impair adaptive control of reaching in humans. Computational models have proposed that internal models that help control movements form in the cerebellum and influence planned motor output through the cerebello-thalamo-cortical pathway. However, lesion studies of the cerebellar thalamus have not consistently found impairment in reaching or adaptation of reaching. To elucidate the role of the cerebellar thalamus in humans, we studied a group of essential tremor (ET) patients with deep brain stimulation (DBS) electrodes placed in the cerebellar thalamus. The stimulation can be turned on or off remotely and is thought to reduce tremor by blocking the spread of the pathological output from the cerebellum. We studied the effect of thalamic DBS on the ability to adapt arm movements to novel force fields. Although thalamic DBS resulted in a dramatic and significant reduction of tremor in ET, it also impaired motor adaptation: the larger the stimulation voltage, the greater the reduction in rates of adaptation. We next examined ET patients that had undergone unilateral thalamotomy in the cerebellar thalamus and found that adaptation with the contralateral arm was impaired compared with the ipsilateral arm. Therefore, although both lesion and electrical stimulation of the cerebellar thalamus are highly effective in reducing tremor, they significantly impair the ability of the brain to form internal models of action. Adaptive control of reaching appears to depend on the integrity of the cerebello-thalamo-cortical pathway.

Adaptation, Physiological↗
PubMed2006

Rapid face-selective adaptation of an early extrastriate component in MEG.

Adaptation paradigms are becoming increasingly popular for characterizing visual areas in neuroimaging, but the relation of these results to perception is unclear. Neurophysiological studies have generally reported effects of stimulus repetition starting at 250-300 ms after stimulus onset, well beyond the latencies of components associated with perception (100-200 ms). Here we demonstrate adaptation for earlier evoked components when 2 stimuli (S1 and S2) are presented in close succession. Using magnetoencephalography, we examined the M170, a "face-selective" response at 170 ms after stimulus onset that shows a larger response to faces than to other stimuli. Adaptation of the M170 occurred only when stimuli were presented with relatively short stimulus onset asynchronies (< 800 ms) and was larger for faces preceded by faces than by houses. This face-selective adaptation is not merely low-level habituation to physical stimulus attributes, as photographic, line-drawing, and 2-tone face images produced similar levels of adaptation. Nor does it depend on the amplitude of the S1 response: adaptation remained greater for faces than houses even when the amplitude of the S1 face response was reduced by visual noise. These results indicate that rapid adaptation of early, short-latency responses not only exists but also can be category selective.

Adaptation, Physiological↗
PubMed1995

The perception of saltiness is eliminated by NaCl adaptation: implications for gustatory transduction and coding.

The tastes of salts to humans are complex. NaCl is the most purely salty of all salts, but even this stimulus tastes sweet at low concentrations and somewhat sour at mid-range intensities. Other salts taste significantly sour or bitter in addition to salty. Previous studies have shown that the saltiness of simple halide salts is reduced by adaptation to NaCl, suggesting that a single mechanism might be responsible for the salty taste of these stimuli. In electrophysiological studies in rodents, the response to NaCl is reduced by application to the tongue of the Na(+)-channel blocker amiloride. Organic Na+ salts are more heavily dependent on this amiloride-sensitive transduction component than NaCl, and are generally less salty and more sour. In order to investigate the relationship between NaCl saltiness and that evoked by other salts, we adapted the tongue to distilled H2O and to 0.1 M NaCl and obtained direct magnitude estimates of the taste intensity of 15 organic and inorganic Na+, Li+, K+ and Ca2+ salts, matched for total intensity. Subjects divided these magnitude estimates among the component taste qualities. Adaptation to NaCl abolished the taste of NaCl and LiCl, and eliminated the saltiness of all other salts. The magnitude estimates of the bitterness and sourness of many salts increased after NaCl adaptation. Since recent biophysical data suggest that adaptation in taste receptors may involve whole-cell mechanisms, we propose that saltiness is reduced by NaCl adaptation because it originates in the subset of taste receptors responsive to NaCl. This implies that saltiness is coded within the CNS in cells whose receptive fields include the NaCl-sensitive receptor cells and that the degree to which any salt tastes salty is determined by its ability to drive these receptors. This model proposes, for example, that KCl has a salty component because it stimulates some of the same receptor cells as NaCl, even though the transduction mechanisms for KCl are different than those engaged by NaCl. Adaptation to NaCl blocks the saltiness of KCl and other salts because they stimulate NaCl-sensitive receptor cells.

Adaptation, Psychological↗
PubMed1997

Genome-wide hypermutation in a subpopulation of stationary-phase cells underlies recombination-dependent adaptive mutation.

Stationary-phase mutation in microbes can produce selected ('adaptive') mutants preferentially. In one system, this occurs via a distinct, recombination-dependent mechanism. Two points of controversy have surrounded these adaptive reversions of an Escherichia coli lac mutation. First, are the mutations directed preferentially to the selected gene in a Lamarckian manner? Second, is the adaptive mutation mechanism specific to the F plasmid replicon carrying lac? We report that lac adaptive mutations are associated with hypermutation in unselected genes, in all replicons in the cell. The associated mutations have a similar sequence spectrum to the adaptive reversions. Thus, the adaptive mutagenesis mechanism is not directed to the lac genes, in a Lamarckian manner, nor to the F' replicon carrying lac. Hypermutation was not found in non-revertants exposed to selection. Therefore, the genome-wide hypermutation underlying adaptive mutation occurs in a differentiated subpopulation. The existence of mutable subpopulations in non-growing cells is important in bacterial evolution and could be relevant to the somatic mutations that give rise to cancers in multicellular organisms.

Adaptation, Biological↗
PubMed1995

The effect of adaptive mutagenesis on genetic variation at a linked, neutral locus.

Based on recent studies in single-celled organisms, it has been argued that a fitness benefit associated with a mutation will increase the probability of that mutation occurring. This increase is independent of mutation rates at other loci and is called adaptive mutagenesis. We modeled the effect of adaptive mutagenesis on populations of haploid organisms with adaptive mutation rates ranging from 0 to 1 x 10(-5). Allele frequencies at the selected locus and a neutral linked locus were tracked. We also observed the amount of linkage disequilibrium during the selective sweep and the final heterozygosity after the sweep. The presence of adaptive mutagenesis increases the number of genetic backgrounds carrying the new fitter allele, making the outcomes more representative of the population before the selection. Therefore, more neutral genetic variation is preserved in simulations with adaptive mutagenesis than in those without it due to hitchhiking. Since adaptive mutagenesis is time-dependent, it can generate mutants when other mechanisms of mutation cannot. In addition, adaptive mutagenesis has the potential to confound both phylogeny construction and the detection of natural selection from patterns of nucleotide variation.

Adaptation, Physiological↗
PubMed1999

Increased episomal replication accounts for the high rate of adaptive mutation in recD mutants of Escherichia coli.

Adaptive mutation has been studied extensively in FC40, a strain of Escherichia coli that cannot metabolize lactose (Lac-) because of a frameshift mutation affecting the lacZ gene on its episome. recD mutants of FC40, in which the exonuclease activity of RecBCD (ExoV) is abolished but its helicase activity is retained, have an increased rate of adaptive mutation. The results presented here show that, in several respects, adaptive mutation to Lac+ involves different mechanisms in recD mutant cells than in wild-type cells. About half of the apparent increase in the adaptive mutation rate of recD mutant cells is due to a RecA-dependent increase in episomal copy number and to growth of the Lac- cells on the lactose plates. The remaining increase appears to be due to continued replication of the episome, with the extra copies being degraded or passed to recD+ recipients. In addition, the increase in adaptive mutation rate in recD mutant cells is (i) dependent on activities of the single-stranded exonucleases, RecJ and ExoI, which are not required for (in fact, slightly inhibit) adaptive mutation in wild-type cells, and (ii) enhanced by RecG, which opposes adaptive mutation in wild-type cells.

Adaptation, Physiological↗
PubMed1997

Retinoic acid stimulates early cellular proliferation in the adapting remnant rat small intestine after partial resection.

Following loss of small bowel surface area, the remnant intestine undergoes a remarkable adaptive response. To define more fully the underlying molecular mechanisms, we have identified genes that are specifically induced in the adapting remnant after partial small bowel resection. Several of these, including cellular retinol binding protein II (CRBP II) and apolipoprotein (apo) AI, participate in vitamin A and lipid trafficking. The CRBP II and apo A-I promoters contain response elements for the nuclear retinoid X receptor RXR-alpha. It is well established that vitamin A is essential for normal cell growth, differentiation and maintenance of epithelial tissues and that CRBP II functions to facilitate intestinal vitamin A absorption and metabolism. On the basis of these considerations, changes in CRBP II and apo A-I mRNA levels could reflect a role for retinoids in modulating the intestinal adaptive response. To explore this hypothesis, we used a rat resection model of intestinal adaptation to examine the temporal patterns of CRBP II, apo A-I and RXR-alpha expression postresection. CRBP II and apo A-I mRNA levels were increased in the remnant intestine in distinct temporal patterns, whereas RXR-alpha expression was unchanged. To address directly the effects of vitamin A in adaptation, retinoic acid or vehicle was administered intravenously to rats immediately after 70% small bowel resection. Compared with vehicle, all-trans-retinoic acid significantly stimulated crypt cell proliferation in the adapting remnant intestine by 6 h after surgery. These data suggest that retinoic acid acts to modulate intestinal proliferation in the adapting small intestine after loss of functional small bowel surface area.

Adaptation, Physiological↗
PubMed2000

Induction of cell-proliferation hormesis and cell-survival adaptive response in mouse hematopoietic cells by whole-body low-dose radiation.

Hormesis and a cytogenetic adaptive response induced by low-dose radiation (LDR) have been extensively documented. However, few studies have investigated the induction of an adaptive response by LDR for cell survival in vitro. In the present study, we investigated whether LDR could induce hormesis in hematopoietic cells and the adaptive response of these cells to subsequent high-dose radiation-induced cytotoxic effects. Mice were exposed in whole-body to 0 (as control), 0.05, 0.25, 0.50, 0.75, and 1.00 Gy of X-rays. They were killed 12, 24, 48, and 72 h later to observe the stimulating effect of LDR on total bone marrow cells per femur and bone marrow progenitor, colony-forming unit-granulocyte-macrophage (CFU-GM). Exposure to 0.5 Gy of X-rays resulted in significantly stimulating effects on both parameters with a maximum effect at 48 h, showing a cell-proliferation hormesis. In the next experiment, mice were irradiated by 0.5 Gy X-rays as an adaptive exposure (D1), and 6, 12, 24, 48, and 72 h later, they were exposed to 6 Gy X-rays as a challenging exposure (D2). Forty-eight h after D2, cytotoxic effects were analyzed using peripheral blood cells (red blood cells, white blood cells, and platelets) and bone marrow cells (total bone marrow cells of the femur, and bone marrow progenitors such as CFU-GM and erythroid burst-forming unit, BFU-E). An adaptive response to D2-induced cytotoxic effect, named as the cell-survival adaptive response, was found in both peripheral blood cells and bone marrow cells when D1 and D2 exposures were given at intervals of 24-48 h. These results suggested that LDR could induce both cell-proliferation hormesis and cell-survival adaptive response to subsequent high-dose radiation in bone marrow cells. It may be of potential importance, if this phenomenon is confirmed clinically, since it may be applied to reduce the adverse effect of radiotherapy.

Adaptation, Physiological↗
PubMed1999

Visuomotor adaptation during inactivation of the dentate nucleus.

Recent experiments have suggested that the process of visuomotor adaptation depends on how a visual distortion is introduced. The cerebellum is thought to be involved in adapting to rapidly introduced visual distortions; however its role in adapting to a gradually introduced distortion is unknown. We tested adaptation to a sudden or a gradual introduction of a visual distortion, during reversible inactivation of a monkey's dentate nucleus. There was significant adaptation in both of these tasks without any lignocaine infusion and during saline infusions. However after inactivation the ability to adapt to either visual distortion was slightly impaired. This dysfunction was significant when the visuomotor distortion was introduced over several trials, suggesting that the cerebellum has a differential contribution to visual adaptation depending on the type of visuo-motor disturbance encountered.

Adaptation, Physiological↗
PubMedDate not supplied

Adaptation of the acoustic reflex.

Acoustic reflex adaptation is reviewed in normal and abnormal auditory systems. The measurement variables affecting the acoustic reflex threshold are discussed with reference to the intensity level above reflex threshold at which the adaptation is measured. The effects of the activator frequency and activator intensity level on the time course of normal reflex adaptation are reviewed. The diagnostic application of acoustic reflex adaptation is discussed with reference to the different definitions of abnormality found in the literature. The acoustic reflex patterns, including absence, threshold, and adaptation of the reflex, are reported in patients with different degrees of hearing loss, in order to identify the false-positive rates associated with cochlear hearing losses. Finally, the diagnostic accuracy of acoustic reflexes is discussed in subjects having lesions of the CNVIII, brain stem, CNVII, and neuromuscular systems. In summary, a method is advocated for measuring acoustic reflex adaptation over 10 seconds, which allows analysis at both 5 and 10 seconds. Further research is needed on procedural variables including activator intensity level and ipsilateral recording methods, which may increase the diagnostic accuracy of acoustic reflex adaptation.

Adaptation, Physiological↗
PubMedDate not supplied

Predictors of adaptation in Icelandic and American families of young children with chronic asthma.

The purposes of this international study were to determine the predictors of adaptation and to assess potential moderating effects of parents' sense of coherence and family hardiness on the relationship of severity of illness of a child with asthma and family and caregiving demands as predictors of family adaptation. For both parents, sense of coherence and family hardiness predicted family adaptation. Icelandic mothers perceived their family's adaptation more favorably than did their American counterparts. For the fathers, family demands predicted adaptation. Sense of coherence moderated the effect of family demands on adaptation for both parents. These findings underscore the importance of strengthening individual and family resiliency as a mechanism for improving family adaptation.

Adaptation, Psychological↗