Dynamics of cellular homeostasis: recovery time for a perturbation from equilibrium

In the collecting duct in vivo, the principal cell encounters a wide range in luminal flow rate and luminal concentration of NaCl. As a consequence, there are substantial variations in the transcellular fluxes of Na+ and Cl-, conditions which would be expected to perturb cell volume and cytosolic concentrations. Several control mechanisms have been identified which can potentially blunt these perturbations, and these entail cellular regulation of the luminal membrane Na+ channel and peritubular membrane K+ and Cl- channels. To illustrate the impact of these regulated channels, a mathematical model of the principal cell of the rat cortical collecting duct has been developed, in which ion channel permeabilities are either constant or regulated. In comparison to the model with fixed permeabilities, the model with regulated channels demonstrates enhanced cellular homeostasis following steady-state variation in luminal NaCl. However, in the transient response to a cytosolic perturbation, the difference in recovery time between the models is small. An approximate analysis is presented which casts these models as dynamical systems with constant coefficients. Despite the presence of regulated ion channels, concordance of each model with its linear approximation is verified for experimentally meaningful perturbations from the reference condition. Solution of a Lyapunov equation for each linear system yields a matrix whose application to a perturbation permits explicit estimation of the time to recovery. Comparison of these solution matrices for regulated and non-regulated cells confirms the similarity of the dynamic response of the two models. These calculations suggest that enhanced homeostasis by regulated channels may be protective, without necessarily hastening recovery from cellular perturbations.     

Changes in heart beat timing: reactivity, resetting, or perturbation?

A fifth annual survey was carried out by mail in February 1997 to investigate the current trends in cataract and refractive surgery in Japan. A questionnaire was sent to 867 ophthalmologist members of the Japanese Society of Cataract and Refractive Surgery. Data received from 440 (50.7%) of the recipients were cross-analyzed and compared with those from the previous surveys. For cataract surgery, there have been trends toward more surgical procedures performed by a surgeon, shorter period of hospitalization, and increased number of outpatient surgery. Ninety percent of respondents preferred phacoemulsification, 42% employed the self-sealing wound closure technique, and 26% used topical anesthesia for phacoemulsification. As for refractive surgery, surgeons remained rather conservative, with 26% and 6% of surgeons doing astigmatic keratotomy and refractive keratotomy, respectively.     

Rate and timing precision of motor coordination in developmental dyslexia.

Temporal characteristics of interlimb coordination were examined in adolescents and young adults with developmental dyslexia, matched normal control subjects, and matched learning disabled adolescent students without reading difficulties. Subjects were asked to tap in time to an entraining metronome at each of 3 prescribed rates by moving the index fingers of both hands in unison, in rhythmical alternation, or in more complex bimanual patterns. Dyslexic subjects showed significant deficits of timing precision on bimanual tasks that required the integration of asynchronous responses, but not when they moved the fingers in unison. Findings are discussed in terms of both the hypothesis that impaired temporal resolution in dyslexia reflects an underlying deficit of left-hemisphere function and an alternative hypothesis that functional deficits in developmental dyslexia are associated with impaired interhemispheric communication. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The prevalence and timing of obstetric ultrasound in Victoria 1991-1992: a population-based study

Viscosupplementation is a relatively new treatment concept for restoring the normal rheologic properties of the osteoarthritic joint. It is most often used for the knee. This case report describes results that were achieved using viscosupplementation following conservative therapy for a patient with osteoarthritis of the temporomandibular joint. Further study in the use of these treatments is recommended.     

Obstructive defecation: a failure of rectoanal coordination

OBJECTIVE: The pathophysiology of obstructive defecation is unclear. We investigated whether impaired rectoanal coordination causes obstructive defecation and if this dysfunction can be corrected by biofeedback therapy. METHODS: We prospectively studied 25 healthy subjects and 35 consecutive patients with constipation (>1 year) with anorectal manometry and balloon expulsion test. Symptoms were assessed from diary cards. Patients found to have obstructive defecation were offered biofeedback therapy. After treatment, their defecation dynamics and symptoms were reassessed. RESULTS: Eighteen patients had obstructive defecation and 17 had normal defecation dynamics (nonobstructive). Five normals (20%) exhibited obstructive pattern but only one failed to expel balloon. In the obstructive group, during straining, the intrarectal pressure and defecation index were lower (p < 0.05), and anal residual pressure was higher (p < 0.01) when compared with the nonobstructive group or normals. After biofeedback therapy, the intrarectal pressure and defecation index increased (p < 0.02) and anal residual pressure decreased (p < 0.001); stool frequency, degree of straining, and bowel satisfaction scores improved (p < 0.05); 67% stopped laxatives and 11 patients discontinued stooling with digitation. CONCLUSION: Patients with obstructive defecation showed impaired rectal contraction, paradoxical anal contraction, or inadequate anal relaxation. These features suggest that rectoanal coordination was impaired. Biofeedback therapy rectified these pathophysiological disturbances and improved constipation.     

The timing of congenital brachial plexus injury: a study of electromyography findings in the newborn piglet

OBJECTIVE: Permanent congenital brachial plexus palsy is a recognized serious complication associated with shoulder dystocia. The timing and etiology of this injury remains controversial. Previous authorities have used adult-derived, non-brachial plexus data to extrapolate the anticipated timing for electromyographic denervation changes to date such injuries in the newborn. With use of a domestic swine model, this investigation tests the hypothesis that electromyographic evidence of brachial plexus denervation in the newborn is temporally different than that in the adult. STUDY DESIGN: Five healthy 2-day-old and two adult pigs underwent unilateral sharp transection of the brachial plexus. Daily electromyographic studies were performed in brachial plexus innervated muscle groups on the involved and contralateral (control) front limbs. Postmortem measurements of the transected nerve segments were obtained in one piglet and one adult animal. Representative hard copy recordings of individual electromyographic studies were collected. RESULTS: Immediately after surgical transection of the brachial plexus, no electromyographic evidence of denervation was observed. Uniformly in the newborn piglets, at 24 hours after transection, denervation in the form of fibrillation potentials, positive sharp waves, and complex repetitive discharges was seen. Serial testing demonstrated proximal to distal gradients of denervation over the next 24 to 48 hours. A delay in electromyographic evidence of denervation was observed in the two adult pigs until days 5 and 8, respectively. Control limb studies remained normal throughout the study period. Nerve length measurements for individual muscle groups were as follows for the adult and newborn pigs, respectively: deltoid 11.4 cm, 2.5 cm; cleidobrachialis 16.0 cm, 4.0 cm; triceps 15.5 cm, 4.5 cm; forelimb flexors 26.0 cm, 6.5 cm; and extensor carpi radialis 31.0 cm, 9.0 cm. CONCLUSION: Electromyographic evidence of brachial plexus denervation after surgical transection differs between the newborn and the adult pig. Consistent with wallerian degeneration, a correlation exists between length of the distal nerve segment and timing for electromyographic signs of denervation. These findings suggest it would be inappropriate to extrapolate the anticipated timing for electromyographic changes in the newborn on the basis of previously established adult non-brachial plexus data.     

Dynamics of tremor-related oscillations in the human globus pallidus: a single case study

Physiological evidence indicates that the resting tremor of Parkinson's disease originates in oscillatory neural activity in the forebrain, but it is unknown whether that activity is globally synchronized or consists of parallel, independently oscillating circuits. In the present study, we used dual microelectrodes to record tremor-related neuronal activity from eight sites in the internal segment of the globus pallidus (GPi) from an awake Parkinson's disease patient undergoing stereotaxic pallidotomy. We utilized spectral analysis to evaluate the temporal correlations between multiunit activity at spatially separated sites and between neural and limb electromyographic activity. We observed that some GPi neural pairs oscillated synchronously at the tremor frequency, whereas other neural pairs oscillated independently. Additionally, we found that GPi tremor-related activity at a given site could fluctuate between states of synchronization and independence with respect to upper limb tremor. Consistent with this finding, some paired recording sites within GPi showed periods of transient synchronization. These observations support the hypothesis of independent tremor-generating circuits whose coupling can fluctuate over time.     

Towards a neural model of timing

This paper describes a neural model of interval timing, which reproduces the duration discrimination experiments of Wearden, J.H., 1992, J. Exp. Psychol. 18, 134-144. The model comprises three layers of neural units. The units in the first layer represent clusters of neurons with probabilistic internal feedback that maintains self-sustained (short-term memory) activity for a random time. The unit in the second layer is a spiking neuron that fires as long as a sufficient number of input clusters are active. The unit in the third layer detects the offset of firing in the previous layer by producing a short burst of spikes. Analysis and simulation of the model shows spikes produced at random times with a distribution determined by the number of units in the first layer, their survival time constant, and the threshold of the unit in layer 2. Interval times can be learned with any of these parameters but lead to different Weber law relations. A variable threshold in layer 2 predicts S-shaped Weber curves, a variable number of units in layer 1 leads to a saturation of the Weber curve (decreasing Weber fraction) and a variable time constant in layer 1 causes a linear Weber curve.     

Dynamics of myocardial contrast enhancement: an in vivo computed-tomographic study

Enhancement of normal functioning myocardium was quantitated in 15 dogs by serial computed transmission tomographic (CT) images during the bolus (10 ml/sec.) or slow (1 ml/sec.) intravenous injection of diatrizoate contrast media (1 ml/kg body weight) in concentrations of 37, 18.5, or 9.25 g iodine (I)/dl. Homogenous images of myocardial enhancement were obtained. However, major streak artifacts were observed frequently when contrast material was injected as a bolus, and myocardial edges were not defined clearly when contrast material with a concentration of 9.25 g I/dl was injected slowly. Time-attenuation curves of normal myocardial enhancement constructed from serial CT images demonstrated a peak in contrast enhancement (delta Hounsfield units, 22-45) followed by a period of deterioration that lasted two to three minutes. These results can be applied to make optimal use of both single (static) and serial (dynamic) myocardial CT images.     

Correlations for timing consistency among tapping and drawing tasks: Evidence against a single timing process for motor control.

Three experiments were conducted to examine whether timing processes can be shared by continuous tapping and drawing tasks. In all 3 experiments, temporal precision in tapping was not related to temporal precision in continuous drawing. There were modest correlations among the tapping tasks, and there were significant correlations among the drawing tasks. In Experiment 3, the function relating timing variance to the square of the observed movement duration for tapping was different from that for drawing. The conclusions drawn were that timing is not an ability to be shared by a variety of tasks but instead that the temporal qualities of skilled movement are the result of the specific processes necessary to produce a trajectory. These results are consistent with the idea that timing is an emergent property of movement. (PsycINFO Database Record (c) 2010 APA, all rights reserved)