Contributions of the input signal and prior activation history to the discharge behaviour of rat motoneurones.

The principal computational operation of neurones is the transformation of synaptic inputs into spike train outputs. The probability of spike occurrence in neurones is determined by the time course and magnitude of the total current reaching the spike initiation zone. The features of this current that are most effective in evoking spikes can be determined by injecting a Gaussian current waveform into a neurone and using spike-triggered reverse correlation to calculate the average current trajectory (ACT) preceding spikes. The time course of this ACT (and the related first-order Wiener kernel) provides a general description of a neurone's response to dynamic stimuli. In many different neurones, the ACT is characterized by a shallow hyperpolarizing trough followed by a more rapid depolarizing peak immediately preceding the spike. The hyperpolarizing phase is thought to reflect an enhancement of excitability by partial removal of sodium inactivation. Alternatively, this feature could simply reflect the fact that interspike intervals that are longer than average can only occur when the current is lower than average toward the end of the interspike interval. Thus, the ACT calculated for the entire spike train displays an attenuated version of the hyperpolarizing trough associated with the long interspike intervals. This alternative explanation for the characteristic shape of the ACT implies that it depends upon the time since the previous spike, i.e. the ACT reflects both previous stimulus history and previous discharge history. The present study presents results based on recordings of noise-driven discharge in rat hypoglossal motoneurones that support this alternative explanation. First, we show that the hyperpolarizing trough is larger in ACTs calculated from spikes preceded by long interspike intervals, and minimal or absent in those based on short interspike intervals. Second, we show that the trough is present for ACTs calculated from the discharge of a threshold-crossing neurone model with a postspike afterhyperpolarization (AHP), but absent from those calculated from the discharge of a model without an AHP. We show that it is possible to represent noise-driven discharge using a two-component linear model that predicts discharge probability based on the sum of a feedback kernel and a stimulus kernel. The feedback kernel reflects the influence of prior discharge mediated by the AHP, and it increases in amplitude when AHP amplitude is increased by pharmacological manipulations. Finally, we show that the predictions of this model are virtually identical to those based on the first-order Wiener kernel. This suggests that the Wiener kernels derived from standard white-noise analysis of noise-driven discharge in neurones actually reflect the effects of both stimulus and discharge history.

SAAM II: Simulation, Analysis, and Modeling Software for tracer and pharmacokinetic studies.

Kinetic analysis and integrated systems modeling have contributed substantially to our understanding of the physiology and pathophysiology of metabolic systems and the distribution and clearance of drugs in humans and animals. In recent years, many researchers have become aware of the usefulness of these techniques in the experimental design. With this has come the recognition that the discipline of kinetic analysis requires its own expertise. The expertise can impact experimental design in many ways, from the collaborative and service activities in which individuals interact in formal ways to the development of software tools to aid in kinetic analysis. The purpose of this report is to describe one such software tool, Simulation, Analysis, and Modeling Software II (SAAM II). In the first part, we describe in general how the user can take advantage of the capabilities of the software system, and in the second part, we give three specific examples using multicompartmental models found in lipoprotein (apolipoprotein B [apoB] kinetics) and diabetes (glucose minimal model) research.

Survival following blunt chest impact-induced cardiac arrest during sports activities in young athletes.

Blunt chest impact-induced cardiac arrest on the athletic field (commotio cordis) is not necessarily fatal. The 3 survivors reported here emphasize the importance of recognizing this syndrome so that emergency resuscitative measures are more likely to be implemented promptly, and such catastrophes avoided.

Impact of an ambulatory care clerkship on the attitudes of students from five classes (1985-1989) toward primary care.

This study was designed to evaluate the effects of a required ambulatory care clerkship during the fourth year of medical school on the students' knowledge of primary care medicine and their subsequent career choices. A questionnaire was mailed to the graduates of the Albert Einstein College of Medicine classes of 1985-1989. Fifty-six percent (408) of the questionnaires were returned. An average of 74% of the respondents answered yes to six items that asked whether the clerkship had successfully introduced them to basic concepts and practices of outpatient medicine (range from 58% to 93%, depending on the item). In addition, 90% reported that the clerkship had enhanced their understanding of primary care medicine, while 41% indicated that the rotation had wholly or partially influenced their career choices. The survey findings suggest that exposure to outpatient medicine during medical school plays an important role in facilitating students' knowledge of primary care medicine and influencing their career choices.

Fossilized viscera in primitive echinoderms.

Fossilized visceral organs in ancient echinoderms have anatomical features that were not inferred from the anatomy of extant relatives. The unique visceral anatomy of extinct echinoderms demonstrates the need to integrate paleontological and neontological analyses in order to fully evaluate this and other morphologically complex phyla. Comparative analysis of the visceral anatomy of extinct and extant echinoderms necessitates regrouping these animals into three new subphyla. Similar analyses for other metazoan groups could yield equally significant results.