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L Barr

Publications and source records attributed to L Barr.

72 records · Page 4Linked to original sources
PubMed1977

The resistance of the septum of the median giant axon of the earthworm.

It is generally thought that nexuses constitute low-resistance pathways between cell interiors in epithelial, neural, muscular, and even connective tissues. However, there are no reliable estimates of the specific resistance of a nexus. The reason for this is that in most cases the surfaces of nexuses between cells are geometrically complex and therefore it has been very hard to accurately estimate nexal areas. However, the septa of the median giant axon have a relatively simple shape. Moreover, in this preparation, it is possible to make a measuring current flow parallel to the axon axis so that from the voltage difference appearing between intracellular electrodes during current flow, the specific septal membrane resistance could be calculated. The average specific nexal resistance obtained was 5.9 omega cm(2) if one assumes that 100 percent of the septum is nexus. The steady state I-V curve for the septum is linear (+/- 10 mV). Placement of electrodes was validated by septa even though the septa were found to be permeable to fluorescein and TEA. Exposure of the axon to hypertonic saline impedes the movement of fluorescein across the septa. By analogy with other tissues it is concluded that hypertonic solutions disrupt nexuses.A mathematical model was derived which predicts the steady- state transmembrane potential vs. distance from a point source of intracellular current. When the specific nexal membrane resistance is 5.9 omega cm(2), the prediction closely approximates the fall of transmembrane potential vs. distance in an ordinary infinite cable. This is commensurate with the electrophysiological behavior of this multicellular "axon."

Animals↗
PubMed1975

Electrical characteristics of frog atrial trabeculae in the double sucrose gap.

The electrical behavior of small single frog atrial trabeculae in the double sucrose gap has been investigated. The currents injected during voltage clamp experiments did not behave as predicted from the assumption of spatial uniformity of the voltage across a Hodgkin-Huxley membrane. Much of the difference is due to the geometrical complexities of this tissue. Nonetheless, two transient inward currents have been identified, the faster of which is blocked by tetrodotoxin (TTX). The magnitude of the slower transient varies markedly between preparations but always increases in a given preparation with increase of external calcium. The fast transient current traces, at small to intermediate depolarizations, are often marred by the presence of notches and secondary peaks due most probably to the loss of space clamp conditions. In many preparations these could be removed by reducing the current magnitude through application of a partially-blocking dose of TTX. Conversely, in the preparations whose fast transient was fully blocked by TTX, notches and secondary peaks in the slow transient could by induced through increasing calcium concentration and thereby the slow current magnitude. Previously used techniques for the measurement of the reversal potential of the fast inward transient have been shown to be invalid. In so far as they can be measured, the reversal potentials of the fast and slow inward transient are in the same neighborhood, i.e. around 120 mV from rest. The true values may be quite a bit apart. The total charge flow in the capacitive transient was measured for different sized nodes and preparations. From these data and estimates of plasma membrane area per unit trabecular volume, specific membrane capacitances of around 3 muF/cm2 were calculated for small bundles. The apparent ion current densities on this basis are approximately 1/10 of those measured in axons. The capacitive current occurring in small bundles decayed as the sum of at least three exponential functions of time. On the basis of these data and the anomalously large stable node widths, we suggest a coaxial core model of the preparation with the inner elements in series with an additional large extracellular resistance.

Animals↗
PubMed1975

An equivalent circuit for small atrial trabeculae of frog.

An equivalent electrical circuit has been constructed for small atrial trabecula of frog in a double sucrose gap voltage clamp apparatus. The basic strategy in constructing the circuit was to derive the distribution of membrane capacitance and extracellular resistance from the preparation's response to small voltage displacements near the resting condition, when the membrane conductance is presumably quite low. Then standard Hodgkin-Huxley channels were placed in parallel with the capacitance and the results of voltage clamp experiments were simulated. The results suggest that the membranes of the preparation cannot in fact be clamped near the control voltage nor can the ionic currents be measured directly with reasonable accuracy by axon standards. It may or may not be a realizable goal in the future to define the preparation's electrical behavior well enough to permit the ultimate quantitative description of the membrane's specific ion conductances. The result of this paper suggest that if this goal is achieved using the double sucrose gap voltage clamp, it will be by a detailed quantitative accounting for substantial irreducible errors in voltage control, rather than by experimental achievement of good voltage control.

Animals↗
PubMed1968

Electrical transmission at the nexus between smooth muscle cells.

The hypothesis that nexuses between cells are responsible for the core conductor properties of tissues was tested using smooth muscle preparations from the taenia coli of guinea pigs. Action potentials recorded from small diameter preparations across a sucrose gap change from monophasic to diphasic when a shunt resistor is connected across the gap. This indicates that transmission between smooth muscle cells is electrical, because the resistor only allows current to flow. Nexal fusion of cell membranes occurs especially where one cell sends a large bulbous projection into a neighbor. Hypertonic solutions rupture the nexuses between smooth muscle cells. Hypertonicity also increases the resistance of a bundle across the sucrose gap and blocks propagation of action potentials. Thus the structural and functional changes in smooth muscle due to hypertonicity correlate with the hypothesis.

Action Potentials↗
PubMedDate not supplied

Ready or not: assessing youths' preparedness for independent living.

This article discusses the utility of the Ansell-Casey Life Skills Assessment (ACLSA) in assessing life skills necessary for living successfully in the community upon emancipation from out-of-home care. ACLSA, completed by youths and their caregivers, identifies skills that have been mastered and those yet to be learned. Assessment information can be used for goal setting, strength identification, and relationship building, as well as to direct program planning and training in self-sufficiency services.

Activities of Daily Living↗
PubMedDate not supplied

Feasibility of screening young children in day care centers--a preliminary investigation.

A preliminary investigation of a method of providing health screening in day care centers was conducted. Ninety-four children, birth to 6 years, attending two day care centers were screened for health and developmental problems. A nurse trained day care staff to conduct the screenings, supervised their activities, rescreened children with questionable results, and conducted an interrater reliability study as well as referral and followup activities. The nurse also did assessments of environmental characteristics of the programs, their policies, procedures, and activities and assessed staff and parent information needs concerning child development, health practices, and health needs of children. Thirty-nine problems were identified in 33 children. Followups of 29 problems were completed, and 16 of these were verified. The 29 problems resulted in a total of 35 visits to primary health care providers. Among parents of children with verified problems, only three had been aware of the problem. The overreferral rate was 47 percent. Interrater percentages of agreement on most screenings were more than 80 percent. The findings suggested that the screenings were feasible with specific modifications. The screening activities were acceptable to parents, their physicians, and center staff. Centers were responsive to staff and parent needs identified in the screening process but not to recommendations for change within the environment and in operating procedures, partly because of fiscal implications. Screenings were adequate to identify a number of health problems prevalent in children under 6, and interrater reliabilities were acceptable.

Child↗