Search PubMed⌕ Search

Biomedical subjects

A Silberberg

Publications and source records attributed to A Silberberg.

At least 55 records · Page 3Linked to original sources
PubMed1984

Kinetorheological aspects of biorheology.

The relationship between stress and strain is the rheological equation of state. In the case of sophisticated systems such as biological tissue, this is rarely a simple relationship. The relationship is seen to be even more complex when it is recalled that in most living tissues, the tissue is not in chemical equilibrium, but is at best in some controlled steady state. At worst, it is undergoing major fluctuations or transitions because the chemical reactions or fluxes are altering the system. It is shown, in particular, that in addition to the changes in composition, the effective rheological relaxation times of the system are shortened due to contributions deriving from the reaction rate constants. These and other points are illustrated by considering a process of irreversible monomolecular degradation of a large macromolecular species.

Aging↗
PubMed1983

Macromolecular surface films at a solid/liquid interface.

Macromolecules generally adsorb well to a solid/liquid interface but do not produce more than a monolayer of molecules in cases where the molecules are reasonably soluble in the liquid medium. This is because for second or higher layer adsorption, the interaction would have to be between segments of macromolecules in solution and segments of macromolecules on the surfaces. Such interactions between segments of soluble macromolecules in solution do not, however, lead to adhesions. Some of the segments of surface held macromolecules would have to have been altered in their reactivity in order that second and, progressively, multilayer formation could occur. This is not too hard to accept in the case where the adsorbing macromolecule is a protein. A decline of reactivity with layer number could be a factor in limiting the growth of the surface multilayer or gel. The presence of a weakly elastic gel at the boundary with a flowing suspension will strongly influence the flow pattern in its vicinity. Effectively flow is slowed down and could essentially be stopped even without having to seal off the vessel totally. It is thus possible to conceive of mechanisms by which an endoendothelial fibrin layer is not only built up but would also function as proposed by Copley.

Adsorption↗
PubMed1983

Biorheological matching: mucociliary interaction and epithelial clearance.

In mucociliary clearance mucus has been found to be essential as coupler between ciliary beat and transport. The rheological properties of mucus are carefully matched to this task. It has to be an incipient gel and deviations from this state, to a sol or to a stiff gel, both render it defective in function. The known rheological properties of normal mucus are shown to be consistent with our understanding of how a periodically interacting set of cilia produce mucus layer flow. A dual requirement has to be met: Adequate tip penetration and adequate force transfer laterally. As a result any biorheologically properly matched system can replace mucus. Mucociliary clearance is a clear cut, rather well understood case of a general principle of biorheological matching.

Biological Transport↗
PubMed1983

Separation flow in matrices - redefinition of friction coefficients.

Separation flow through matrices or membranes is generally characterized by three phenomenological coefficients. Of these the hydraulic permeability Lp and the reflection coefficient sigma are the most commonly discussed. Their theoretical interpretation is much aided by the use of friction coefficients flm defined to characterize the relative interaction in flow of the thermodynamic component l in its movement past m. It is shown that this definition can be further simplified and that as a result simplified expressions notable for Lp result.

Biological Transport↗
PubMed1982

Structure and function of mucus.

Discussing the available evidence a fairly strong case can be made for the existence of a basic glycoprotein unit, characterized by what may be a common protein backbone (Fig. 1). This is far less likely for the carbohydrate portion. The considerably more variability in the amount and composition of the carbohydrate coat and species and organ differences may arise because of this fact. Very large aggregates are built up from the basic unit using cross-links of disulfide bonds either intermolecularly, i.e. directly, or intramolecularly, i.e. indirectly via a possible lectin-like structure which forms its bond with some of the carbohydrate side chains. Structures of the order of 10-100 million molecular weight are to be expected which, being heavily entangled, give rise to the special rheological character of the mucus. In most instances mucus behaves rheologically like a gel. The concentration of glycoprotein in the mucus may be the most important parameter which determines the special rheological features required in a special functional context. A unified point of view, when discussing mucus structure and function, was taken. On the evidence available, it seems well justified to continue to do so.

Chemical Phenomena↗
PubMed1982

The effects of concurrent responding and reinforcement on behavioral output.

Four birds key pecked on concurrent variable-interval one-minute variable-interval four-minute schedules with a two-second changeover delay. Response rates to the variable-interval one-minute key were then reduced by signaling its reinforcer availability and later by providing its reinforcers independently of responding. Each manipulation increased response rates to the variable-interval four-minute key even though relative reinforcement rates were unchanged. In a final phase, eliminating the variable-interval one-minute key and its schedule produced the highest rates of all to the variable-interval four-minute key. These results show that both reinforcement and response rates to one schedule influence response rates to another schedule. These results join those of Guilkey, Shull, & Brownstein (1975) in failing to replicate Catania (1963). Moreover, they violate the predictions of the equation for simple action (de Villiers & Herrnstein, 1976). In terms of a median-rate measure (reciprocal of the median interresponse time), rates to the variable-interval four-minute key were high when responding was not reduced to the variable-interval one-minute key and were low when it was reduced. This rate difference suggests a process difference between concurrent-schedule procedures that maintain high concurrent response rates versus those that do not. This process difference jeopardizes attempts to integrate single- and concurrent-operant performances within a single formulation.

Journal Article↗
PubMed1982

The mechanics and thermodynamics of separation flow through porous, molecularly disperse, solid media - the Poiseuille Lecture 1981.

Steady flow of a multicomponent, incompressible fluid through a connected, solid-like network is considered. The network acquires a finite deformation, bears the extra stresses required for mechanical balance, but does not flow. It constitutes at least one thermodynamic component and, in terms of the thermodynamics of irreversible processes, the simplest case of the flow of a binary solution through the network creates a three-component system with three independent cross-coefficients to be determined. The number of coefficients to be determined in the case of more than three components tends to become prohibitive. Hence, the formalism, developed for three components, is often applied, justifiably and unjustifiably, to practical problems in biology, where the system is much more complex. The conditions under which this is permissible are given and discussed. For such a case, the questions of volume and separation flows is also considered. Relationships are given in terms of the friction coefficients between the components. The important biorheological and thermodynamic role of the matrix is stressed throughout.

Animals↗
PubMed1978

Structure and function of mucus.

A glycoprotein building block is common to mammalian mucins. This structure is composed of several protein chains having the same sequence. The carbohydrate side chains, which constitute over three-quarters of the weight, coat only some two-thirds of the backbone chain. The bare protein chains are linked by disulphide bridges and can be digested away with trypsin. Either procedure rapidly solubilizes mucus and results in a structural unit of about 500 000 molecular weight. Mucus solubilizes spontaneously. The first size unit which reaches solution is about 15 X 10(6) molecular weight but continues to break down further. Mechanical agitation considerably speeds up this process. The gel-like character which is an essential feature of mucus--which cannot otherwise act as transport coupler--is thus a transient phenomenon. The problem of how such a structure can arise from the building blocks known to be available is discussed.

Amino Acid Sequence↗
PubMed1978

Contrast and autoshaping in multiple schedules varying reinforcer rate and duration.

Thirteen master pigeons were exposed to multiple schedules in which reinforcement frequency (Experiment I) or duration (Experiment II) was varied. In Phases 1 and 3 of Experiment I, the values of the first and second components' random-interval schedules were 33 and 99 seconds, respectively. In Phase 2, these values were 99 seconds for both components. In Experiment II, a random-interval 33-second schedule was associated with each component. During Phases 1 and 3, the first and second components had hopper durations of 7.5 and 2.5 seconds respectively. During Phase 2, both components' hopper durations were 2.5 seconds. In each experiment, positive contrast obtained for about half the master subjects. The rest showed a rate increase in both components (positive induction). Each master subject's key colors and reinforcers were synchronously presented on a response-independent basis to a yoked control. Richer component key-pecking occurred during each experiment's Phases 1 and 3 among half these subjects. However, none responded during the contrast condition (unchanged component of each experiment's Phase 2). From this it is inferred that autoshaping did not contribute to the contrast and induction findings among master birds. Little evidence of local contrast (highest rate at beginning of richer component) was found in any subject. These data show that (a) contrast can occur independently from autoshaping, (b) contrast assays during equal-valued components may produce induction, (c) local contrast in multiple schedules often does not occur, and (d) differential hopper durations can produce autoshaping and contrast.

Journal Article↗
PubMed1977

Structural investigation of loose connective tissue by using a series of dextran fractions as non-interacting macromolecular probes.

The ability of the uncharged open-coil dextran molecules to penetrate tissue space, without coil-shape change, was utilized to probe (by partitioning experiments) the structural arrangement of the collagen-fibre network and the proteoglycan system. Hyaluronidase digests most of the proteoglycans away and enables the respective contributions to the exclusion volume to be evaluated by using a series of different-molecular-weight dextrans. It appears that the major part of the exclusion volume is due to the collagen-fibril as a rod and the dextran coil as an impenetrable sphere. The additional exclusion due to the proteoglycans could be accounted for by a set of points (regions of high proteoglycan-segment density) over which the dextran coild cannot pass. These points are an average of 50 nm apart and are indicative of local extensive entanglement of high-molecular-weight proteoglycans with each other. Reasons are given why these entanglements could not act as cross-links in long-term elastic loading of the tissue.

Connective Tissue↗