55 research outputs found
On the location of the surface-attached globule phase in collapsing polymers
We investigate the existence and location of the surface phase known as the
"Surface-Attached Globule" (SAG) conjectured previously to exist in lattice
models of three-dimensional polymers when they are attached to a wall that has
a short range potential. The bulk phase, where the attractive intra-polymer
interactions are strong enough to cause a collapse of the polymer into a
liquid-like globule and the wall either has weak attractive or repulsive
interactions, is usually denoted Desorbed-Collapsed or DC. Recently this DC
phase was conjectured to harbour two surface phases separated by a boundary
where the bulk free energy is analytic while the surface free energy is
singular. The surface phase for more attractive values of the wall interaction
is the SAG phase. We discuss more fully the properties of this proposed surface
phase and provide Monte Carlo evidence for self-avoiding walks up to length 256
that this surface phase most likely does exist. Importantly, we discuss
alternatives for the surface phase boundary. In particular, we conclude that
this boundary may lie along the zero wall interaction line and the bulk phase
boundaries rather than any new phase boundary curve.Comment: slightly extended versio
Pulling absorbing and collapsing polymers from a surface
A self-interacting polymer with one end attached to a sticky surface has been
studied by means of a flat-histogram stochastic growth algorithm known as
FlatPERM. We examined the four-dimensional parameter space of the number of
monomers up to 91, self-attraction, surface attraction and force applied to an
end of the polymer. Using this powerful algorithm the \emph{complete} parameter
space of interactions and force has been considered. Recently it has been
conjectured that a hierarchy of states appears at low temperature/poor solvent
conditions where a polymer exists in a finite number of layers close to a
surface. We find re-entrant behaviour from a stretched phase into these
layering phases when an appropriate force is applied to the polymer. We also
find that, contrary to what may be expected, the polymer desorbs from the
surface when a sufficiently strong critical force is applied and does
\emph{not} transcend through either a series of de-layering transitions or
monomer-by-monomer transitions.Comment: 4 pages, 4 figure
Influence of long-range correlated surface and near the surface disorder on the process of adsorption of long-flexible polymer chains
The influence of long-range correlated surface and decaying near surface
disorder with quenched defects is studied. We consider a correlation function
for the defects of the form , where and
being the coordinate in the direction perpendicular to the surface and
denotes the distance parallel to the surface. We investigate the process of
adsorption of long-flexible polymer chains with excluded volume interactions on
a "marginal" and attractive wall in the framework of renormalization group
field theoretical approach up to first order of perturbation theory in a double
(,)- expansion (, ) for the
semi-infinite model with the above mentioned type of
surface and near the surface disorder in the limit . In particular we
study two limiting cases. First, we investigate the scenario where the chain's
extension it much larger then . Second, we consider the case where the
chain's extension is of the order of . For both cases we obtained series
for bulk and the whole set of surface critical exponents, characterizing the
process of adsorption of long-flexible polymer chains at the surface. The
polymer linear dimensions parallel and perpendicular to the surface and the
corresponding partition functions as well as the behavior of monomer density
profiles and the fraction of adsorbed monomers at the surface and in the volume
are studied.Comment: 31 pages, 5 figures, 2 table
A hierarchy of determining factors controls motoneuron innervation
Quail leg buds were grafted in place of chick leg buds or chick wing buds and vice versa at stages 18 to 21 after colonization by muscle precursor cells had been completed. Motor endplate pattern in the plantaris muscle of the grafts was analyzed before hatching by means of esterase and acetylcholinesterase staining techniques. Muscle fibre types were made visual using the myosin ATPase reaction. Investigations are based on the species-specific endplate pattern of the plantaris muscle: multiply innervated fibres in the chick and focally innervated fibres in the quail. Muscle pieces isolated from the adjacent medial gastrocnemius muscle of the grafted legs were histologically examined to judge their species-specific composition. Horseradish peroxidase was injected into the plantaris muscles of both the grafted and the opposite leg as well as in the plantaris muscle of normal quail embryos, in order to be sure that the plantaris muscle of the graft is innervated by appropriate motoneurons. This procedural design offers for the first time a possibility to test experimentally the influences of motoneurons on endplate pattern formation under conditions corresponding to those in normal ontogenesis. It is shown that such appropriate motoneurons of one species which project to the plantaris muscle of the other species dictate the endplate pattern. When the plantaris muscle is innervated by inappropriate motoneurons, the endplate pattern inherent in the muscle primordium itself becomes realized. A sequence of hierarchically acting factors is proposed to bring different results in line. According to this, the neuronally set programme has priority compared with that set in the muscle. This is true for the normal development and might generate the high neuro-muscular specificity. If under experimental conditions the neuronal programme and the peripheral programme differ, the axons and muscle fibres selectively interact with respect to their inherent characteristics and the muscle-specific programme becomes expressed. If there is a lack of a certain axon type, muscle fibres might become innervated by non-corresponding motoneurons which alter the muscle fibre type.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47522/1/429_2004_Article_BF00309770.pd
Elimination of polyneuronal innervation in a fast muscle of normal and dystrophic mice.
The changes in the pattern of innervation of extensor digitorum longus (e.d.l.) during post-natal development was studied in normal and dystrophic mice. As in other mammals, individual muscle fibres of new-born mice are supplied by more than one axon. Up to 10 days after birth there was no difference in the extent of this polyneuronal innervation between normal and dystrophic muscle fibres. During post-natal development the polyneuronal innervation gradually disappeared. In normal e.d.l. muscles the rate of the elimination of polyneuronal innervation was faster during the first 10 post-natal days and then slowed down. By 16 days the final value of less than 10% of muscle fibres receiving more than one input was reached. In the dystrophic muscles the rate of elimination was similar to normal up to 10 days of age, but continued to decrease rapidly so that already by 11 days of age polyneuronal innervation was reduced to its final level of less than 10%. Thus the elimination of polyneuronal innervation was completed at least 3 days earlier in the dystrophic animals. It is suggested that the increased nerve activity said to be present in dystrophic mice could account for this finding
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