1,601 research outputs found
Similarities between action potentials and acoustic pulses in a van der Waals fluid
An action potential is typically described as a purely electrical change that
propagates along the membrane of excitable cells. However, recent experiments
have demonstrated that non-linear acoustic pulses that propagate along lipid
interfaces and traverse the melting transition, share many similar properties
with action potentials. Despite the striking experimental similarities, a
comprehensive theoretical study of acoustic pulses in lipid systems is still
lacking. Here we demonstrate that an idealized description of an interface near
phase transition captures many properties of acoustic pulses in lipid
monolayers, as well as action potentials in living cells. The possibility that
action potentials may better be described as acoustic pulses in soft interfaces
near phase transition is illustrated by the following similar properties:
correspondence of time and velocity scales, qualitative pulse shape, sigmoidal
response to stimulation amplitude (an `all-or-none' behavior), appearance in
multiple observables (particularly, an adiabatic change of temperature),
excitation by many types of stimulations, as well as annihilation upon
collision. An implication of this work is that crucial functional information
of the cell may be overlooked by focusing only on electrical measurements.Comment: 8 pages, 5 figure
Stripes of Partially Fluorinated Alkyl Chains: Dipolar Langmuir Monolayers
Stripe-like domains of Langmuir monolayers formed by surfactants with
partially fluorinated lipid anchors (F-alkyl lipids) are observed at the
gas-liquid phase coexistence. The average periodicity of the stripes, measured
by fluorescence microscopy, is in the micrometer range, varying between 2 and 8
microns. The observed stripe-like patterns are stabilized due to dipole-dipole
interactions between terminal -CF3 groups. These interactions are particularly
strong as compared with non-fluorinated lipids due to the low dielectric
constant of the surrounding media (air). These long-range dipolar interactions
tend to elongate the domains, in contrast to the line tension that tends to
minimize the length of the domain boundary. This behavior should be compared
with that of the lipid monolayer having alkyl chains, and which form spherical
micro-domains (bubbles) at the gas-liquid coexistence. The measured stripe
periodicity agrees quantitatively with a theoretical model. Moreover, the
reduction in line tension by adding traces (0.1 mol fraction) of cholesterol
results, as expected, in a decrease in the domain periodicity.Comment: 20 pages, 4 fig
On measuring the acoustic state changes in lipid membranes using fluorescent probes
Ultrasound is increasingly being used to modulate the properties of
biological membranes for applications in drug delivery and neuromodulation.
While various studies have investigated the mechanical aspect of the
interaction such as acoustic absorption and membrane deformation, it is not
clear how these effects transduce into biological functions, for example,
changes in the permeability or the enzymatic activity of the membrane. A
critical aspect of the activity of an enzyme is the thermal fluctuations of its
solvation or hydration shell. Thermal fluctuations are also known to be
directly related to membrane permeability. Here solvation shell changes of
lipid membranes subject to an acoustic impulse were investigated using a
fluorescence probe, Laurdan. Laurdan was embedded in multi-lamellar lipid
vesicles in water, which were exposed to broadband pressure impulses of the
order of 1MPa peak amplitude and 10{\mu}s pulse duration. An instrument was
developed to monitor changes in the emission spectrum of the dye at two
wavelengths with sub-microsecond temporal resolution. The experiments show that
changes in the emission spectrum, and hence the fluctuations of the solvation
shell, are related to the changes in the thermodynamic state of the membrane
and correlated with the compression and rarefaction of the incident sound wave.
The results suggest that acoustic fields affect the state of a lipid membrane
and therefore can potentially modulate the kinetics of channels and proteins
embedded in the membrane
On cell surface deformation during an action potential
The excitation of many cells and tissues is associated with cell mechanical
changes. The evidence presented herein corroborates that single cells deform
during an action potential (AP). It is demonstrated that excitation of plant
cells (Chara braunii internodes) is accompanied by out-of-plane displacements
of the cell surface in the micrometer range (1-10 micron). The onset of
cellular deformation coincides with the depolarization phase of the AP. The
mechanical pulse (i) propagates with the same velocity as the electrical pulse
(within experimental accuracy; 10 mm/s), (ii) is reversible, (iii) in most
cases of biphasic nature (109 out of 152 experiments) and (iv) presumably
independent of actin-myosin-motility. The existence of transient mechanical
changes in the cell cortex is confirmed by micropipette aspiration experiments.
A theoretical analysis demonstrates that this observation can be explained by a
reversible change in the mechanical properties of the cell surface
(transmembrane pressure, surface tension and bending rigidity). Taken together,
these findings contribute to the ongoing debate about the physical nature of
cellular excitability
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