Electric Gradient Field Applied to Lipid Monolayers - Membrane

CHAPTER 1. INTRODUCTION 3
Fig. 1.2: Left: (Top trace) Thermal responds of the non-myelinated garfish olfactory nerve in phase
with the (lower trace) recorded electric impulse measured at the same point. Adapted from [Tasaki1989]
Right: The non-myelinated giant squid axon is thickened by 0.5Å and (top trace) at the same point where
the recorded electrical impulse pass. (lower trace). Adapted from [Tasaki1980]
with the electrical signal. The magnitudes of the two signals made it difficult to detect
and first in 1958 - 1960 Abbott managed to setup a heat sensor of the required accuracy.
This and following experimental results were considerably different from the predicted heat
production by ion fluxes. At the raise of the impulse, heat was released from the nerve
but absorbed again in an equal amount [Abbott1960]. Experiments like these have been
performed mainly by Tasaki and co., who worked on both myelinated nerves and nonmyelinated
nerves, fig. 1.2 [Tasaki1989] [Tasaki1992]. In 1980 a mechanical swelling of a
nerve was detected to have a displacement of the nerve cross section in the order of 0.5Å,
and this effect was found in a variety of animals [Tasaki1980] [Tasaki1989] [Tasaki1999].
The results imply an electro-mechanical coupling of the compounds in a nerve membrane,
neither the thermal response nor the swelling can be deduced from the Hodgkin-
Huxley model. If the selective ion channels cannot explain these two detectable effects,
logic claims that, the rest of the membrane i.e. the lipids must be involved and therefor be
examined. However, the questioning of electro-physiology shows the relevance to the initial
question How does electrical stimulation of a nerve affect the main membrane components
- the lipids The thermal-mechanical coupling has been approached in [Heimburg2005]
by introducing a mechanical wave (a soliton) in the lipid membrane and include the melting
transition of the membrane. During the melting transition lipid membranes display
changes in volume, heat capacity and area compressibility. The mechanical wave pushes
the liquid membrane about 85% into the transition.
1.3 Ordered Phosphorlipid Monolayers on aqueous subphase
The distribution of lipid species in nerve membranes wary from nerve to nerve but for nonmyelinated
nerves, the main part is composed of phospholipids with head groups carrying
a charge or has a dipole moment. As an example, the distribution of lipids in a brain nerve
from a rat is given in mass percentage, fig. 1.3 [Sackmann1995].
Monolayers composed of oily substances on water have been reported since the ancient