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Neurophysiology
Three areas of research are currently active in my lab: copepod
neuroecology, computational properties of network neurons, and computational
studies of space clamp errors in point-clamp experiments.
Copepod neuroecology
In this project on the neuroethology and neuroecology of zooplankton,
we are examining the relation between physiological and morphological
properties of a zooplankter's sensory systems (specifically mechano-
and chemoreception in copepods) and the animal's behavior and ecology.
The sensory systems reflect unusual adaptations to pelagic life
when compared to similar systems in benthic and nektonic forms.
We have been finding that the antennules in certain copepod groups
have two pairs of giant mechanoreceptor neurons, which are exceptionally
sensitive to water-borne disturbances. They have peak sensitivities
to vibrations at frequencies well above those of other aquatic invertebrates.
Behavioral studies are showing that sensitivities for triggering
rapid escape "jumps" parallel those for receptor activation.
The evidence suggests that one of the keys to the success of copepods
as a group (they are more numerous than insects) is a very rapid
mechanically-triggered activation of a swim motor pattern generator
tuned to signals produced by predatory attack. We have found that
different copepod groups show markedly different reaction times
to stimuli mimicking predatory attack. The animals with the more
rapid reactions belong to more recently-evolved groups and inhabit
a wider range of ecological habitats than do slower animals. In
electron microscopic studies, we have discovered that the faster
animals have evolved a myelin sheathing that surrounds most of the
large axons in their nervous systems. As in vertebrates, we believe
that this insulating sheath is responsible for speeding up communication
in the copepod nervous system and can explain much of the improvement
in reaction times of the more advanced species. We are now testing
this hypothesis by examining a range of species from different phyletic
groups, extending our analysis of both physiological and behavioral
properties of myelinated and non-myelinated copepods and their relations
to ecological factors.
Computational properties of network neurons
Computational approaches are becoming increasingly useful for attacking
problems in neuroscience, including problems dealing with the computational
properties of the nervous system itself. My lab is currently applying
computational approaches to the study of local computation in "dendritic"
trees of reidentifiable neurons. Motor neurons in the stomatogastric
ganglion (a model motor pattern generator found in decapod crustaceans)
are dye-injected, imaged with a confocal microscope and reconstructed
in 3D with computer software. Quantitative measurements on the reconstructed
dendritic trees are placed in a computer model simulating the spread
of signals, active and passive, throughout the tree and along axons.
Assessment is made of the effects of inputs placed at various points
in the tree on the expected outputs from other regions of the tree.
The model predicts that outputs from some regions differ qualitatively
and quantitatively from those of others. This has led to the hypothesis
that the tree is spatially differentiated in computational properties.
We are working to 1) refine physiological measurements made in the
cells to improve the reliability of the simulations; 2) investigate
the postsynaptic targets for different tree regions to determine
the potential ramifications for the neural network of regional computational
heterogeneity; 3) extend the modeling studies to other cell types
within the ganglion.
Computational studies of space clamp errors in point-clamp experiments
"Point clamping" with microelectrodes has become a standard
method for identifying and characterizing the various ion channels
which are responsible for the computational properties of nerve
cells. Unfortunately, the technique is only accurate in spherical
cells, which few neurons are. Using computer-simulation approaches,
we are studying the properties of the errors that occur when nerve
cells are not spherical. The goal of the work is to provide correction
factors that can be applied to the flawed measurements made with
current technology to determine true values for physiological parameters
of ion channels. Specifically, we are working to first establish
correction factors for a set of different conditions in nerve cells
of simple form having a single active ion channel. We will then
extend the work to encompass more complex cells with ramifying dendritic
arbors and multiple active channels.
Representative
publications
Copepod neuroecology
Buskey, E.J. and Hartline, D.K. 2003. High speed video analysis
of the escape responses of the copepod Acartia tonsa to shadows.
Biol. Bull. 204: 28-37
Buskey, E.J., Lenz, P.H. and Hartline, D.K. 2002. Escape behavior
of planktonic copepods to hydrodynamic disturbances: High speed
video analysis. Mar. Ecol. Progr. Ser. 235: 135-146
Lenz PH, Hartline DK. 1999. Reaction times and force production
during escape behavior of a calanoid copepod, Undinula vulgaris.
Mar Biol 133: 249-258.
Hartline DK, Buskey EJ, Lenz PH .1999. Rapid jumps and bioluminescence
by controlled hydrodynamic stimuli in a mesopelagic copepod, Pleuromamma
xiphias. Biol Bull 197: 132-143.
Computational and cellular neuroscience
Hartline, D.K. and Castelfranco, A.M. 2003 Simulations of voltage
clamping poorly space-clamped voltage-dependent conductances in
a uniform cylindrical neurite. J. comput. Neurosci. 14: 253-269
Hartline DK, Gassie DV, Jones BR. 1993. Effects of soma isolation
on outward currents measured under voltage clamp in spiny lobster
stomatogastric neurons. J. Neurophysiol 69: 2056-2071.
Hartline DK, Graubard K. 1992. Cellular and synaptic properties
in the crustacean stomatogastric nervous system. In: Harris-Warrick
R, Marder E, Selverston AI, editors. Dynamic Biological Networks:
The Stomatogastric Nervous System. Cambridge: MIT Press. p 31-85.
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