Transcript View Lymnea Poster - Wellesley College

What makes a neuron a neuron? Using pond snails to explore intracellular properties of
neurons.
Stephen
1Department
1
Hauptman ,
Carol Ann
2
Paul ,
Patsy
1
Dickinson ,
and Bruce
25.13
3
Johnson
2
04011, Department
of Biology, Bowdoin College, Brunswick, ME
of Biological Sciences, Wellesley College, Wellesley, MA 02171
3Department of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853
Introduction
To demonstrate the functional properties of neurons to students, we
use the criteria enumerated by Henriette van Praag et al.1 to
determine whether newborn neurons behaved as functional neurons,
as defined by their physiological properties:
The Pond Snail Preparation
The pond snail brain is
located just posterior to
the buccal mass. There
are large, easily
penetrable cells in the
parietal, visceral and
buccal ganglia.
Students can easily master the dissection with the
assistance of the instructional videos available at
http://www.wellesley.edu/Biology/Concepts/Htm
l/theneuronconnection.html.
In this student lab, we use and
expand on these criteria in pond snail
species such as Lymnaea or
Helisoma. We have developed a set
of exercises based on these snails,
which can also be done with any
intracellular prep.
Through these exercises, students can start to answer the question:
what makes a neuron a neuron?
We use pond snails because they are easy to maintain, moderately
easy to dissect, cells are amazingly easy to visualize and cells are
extremely active. Many of the cells have also been identified and
mapped. The cells in pond snail preps show a range of spontaneous
activity patterns, from cells that are quiet at rest, to beating cells, to
cells showing regular bursts.
These exercises may take several weeks in the lab. Students who
do these exercises end up not only with an excellent understanding
of physiological properties of a neuron, but also with a thorough
grounding in the use of electrophysiological equipment.
1
van Praag H, Schinder A, Christie B, Toni N, Palmer T & Gage F. 2002, Functional
neurogenesis in the adult hippocampus Nature 415: 1030-1034.
http://www.wellesley.edu/Biology/Concepts/Html/theneuronconnection.html
Active Properties
Synaptic Properties
Action potential
frequency can be
plotted as a
function of current
pulse amplitude.
Students can
consider why the
frequency levels
off and the action
potentials diminish
in amplitude.
Cells in the buccal ganglia are part of the snail’s feeding pattern and show
regular spontaneous postsynaptic potentials.
To get microelectrodes through the sheath that surrounds the
ganglia, the ganglia must first be treated with 0.5% protease (45
seconds for buccal ganglia, one minute for ring ganglia).
epsps
ipsp
s
epsps
Cells showing spontaneous psps can be hyperpolarized to find the reversal
potential. Psp amplitude can be plotted as a function of membrane potential.
The x-intercept of the resulting line is the reversal potential.
Passive Properties
Resting membrane
potential can be
established upon
successful impalement.
The time constant can be
determined by injecting a
current pulse and
calculating the time it takes
to reach 63% of the voltage
change.
Input resistance can be calculated at a given membrane potential
by calculating V/I. The overall capacitance of the cell can be
determined by calculating Tau/R.
Input resistance can be
determined over a range
of membrane potentials by
creating a V-I curve for
both hyperpolarizing and
depolarizing current
pulses, and determining
the slope. Notice in this
example that input
resistance decreases once
threshold is reached (as
indicated by the
decreased slope),
indicating the opening of
voltage-gated channels.
The action potential threshold can be determined
by finding the membrane potential at which any
further depolarization triggers an action potential.
In this example, the threshold is -52 mV.
Many cells show post-inhibitory rebound. This can
be demonstrated by hyperpolarizing the cell and
then releasing the inhibition.
+
-
Membrane conductivity can be compared
during psps and between psps by
injecting current pulses long enough to
come to a new steady-state voltage. The
amplitudes are compared. Smaller
amplitude indicates reduced resistance
and increased conductance. In this way,
students can determine whether channels
are opening or closing during the psps.
Time constants can also be compared as
another measure of changed
conductance.
During psp
Between psps
Conclusion
These exercises clearly reinforce the idea that neurons are special types of cells.
This study of snail passive and active membrane properties is useful in helping
students define what properties make neurons special and different from other cell
types. It answers the question: what makes a neuron a neuron?
Acknowledgements
We would like to thank the students in Bowdoin’s Bio 253, Neurophysiology, for the
use of data collected in that class.
Support Contributed by: NSF Due - 0231019 and DEB-0336919