F Dec. 3,

Steve Baer, Department of Mathematics and Statistics, ASU


Dynamic spines: The impact of time-dependent changes in
spine morphology on the input-output properties of a dendritic branch

  Dendritic spines, the postsynaptic targets of over 90% of all excitatory
synapses in the central nervous system, are abundant in brain regions
associated with learning and memory. A single dendritic tree may be
populated with hundreds, if not thousands, of spines with different sizes
shapes and configurations. Direct observation of living spines with
fluorescent probes has allowed us to see that their shapes can change with
remarkable rapidity, within seconds. Growth and movement of filopodia or
spines can occur within minutes, either as a developmental phenomenon or
as a result of stimulation.  Recent experiments implicate intraspine
calcium level as a mediator for changes in dendritic spine structure.
Release of calcium from internal stores, in response to pulse applications
of caffeine, induced a small transient rise in Ca++ (200-400nM), and and
increase in the length of spine stems in less than 5 minutes (Korkotian
and Segal 1999). conversely, Halpain et al. (1998) induced a rapid
collapse of dendritic spine stems (also within 5 minutes) by stimulating
cultured neurons with glutamate. This caused maximal calcium influx,
raising intraspine calcium to much higher levels. In this talk, I
formulate a mathematical model based on an interpretation of the above
experiments by Harris (1999).  In this model, a moderate amount of
synaptic activation results in spine stem elongation, whereas, a high
level of activity causes too much calcium influx which induces spine stem
shortening. I explore the consequences of such changes on the input-output
properties of a dendritic branch.