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The Journal of Neurophysiology Vol. 87 No. 3 March 2002, pp. 1586-1602
Copyright ©2002 by the American Physiological Society
Biology Department, Emory University, Atlanta, Georgia 30322
Hill, Andrew A. V.,
Mark
A. Masino, and
Ronald L. Calabrese.
Model of Intersegmental Coordination in the Leech Heartbeat
Neuronal Network. J. Neurophysiol. 87: 1586-1602, 2002. We have created a computational model of
the timing network that paces the heartbeat of the medicinal leech,
Hirudo medicinalis. The rhythmic activity of this network
originates from two segmental oscillators located in the third and
fourth midbody ganglia. In the intact nerve cord, these segmental
oscillators are mutually entrained to the same cycle period. Although
experiments have shown that the segmental oscillators are coupled by
inhibitory coordinating interneurons, the underlying mechanisms of
intersegmental coordination have not yet been elucidated. To help
understand this coordination, we have created a simple computational
model with two variants: symmetric and asymmetric. In the symmetric model, neurons within each segmental oscillator called oscillator interneurons, inhibit the coordinating interneurons. In contrast, in
the asymmetric model only the oscillator interneurons of one segmental
oscillator inhibit the coordinating interneurons. In the symmetric
model, when two segmental oscillators with different inherent periods
are coupled, the faster one leads in phase, and the period of the
coupled system is equal to the period of the faster oscillator. This
behavior arises because, during each oscillation cycle, the oscillator
interneurons of the faster segmental oscillator begin to burst before
those of the slower oscillator, thereby terminating spike activity in
the coordinating interneurons. Thus there is a brief period of time in
each cycle when the oscillator interneurons of the slower segmental
oscillator are relieved of inhibition from the coordinating
interneurons. This "removal of synaptic inhibition" allows, within
certain limits, the slower segmental oscillator to be sped to the
period of the faster one. Thus the symmetric model demonstrates a
plausible biophysical mechanism by which one segmental oscillator can
entrain the other. In general the asymmetric model, in which only one
segmental oscillator has the ability to inhibit the coordinating
interneurons, behaves similarly, except only one segmental oscillator
can control the period of the system. In addition, we simulated
physiological experiments in which a "driving" stimulus, consisting
of alternating positive and negative current steps, was used to control
a single oscillator interneuron and thereby entrain the activity of the entire timing network.
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S. H. Jezzini, A. A. V. Hill, P. Kuzyk, and R. L. Calabrese Detailed Model of Intersegmental Coordination in the Timing Network of the Leech Heartbeat Central Pattern Generator J Neurophysiol, February 1, 2004; 91(2): 958 - 977. [Abstract] [Full Text] [PDF]
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A. Wenning, A. A. V. Hill, and R. L. Calabrese Heartbeat Control in Leeches. II. Fictive Motor Pattern J Neurophysiol, January 1, 2004; 91(1): 397 - 409. [Abstract] [Full Text]
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M. A. Masino and R. L. Calabrese Phase Relationships Between Segmentally Organized Oscillators in the Leech Heartbeat Pattern Generating Network J Neurophysiol, March 1, 2002; 87(3): 1572 - 1585. [Abstract] [Full Text] [PDF]
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M. A. Masino and R. L. Calabrese Period Differences Between Segmental Oscillators Produce Intersegmental Phase Differences in the Leech Heartbeat Timing Network J Neurophysiol, March 1, 2002; 87(3): 1603 - 1615. [Abstract] [Full Text] [PDF]
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