RECURRENT AXON COLLATERALS OF THE LUMBAR MOTONEURONS IN THE TURTLE
TESTUDO HORSFIELDI
N. M. Chmykhova,1 O. A. Karamian,1 V. M. Kozhanov,1
H.-P. Clamann 2
1 The I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry RAS,
St. Petersburg, Russia, and 2 Institute of Physiology, Bern University, Switzerland;
1 e-mail: nch@iephb.ru
A combined morphophysiological study was made of connections between motoneurons on the superfused isolated lumbar
spinal cord of Testudo horsfieldi. Postsynaptic potentials of motoneurons, followed by antidromic stimulations of ventral root
ilaments (VR-PSPs), were recorded intracellularly. Depolarizing VP-PSPs had short latencies (1.0-1.5 mc) and amplitudes in the range
of 0.3-3.0 mV. At the constant stimulus intensity, the fluctuations of amplitudes were recorded. In some motoneurons, hyperpolarizing
VP-PSRs with the latencies 2.5-3.0 mc were observed. A possible structural basis of VR-PSPs was studied by the horseradish
peroxidase (HRP) method. After HRP application on thin ventral root filaments the retrograde staining of motoneurons revealed recurrent
axon collaterals of labeled motoneurons. Three-dimensional computer reconstructions showed one to three collaterals given off by
motoneuron axons. There were up to 19 points of branching in a single collateral. In some cases the full length of collateral trees reached
4.0 mm. The collateral branches had up to 72 "en passant" and terminal axon swellings. The swellings (presumed contacting boutons)
were distributed in the ventral and intermedial gray matter and in the ventromedial while matter and revealed on motoneurons and
inerneurons. These data suggest the participation of the motor axon collaterals in the motoneuron - motoneuron communication in the
turtle spinal cord whereas only dendro-dendritic contacts had been discussed earlier.
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