Journal of Neurophysiology, Vol 60, Issue 5 1678-1694, Copyright © 1988 by APS
Acute effects of spinal transection on EPSPs produced by single homonymous Ia-fibers in soleus alpha-motoneurons in the cat
T. C. Cope, K. R. Hickman and B. R. Botterman
Department of Cell Biology and Anatomy, University of Texas Health Science Center, Dallas 75235.
1. Excitatory postsynaptic potentials (EPSPs) generated in soleus
motoneurons by single homonymous Ia-fibers were measured using
intracellular recording and the spike-triggered averaging technique. Two
groups of barbiturate-anesthetized adult cats were studied: one with the
spinal cord intact and the other with the spinal cord severed at thoracic
segment 13 (T13) several hours prior to recording. 2. In cord-transected
cats, single homonymous Ia-fibers produced EPSPs in soleus motoneurons that
were, on average, larger and faster rising relative to normal, as they are
for those produced in medial gastrocnemius (MG) motoneurons (8, 12, 13,
40). Specifically, mean EPSP amplitude and rise time were, respectively,
261 +/- 22 microV and 0.65 +/- 0.05 ms for the transected group vs. 160 +/-
21 microV and 0.96 +/- 0.08 ms for the intact group. The group means for
each parameter were significantly different (P less than 0.005). 3. The
group difference in EPSP amplitude was largely due to a decrease in number
of small EPSPs in the transected group (11% less than 100 microV compared
with the normal 41%) and not due to the occurrence of unusually large ones.
Ratios of the largest to smallest amplitude EPSPs produced in the same
motoneuron were similarly distributed for intact and transected groups,
implying that the effect of transection on EPSP size was uniform across
different Ia-fiber synapses made with the same motoneuron. Mean EPSP
amplitude for each transected cat (n = 5) was larger than normal, but in
some cases the increase took greater than 10 h to express itself. 4. The
normal tendency for EPSP rise time to decline on average with amplitude was
absent in the transected group, wherein rise time was reduced to similar
average values in all amplitude categories. This suggests that the decrease
in rise time occurred independently of the increase in amplitude. In
contrast, EPSP half-width, which tended tow ward lower than normal values
[5.63 +/- 0.36 (SE) ms vs. 6.51 +/- 0.44 ms; P greater than 0.10],
decreased in proportion with rise time as evidenced by the preservation of
the normal relation between those parameters in transected cats.
Normalizing EPSPs by motoneuron time constant (tau) reduced the group
differences in rise time and half-width, suggesting that a fall in tau
contributes to the abbreviation of EPSP time course. 5. The condition of
the spinal cord best accounted for differences in synaptic strength between
groups.(ABSTRACT TRUNCATED AT 400 WORDS)
