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J Neurophysiol (January 1, 2003). 10.1152/jn.00137.2002
Submitted on Submitted 25 February 2002; accepted in final form 3 September 2002
Departments of Physiology and Physical Medicine and Rehabilitation and The Neuroscience Institute, Northwestern University Medical School, Chicago, Illinois 60611
Lee, R. H.,
J. J. Kuo,
M. C. Jiang, and
C. J. Heckman.
Influence of Active Dendritic Currents on Input-Output
Processing in Spinal Motoneurons In Vivo. J. Neurophysiol. 89: 27-39, 2003. The extensive dendritic tree of
the adult spinal motoneuron generates a powerful persistent inward
current (PIC). We investigated how this dendritic PIC influenced
conversion of synaptic input to rhythmic firing. A linearly increasing,
predominantly excitatory synaptic input was generated in triceps ankle
extensor motoneurons by slow stretch (duration: 2-10 s) of the
Achilles tendon in the decerebrate cat preparation. The firing pattern
evoked by stretch was measured by injecting a steady current to
depolarize the cell to threshold for firing. The effective synaptic
current (IN, the net synaptic current
reaching the soma of the cell) evoked by stretch was measured during
voltage clamp. Hyperpolarized holding potentials were used to minimize
the activation of the dendritic PIC and thus estimate stretch-evoked
IN for a passive dendritic tree
(IN,PASS). Depolarized holding
potentials that approximated the average membrane potential during
rhythmic firing allowed strong activation of the dendritic PIC and thus
resulted in marked enhancement of the total stretch-evoked
IN
(IN,TOT). The net effect of the
dendritic PIC on the generation of rhythmic firing was assessed by
plotting stretch-evoked firing (strong PIC activation) versus
stretch-evoked IN,PASS (minimal PIC
activation). The gain of this input-output function for the neuron
(I-ON) was found to be ~2.7 times as high as
for the standard injected frequency current (F-I) function
in low-input conductance neurons. However, about halfway through the
stretch, firing rate tended to become constant, resulting in a sharp
saturation in I-ON that was not present in
F-I. In addition, the gain of I-ON
decreased sharply with increasing input conductance, resulting in much
lower stretch-evoked firing rates in high-input conductance cells. All
three of these phenomena (high initial gain, saturation, and
differences in low- and high-input conductance cells) were also readily
apparent in the differences between stretch-evoked
IN,TOT and IN,
PASS and thus could be accounted for by the activation of
the dendritic PIC. As a result, stretch-evoked
IN,TOT and F-I provided an
accurate prediction of the overall change in stretch-evoked firing.
However, in about half of the low-input conductance cells, the rate of rise of firing in response to stretch was not smoothly graded but
instead consisted of a rapid surge. Stretch-evoked
IN,TOT was always smoothly graded.
This suggests that although stretch-evoked IN,TOT can be used to predict the
overall change in firing, prediction of the dynamics of firing may be
less accurate.
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