Download Harding, G. W. and A. L. Towe. 1995. Neuron Response to Direct

Research note:
Neuron Response to Direct Sensorimotor Cortex Stimulation in Cats: Local and
Interareal Correlation With Wide-Field Modulation
G.W. Harding1, A.L. Towe2
1Departments
of Otolaryngology and Neurology – Neurological Surgery, Washington
University School of Medicine, St. Louis, MO 63110 (USA) and 2Department of
Physiology and Biophysics, University of Washington School of Medicine, Seattle,
WA 98195(USA).
Key words: Sensorimotor cortex - Single neurons - Wide-field Modulation - Direct
cortical response - Primary Evoked Response - Cat
Running Title: Wide-field neurons and direct cortical stimulation
Direct correspondence to:
Gary W. Harding
Box 8115
Washington University Medical School
St. Louis, MO 63110
(314) 362-7501
14 Pages; 0 Tables; 1 Figure
Abstract:
Single neurons were studied extracellularly in postcruciate cerebral cortex of
cats using both cutaneous and cortical-surface stimulation. Neurons were
classified according to the pattern of their response to electrical stimulation
of each of the four paws. In addition, their responsiveness to strong corticalsurface stimulation, applied both 1.5 mm away from the recording site in area 4r
(near stimulation) and 3.0 mm away in area 3a far stimulation), was assessed.
The cutaneous response latencies were used in a model which deduces facilitatory
and inhibitory modulation of wide-field neurons by small-field neurons. Several
different patterns of response to cortical stimulation were observed: response
to both near and far (65%); response to near but not far (5%); response to far
but not near (9%); and response to neither (21%). Most neurons which responded
to both near and far stimulation responded earlier to near than to far; a few
responded earlier to far stimulation. Some neurons responded late to far
stimulation and early or not at all to near. Others responded early to near
stimulation and both early and late to far. These complex patterns of response
to cortical stimulation coincided with the model's prediction of their
peripheral facilitatory and inhibitory modulation. It was also found that
facilitated and inhibited wide-field neurons were mixed randomly through
cortical depth.
Electrical stimulation at the surface of postcruciate cortex excites some
neurons directly and many synaptically, and these neurons produce excitatory or
inhibitory effects on their post-synaptic targets. The excitatory effects are
evident in the radially-spreading direct cortical response, or DCR (Adrian,
1936), in the caudally-directed pyramidal tract discharge, recorded as D and I
waves (Patton and Amassian, 1954), and in evoked muscle contractions. The
inhibitory effects are less obvious: some are direct and some recurrent. Direct
inhibition was addressed by Towe, et al. (1976) through a model for estimating
the sign and magnitude of the modulation of wide-field neuron excitability which
is produced by local small-field neurons after skin stimulation. The model
predicts that electrical stimulation of the nearby cortical surface will excite
those m neurons which receive facilitatory modulation, but will not excite those
that receive inhibitory modulation. The observations presented in this note are
consistent with this prediction.
Materials and Methods:
Extracellular microelectrode recordings were taken from single neurons in the
forepaw region of the postcruciate gyrus (field 4r; 2 mm posterior to lateral
tip of Cruciate sulcus). The methods used were identical to those described by
Towe, et al. (1976) for chloralose-anesthetized cats. Also, two pairs of
bipolar silver wire stimulating electrodes, with tips separated 1.5 mm, were
placed on the cerebral cortex 1.5 mm posterior (near to microelectode in area
4r, N) and 3.0 mm posterior (far from microelectrode in area 3a, F) to the
recording site at the postcruciate dimple. Square pulses (0.01-0.05 ms, 2-6 ma)
were applied to the pial surface, and the shock artifact was minimized by
orienting the stimulating electrodes to the recording microelectrode to form two
isosceles triangles. Bipolar needle electrodes were inserted into the glabrous
skin of each paw and 6 mA, 0.1 ms pulses were delivered at once per second to
assess each neuron's response to peripheral stimulation. The neurons were thus
classified as small-field (respond to contralateral forepaw only, s); wide-field
(respond to all four paws, m); or mute (do not respond to paws, n). A blunt
recording electrode was also placed on the cortical surface adjacent to the
microelectrode to monitor the DCR and primary evoked response.
Results:
A sample of 57 neurons was obtained in 11 electrode tracks in two adult cats; 8
others were lost before sufficient study. Of the 57 neurons, 35 were m, 21 were
s and one was n. The responses of these neurons to skin stimulation were
similar to those described by Towe et al. (1976). The responses to surface
cortical stimulation were complex: 23 m, 13 s, and one n neuron responded to
stimulation at both the N and F sites, 7 m and 5 s did not respond to
stimulation at either site, and the remainder responded only to N (1 m, 2 s) or
only to F (4 m, 1 s) stimulation. The distributions of response latency to
cortical stimulation are shown in Fig. 1A and 1B as a function of depth of
isolation; latencies greater than 10 ms, along with non-responding neurons, are
shown in the box at the right of each graph. Only s neurons were isolated in
the first 0.9 mm of cortex, in layers II and III. The mean response latencies
to N stimulation did not differ significantly through depth, although they had
the shortest and longest latencies in the middle third of cortex (lower layer
III through V). One s neuron responded at 0.8 ms with constant latency, as if
it was activated directly; but all others responded as if they were synaptically
activated. The mean latency for each electrode track to stimulation at the N
site did not differ significantly from the overall mean of 2.99 ms (SE=0.22 ms)
or from one another (t-test). The mean response latency was longer for
stimulation at the F site, which was twice as far away from the recording
location as the N site. However, the joint variation in response latencies to N
and F stimulation, shown in Fig. 1C, reveals that two neurons responded sooner
to F than to N stimulation and that several responded at about the same latency.
The s and m response latencies defined two ellipses with slopes of 0.432 and
0.396 with two correlations of .69 and .73 respectively. The third ellipse
identifies 10 neurons that responded late to F stimulation (half with low
response probability), including 4 which also responded early. Clearly, there
are several different routes by which neurons in a given track may be excited
after cortical stimulation, one of which involves s neuron activation of m
neurons.
Towe et al. (1976) described a method to estimate the nature of the excitability
change produced on m neurons by local s neurons. The expected mean response
latency after contralateral forepaw stimulation, as calculated via this
coadunate model, was divided into the observed mean response latency to obtain a
modulation ratio (MR). Ratios less than one indicated facilitation, ratios
greater than one indicated inhibition. However, because of sampling errors,
ratios between 0.9 and 1.1 were treated as indeterminate. By these rules, 75%
of the m neurons were facilitated and 15% were inhibited. The distribution of
modulation ratios obtained in the present small sample was similar: a 7-bin X2
test yielded p>0.5, with 88% of the variance in the indiscriminate range.
The modulation ratios accurately identified the responsiveness of the m neurons
to cortical stimulation: all neurons with ratios less than 0.9, 4 of 10 neurons
in the indeterminate range, and no neurons with ratios greater than 1.1
responded to stimulation at the N site, 1.5 mm away. The mean modulation ratio
for the neurons responding to N stimulation was 0.70 (SE=0.04) and for those not
responding was 1.06 (SE=0.04). Student's t-test yielded p<0.001 (32 df). All
but one neuron with a ratio less than 0.9, 7 of 10 neurons in the indeterminate
range, and no neurons with ratios greater than 1.1 responded to stimulation at
the F site, 3.0 mm away. Again, the mean ratios differed (p<0.001; 31 df).
Towe et al. (1976) found that the mean number of spikes/response and the
interspike intervals changed systematically as a function of the modulation
ratio. Sorting by these two criteria yielded mean modulation ratios of 0.68
(SE=0.04) for 16 clearly facilitated neurons and of 1.12 (SE=0.06) for 5 clearly
inhibited neurons; a mean of 0.84 (SE=0.06) was obtained for those not clearly
classifiable by these criteria. Again, the mean ratios differed (p<0.001; 19
df).
The responses of 23 of the 35 m neurons were entirely consistent with the model:
they either responded to both N and F input and had modulation ratios less than
0.9, or failed to respond to either input and had modulation ratios greater than
1.1. Ten neurons with modulation ratios in the indeterminate range either
responded to both inputs (n=3); to neither input (n=3); or only to the F input
(n=4; 2 at long latency). One m neuron with MR=0.80 responded to the N, but not
the F stimulation.
The patterns of response to cortical stimulation differed among neurons within
any track. For example, of the 11 well-documented m neurons in the sixth track,
6 responded to both N and F inputs, 3 responded to F, but not N input, and 2 did
not respond to either input. Their modulation ratios ranged from 0.57 to 1.09.
Of the 11 electrode tracks, the neurons in only 2 tracks (one with 2 and the
other with 6 neurons) showed a common response pattern. The modulation ratios
varied widely in each track. Of the 7 pairs of neurons recorded simultaneously,
3 showed a common response pattern, responding to both N and F inputs, one
showed an opposite pattern, and 3 showed a mixed pattern. Using the proportions
in which each response pattern occurred in the sample, a random distribution
model predicted that 2.95 of the 7 pairs would show the same response pattern,
0.95 would show an opposite pattern, and 3.10 would show a mixed pattern.
Discussion:
Evidently, not all neurons that respond to skin stimulation also respond to
stimulation of the cortical surface: those m neurons which receive an inhibitory
influence from local s neurons, as estimated from their modulation ratios, do
not respond to such stimulation. Some s neurons also fail to respond to this
stimulation. Li and Chou (1962) and Sugaya et al. (1964) also found that many
intracellularly recorded neurons in cat sensorimotor cortex did not respond to
cortical-surface stimulation.
After surface stimulation, neurons first become active in the middle third of
the cortex, although activity begins quickly at all depths. The latencies of
response to the two stimulation sites are positively correlated, with about 50%
of the variance from one site being accounted for by that from the other. Even
so, there are several possible conduction routes from each stimulus site to each
neuron, and the existence of both early and late discharges in the same neuron
suggests that input from different routes converges onto the same neuron.
The late responses from 10 to 12 ms occurred at a time which is consistent with
the second negative potential of the DCR and those from 20 to 33 ms coincide
with DCR after-positivity (Goldring et al., 1994). The second negative
potential is abolished by barbiturates (Goldring and O'Leary, 1960), as are the
responses of m, but not s, neurons (Harding et al., 1979). After-positivity is
reduced by barbiturates (Goldring and O'Leary, 1960), consistent with the mix of
s and m neurons responding later than 20 ms. Stohr et al. (1963) reported that
spontaneously active neurons, deep in sensorimotor cortex of barbiturateanesthetized cats were inhibited during after-positivity.
Li and Chou (1962) found that when strong cortical-surface stimuli are used,
the brief initial depolarization is followed by inactivity and
hyperpolarization. However, Sugaya et al. (1964) showed that with strong
stimuli, many neurons deep in cat sensorimotor cortex show depolarization during
the initial response and the descending part of the primary negative potential
of the DCR. These apparently contradictory observations are resolved by the
finding that facilitation and inhibition may occur on neighboring neurons.
Responses to near and far stimulation are minimal during the primary negative
potential of the DCR, although a few neurons still responded (as also reported
by Li and Chou (1962) and Stohr et al. (1963)).
The response properties of the 4 neurons with modulation ratios in the
indeterminate range which responded only to the far stimulus may be understood
if they received an inhibitory input from near stimulation of local s neurons,
but the far site was too far away to activate these s neurons. On the other
hand, they could have been activated by some less direct excitatory route (Chen
and Towe, 1985).
The response pattern of the m neuron and the 2 s neurons that
responded to near, but not far stimulation, may be explained if the far site was
too far away to activate those neurons by any route.
There is a thorough mix of inputs, such that neurons with strong inhibitory
modulation reside near neurons with strong facilitatory modulation, and neurons
that respond early to surface stimulation reside near those that respond late,
or not at all, to the same stimulus. The simplicity of the response which is
obtained near the site of strong surface stimulation, where most neurons may be
directly activated, quickly gives way to a more complex pattern of responses as
activation spreads upward at and outward from that site (Harding, 1992).
There has been controversy about whether or not 3a is a distinct
cytoarchitectonic area in cats (Jones and Porter, 1980). It may be an overlap
zone between somatosensory and sensorimotor cortex and its corticocortico
connections are probably short-range and non-specific. The connections between
3a and the adjacent sensorimotor part of area 4 are characterized by the fanning
out of axon collaterals from pyramidal cells. In monkeys, area 3a receives
projections from area 4, but appearently does not project to it.
In rats, although the hindlimb representation resembles hindlimb 3a of cats and
in part, that of monkeys (Jones and Porter, 1980), there does not appear to be a
forelimb area 3a (Gioanni, 1987). Many excited cells in rat forelimb
sensorimotor cortex are multi-modal and intracellular recordings show a
distribution of excitation and inhibition similar to that of the facilitory and
inhibitory modulation ratios for m neurons in forepaw area 4 of cats.
Studies to determine corticocortico connections between forepaw SI areas and
area 4 of cats using anatomical fiber tracing and in-vitro electrophysiological
techniques have shown that horizontal axon collaterals of pyramidal cells
project more than 2 mm (Keller, 1993). Locally within area 4, axon collaterals
of pyramidal cells project vertically from layer V to other pyramidal cells in
more superficial layers and vise versa. Long-range and intrinsic-descending
excitatory collaterals would be likely candidates for facilitory modulation of m
neurons.
Most non-pyramidal cells in the immediate vicinity of pyramidal neurons appear
to be intrinsic inhibitory interneurons (Keller, 1993). Horizontal inhibitory
connections by some of these cells probably extend more than 1 mm. Activation
of both mid-range and intrinsic non-pyramidal cell connections could account for
inhibitory modulation of m neurons.
Strong cortical-surface stimulation in area 3a probably activates fibers passing
through it to area 4 as well, particularly any from areas 2, 1 and 3b. The
results presented here add some clues about the modulation that occurs in area 4
of cats via intrinsic and interareal circuits.
Acknowledgements:
This work was supported by USPHS research grants NS00396 and NS05136 to the
University of Washington, Seattle, WA from NINCDS and the McDonnell Center for
Studies of Higher Brain Function, Washington University, St. Louis, MO.
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Figure and Legend:
Fig. 1 Distributions of first spike latency (A&B) with depth for cortical
responses to near (A) and far (B) cortical stimulation. Neurons in box at right
either did not respond (circles) or responded late (triangles). Horizontal
dashed lines separate upper, middle and lower thirds of cortex. Joint
distribution of responses to near and far (C) cortical stimulation. Solid
diagonal line separates N<F and N>F. Early response ellipses s and m enclose
75% of joint variance and late response ellipse 50%. Open symbols - s; filled
symbols - m; circles - early responses; triangles - only late responses to far
stimulation; diamonds - both early and late responses to far stimulation; square
- no response to peripheral stimulation.