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The Journal of Neurophysiology Vol. 82 No. 4 October 1999, pp. 1876-1882
Copyright ©1999 by the American Physiological Society
1Department of Internal Medicine,
Al-Chaer, Elie D.,
Yi Feng, and
William D. Willis.
Comparative Study of Viscerosomatic Input Onto Postsynaptic
Dorsal Column and Spinothalamic Tract Neurons in the Primate.
J. Neurophysiol. 82: 1876-1882, 1999.
The purpose of the present investigation was to examine, in the
primate, the role of the postsynaptic dorsal column (PSDC) system and
that of the spinothalamic tract (STT) in viscerosensory processing by
comparing the responses of neurons in these pathways to colorectal
distension (CRD). Experiments were done on four anesthetized male
monkeys (Macaca fascicularis). Extracellular recordings
were made from a total of 100 neurons randomly located in the
L6-S1 segments of the spinal cord. Most of
these neurons had cutaneous receptive fields in the perineal area, on
the hind limbs or on the rump. Forty-eight percent were PSDC neurons
activated antidromically from the upper cervical dorsal column or the
nucleus gracilis, 17% were STT neurons activated antidromically from
the thalamus, and 35% were unidentified. Twenty-one PSDC neurons, located mostly near the central canal, were excited by CRD and three
were inhibited. Twenty-four PSDC neurons, mostly located in the nucleus
proprius, did not respond to CRD. Of the 17 STT neurons, 7 neurons were
excited by CRD, 4 neurons were inhibited, and 6 neurons did not respond
to CRD. Of the unidentified neurons, 23 were excited by CRD, 7 were
inhibited, and 5 did not respond. The average responses of STT and PSDC
neurons excited by CRD were comparable in magnitude and duration. These
results suggest that the major role of the PSDC pathway in
viscerosensory processing may be due to a quantitative rather than a
qualitative neuronal dominance over the STT.
The dorsal column (DC) plays an important role in
relaying noxious visceral information to the ventral posterolateral
(VPL) nucleus of the thalamus (Al-Chaer et al. 1996a Spinothalamic tract (STT) neurons are also known to be involved in
visceral sensory processing. The spinothalamic tract arises largely
from neurons in the dorsal horn of the spinal cord. The locations of
these cells have been mapped in rats, cats, and monkeys using
retrograde tracing and antidromic activation methods (see reviews by
Hodge and Apkarian 1990 On the other hand, several research groups have argued for an important
role of the DC in visceral input into the VPL nucleus (Apkarian
et al. 1995 The purpose of this study was to compare the responses of PSDC and STT
neurons in the monkey lumbosacral spinal cord to visceral and somatic
inputs. To do this, recordings were made from single, randomly isolated
neurons in the L6-S1 spinal segments. These neurons were examined for projections in the DC or the STT and were
tested with graded CRD and cutaneous stimuli. Preliminary results of
this study have been reported in abstract form (Al-Chaer et al.
1998d Experiments were done on four adult male monkeys (Macaca
fascicularis) weighing between 2 and 2.5 kg. The monkeys were
sedated with ketamine (10 mg/kg im) and then anesthetized with 4%
halothane in a mixture of oxygen and nitrous oxide (30:70%). Tracheal
and intravenous cannulae were inserted, and anesthesia was maintained initially by an infusion of Stimulation
The visceral stimulus used was CRD. It was adapted from the
model used in rats (Gebhart and Sengupta 1996 The cutaneous stimuli employed were brushing (BR) of the receptive
field using a camel hair brush, an innocuous stimulus; pressure (PR),
using a large arterial clip applied to a fold of skin, a stimulus that
causes a sense of near painful pressure if applied to human skin; and
pinch (PI) using a small arterial clip that exerts a force of 550 gm/mm2, a distinctly painful stimulus if applied
to human skin.
Central stimulation consisted of electrical pulses (200 µs in
duration and up to 500 µA in intensity) at a frequency of 2 Hz
applied to the CNS to activate projection neurons antidromically. The
VPL nucleus was stimulated to identify STT cells. The upper cervical DC
or the nucleus gracilis (NG) was stimulated to identify PSDC cells. For
VPL nucleus stimulation, a tungsten microelectrode was introduced
stereotaxically into the VPL nucleus while recording from the
electrode. The tip of the electrode was advanced ventrally while
tapping on the perineal area of the body until multiunit activity
became distinctly audible on an audio monitor. The coordinates were
noted and the electrode was then connected to the stimulator. For
stimulation of the upper cervical DC, a ball electrode was placed on
the fasciculus gracilis (FG) near the midline, under view through a
surgical microscope. Viscerosensitive cells in the sacral cord were
isolated and checked for projection in the DC. In one preparation (11 PSDC cells), an extra fine (25 µm tip) tungsten microelectrode was
used to map the DCN for terminals or fibers of passage of PSDC cells.
This was done to ensure that these neurons were indeed PSDC cells. The
stimulating electrode was systematically moved laterally from the
midline in increments of 100 µm at different transverse planes
separated by 100 µm and spanning a rostrocaudal length of 8 mm
centered at the obex. The electrode was advanced ventrally in 100-µm
steps until a minimal antidromic threshold was recorded. The antidromic
threshold at each site in the penetration was defined as the current
that produced an action potential in ~50% of the trials. The
electrode was moved until the PSDC unit was activated antidromically
with currents Recordings
Recordings were made from single units isolated at the
L6-S1 segmental level
using carbon fiber glass microelectrodes. Small holes were made in the
pia mater, under view through a surgical microscope and using fine
forceps to facilitate the introduction of the recording microelectrode.
All units were examined for projections into the DC or the VPL nucleus
and also for peripheral somatic and visceral inputs. During the search
process, the recording electrode was driven into the spinal cord while
electrical stimuli were simultaneously applied in the VPL nucleus (to
activate STT neurons antidromically) and to the DC (to activate PSDC
neurons antidromically). Every neuron encountered was examined for
possible antidromic activation from either the VPL nucleus or the DC
separately. All neurons recorded in this sample were checked for input
from the colon using CRD as the stimulus. Then all neurons were
examined for cutaneous input using three different modalities (Brush,
Press, and Pinch). Units that responded to CRD were classified as
viscerosensitive, units that were antidromically activated from the DC
or the DCN were considered to be PSDC neurons, and units antidromically
activated from the VPL nucleus were regarded as STT neurons. Units that were not activated antidromically from either the DC or the VPL nucleus
were considered to be unidentified neurons.
The extracellular action potentials recorded were fed into a window
discriminator and displayed on an oscilloscope screen. The output of
the window discriminator was led into a data collection system (CED
1401+) and a personal computer to compile rate histograms or wavemark
files using the Spike 2 software program. The response to each
intensity of CRD was stored separately. Twenty seconds of baseline
activity preceded the application of a distension stimulus. Each
stimulus lasted 20 s. Four minutes were allowed to elapse between
two consecutive CRDs. The responses are expressed as the average rate
of firing of the cell during a particular stimulus minus the average
baseline rate.
Histology
At the end of each experiment, a continuous current (1 mA for
20 s) was passed to mark the stimulation sites in the VPL nucleus and in the NG. The tip of the carbon fiber glass microelectrode was cut
off and left at the recording site. The spinal cord at the level of the
recordings and the brain were removed and put into a neutral Formalin
solution (4%). The tissues were stored in 20% sucrose before frozen
sectioning at 50 µm. The recording and stimulation sites were later
identified histologically.
Statistics
The CRD data were analyzed using a repeated measures ANOVA. A
model with the repeated factor of intensity, group, and the (intensity × group) interaction showed that there was a
significant intensity effect (P < 0.001), no group
effect (P = 0.67) and no (intensity × group)
interaction (P = 0.12). The interpretation of these
results is that there was a significant increase in the magnitude of
neuronal responses across levels of CRD for all three groups. The
nonsignificant group and interaction effects mean that responses in all
three groups increased in a similar fashion.
The responses to cutaneous stimuli (Brush, Press, and Pinch) were first
analyzed using an overall test (Hotelling's T2
test) to assess the extent to which the distributions of these three
variables differed across all three groups (PSDC, STT, and Unidentified), when considered simultaneously. This test showed a
significant difference across groups (P = 0.038), and
subsequent one-way ANOVAs showed that only the variable Press showed an
overall difference among the three groups (P = 0.002).
Tukey's Studentized range test showed that groups STT and PSDC
differed significantly, with a difference of means of 33.6 spikes/s
(95% CI: 12.9, 54.2).
A total of 100 neurons were isolated in the
L6-S1 segments of the
spinal cord. The neurons were distributed between 400 µm and 2 mm in
depth measured from the surface of the spinal cord. The mediolateral
distribution extended from near the dorsal median septum to the edge of
the dorsolateral funiculus. The recording sites were reconstructed by
extrapolating their locations from distance parameters recorded in
relation to marked sites as shown in Fig.
1.
ABSTRACT
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ABSTRACT
INTRODUCTION
METHODS
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DISCUSSION
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INTRODUCTION
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
,c,
1998b; Feng et al. 1998). A study done on
primates has shown that a lesion of the DC blocks much of the
colorectal input into the VPL nucleus (Al-Chaer et al.
1998b). Furthermore, recent results obtained using functional
magnetic resonance imaging (fMRI) show that a DC lesion placed at the
T10 spinal level dramatically reduces the increases in blood volume
observed in thalamic and other brain areas in response to noxious
colorectal distension (CRD) (Al-Chaer et al. 1998a,e).
Earlier studies done on rats demonstrated that the DC-mediated colon
input is largely conveyed in a newly identified component of the
postsynaptic dorsal column (PSDC) system, whose cells of origin are in
the vicinity of the central canal and whose axons project in the DC
(Al-Chaer et al. 1996a,b, 1997b;
Hirshberg et al. 1996; C. C. Wang, W. D. Willis, and K. N. Westlund, unpublished observations). Neurons of
this newly identified component of the PSDC can be activated by CRD or
colon inflammation (Al-Chaer et al. 1996b,
1997b).
; Willis and Coggeshall
1991). Most STT cells studied have cutaneous receptive fields
and usually respond to noxious and often also to innocuous mechanical
stimulation of the skin (Katter et al. 1996;
Owens et al. 1992; Willis et al. 1974).
Visceral afferent fibers can activate many STT cells (Ammons et
al. 1985; Foreman et al. 1981;
see Foreman 1989). STT neurons can be excited by
distension of the gall bladder (Ammons et al. 1984),
kidney (Ammons 1989a), ureter (Ammons
1989b), or urinary bladder (Milne et al.
1981). Milne et al. (1981) also recorded
responses of STT cells in the upper lumbar and sacral segments of the
monkey spinal cord to noxious testicular stimulation. These
observations form the experimental basis for the assumption that the
STT is the major ascending spinal cord tract that conveys information
about noxious visceral events to lateral thalamic nuclei.
; Berkley and Hubscher 1995;
Chandler et al. 1998). In fact, Al-Chaer et al.
(1996a, 1997a,c, 1998b) have argued that the DC plays a more important role than that of the STT in
relaying information about noxious and innocuous visceral activity into
the VPL nucleus in rats and monkeys, because a DC lesion largely blocks
colonic input into the VPL nucleus of the thalamus, whereas a lesion
that interrupts the STT has much less effect (Al-Chaer et al.
1996a, 1998b).
).
METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
-chloralose (60 mg/kg iv) followed by a
continuous infusion of pentobarbital sodium (5 mg · kg
1 · h1). The
animals were paralyzed with pancuronium (0.2-0.25 mg · kg1 · h1) and
artificially respired. A lumbosacral laminectomy was performed to
expose the L5-S2 segments
of the spinal cord. Cervical laminectomy and removal of part of the
occipital bone exposed the upper cervical spinal cord and the caudal
medulla. Animals were then transferred to a stereotaxic frame in a
shielded recording room, the skin flaps overlying the cord were tied
back to form a pool, the dura was cut and reflected to expose the cord,
and the pool filled with warmed mineral oil. A craniotomy exposed the
cortex above the thalamus. The head was flexed ventrally, the muscles
of the neck were retracted, and the dura was cut and reflected to
expose the medulla. In one experiment, part of the cerebellum was
removed by suction to gain better access to the nucleus gracilis. The adequacy of the depth of anesthesia was evaluated by the examination of
the pupillary reflexes, monitoring of the electrocardiogram, and
assessing the stability of the expired CO2. A
pneumothorax was performed to minimize respiratory movements during
recording. End tidal CO2 was maintained at 4 ± 0.5%, and body temperature was kept near 37°C by a
thermostatically controlled heating blanket. The arterial blood oxygen
saturation level was monitored using a laser oxymeter rectal probe and
was kept at 98 ± 2%.
; see also
Al-Chaer et al. 1996a,b). The stimulus was applied using
an inflatable balloon inserted rectally. The balloon was constructed
from a latex glove finger attached to a length of a tygon tubing (10 cm). CRD consisted of consecutive inflations of the balloon to pressures ranging between 20 and 80 mmHg, applied in increments of 20 mm for 20 s every 4 min. CRD stimuli having an intensity above 40 mmHg are considered noxious in rats and painful in humans (Ness
and Gebhart 1988; Ness et al. 1990).
30 µA. Current pulses of 30 µA spread <400 µm
(Ranck 1975). The locations from which PSDC neurons were
activated antidromically with currents 30 µA were marked by an
electrolytic lesion. Responses were considered to be antidromic if they
fulfilled three criteria: constant stimulus-response latency, following
of high-frequency stimuli (300-500 Hz), and collision of the evoked
spike with orthodromic spikes at appropriate intervals.
RESULTS
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ABSTRACT
INTRODUCTION
METHODS
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DISCUSSION
REFERENCES
View larger version (20K):
[in a new window]
Fig. 1.
Drawing of a cross-section through S1 illustrating recovered recording
sites. 
, postsynaptic dorsal column (PSDC) neurons that
responded to colorectal distension (CRD); 
, PSDC
neurons that did not respond to CRD; 
, spinothalamic
tract (STT) neurons that responded to CRD; 
, STT
neurons that did not respond to CRD.
Neuronal characteristics
All isolated neurons were tested for projections into the DC and into the VPL nucleus. They were also tested for responses to cutaneous stimulation and to CRD. The neurons were then grouped into three categories based on whether they could be antidromically activated by DC stimulation, by stimulation of the VPL nucleus or by neither. The responses of neurons within each category were analyzed separately. Table 1 illustrates the number of neurons in each category according to their responses to CRD.
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Neurons activated by CRD exhibited an increase in their responses that correlated with the increase in stimulus intensity. The population stimulus response curve was approximated by a linear regression for all the neurons in each category. These regression lines show that the rate of change (slope) was similar across neuronal types (Fig. 2: PSDC, STT, and Unidentified). This graded response was observed regardless of the threshold of activation of the VPL unit by CRD. The threshold of activation was defined as the lowest intensity of CRD that evoked a neuronal response and was determined with increasing steps of 20 mmHg of intracolonic pressure for the cells tested. The accuracy is estimated to be 10 mmHg across the pressure spectrum (20-80 mmHg). The average thresholds of activation for neurons of each category are illustrated in Fig. 3.
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PSDC NEURONS. Of 100 neurons isolated, 48 were antidromically activated by stimulation of the DC. In one preparation, nine PSDC neurons were antidromically activated by low-intensity stimulation in the NG (as low as 10 µA). The PSDC neurons could be classified into two clearly separable populations. One group of relatively shallow neurons (n = 24; <1,400 µm in depth) located mostly near the lateral edge of the pia hole was unresponsive to CRD. A second group of relatively deep neurons (n = 24; >2 mm) located mostly near the midline responded to CRD. Twenty-one neurons were excited and three neurons were inhibited (Table 1). Of the 24 viscerosensitive PSDC neurons, 22 had cutaneous receptive fields located on the inner and posterior aspects of the thigh, on the rump, or on the scrotum. Two PSDC neurons excited by CRD could not be activated by skin stimulation. Of the 24 PSDC neurons that did not respond to CRD, 12 neurons had cutaneous receptive fields located over the outer aspect of the leg, the upper thigh, and the rump, and 12 could not be activated by somatic stimulation. The responses of viscerosensitive PSDC neurons to CRD were graded with stimulus intensity (Fig. 2A). The mean threshold for activation of PSDC neurons was 34 ± 3.9 (SE) mmHg (Fig. 3). The average response to 20 mmHg was 4.6 ± 1.9 spikes/s, and the average response to 80 mmHg was 17.4 ± 3.5 spikes/s (Fig. 4). The average responses to cutaneous stimulation did not show any major differences between types of stimuli, although there was a trend for the responses to Brush stimuli to be greater than those to Press or Pinch stimuli (Fig. 5). Figure 6 illustrates the site of recording of a PSDC neuron, the site of stimulation for antidromic activation in the NG, the antidromic spikes, and the responses of the PSDC neuron to CRD and cutaneous stimulation.
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STT NEURONS. Seventeen isolated neurons were antidromically activated by stimulation in the VPL nucleus. The depth of these neurons ranged between 1 and 2 mm from the surface of the pia hole, and most of them were best isolated at the lateral edge of the pia hole near the dorsolateral funiculus. Of the 17 STT neurons, 7% were excited by CRD, 4% were inhibited, and 6% did not respond to CRD. Fifteen STT neurons could be activated by cutaneous stimulation applied to the back and inner aspect of the thigh, to the scrotum, and to the perineal area. The responses of viscerosensitive STT neurons to CRD were graded with stimulus intensity (Fig. 2B). The mean threshold for activation of STT neurons was 31.4 ± 5.9 mmHg (Fig. 3). The average response to 20 mmHg was 6.8 ± 4 spikes/s, and the average response to 80 mmHg was 20.6 ± 7.2 spikes/s (Fig. 4). The STT cells responded better to Press or Pinch than to Brush stimuli applied to the cutaneous receptive field (Fig. 5). Figure 7 illustrates the site of recording of an STT neuron, the site of stimulation for antidromic activation in the VPL nucleus, the antidromic spikes, and the responses of the STT neuron to CRD and cutaneous stimulation.
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UNIDENTIFIED NEURONS. Thirty-five neurons encountered could not be activated antidromically from either the VPL nucleus or the DC and were classified as unidentified neurons. Twenty-three of these neurons were excited by CRD, seven were inhibited, and five did not respond to CRD. The responses of these neurons to CRD were comparable to the responses of PSDC and STT neurons. Twenty-four of these neurons could be activated by cutaneous stimulation applied to the back of the leg, the inner thigh, the rump, or the perineal area. The responses of the viscerosensitive unidentified neurons to CRD were graded with stimulus intensity (Fig. 2C). The mean threshold for activation of these neurons was 43.4 ± 3.9 mmHg (Fig. 3). The average response to 20 mmHg was 3.3 ± 1.4 spikes/s, and the average response to 80 mmHg was 21.6 ± 3.8 spikes/s (Fig. 4). There was a tendency for the responses to Press and Pinch stimuli to exceed those to Brush stimuli (Fig. 5).
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ABSTRACT
INTRODUCTION
METHODS
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DISCUSSION
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The results obtained in this study do not reveal any major
differences between the nature and characteristics of the responses of
PSDC, STT, and unidentified neurons to CRD. The average responses and
the threshold for activation of each neuronal category were not
significantly different. Individual responses were stimulus-bound and
graded with stimulus intensity. The only major difference noticed was
the number of neurons isolated within each category. The neurons were
isolated on a random encounter basis during simultaneous search
stimulation of the DC and of the VPL nucleus. They were then tested for
visceral and cutaneous responses and for specific projections into the
VPL nucleus or into the DC. No neurons with double projections were
encountered. The PSDC neurons isolated at
L6-S1 spinal levels
outnumbered the STT cells and the unidentified neurons. This numerical
predominance of PSDC neurons that respond to CRD may underlie the
greater role of the DC in transmitting colonic input into the VPL
nucleus than the STT as was shown in a recent study by Al-Chaer
et al. (1998b). A lesion of the DC at the
T10 segmental level dramatically reduced the
responses of VPL neurons to CRD, whereas the effects of anterolateral
or dorsolateral spinal lesions were not as consistent and dramatic as
those of a DC lesion. Earlier observations in the rat (Al-Chaer et al. 1996a) showed that the visceral input in the DC is
conveyed largely by the axons of PSDC neurons (Al-Chaer et al.
1996b), whose cell bodies are located mainly around the central
canal (Hirshberg et al. 1996). These axons converge onto
gracilothalamic neurons (Al-Chaer et al. 1996b), which
presumably relay the majority of pelvic visceral input from the colon
to the VPL nucleus (Al-Chaer et al. 1997a).
In most mammalian species, the VPL nucleus receives sensory information
arising from stimulation of the skin, deep tissues, and viscera and
conveyed over two major ascending tracts: the dorsal column pathway and
the spinothalamic tract. Some of this information can also be conveyed
through the spinocervical tract and also other indirect pathways
relaying through neurons of the brain stem reticular formation that
receive input from fibers traveling in the ventrolateral quadrant
(Willis and Coggeshall 1991). Although spinoreticular
neurons may respond to visceral stimuli (e.g., Blair et al.
1984), it is not known whether visceral input can be
transmitted indirectly by this pathway to the VPL nucleus.
A considerable amount of research has been done on the response
characteristics of STT neurons and of PSDC neurons and neurons of the
DC nuclei (Willis and Coggeshall 1991). However, very
little has been done to characterize potential differences in the
response properties of these different populations of projection
neurons (Brown et al. 1986; Chandler et al.
1998). In a recent study in monkeys, Chandler et al.
(1998) described differences in the evoked discharge rates,
latencies to activation and duration of peristimulus histogram peaks
between cuneothalamic and STT neuronal responses to cardiopulmonary
sympathetic input. They suggested that dorsal and ventrolateral
pathways to the VPL nucleus of the thalamus play different roles in the
transmission and integration of nociceptive cardiac information.
Cuneothalamic neurons are not the anatomic equivalent of STT neurons.
Therefore the differences seen by Chandler et al. may be due to
different organizations of a spinal pool of neurons (STT) and a
supraspinal pool (DCN). In this study, we compare two different
populations of projection neurons, both of which are located in the
spinal cord, at the level of convergence of colon afferents. In
addition, there may be differences in the nature of the input received
by each pathway. These may arise out of differences between the stimuli
used (natural colon stimulation vs. electrical stimulation of
cardiothoracic afferents) or differences in the type of receptors
innervating the colon and the heart.
In the present study, the location of viscerosensitive PSDC neurons
corresponded well with the location of viscerosensitive PSDC neurons
observed previously in the rat (Honda 1985; Ness and Gebhart 1987). The vast majority of these neurons were in and around the central canal. PSDC neurons isolated elsewhere in the
spinal cord, mainly in more superficial layers of the dorsal horn, were
not responsive to CRD. The distributions of visceroceptive PSDC neurons
correlate with the site of convergence of pelvic nerve terminals
(Nadelhaft et al. 1983). The STT neurons encountered were not as abundant as PSDC neurons. The distribution of STT neurons
corresponded with findings of previous studies from this laboratory
(Willis et al. 1979). Unidentified viscerosensitive neurons were also distributed throughout the area of gray matter explored.
Careful analysis of the response characteristics of each group of
neurons to CRD did not reveal any differences in the attributes of
these responses. The responses were stimulus bound and graded with the
stimulus intensity. No significant differences in the amplitude of the
responses to each individual stimulus were seen; in addition, the
slopes of the stimulus-response curve for each type of neuron were not
significantly different. These observations reduce the likelihood that
the predominant role of the DC in visceral input into the thalamus can
be attributed to stronger responses of individual viscerosensitive PSDC
neurons and argues for a stronger collective input of the PSDC system
compared with other ascending spinal pathways. Even though the
characteristics of the neuronal responses could differ were we to use a
different stimulus (Chandler et al. 1998), the effect of
a DC lesion remains consistent with a stronger DC input.
Putative roles for each system
These various pathways may be anatomically separate, but they are
by no means independent (Zhang et al. 1996). Disruption of the information flow in one of them may alter the function of the
other (Saadé and Jabbur 1984). So how could one
extrapolate a limited physiological function into a global sensory
role? In a situation where the perceived stimulus is localized in time and brief in duration, a linear flow of information is a plausible hypothesis even though it may be incomplete. However, when dealing with
situations of chronic pain or sensory loss, many variables come into
play, among them the dynamic interplay between the various neuronal
components (Berkley et al. 1993; Chandler et al.
1996; Le Bars et al. 1979; see also
Bouhassira et al. 1998), the wind-up of activity within
each component (Al-Chaer et al. 1996c,
1997a,b; Roza et al. 1998); and the
rewiring of the nervous hardware when necessary. The abundance of PSDC
neurons in lumbosacral segments of the cord affords the PSDC system the
ability to manage the peripheral input at that level and to relay it to
other neurons within those segments and to higher neuronal structures.
In summary, the results of this study show that the PSDC system plays
an important role in the processing of innocuous and noxious colorectal
information in the monkey. This role may be based largely on the number
of viscerosensitive PSDC neurons encountered at
L6-S1 segments. This is
not to discount the role of other ascending pathways such as the STT,
but the data obtained suggest that STT cells are less abundant in those
segments than PSDC cells. We can only speculate about the role of
unidentified neurons. Therefore similar to what we have seen in rats
(Al-Chaer et al. 1996b), we conclude that the neuronal
basis for the role of the DC in colon pain (Al-Chaer et al.
1998b) rests largely on the PSDC system.
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ACKNOWLEDGMENTS |
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The authors thank K. Gondesen for assistance with the surgical procedures and G. Gonzales for assistance with the artwork.
The experiments were supported by National Institute of Neurological Disorders and Stroke Grants NS-11255 and NS-09743.
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FOOTNOTES |
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Address for reprint requests: W. D. Willis, Dept. of Anatomy and Neurosciences, Marine Biomedical Institute, 301 University Blvd., Galveston, TX 77555-1069.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 29 January 1999; accepted in final form 29 June 1999.
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ABSTRACT
INTRODUCTION
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DISCUSSION
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