Department of Physiological Sciences, College of Veterinary Medicine and University of Florida Brain Institute, University of Florida, Gainesville, Florida 32610-0144
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ABSTRACT |
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Hubscher, Charles H. and Richard D. Johnson. Effects of Acute and Chronic Midthoracic Spinal Cord Injury on Neural Circuits for Male Sexual Function. II. Descending Pathways. J. Neurophysiol. 83: 2508-2518, 2000. In normal animals, microstimulation of the medullary reticular formation (MRF) has two effects on efferent neurons in the motor branch of the pudendal nerve (PudM). MRF microstimulation depresses motoneuron reflex discharges (RD) elicited by dorsal nerve of the penis (DNP) stimulation and produces long latency sympathetic fiber responses (SFR). The midthoracic spinal location of these descending MRF-PudM projections was studied electrophysiologically using a variety of acute and chronic lesions. Chronic lesions, in 27 mature male rats, included dorsal (DHx) or lateral (LHx) hemisections or moderate/severe contusions (Cx) at spinal level T8. Behavioral data (sexual reflex latency, bladder voiding) obtained throughout the recovery period revealed a significant impairment of urogenital function for the DHx and severe Cx groups of animals. Microstimulation-induced PudM-RDs and PudM-SFRs, obtained in terminal electrophysiological experiments 30 days postinjury in the same 27 rats (urethan-anesthetized), were tested for a combined total of 1,404 bilateral MRF sites. PudM-RD was obtained for LHx and moderate Cx groups of animals but not for DHx or severe Cx groups. PudM-SFRs were obtained for LHx, DHx (although significantly weakened) and moderate Cx groups but not for those having received either an over-DHx or a severe Cx injury. PudM responses also were tested for 6 MRF sites in six intact control rats both before and after various select acute spinal cord lesions. For MRF sites producing a robust PudM-RD and PudM-SFR, acute bilateral lesions confined to the dorsolateral quadrant (DLQ) eliminated the PudM-RD but failed to eliminate PudM-SFRs. A deeper lesion encompassing additional white matter located dorsally in the ventrolateral quadrant (VLQ) was necessary to eliminate PudM-SFRs. Overall, these electrophysiological results provide evidence for descending projections conveying information between MRF and the lower thoracic/lumbosacral male urogenital circuitry within the DLQ and the dorsal-most aspect of VLQ at the midthoracic level of spinal cord. The alterations of supraspinal projections observed after chronic injury are likely of important clinical significance for functional recovery in cases of clinically incomplete spinal cord injury at midthoracic spinal cord.
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INTRODUCTION |
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The sensory limb of segmental reflexes for
erection and ejaculation (Johnson and Halata 1991;
Johnson and Murray 1992
; McKenna and Nadelhaft
1986
; Nunez et al. 1986
; Steers et al.
1988
), produce bilateral (crossed and uncrossed) reflex
facilitation of motoneurons in the pudendal motor branch (PudM)
(Johnson and Hubscher 1998
; McKenna and Nadelhaft
1989
). The synaptic efficacy of dorsal nerve of the penis (DNP)
afferents and associated interneurons onto PudM motoneurons is reduced
progressively in rats with chronic spinal cord injuries (Johnson
1995
). The enhanced erectile and depressed ejaculatory reflexes
observed in rats 30 days after midthoracic transection (Hart and
Odell 1981
; Mas et al. 1987
; Sachs and
Garinello 1979
) is likely due to a loss of supraspinal influences. Thus DNP facilitation of ejaculation may require a brain
stem loop, much like the control of micturition (deGroat et al.
1981
). Previous studies show that medullary reticular formation (MRF) neurons, particularly those within the lateral
paragigantocellular reticular nucleus (LPGi), are likely involved in
this supraspinal loop (Marson and McKenna 1990
;
Marson et al. 1993
; Tanaka and Arnold
1993
; Yells et al. 1992
). Many MRF neurons
respond to bilateral DNP stimulation (Hubscher and Johnson
1996
). Bilateral electrical stimulation of specific
DNP-responsive MRF subregions, particularly in and medial to LPGi,
produce a reflex (DNP-elicited) depression of pudendal motoneurons
(PudM-RD) (Johnson and Hubscher 1998
) and produces a
sympathetic fiber response (PudM-SFR) (Johnson and Hubscher
2000
).
Using select acute and chronic lesions, our current focus is the
investigation of the midthoracic white matter location of projections
comprising the supraspinal loop between the lumbosacral ejaculatory
circuitry and MRF. The location of the ascending spinal projections is
presented in a companion article (Hubscher and Johnson
1999b). In the present paper, the location of descending projections within the midthoracic white matter is presented along with
behavioral data (sexual reflex latency, bladder voiding) obtained
during the 30 day postinjury recovery period.
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METHODS |
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Spinal cord lesions
Twenty-seven male Wistar rats received either a dorsal (DHx) or
lateral (LHx) hemisection, or contusion (Cx) injury (severe or
moderate) at spinal level T8. These same animals
also provided data for two other articles (Hubscher and Johnson
1999a,b
). Terminal electrophysiological experiments were
performed 30 days postinjury (see following text).
Lesion surgeries were performed under aseptic conditions using ketamine
(80 mg/kg ip)/xylazine (10 mg/kg ip) anesthesia. A long acting
antibiotic (Flo-Cillin: 0.5 ml; Fort Dodge Laboratories, Fort Dodge,
IA) was administered before surgery. The spinal cord was exposed at
T8 via removal of the overlying
T7 vertebral lamina. Hemisections (as well as
acute lesions) were made with microdissecting scissors; contusions were
performed by rapid compression with a concave probe having a
radius/size matching the overlying lamina (see Hubscher and
Johnson 1999b for further details).
After closure and recovery, the animals were encouraged to eat with apple slices (1st postoperative day). Each animal was tended to three times daily throughout the recovery period. Daily logs were kept to note the status of bladder function. In the initial postsurgical period, the distended bladder was expressed manually by gentle squeezing every 8 h, lasting until "reflex" voiding onset. Initiation of "reflex" micturition, which required only gentle pressure or stroking of the perineum, was performed until "automatic" micturition occurred; i.e., the bladder periodically voided through contact with the cage bedding (without caretaker assistance).
Sexual reflexes were tested once during the second and once during the
third weeks postinjury, using standard procedures (see Sachs and
Garinello 1979; Sachs and Meisel 1988
). Each rat
was placed in dorsal recumbency, and the penis was exteriorized by manually retracting the prepuce. To document the behavior, each rat was
videotaped for 20 min. The latency to the first penile reflex (penile
erection or dorsiflexion) (Sachs and Garinello 1979
) was
measured. Statistical comparison of injury groups was performed
(Mann-Whitney U test).
Surgical preparation for terminal electrophysiological experiments
Six male Wistar rats at ~90-120 days of age were used in the
acute lesion experiments. The remaining 27 rats were subjected to a
terminal electrophysiological experiment after a 30-day chronic lesion.
All animals were anesthetized with urethan (1.2 g/kg ip) and
prepared/maintained as previously described (Hubscher and Johnson 1999b). The head was clamped in a stereotaxic holder
and the dorsal surface of the brain stem exposed (see Hubscher
and Johnson 1996
). Specially fabricated bipolar silicon cuff
microelectrodes were placed bilaterally around the dorsally exposed
pelvic nerve, DNP, and motor branch of the pudendal (i.e., PudM - also
known as deep perineal) nerves (see experimental setup in Fig. 1 of Hubscher and Johnson 1996
).
MRF stimulation protocol
Two glass-coated platinum-plated tungsten microelectrodes
with a 20-µm exposed tip (Merrill and Ainsworth 1972),
attached to a stepping microdrive, were set for bilateral penetration
of the MRF (see Hubscher and Johnson 1996
;
Johnson and Hubscher 1998
). Each terminal experiment
included initial MRF neuronal recordings in four rostral tracks to
assess the ascending circuitry from the male genitalia (see
Hubscher and Johnson 1999a
,b
). Responses were then
measured in left and right PudMs to unilateral and bilateral microstimulation at an average of 52 MRF sites located caudal (2,400-3,200 µm rostral to obex) to the MRF recordings. These stimulation sites comprised the portion of the microstimulation matrix
in intact controls that produced robust PudM-RD and PudM-SFRs (Johnson and Hubscher 1998
, 2000
).
A monopolar stimulating current (20 µA) was passed through the
microelectrodes either individually or simultaneously to unilaterally or bilaterally microstimulate MRF regions. The current intensities were
subsequently either decreased or increased in rostral tracks only to 10 and 30 µA to just cause contraction of the facial muscles (i.e., when
the electrode was next to the facial nucleus) (Zhuo and Gebhart
1991). Reflex discharges recorded simultaneously in both PudMs
in response to the DNP test stimulus (single 0.1-ms duration pulses of
30-50 µA set at 2.5 times reflex threshold) were recorded before and
after conditioning stimulation of the brainstem (50-ms train of 200 pulses/s, 0.2-ms pulse duration, given at a train rate of 0.5 trains/s)
to determine if that particular MRF site was capable of modulating the
polysynaptic pudendal segmental circuitry. Based on previous work
(Johnson and Hubscher 1998
), the short conditioned test
interval (<80 ms) reflex depression (Pud-RD) consisted of a decrease
in amplitude and an increase in latency of the DNP-elicited reflex. In
addition, direct long latency (>150 ms) activation of sympathetic
postganglionic fibers (PudM-SFR) in response to conditioning
stimulation alone was determined for each MRF site (Johnson and
Hubscher 2000
).
Three matrix rows extended from 400 to 2,000 µm laterally from
midline. Tandem (i.e., left and right) electrode tracks covered a
900-µm depth zone in ventral MRF (2,400-3,300 µm below the brain stem surface at depth increments of 300 µm) (Paxinos and
Watson 1997). In the six intact animals, a single robust
PudM-RD and PudM-SFR responsive uni/bilateral region of MRF was found
and tested both before and after a variety of acute spinal lesions at
T8.
Data collection and analysis
At each microstimulation site in the assay, two measures were
taken for the short-latency PudM-RD and the long-latency PudM-SFR. The
first measure was the determination of the presence or absence of
responses to bilateral MRF stimulation. If responses were present on
one or both PudMs, the effect of unilateral MRF microstimulation was
tested on both the crossed and uncrossed PudM. The second measure was
the magnitude of response (if present). Each PudM response was assigned
to one of three strengths. For the short-latency PudM-RD, responses
were categorized (Johnson and Hubscher 1998) as being
either slight (barely visible reduction), partial (easily visualized
reduction), or complete (approximately a flat line). For statistical
analyses and comparison of the data, these response strengths were
later assigned values on an ordinal scale of 1, 2, and 3, respectively
(a 0 score was assigned if no responses to bilateral MRF
microstimulation were detected). For the long-latency PudM-SFR,
responses were categorized as being either slight (barely detectable
above baseline), moderate (easily discriminated above baseline), or
strong (very large burst) (see Johnson and Hubscher 2000
). These response strengths also were assigned values (1, 2, and 3, respectively) on an ordinal scale (0 for no response).
Histology
At the end of the experiment, the animal was euthanized with an
anesthetic overdose and perfused transcardially with 0.9% saline
followed by 10% formalin. The block of brain stem tissue containing
the stimulation sites was removed and stored in 10% formalin/30%
sucrose. Stimulation sites were visualized in 50-µm vibratome
sections stained with cresyl violet and reconstructed under light and
dark field illumination (Paxinos and Watson 1997). The
perfused spinal cord was analyzed histologically (paraffin sections)
for confirmation of acute or chronic lesion extent (see photomicrograph
examples in Fig. 3 of Hubscher and Johnson 1999b
). Spinal cord tissue sections were stained with both luxol fast blue and
cresyl violet (Kluver-Barrera stain).
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RESULTS |
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Acute control spinal cord lesions
In each of six unoperated rats, a microstimulation MRF site producing both a robust PudM-RD and PudM-SFR (unilateral and bilateral MRF effects on both PudMs) was tested before and after selective acute unilateral midthoracic spinal lesions. The results from these experiments are summarized in Fig. 1. A unilateral lesion encompassing ventral portions of the dorsal lateral quadrant (DLQ) eliminated left and right PudM-RDs produced by conditioning microstimulation of the lesion-side MRF (for all the different lesions shown in sections 2 and 3 of Fig. 1A) but failed to eliminate left and right PudM-RD produced by microstimulating the intact-side MRF. These results support the existence of both a crossed and uncrossed descending projection below the level of the lesion and only an uncrossed projection above (for intact controls). The example in Fig. 2 illustrates this point; i.e., the reflex depression is maintained from the right MRF (left dorsal quadrant lesion) to both PudMs but lost from the left MRF (lesion side) to both PudMs.
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In contrast, a unilateral lesion eliminated PudM-SFRs on the lesion side from left and right MRF. PudM-SFRs were maintained (on the intact side) from both the left and right MRF, supporting the existence of a crossed and uncrossed descending projection above the level of lesion and only an uncrossed projection below (for intact controls). In contrast with PudM-RD, a deeper unilateral lesion encompassing additional white matter in the dorsal-most aspect of the ventrolateral quadrant (VLQ) was necessary to eliminate PudM-SFRs (i.e., for all the different lesions shown in sections 2 and 3 of Fig. 1B).
Chronic spinal cord lesions
A total of 27 male rats received one of three types of chronic
midthoracic spinal cord lesions; i.e., either a DHx, LHx, or Cx.
Bladder function data and sexual reflex response latency data (time to
first penile reflex penile erection or dorsiflexion) obtained
during the 30-day recovery period in the awake behaving animal are
presented in Fig. 3. In addition, there
were significantly more penile reflex clusters and penile dorsiflexion
(flips) in the DHx and Cx animals as compared with the LHx animals.
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In terminal electrophysiological experiments performed 30 days postinjury, the effects of various chronic T8 lesions on MRF microstimulation-elicited effects on PudMs (RD and SFR) was determined. A summary of responses is provided in Table 1 and Fig. 4.
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CHRONIC CONTUSION INJURIES.
In six of the rats with a severe Cx injury (see photomicrograph
example in Fig. 3A of Hubscher and Johnson
1999b), no PudM-RD or PudM-SFRs were found (see Table 1 and
Fig. 4). In four rats with moderate Cx (see photomicrograph example in
Fig. 3B of Hubscher and Johnson 1999b
), no
overall significant difference was found with intact controls for
PudM-RD (see Table 1). However, a difference was observed for the one
animal with the least amount of spared tissue, i.e., a narrow
unilateral rim remaining in the left lateral funiculus. Whereas a
partial PudM-RD was elicited from the left side of the MRF, only a
slight PudM-RD was elicited from right MRF (mainly on the crossed left
PudM
see Fig. 5). For Pud-SFR, a
significant difference (
2, P < 0.01) was found between the moderate Cx and intact control groups,
although this difference was nowhere near that obtained for the severe
Cx versus intact control group (see Table 1).
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CHRONIC DORSAL HEMISECTIONS.
In four animals with complete DHx (see photomicrograph example in Fig.
3C of Hubscher and Johnson 1999b), MRF
microstimulation-induced PudM-RD was eliminated bilaterally, except in
one case where some sparse sparing produced a slight effect (see Fig.
6). In five animals with complete DHx,
the PudM-SFR was significantly weakened, but not eliminated (see Fig.
4). This outcome appeared dependent on the presence or absence of the
dorsal-most aspect of VLQ. Note that for one animal, PudM-RD data were
not obtained. In four cases with incomplete DHx, PudM-RD and PudM-SFRs
were not eliminated (see typical examples in Fig.
7, A and B,
respectively).
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CHRONIC LATERAL HEMISECTIONS.
In eight animals with chronic LHx (see photomicrograph example in
Fig. 3D of Hubscher and Johnson 1999b), no
overall significant differences in PudM-RD and PudM-SFR were found with
intact controls. The rostral-to-caudal, medial-to-lateral shift in
PudM-RD response strength was still evident after LHx (see Fig. 4,
top) as was the increase in response strength as the
electrode shifted ventrally (Johnson and Hubscher 1998
).
However, important side-to-side differences were found. For PudM-RD,
responses from conditioning microstimulation of the lesion-side MRF
were either substantially weak or absent to both PudMs (see example in
Fig. 8A). The presence of some
weak responses from the lesion-side MRF to both PudMs supports the existence of both a crossed and uncrossed descending projection above
as well as below the level of a chronic lesion.
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DISCUSSION |
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The results of the present study demonstrate the presence of descending brain stem PudM-RD after either a chronic midthoracic LHx or a moderate Cx injury and the absence of these descending influences after either a severe chronic Cx injury or a complete chronic DHx. The results also demonstrate the presence of brain stem-elicited PudM-SFR after either a chronic LHx or a complete DHx or a moderate Cx injury and the absence of responses after either a severe chronic Cx injury or an over-DHx. When taken together, the results indicate that the two types of descending spinal projections between the MRF and circuits involving the PudM differ both in terms of laterality and location. Most of the results of these chronic lesion experiments were confirmed with select acute lesions; however, differences between chronic LHx and acute uni-DHx/LHx suggest that some reorganization in the neural circuitry for sexual reflexes had occurred after chronic injury (discussed further in the following text). In addition, the present electrophysiological results, particularly those for PudM-RD, are consistent with the behavioral reflex testing (i.e., impairment of reflex function after either complete DHx or severe Cx). When combined, the results indicate the presence of descending projections between MRF and the lower thoracic/lumbosacral male urogenital circuitry within the DLQ and the dorsal-most aspect of the VLQ at the midthoracic level of spinal cord.
Pudendal motoneuron reflex depression
The most robust depression of the DNP-elicited PudM reflex is
produced by a conditioning microstimulation of the MRF bilaterally and
ventrally in or medial to LPGi (Johnson and Hubscher
1998). This PudM-RD (decrease in amplitude, increase in
latency) was not found after a complete chronic DHx or severe Cx
injury, suggesting that the projections descend within the DLQ at the
midthoracic spinal cord. These observations are consistent with
neuroanatomic transport studies in the rat (Basbaum and Fields
1979
; Martin et al. 1985
) as well as behavioral
and physiological investigations that also demonstrate the location of
descending pathways originating from caudal brain stem in the DLQ using
select lesions (Basbaum et al. 1977
; Watkins et
al. 1984
).
The PudM-RD was maintained from both sides of the MRF after a chronic
LHx, suggesting that descending projections are both crossed and
uncrossed above, as well as below, the level of the midthoracic lesion
(see left side of summary diagram, Fig.
10). These results are in contrast with
those observed after an acute unilateral lesion of the DLQ, where
reflex modulation was only maintained from the intact-side MRF. The
acute lesion results indicate that the descending projections are
uncrossed above the level of injury and are both crossed and uncrossed
below. The reasons for this difference in the results is unclear.
Possibilities include an unmasking of already present projections after
injury, sprouting of new collaterals across the midline, or differences due to a sampling of only a single robust site in the acute controls. The latter possibility is unlikely because the most robust site producing PudM-RD was used in all six cases, so that even a weak response, if present, should have been detected at that location. The
first possibility seems most likely because it is known that local
circuits projecting across the midline exist within the MRF
(Jones 1995; Robbins et al. 1992
).
Perhaps in a normal state, this circuitry is only involved in
processing of information locally, and only under extreme conditions,
such as after spinal cord injury, do they play an additional role to
compensate for the loss of long descending projections after an
incomplete injury.
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Pudendal motor nerve sympathetic fiber response
The activation of sympathetic fibers in the PudMs is produced by
microstimulating the MRF bilaterally and ventrally in or medial to LPGi
(Johnson and Hubscher 2000). These PudM-SFRs were not
found in the present study after a complete chronic over-DHx or severe
Cx injury but were found after a complete chronic DHx (although
responses were weak), suggesting that the projections descend within
the ventral DLQ and the dorsal-most aspect of the VLQ (i.e.,
intermediate zone of white matter) at the midthoracic spinal cord.
These findings are consistent with previous studies showing the
location of descending sympathetic axons in the intermediate zone of
the spinal white matter, with terminal labeling in the intermediolateral cell column with injections of anterograde tracer into the LPGi (Martin et al. 1985
).
The PudM-SFR was maintained on both sides after chronic LHx, suggesting
that the descending projections are both crossed and uncrossed above,
as well as below, the level of the midthoracic lesion (see right
side of summary diagram, Fig. 10). These results are in contrast
to those observed after an acute unilateral lesion of the DLQ, where
responses were only maintained for the PudM on the intact-side. The
acute lesion results indicate that the descending projections are both
crossed and uncrossed above the level of injury and are only uncrossed
below (the opposite was found for PudM-RD). The reasons for this
difference in the results are unclear, but possibilities include those
outlined in the preceding text for PudM-RD. The likelihood of sprouting
of new connections across the midline is supported by previous work
demonstrating sprouting of unmyelinated afferent fibers after spinal
transection (Krenz and Weaver 1998). These new
connections are believed to provide new sources of innervation for
autonomic preganglionic neurons, which may account for the increased
sympathetic reflex responses to sensory stimulation that cause
autonomic dysreflexia (Krenz and Weaver 1998
).
Location of descending pathways
The reticular formation as a whole covers an enormous territory,
covering several rostral/caudal levels of the brain stem. Because of
this large extent, there are many different descending reticulospinal
projections with different spinal white matter locations (just as there
are several different locations for ascending spinoreticular
projections) (see discussion in Hubscher and Johnson 1999b). In the present study, the focus was on one subset of
reticulospinal projections, those that convey information to the male
urogenital tract via projections originating from MRF to the lower
thoracic and lumbosacral spinal cord. When combined, the results of the present study shows different locations for two long descending pathways between ventromedial MRF and the neural circuitry innervating the male urogenital tract. One pathway, whose activation produces a
depression of the DNP-elicited PudM discharge, is located somewhere in
the mid to lower region of the DLQ (see Figs. 10 and
11, left). This pathway
overlaps with the ascending one conveying high-threshold sensory inputs
originating from the male urogenital tract to MRF (see Fig. 11) (see
also Hubscher and Johnson 1999b
). The other descending
pathway, whose activation produces firing bursts of sympathetic
postganglionic neurons in the PudMs, is located somewhere in the
intermediate zone of the midthoracic white matter, either deep within
the DLQ and/or within the dorsal-most aspect of VLQ (see Figs. 10 and
11, right). When combined with the results from our
accompanying paper (Hubscher and Johnson 1999b
) (see
Fig. 11), the results demonstrate a bilateral spino-bulbo-spinal
circuitry confined, for the most part, to the dorsal half of the spinal white matter at T8.
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The location of descending reticulospinal projections within the DLQ
has been demonstrated in a number of other studies using rats, focusing
on the same subnuclei (i.e., nucleus reticularis gigantocellularis pars
alpha and LPGi) and looking at terminations within the
intermediolateral cell column and lumbosacral spinal cord (Loewy
et al. 1981; Martin et al. 1985
; Watkins
et al. 1984
; Zemlan et al. 1984
). Descending
reticulospinal projections from other brain stem regions also are found
within the DLQ (Basbaum and Fields 1979
; Basbaum
et al. 1977
; Leichnetz et al. 1978
; Loewy et al. 1979
; Martin et al. 1979
; Niemer
and Magoun 1947
; Watkins and Mayer 1982
;
Zemlan et al. 1979
). It is unknown whether these projections also participate in the control of pudendal reflexes. The
variety of descending reticulospinal projections that exist, including
those within the ventral quadrant, subserve many different functions.
These functions may depend on cell body location (for example, level of
pons vs. rostral medulla vs. caudal medulla; ventromedial medulla vs.
ventrolateral medulla), terminal field location (for example,
terminations within different spinal laminae and at different spinal
levels), and the motoneuronal pool being affected (for example,
locomotor vs. pelvic floor musculature) (Basbaum and Fields
1979
; Cohen et al. 1987
; Jones
1995
; Kow et al. 1977
; Murphy et al.
1996
; Nyberg-Hansen 1965
; Zemlan et al. 1979
, 1984
).
Functional implications
The MRF is a multifunctional zone, with multiple ascending and
descending inputs and outputs. The descending reticulospinal projections within DLQ have been shown to be part of a control mechanism for reflexive reactions to noxious (Basbaum and Fields 1979; Watkins and Mayer 1982
; Watkins et
al. 1984
; Wei et al. 1999a
,b
) and mating stimuli
(Watkins et al. 1984
). The disruption of the descending
MRF-lumbosacral pathway in the present study that results in the loss
of the supraspinal control (i.e., depression) of the DNP-elicited PudM
reflex likely contributes to the incoordination of perineal muscle
contractions that occurs after chronic spinal cord injury, and thus the
ensuing sexual dysfunction and bladder sphincter dyssynergia (see
discussion in Hubscher and Johnson 1999b
; Johnson
and Hubscher 1998
; Kruse et al. 1993
).
Both sexual and bladder dysfunction are due to an incoordination of
perineal muscle contractions mediated by pudendal motoneuron activity
and segmental reflexes. As discussed previously (Johnson and
Hubscher 1998), the function of the descending MRF-PudM-RD pathway in uninjured animals may be to coordinate, through the periodic
inhibition of DNP inputs, the phasic timing and sequence of propulsive
contraction bursts required for normal micturition and sexual function.
The loss of this MRF control after spinal cord injury to the dorsal
half of the midthoracic spinal cord, either through the elimination of
the descending pathways to the pudendal reflex circuitry and/or the
ascending sensory pathways of the DNP (Hubscher and Johnson
1999b
) likely contributes to the inappropriate tonic
contractions of the perineal musculature. In addition, disruption of
the descending MRF-PudM pathway in ventral DLQ and the dorsal-most
aspect of VLQ that produces an activation of postganglionic sympathetic
fibers in PudMs also may contribute to the disruption of sexual
function after chronic spinal cord injury. Although the function of
this pathway is unknown, it also may have an inhibitory role in
modulating sexual reflexes. Sympathetic fibers in the PudM have been
shown previously to innervate cavernosal erectile tissue and promote
detumescence (Galindo et al. 1997
). Conversely, PudM
sympathetic fibers may innervate glands in the distal urogenital tract
which participate in the ejaculatory response.
Our data on these ascending and descending pathways demonstrates that they are organized bilaterally with some redundancy so that damaging only one side of the spinal cord should not alter sexual and bladder reflex control. The behavioral data from LHx animals in the present study, that is, inhibition of penile reflex initiation and no signs of bladder-sphincter dyssynergia, supports this conclusion. Only when these pathways are disrupted bilaterally through DHx or severe Cx lesions were behavioral signs of dysfunction observed.
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ACKNOWLEDGMENTS |
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We thank V. Dugan and R. Odama for excellent technical assistance and data collection.
This study was supported by the American Paralysis Association, the Brain and Spinal Cord Rehabilitation Trust Fund of Florida, and Grant NS-35702 from the National Institute of Neurological Disorders and Stroke.
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FOOTNOTES |
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Address reprint requests to: C. H. Hubscher.
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 1 September 1999; accepted in final form 11 January 2000.
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REFERENCES |
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