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SACRAL MONITORING
Confusing
anatomy is often encountered during operations on complex
dysraphic lesions in the lumbosacral canal. It is common to see
nerve roots embedded in lipoma or scar tissue, or they may not
be easily distinguishable from fibrous adhesion bands. Sometimes
nerve roots that are bundled tightly by abnormally thickened
arachnoid can look like a thickened filum terminale. Also, the
transition between a functional but structurally deformed conus
and an intramedullary lipoma is not always visually apparent.
Thus, some objective means to identify the sacral nerve roots
and the conus is necessary to ensure preservation of these
neuronal structures. In addition, in some cases of complex
transitional lipomas, the tip of the conus is tautly suspended
by low sacral roots that are short, stout, and fibrotic. An
assessment of their functional integrity is useful for
determining whether dividing them, in order to complete the
untethering process, would lead to unacceptable loss of
sphincter function.
The first
sacral and lower lumbar roots are recognized readily by
intraoperative nerve stimulation while palpating for
contractions of the respective segmental muscle groups through
the surgical drapes. Identification of the lower sacral roots
and functional quantification of these roots and their
corresponding medullary connections, however, require some
objective assessment of perineal sensation and sphincter
function.
Modality for Sensory Monitoring of S2-4 Segments
The
assessment of evoked responses generated by directly stimulating
parts of the sex organs, urethra, and anal canal constitutes the
mainstay of sensory monitoring of the lower sacral segments.
Monitoring of such responses is most useful when the distal
conus or dorsal nerve roots are being rather strenuously
handled, as in certain difficult resections of large
transitional lipomas or during removal of the median fibrous
sleeve of a Type I split cord malformation. The latency and
amplitudes of the waveforms are exquisitely sensitive to
structural deformation and ischemic changes to the central
sensory pathways. Sensory evoked response monitoring is less
useful in the identification of sacral sensory roots, because
the responses are generated by end organ stimulation. Cortical
responses generated by direct dorsal root stimulation give much
less predictable waveforms, which are not stable enough for
foolproof identification purposes.
Anatomy
The
peripheral nerves that supply the bladder, anal canal, and
perineal skin, all potentially available for stimulation, are
divided into three main groups.
1.The
pudendal nerve is the primary somatic nerve to this region. The
pudendal motor neurons innervating the external sphincter and
pelvic floor originate from Onuf's nucleus in the anterior horn
of the S2 to S4 cord segments. The sensory fibers come from the
corresponding dorsal root ganglia. The mixed fibers course via
the S2, S3, and S4 roots to exit the spinal canal through the
sacral foramina (Figure-1). Somatosensory impulses travel in this nerve
from receptors located in the skin of the genitalia and
perineum, the pelvic floor, and bulbocavernosus muscles, as well
as in the mucosa of the distal urethra and anus. Motor fibers in
the pudendal nerve innervate the bulbocavernosus muscle,
external urethral sphincter, external anal sphincter, and pelvic
floor muscles. The pudendal nerve is the most easily accessible
nerve for evoked response testing.
Fig-1:
Schematic representation of the pudendal nerve and branching.
3.The
pelvic splanchnic nerves supply the sacral parasympathetic
innervation to the pelvic organs. The motor neurons in this
nerve originate in the S2 to S4 cord segments, slightly more
caudal than the pudendal motor neurons. The fibers are
distributed to the pelvic organs via the S2 to S4 nerve roots
and inferior epigastric plexus. The pelvic nerve carries sensory
afferents from the proximal urethra, bladder wall, prostate,
seminal vesicles, and rectum. Motor innervation is primarily to
the detrusor muscles, the corpus cavernosus, the rectum, and
probably the upper smooth-muscle portion of the external
urethral sphincter. Evoked responses can be elicited on
stimulation of the proximal urethra and bladder, presumably due
to activation of the pelvic sensory fibers.
3.The
hypogastric nerve plexuses carry autonomic (sympathetic) fibers
from the intermediolateral cell column of the T11-L2 spinal cord
segments. The preganglionic fibers course via the paravertebral
sympathetic chain ganglia, inferior mesenteric plexus, superior
hypogastric plexus, and finally the inferior hypogastric plexus.
The postganglionic fibers are distributed to the smooth muscles
of the bladder neck, the smooth-muscled internal urethral
sphincter, the parasympathetic intramural ganglia of the
detrusor muscles and probably the intrinsic portion of the
external urethral sphincter. The postganglionic fibers also
share connections with plexuses around the rectum and anal
canal, seminal vesicles, ductus deferens, prostate, and corpus
cavernosus in the male, and vagina in the female. It is
uncertain how much the afferent component of the hypogastric
nerves contributes to the evoked response in humans.
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Cortical Sensory Evoked Response
Standard recording of the cortical
evoked response is made by 5-mm silver or
gold-plated cup electrodes or dermal needle
electrodes sutured to the scalp. The electrode
impedance should be kept below 2000
Ω. The active
recording electrode is placed in the midline,
approximately 2 cm behind the Cz
electroencephalographic recording site according to
the International 10-20 Electrode Placement System.
This has been demonstrated to give maximum cortical
response on stimulation of the penile and clitoral
skin. The reference electrode can be placed at a
number of sites, although the forehead (Fpz) is
convenient and gives a good waveform. Stimuli are
delivered at a rate of 3.5 to 5.0 per second, with
approximately 2.5 to 3.0 times the threshold
intensity. The recording console consists of high-
and low-frequency filters to keep the band pass at
30 to 1000 Hz. The sensitivity of the signal
amplifier is usually set at 2 to 10
µV per division.
About 250 to 350 responses are averaged to ensure
reproducibility of the reading, but weak and
unstable signals from severely damaged conuses may
require up to 1000 responses to generate an
interpretable waveform.
Pudendal Dermatomal Evoked Response
The most commonly used form of
pudendal nerve evoked response utilizes stimuli
applied to the sensory domain of the dorsal genital
nerve. In the male, the dorsal nerve of the penis
can be stimulated either bilaterally or unilaterally
using 5-mm cup electrodes placed 2 to 3 cm apart at
the base of the penis, with the cathode proximal to
the anode. Stimuli up to 3.0 or 3.5 times threshold
are well-tolerated. In the female, the dorsal nerve
of the clitoris is stimulated by 5-mm cup electrodes
or fine dermal needle electrodes fixed bilaterally
to the cleft between the labia major and labia
minor. The anodes are placed adjacent to the
clitoris bilaterally and the cathode approximately 2
cm posterior to the anode.
The averaged cortical pudendal evoked
response has a similar morphology as the responses
obtained from stimulation of the posterior tibial or
peroneal nerve. The response has a fairly
characteristic "M" pattern, with an initial positive
deflection followed by a constant negative,
positive, negative, positive waveform. Injury to the
S2-4 roots or cord segments is manifested by
lengthening of the P1 latency and
decreased amplitude of the triphasic waves (Figure
2). |
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Figure 2. Cortical pudendal nerve
evoked responses obtained from a male child with a
Type I split cord malformation. The neurological
deficits are much worse in the left leg. All
responses are recorded from Pz referenced to Fz.
A)
Tracing obtained by stimulating the right dorsal
nerve of the penis. B) Tracing obtained by
stimulating the left dorsal penile nerve. Note the
significant reduction in amplitude in the left
responses. |
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Urethral Evoked Response
Cortical evoked responses of very similar morphology and
latencies can be obtained using stimulating electrodes
embedded in a catheter inserted into the bladder. The
catheter has a balloon at its tip, which can be pulled
back snugly for anchorage. The location of the urethral
electrodes can be kept reasonably constant to eliminate
movement artifacts and interference.
Anal
Evoked Response
Electrode-bearing catheters can also be inserted into
the anal canal for measurement of anal evoked responses.
The catheter is anchored by double balloons, the inner
one within the anorectal junction and the outer one
wedged at the anal verge. The cortical anal responses do
not differ from the urethral responses or the pudendal
dermatomal responses.
Spinal
Evoked Response
Evoked responses can be recorded by electrodes placed on
the skin over the spine in humans. They reflect the
afferent volley traversing the dorsal columns. The
responses progressively increase in latency at more
rostral recording locations. Spinal evoked responses are
relatively easy to obtain in children, but the
amplitudes and waveform definition decrease with age,
such that by mid-teenage years, these responses are more
difficult to obtain, as in the case of adults. The
response over the mid-tolower lumbar spine consists of
an initially positive triphasic potential, representing
the volley as it ascends the cauda equina. Over the
caudal thoracic spine, the response consists of an
initially positive, predominantly negative triphasic
wave, the negative component of which has several peaks
or inflections. The initial portion of this response
arises in the intramedullary continuation of the dorsal
root fibers, and the subsequent portion reflects
synaptic activity concerned with local reflex mechanism
rather than the propagation of the response to more
rostral cord levels. From the mid-thoracic to the
cervical levels, the response consists of small,
triphasic potentials that are difficult to follow,
presumably arising from multiple ascending pathways
including the dorsal and dorsolateral columns.
The
only consistent spinal pudendal response has been from
stimulation of the dorsal nerve of the penis. The
recording electrodes are usually fixed at the T12-L1
interspinous space. The response has a morphology
comparable to the spinal response from the posterior
tibial and peroneal nerves but with smaller amplitudes
and a much shorter latency (Figure 3). The spinal
pudendal evoked response is sometimes not measurable in
overweight individuals, but its presence yields useful
information concerning peripheral sensory conduction
from the penis since it bypasses the central conduction
pathway rostral to the thoracic levels. Because the
cortical pudendal evoked response has similar latency
with the cortical posterior tibial response, the central
conduction time involved in the pudendal pathways must
be considerably longer than that in the posterior tibial
pathways.
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Figure 3. Spinal pudendal evoked responses recorded over
the T12-L1 interspinous space on stimulation of the
dorsal nerve of the penis (upper), and spinal responses
on stimulation of the posterior tibial nerve (lower).
Note the much shorter latency of the pudendal response.
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Modality for Motor Monitoring of the S2-4 Nerve Roots
Pudendal sensory evoked responses are useful in
monitoring intraoperative injury to the conus and lower
sacral sensory nerve roots, but they are neither
qualitatively nor quantitatively suitable for the
identification of the lower sacral roots (especially the
motor roots) or conus from non-neural elements.
Intraoperative identification requires some way of
measuring the oneto-one stimulus-to-response coupling
of end organ function when the nerve root in question is
being stimulated. For the lower sacral roots, this means
the assessment of sphincter function.
External Anal Sphincter Electromyography
External anal sphincter electromyography (EMG) has long
been found useful as a qualitative tool for studying
anorectal closure function and disorders. The EMG
electrodes are either embedded in an anal plug or anal
balloon, which is placed into the anal canal, or are in
the form of needles inserted directly into the external
anal sphincter transmucosally. The needle electrodes are
more reliable because they are not subject to
dislodgement or to having mechanical artifacts during
contraction of the sphincter itself; however, accurate
and secure placement of the needles requires some
expertise and is initially best done by the
neuro-urologist. The grounding plate is pasted on the
patient's thigh, and EMG recordings are made using a
standard bladder diagnostic unit. The sensitivity of the recording stylus
is adjusted so that minimal deflection occurs at rest.
With stimulation of the lower sacral motor roots, the
stylus gives a discrete one-to-one spike-deflection,
much different than the baseline.
External Anal Sphincter Pressure Monitor
The
external anal sphincter EMG requires bulky and expensive
equipment, as well as the availability of someone expert
in the accurate placement of the needle electrodes. An
alternate method of monitoring anal sphincter function
is the direct measurement of the "squeeze pressure"
induced by sacral root stimulation using a
pressure-sensitive balloon inserted in the anal canal.
This technique is simple and noninvasive, requires no
special expertise, utilizes inexpensive, portable
equipment, and produces easily interpretable pressure
waves which are semi-quantitative and virtually
unaffected by other electronic components in the
operating room that are known to cause annoying
baseline noise in an EMG recording.
Physiology
The
relationship between EMG and contractile strength in a
longitudinal muscle was first defined by Lippold, who
found a linear relationship between the integrated
action potentials on the EMG and the tension generated
by voluntary isometric contractions of the human
gastrocnemius. This linearity was explained by the fact
that an increase in contractile strength of
a muscle is brought about either by a spatially random
increase in the number of contracting motor units or by
random increments of discharge frequencies of the active
units; in both situations, the integrated electrical
output of the muscle would increase proportionately. The
same linear relationship was also demonstrated in the
external anal sphincter by Schweiger, who made
simultaneous recordings of sphincter EMG and anal canal
pressures with an anal balloon. These data support the
validity of using squeeze pressure, instead of sphincter
EMG, to monitor the functional status of the lower
sacral motor neurons.
In
order for the anal pressure monitor to be operational,
some sphincter function must be present. Theoretically,
a severely damaged motor nerve with only enough viable
axons to generate a barely visible EMG would produce no
measurable squeeze pressure; in such a situation, the
EMG might be more sensitive. However, such a nerve
would not provide useful sphincter function for the
patient, and its preservation is of doubtful value. In
the author's experience, any external anal sphincter
that could generate enough voluntary or reflex (as in
the bulbocavernosus or anal wink reflex in the infant)
contractions to be appreciable by preoperative digital
examination should produce recognizable pressure spikes
on the anal pressure monitor. This anal balloon monitor
is therefore sufficiently sensitive for the practical
purpose of sacral root and conus identification.
Anatomy
The
external anal sphincter consists of a bulky deep part, a
fusiform superficial part, and a subcutaneous part
decussating behind and in front of the anus. It
encloses the lower part of the levator ani, the
anorectal junction, and the anal canal in the shape of a
funnel. The internal anal sphincter arises from the
muscular coats of the rectum and insinuates itself
between the rectal mucosa and the upper portion of the
funnel.
The
external anal sphincter is innervated by the pudendal
nerve. This arises from the anterior division of S2 and
S3 and both divisions of S4, enters the pudendal (Alcock's)
canal through the lesser sciatic foramen, and divides
into two main branches just proximal to the urogenital
diaphragm. The proximal branch, the inferior
hemorrhoidal nerve, supplies the striated muscles of
the external anal sphincter; the distal branch, the
perineal nerve, supplies the external urethral
sphincter. The internal anal sphincter, composed of
smooth muscles, is innervated by the hypogastric nerve,
derived from the intermediolateral (sympathetic)
columns of L1 and L2. Stimulation of the S2, S3, and
S4 roots, therefore, activates only the external and not
the internal anal sphincter. Furthermore, unless there
is localized disease or trauma to the pudendal branches
at the urogenital diaphragm, activity of the external
anal sphincter reflects function of the external
urethral sphincter.
The
anal pressure balloon described here is an elongated
ellipsoid selected specifically to pick up activities
from all three parts of the external sphincter funnel.
Its elongated span also minimizes the possibility of
accidental dislodgement by contractions of the pelvic
musculature induced intraoperatively. Although the
elongated balloon will also pick up contractions of the
internal anal sphincter, the latter is never activated
by the nerve stimulator or by manipulation of the lower
sacral spinal cord or nerve roots because its nerve
supply is from L1 and L2. However, being made up of
smooth muscles, the internal sphincter does have
spontaneous rhythmic contractions that will be
registered by the balloon, and these must be
distinguished from the stimuli-generated pressure
spikes from the external anal sphincter.
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Equipment
The
pressure sensor is a double-lumen balloon catheter ordinarily used for
intraluminal angioplasty (Figure 4). The ellipsoidal
balloon is made of treated polyethylene, which does not
stretch or deform at high inflation pressures, so that a
high degree of sensitivity to circumferential squeezing
can be maintained. The central infusion catheter
concentric with the balloon is not actually being used
in the pressure measurement but functions effectively
as a stent for easy balloon insertion. The balloon comes
in different sizes, but the 3 x 0.8-cm (inflated
diameter) balloon should fit almost any patient, from
infants to large adults.
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Figure 4. Double-lumen polyethylene balloon catheter.
The infusion lumen is not involved in the pressure
measurement but merely serves as a stent. |
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The
balloon is held vertically with the tip down, and is
maximally deflated and inflated several times with water
to expel all air bubbles. It is then connected to a
Bentley Model D240 pressure transducer, which displays the
pressure tracing on a two-channel Datascope Model 870
monitor (Figure
5). Although the baseline pressure of the balloon, which
can be adjusted by varying the amount of water used,
does not affect the actual pressure measurement, it
should be kept within a range that allows the monitor to
give good-sized pressure waves in the usual sensitivity
setting. The optimal condition is when the balloon is
rendered just turgid (with 0.8 ml water for the 3 x
0.8-cm balloon) and when the sensitivity on the Datascope
monitor is set at 25 (1 cm on the screen is
calibrated to 25 torr). The balloon is inserted into the
anal canal until its posterior end is just visible at
the mucocutaneous junction and then taped securely to
the gluteal skin.
One
cutaneous electrocardiography (ECG) electrode is pasted
over each iliac crest and a third on the right upper
thigh. The ECG tracing is displayed continuously on the
second channel of the Datascope screen (Figure 6).
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| Figure-5. Simple
assembly, consisting of the balloon
catheter, the Bentley pressure transducer,
the injecting syringe and stop-cocks and the
2-channel Datascope monitor. |
Figure 6. ECG electrodes are placed over the iliac
crests and the thigh to register the stimulus artifact. |
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Technical Points
Intraoperative nerve stimulation is done with a
disposable monopolar nerve locator-stimulator using 3 V and three
variable current intensities: 0.5, 1, and 2 mA. The
monopolar stimulator is chosen over the bipolar
stimulator because only the former will generate
sufficient volume-conducted current to produce an
obvious stimulus artifact on the ECG when any tissue is
touched by the monopolar electrode. When a lower sacral
root is stimulated, the combined ECG stimulus artifact
and the pressure spike from the external anal sphincter
form an easily recognizable electromechanical couple on
the monitor (Figure 7).
There are two advantages in having this
electromechanical couple. 1) The stimulus artifact
eliminates the possibility of a faulty stimulator or
faulty stimulation technique because even at 0.5-mA
current, the monopolar electrode always produces a
prominent spike on the ECG tracing. If an artifact is
present without a corresponding pressure spike at high
current intensity, the tissue stimulated does not
innervate the external anal sphincter; if neither
stimulus artifact nor pressure wave is obtainable, then
the nerve stimulator is faulty (Figure 8).
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Figure 7. Monopolar stimulations of the S3 root in a
5-year-old child. Note the prominent stimulus artifact
on the ECG tracing (arrows) coupled with large pressure
spike waves measured at 50-torr peak values. Scale in
torr. |
Figure 8. Use of the EGG. A) Stimulation of a
nonfunctional element (fibrous adhesion band) showing
only EGG stimulus artifacts (arrows) but no pressure
response. Scale in torr. B) Recognition of a faulty
nerve stimulator when neither an EGG stimulus artifact
nor a pressure response is detectable. Scale in torr. |
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2)
Involuntary, rhythmic activity of the internal anal
sphincter has been noted as spontaneous 5-to10-torr
waves with a frequency of 10 to 30 per minute, which
may be confused with external anal sphincter activities
except for the fact that these spontaneous waves are
completely out of phase with the ECG stimulus
artifacts (Figure 9).
During nerve stimulation, the cerebrospinal fluid must
be continuously suctioned away from the stimulation site
to prevent current dispersion. With this precaution,
supramaximal stimulation of the small sacral roots of
infants and young children can usually be accomplished
with 0.5 mA. The larger roots of adults sometimes
require higher amperage, as does direct conus
stimulation. Unilateral S2, S3, or S4 stimulation
consistently generates a peak pressure of 40 to 70
torr, in line with recordings reported by Lane of 60 to
125 torr pressures with voluntary (bilateral)
contraction in normal adults. Even in young infants,
peak pressure responses are generally above 40 torr. S3
stimulation produces the strongest and most consistent
response in the external anal sphincter. Direct
stimulation of the conus also results in waves of
comparable peak values but usually with a wider base,
probably because of multilevel and bilateral recruitment of anterior horn cell
units (Figure 10). |
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Figure 9. Use of the EGG. Spontaneous lower pressure
waves «10 torr) from the internal anal sphincter are
completely out of phase with the EGG stimulus artifacts
(arrows). Scale in torr. |
Figure 10. Direct stimulation of the conus in a
3-year-old boy, generating pressure waves with a wide
base and irregular blunted peaks. Stimulus artifacts are
indicated by the arrows. Scale in torr.
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Occasionally, a small pressure wave of less than 7 torr
follows S1 stimulation (which does not innervate the
sphincters) because of compression on the protruding
proximal portion of the balloon by the medial inferior
fibers of the gluteus maximus. Although such "ripple
waves" are easily differentiated from the tall spike
waves of healthy lower sacral roots, they could be
mistakenly construed as the subdued responses seen with
partially damaged S2, S3, and S4 roots. This confusion
is eliminated if care is taken to secure the posterior
end of the balloon just above the mucocutaneous
junction.
Stimulation of the filum terminale and nonneural
tissues always produces a stimulus artifact but not a
pressure wave. Thus, the S2, S3, and S4 roots and the
conus can be distinguished from the S1 and lumbar roots,
the filum, lipoma, fibrous adhesions, and other
nonfunctional fibroneural bands, such as an occult
myelomeningocele. The ECG artifact and pressure wave
relationships are summarized in Table 1.
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Table 1.
Interpretation of Stimulus (ECG) Artifact and
Pressure Response Relationship |
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Stimulus (ECG) Artifact |
Pressure
Response |
Interpretation |
| - |
- |
Faulty stimulator |
| + |
-; spike waves 40-75 torr |
S2,S3,S4, conus medullaris |
| + |
- or ripple waves
(<7 torr) (and plantar flexion) |
S1 |
| + |
- |
Lumbar roots, filum, non-neural
tissues |
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No stimulation |
Rhythmic waves 10-30/min < 10 torr |
Internal anal sphincter
spontaneous activity |
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Clinical Use of the Anal Sphincter Function Monitors
The
anal sphincter function monitors (EMG or pressure balloon)
have been found useful in the following circumstances.
1.
Functional sacral nerve roots embedded in large lipomas may
be detected and traced through a sometimes aberrant course
to their exit foramina. This is particularly useful in
transitional lipomas that involve the dorsal as well as the
ventral portions of the conus
2. Sacral
nerve roots can be distinguished from fibrous adhesion
bands.
3. Atrophic, fibrous distal nerve roots in long-standing
myelodysplastic cases can be holding the conus tautly
against the dura and can thus prevent complete release of
the tethering. If these roots can be shown by the monitors
to have no contribution to sphincteric functions, they
should be cut.
4. Occasionally, the junction between functional conus and
fat is not well demarcated in cases of large transitional
lipomas or the type of terminal lipoma not having an
intervening filum terminale. Direct stimulation proceeding
from the obviously normal portion of the conus in a caudal
direction will identify the lowest extent of pudendal motor
neurons, beyond which sphincter contractions can no longer
be elicited by the nerve stimulator (Figure 11).
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Figure 11. Progressively caudal stimulation of the extremely
stretched-out conus of a 48-year-old patient with adult tethered
cord syndrome. Stimulation on the obviously normal portion of
the conus generated tall spike waves (A), stimulation at the
junctional zone between the conus and the filum produced smaller
waves with a wide base (B), and stimulation just beyond the
caudal extent of the conus elicited a minimal pressure response
(C). Arrows indicate stimulus artifacts. Scale in torr. |
Pelvic Floor EMG
Needle
recording electrodes can be percutaneously inserted into the
"extrinsic" portion of the external urethral sphincter to
monitor activity of this sphincter. This technique is
routinely used by neurourologists to correlate simultaneous
measurements of bladder pressure, urethral pressure, and
external urethral sphincter activities. Pelvic floor EMG can
thus be used for intraoperative sacral root identification
in the same manner as external anal sphincter EMG.
Modality for Sacral Reflex Monitoring
Two
reflexes with centers in the sacral cord can be utilized to
assess the integrity of both the sensory and motor roots as
well as their interconnecting intramedullary components.
Bulbocavernosus Reflex
The
reflex response of the bulbocavernosus muscle to stimulation
of penile nerves can be studied using square wave electrical
stimuli applied through ring electrodes on the penis, and
recorded either by needle electrodes in the muscle or by
surface electrodes fixed to the midline of the perineum,
between the base of the penis and the anus. The averaged
response from 50 to 100 stimuli is usually biphasic with an
initial negative peak. The latency for most healthy adults
is 24 to 42 msec but varies with age and maturation in young
children. The waveform is also distorted significantly in
most cases of myelodysplasia and tends to become "unstable"
with very minor manipulations of the conus. The use of this
monitoring modality is therefore limited and is feasible
only in patients with virtually normal sphincter function
preoperatively.
Urethral to Anal Sphincter Reflex Response
The
urethral to anal sphincter reflex can be measured using
stimulating electrodes similar to those used in eliciting
urethral cortical evoked responses and recording electrodes
used in recording external anal sphincter EMG. The latency
is considerably longer (50 to 70 msec) than the
bulbocavernosus reflex, although their morphologies are
similar. The long latency in the urethral-anal sphincter
reflex is due partly to the slower conducting velocity of
autonomic afferent fibers and partly to a more complex
central polysynaptic reflex organization.
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