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The organization of the central control of micturition in cats and humans
Blok, Bertil Feddo Maarten
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The Organization of the Central Control of Micturition
in Cats and Humans
Anatomical and Physiological Investigations
© 1998, B.F.M. Blok
All rights reserved. No part of this book may be reproduced or transmitted in any form
or by any means, without permission from the author.
ISBN: 90-367-0888-5
Cover design: P.O. Gerrits
Printed by: Joh. Enschedé en Zonen, Amsterdam
This thesis was supported by:
Johan Vermeij Stichting
Medical Measurements Systems
Medtronic Interstim
Remmert Adriaan Laan fonds
Van Leersumfonds KNAW
RIJKSUNIVERSITEIT GRONINGEN
The Organization of the Central Control of Micturition
in Cats and Humans
Anatomical and Physiological Investigations
Proefschrift
ter verkrijging van het doctoraat in de
Medische Wetenschappen
aan de Rijksuniversiteit Groningen
op gezag van de
Rector Magnificus, dr. F. van der Woude
in het openbaar te verdedigen op woensdag 20 mei 1998
des namiddags te 2.45 uur
door
Bertil Feddo Maarten Blok
geboren op 15 december 1962
te Noordwijk
Promotor:
Prof. dr. G. Holstege
Promotion Committee:
Prof. dr. W.C. de Groat
University of Pittsburgh, Pittsburgh
Prof. dr. D. Griffiths
University of Alberta, Edmonton
Prof. dr. R.A. Janknegt
University of Maastricht, Maastricht
Prof. dr. E.A. Tanagho
University of California, San Francisco
Paranymphs:
Drs. L.J. Mouton
Dr. P.O. Gerrits
Voor Wessel en Willemijn
Contents
General Introduction
9
Chapter 1
Ultrastructural Evidence for a Paucity of
Projections from the Lumbosacral Cord to the
Pontine Micturition Center or M-Region in the Cat:
A New Concept for the Organization of the
Micturition Reflex with the Periaqueductal Gray as
Central Relay
Bertil F.M. Blok, Henk de Weerd, and Gert Holstege
J. Comp. Neurol. 359:300-309 (1995)
19
Chapter 2
Ultrastructural Evidence for a Direct Pathway
from the Pontine Micturition Center to the
Parasympathetic Preganglionic Motoneurons
of the Bladder of the Cat
Bertil F.M. Blok, and Gert Holstege
Neurosci. Lett. 222:195-198 (1997)
33
Chapter 3
The Pontine Micturition Center Projects to
Sacral Cord GABA Immunoreactive Neurons
in the Cat
Bertil F.M. Blok, Henk de Weerd, and Gert Holstege
Neurosci. Lett. 233:109-112 (1997)
39
Chapter 4
Electrical Stimulation of the Sacral Dorsal Gray
Commissure evokes Relaxation of the External Urethral
Sphincter in the Cat
Bertil F.M. Blok, Jos T.P.W. van Maarseveen, and
Gert Holstege
Neurosci. Lett., in press
45
Chapter 5
Location of External Anal Sphincter Motoneurons in
the Sacral Cord of the Female Domestic Pig
Bertil F.M. Blok, Gert Roukema, Bas Geerdes, and
Gert Holstege
Neurosci. Lett. 216:203-206 (1996)
49
Chapter 6
The Two Pontine Micturition Centers in the Cat are
not Interconnected; Implications for the Central
Organization of Micturition
Bertil F.M. Blok, and Gert Holstege
J. Comp. Neurol., submitted
53
Chapter 7
Direct Projections from the Periaqueductal
Gray to the Pontine Micturition Center (M-region).
An Anterograde and Retrograde Tracing Study
in the Cat
Bertil F.M. Blok, and Gert Holstege
Neurosci. Lett. 166:93-96 (1994)
63
Chapter 8
A PET Study on Brain Control of Micturition
in Humans
Bertil F.M. Blok, Antoon T.M. Willemsen, and
Gert Holstege
Brain 120:111-121 (1997)
69
Chapter 9
A PET Study on Cortical and Subcortical Control
of Pelvic Floor Musculature in Women
Bertil F.M. Blok, Leontien M. Sturms, and
Gert Holstege
J. Comp. Neurol. 389:535-544 (1997)
83
Chapter 10
Brain Activation during Micturition in Women
Bertil F.M. Blok, Leontien M. Sturms, and
Gert Holstege
Brain, in press
95
General Discussion
105
References
111
Abbreviations
118
Summary
119
Samenvatting
121
Dankwoord
123
List of Publications
125
Curriculum Vitae
127
General Introduction
General Introduction
The kidneys continuously produce urine,
which, for practical reasons, is first collected
in the bladder where it is stored until disposal or micturition is possible. Since the
individual animal is relatively vulnerable
during the release of urine, micturition only
takes place when the environment is relatively safe. Furthermore, in many animals
urine is used as a marker for territorial demarcation or sexual attraction (a female lets
the males know that she is in estrus by leaving a scent trace). Thus, micturition does
not take place at random, but is part of a
rather complicated behavior, directly related
to the survival of the individual or species.
In order to investigate this rather complicated behavior it is necessary to identify and
define the nature of the muscular and neural structures involved in micturition. This
chapter gives a general description of the
central organization of motor control. The
structures involved in micturition will be
introduced, and the aim of the work, described in this thesis, will be explained.
Motor system
Motoneurons
All motor behavior, including micturition,
is the result of the activation of specific sets
of striated and smooth muscles. Striated or
skeletal muscles are innervated by somatic
motoneurons, whereas smooth musculature
is innervated by sympathetic or parasympathetic postganglionic motoneurons,
which, in turn, are innervated by preganglionic motoneurons. The sympathetic and
parasympathetic motoneurons and their fibers form the autonomic nervous system.
Each muscle is innervated by its own group
of motoneurons. Somatic motoneurons innervating muscles of the head are located
in various cell groups in the brainstem, those
of the remaining parts of the body in the
ventral horn of the spinal cord. The axial
muscles of the neck and back are innervated
by motoneurons in the medial part of the
ventral horn throughout the length of the spinal cord, whereas those innervating the
muscles of the extremities are located in the
lateral part of the ventral horn of the cervical and lumbosacral enlargements. Sympathetic preganglionic motoneurons are
present in the lateral horn of the thoracic
and upper lumbar cord, whereas parasympathetic preganglionics are located in certain brain stem nuclei, and in the sacral cord
(see Holstege, 1996 for review). All central
somatic and autonomic motoneuronal cell
groups are controlled by other structures in
the central nervous system (CNS). The motoneurons together with their control structures form the “motor system” as defined
by Holstege (1991; Fig. 1).
The basic premotor interneuronal system
Motoneurons receive afferents from several
sources. Most are excitatory in nature, but
normal motor behavior is not possible without inhibitory afferent input to antagonist
muscles. In the spinal cord most of the
premotor interneurons are located in the intermediate zone of the spinal cord (laminae
V to VIII; Rexed, 1952). Premotor interneurons for the somatic motoneurons of the
brainstem are located in the pontine and
medullary reticular formation, which can be
seen as the rostral extent of the spinal intermediate zone (Holstege et al., 1977). Most
of these interneurons project to motoneurons at the same level or at levels rostral or
caudal to where interneurons are located.
The majority of the premotor interneurons
terminate on motoneurons nearby, but in
some cases they project to motoneurons
much further away, for example C2 interneurons projecting to C8 (see Holstege,
1988), the respiratory interneurons in the
medulla projecting to the phrenic, intercos-
9
General Introduction
Motor system
Emotional motor
system
Voluntary motor
system
Lateral
Medial
Lateral
eye, neck,
axial and proximal
body movements
Medial
specific
emotional
behaviors
independent
movements of
the extremities
gain setting systems
including triggering
mechanisms of
rhythmical and other
spinal reflexes
Basic system
(premotor interneurons)
Motoneurons
Fig. 1. Schematic overview of the three subdivisions of the motor system (from Holstege, 1996).
tal or abdominal motoneurons, and the
nucleus retroambiguus (NRA) interneurons
in the caudal medulla projecting to motoneurons in the lumbosacral cord involved
in mating behavior (see VanderHorst and
Holstege, 1996). The “basic motor system”
consists of all the premotor interneuronal
cell groups in the spinal cord and caudal
brainstem (Fig. 1).
The voluntary or somatic motor system
The influence of the voluntary motor system is evident in hemiplegic patients who
are unable to lift their arm or leg voluntarily on one side of their body. The somatic
motor system is first described by Kuijpers
(for review, see Kuijpers, 1981; Holstege
1991; 1996) and involves structures controlling voluntary non-emotionally directed
movements. It consists of a medial and a
lateral component. The medial component
controls the voluntary control of proximal
musculature of the trunk and back, and is
important for postural control against grav-
10
ity and the coordination of the head-body
movements. This component consists of the
ventral cortico-, interstitio-, tecto-,
vestibulo-, and reticulospinal tracts, which
are located at the level of the spinal cord in
the ventral funiculus. These tracts terminate
bilaterally on premotor interneurons located
medially in the ventral horn, which in turn
project to the motoneurons of back and trunk
muscles.
The lateral component concerns the voluntary control of distal musculature of arms
and legs, and is important for behavior like
grasping objects and typing on a computer.
In most mammals the lateral component
consists of the lateral corticobulbospinal
tract and the rubrospinal tract. The lateral
corticospinal tract originates in the motor
and premotor cortex, passes through the internal capsule, cerebral peduncle and pyramidal tract to cross via its decussation at the
transition between brainstem and spinal
cord. The fibers terminate on premotor interneurons and motoneurons in the lateral
General Introduction
part of the ventral horn. Interruption of the
lateral corticospinal tract by a cerebral stroke
in the internal capsule results in a paralysis
of one or more contralateral limbs. The rubrospinal tract, originating in the nucleus
ruber in the mesencephalon, also plays an
important role in the lateral component, but
not in humans, where it seems to be “overgrown” by the corticospinal tract.
The emotional motor system
Many studies demonstrate that certain parts
of the limbic system give rise to a descending system, which is completely separate
from the somatic motor system. Holstege
(1992; 1996) used the term “emotional motor system” for the other motor system,
originating in parts of the limbic system or
in structures strongly related to it. All emotionally related activities together are crucial for the survival of the individual or its
species. Also the emotional motor system
can be subdivided into a medial and a lateral component (Fig. 1). The medial component, via its projections to the neurons of
origin of the diffuse (sub-)coeruleo-, ventral medullary reticulospinal and raphespinal
pathways, has a global effect on the level of
activity of all the somatosensory and motoneurons by changing their membrane excitability. The medial component has its origin in the medial hypothalamus and mesencephalic periaqueductal gray (PAG). The lateral component represents several distinct
pathways underlying specific emotional behaviors, such as vocalization (Jürgens, 1979;
Holstege, 1989), blood pressure control
(Lovick, 1993; 1996), sexual behavior (Pfaff
et al., 1994; VanderHorst and Holstege,
1995), and micturition (Blok and Holstege,
1996). The lateral component has its origin
in the central nucleus of the amygdala, the
bed nucleus of the stria terminalis, the lateral hypothalamus, but also the mesencephalic PAG. The PAG controls several relatively
uncomplicated primary reactions important
for the survival of the individual, like ag-
gression and defensive behavior, and important for the survival of the species, like maternal and reproductive behavior. Each of
these reactions consists of a specific array
of motor activities and level setting mechanisms. In order to control this specific array
specific parts of the PAG project to specific
premotor interneurons (Fig. 2).
Micturition related motoneurons and
premotor interneurons
Micturition is a coordinated action between
the bladder muscle and the striated external
urethral sphincter (EUS), which closes the
bladder outlet. The EUS is part of the pelvic floor musculature. During the storage
of urine, the detrusor muscle is relaxed and
the EUS is tonically contracted. When micturition takes place, this activation pattern
is reversed: the EUS is relaxed and the bladder contracts, resulting in expulsion of urine.
Although the preganglionic motoneurons of
the bladder and the somatic motoneurons of
the EUS are located in the sacral cord (Fig.
3), their premotor interneurons are located
in the brainstem.
Motoneurons innervating the urinary bladder and external urethral sphincter
The parasympathetic preganglionic motoneurons of the smooth muscle of the bladder (musculus detrusor) are located in the
sacral intermediolateral cell group (IML).
In the cat these neurons are located at the
spinal segments S2-S3 and their axons reach
the bladder via the pelvic nerve (Morgan et
al., 1979). In the rat the parasympathetic
preganglionic motoneurons are located in
the spinal segments L6-S1 (Hancock and
Peveto, 1979; Nadelhaft and Booth, 1984)
and in humans in S2-S4 (Pick, 1970). The
preganglionic fibers terminate on ganglion
cells in the bladder wall. These postganglionic neurons innervate the smooth bladder
muscle.
The majority of the sympathetic preganglionic motoneurons innervating the bladder
11
General Introduction
12
Limbic system
Periaqueductal gray
(PAG)
Pedunculopontine
and cuneiform nuclei
Ventral 1/3 of caudal pontine
and medullary medial tegmentum
sympathetic
sensory neurons
preganglionics
in the
in the
dorsal horn
intermediolateral
cell column
general level
of sympathetic
activity
nociception
control
motoneurons and
premotor interneurons
in the
ventral horn and
intermediate zone
locomotion
Barrington's
nucleus
subretrofacial
nucleus
nucleus
retroambiguus
parasympathetic
preganglionics
in the
sacral cord
sympathetic
preganglionics
in the
intermediolateral
cell column
motoneurons of the
larynx,
pharynx,
soft palate, and
expiratory muscles
motoneurons of the
iliopsoas,
adductor longus,
hamstring,
pelvic floor, and
axial muscles
spinal cord
laminae
VIII and medial VII
T1-T2
intermediolateral
cell column
C4-T8
lamina X
micturition
(this thesis)
cardiovascular
changes
vocalization
receptive
behavior
defensive
posture ?
pupil dilatation ?
?
Fig. 2. Schematic overview of the descending projections from the PAG to different regions in the caudal brainstem and spinal cord, and their possible
functions.
General Introduction
(Onufrowicz, 1899).
(+)
Parasympathetic
motoneurons
Onuf's nucleus
S2
(+)
Bladder
(+)
External
urethral
sphincter
Fig. 3. Schematic overview of the sacral motoneurons involved in micturition.
are located in the intermediolateral cell
group of the caudal thoracic and rostral lumbar cord and send their axons via the
splanchnic nerves to sympathetic ganglion
cells in the lumbosacral sympathetic chain,
inferior mesenteric ganglia and the major
pelvic ganglia (Applebaum et al., 1980;
Andersson and Sjögren, 1982; Vera and
Nadelhaft, 1992). Postganglionic sympathetic fibers run via the pelvic and hypogastric nerves before innervating the bladder. The sympathetic outflow innervates the
smooth muscle fibers of the bladder neck,
and is believed to play a role during the urine
storage phase (de Groat and Saum, 1972;
Vaughan and Satchell, 1992) and during
sexual behavior (Kimura et al., 1975).
The EUS is innervated by the pudendal
nerve. Motoneurons of the EUS in the cat
are located in the ventrolateral part of the
so-called nucleus of Onuf of the ventral horn
at the level of S1 and S2 (Sato et al., 1978;
Kuzuhara et al., 1983). The motoneurons of
the external anal sphincter are located in the
dorsomedial part of the nucleus of Onuf
(Kuzuhara et al., 1983). In humans Onuf’s
nucleus is located in the segments S1-S3
Premotor interneurons involved in micturition
Since the work of Barrington (1925) it is
known that the coordinating component of
the micturition reflex is not located in the
sacral cord, but in the dorsolateral portion
of the pontine tegmental field. Bilateral lesions in this area in the cat result in urinary
retention. The same is true when the fiber
pathways originating in Barrington’s area
are interrupted e.g. at the level of the spinal
cord. Also in humans interruption of the descending fibers from the pons to the sacral
cord, for example in patients with a transection of the spinal cord, results in retention
of urine and finally in dyssynergic micturition. In such patients the contraction of the
bladder is accompanied by simultaneous
contraction of the sphincter. Several studies in various animals have attempted to
identify the area originally described by
Barrington.
Pontine micturition center
Barrington’s area or nucleus is also called
the pontine micturition center (PMC; Loewy
et al., 1979), or M (medial)-region (Holstege
et al., 1986). The latter term was chosen
because another group of neurons in more
lateral parts of the same dorsolateral pontine tegmental field was found to influence
micturition and urinary continence. These
lateral cells are referred to as L (lateral)-region (Holstege et al., 1986). Anterograde
tracing studies in the rat (Loewy et al.,
1979), opossum (Martin et al, 1979), cat
(Holstege et al., 1979, 1986), and monkey
(Westlund and Coulter, 1980) have shown
that neurons in the PMC project, via fibers
in the spinal lateral and dorsolateral funiculi, directly to the sacral intermediolateral
cell group, which contain the parasympathetic preganglionic motoneurons of the urinary bladder. Neurons in the PMC in the cat
project also to the sacral dorsal gray com-
13
General Introduction
Fig. 4. Brightfield photomicrographs of autoradiographs showing [3H] leucine injection areas and
darkfield photomicrographs showing the spinal distribution of labeled fibers after injections in the
M-region (on the left) and in the L-region (on the right) in the cat. Note the dense distribution of
labeled fibers to the sacral intermediolateral (parasympathetic motoneurons) and intermediomedial
cell groups from the M-region (S2 segment on the left). Note also the pronounced projection to the
nucleus of Onuf (S1 segment on the right) from the L-region. Note further the contralateral pathway
in the dorsolateral funiculus, terminating in lamina I, the outer part of II, and laminae V and VI
throughout the length of the spinal cord (from Holstege et al., 1986).
14
General Introduction
missure (DGC) or intermedio-medial cell
group (IMM) , but do not project to the
nucleus of Onuf (Holstege et al.,1979, 1986;
Fig. 4, on the left). Electrical stimulation in
the PMC in the cat produces an immediate
and sharp decrease in the urethral pressure
and pelvic floor electromyogram (EMG),
followed in about 2 seconds by a steep rise
in the intravesical pressure (Holstege et al.
1986), mimicking normal micturition.
L-region
The L-region projects, via fibers in the lateral funiculus, bilaterally to the nucleus of
Onuf (Holstege et al., 1979; 1986; Fig. 4,
on the right). Stimulation in the L-region
results in strong excitation of the pelvic floor
musculature and an increase in the urethral
pressure (Holstege et al., 1986). Bilateral lesions in the L-region give rise to an inability to store urine; bladder capacity is reduced
and urine is expelled prematurely by excessive detrusor activity accompanied by urethral relaxation (Griffiths et al., 1990).
Apart from the afferents from the PMC- and
L-regions, the parasympathetic preganglionic bladder motoneurons and Onuf’s
nucleus motoneurons receive also afferents
from other sources, like the NRA
(VanderHorst and Holstege, 1995) and the
paraventricular nucleus of the hypothalamus
(Holstege, 1987). However, their descending systems are thought to play a role in
other functions as mating behavior, but not
in micturition.
Ascending pathways involved in micturition
Peripheral afferent nerves
Most afferent fibers from the bladder enter
the sacral cord via the pelvic nerve. The peripheral fibers of the dorsal root ganglia
neurons of the pelvic nerve contact the bladder wall mechanoreceptors. The proximal
fibers enter Lissauer’s tract and terminate
mainly in Rexed’s (1954) laminae I, V, VII,
and X of the lumbosacral spinal cord at seg-
ments L4-S2 (Morgan et al., 1979 in the cat).
The majority of these afferents are thin myelinated and unmyelinated axons, and their
conduction velocities are in the A∂ and Cfiber range, respectively (Hulsebosch and
Coggeshall, 1982). Most A∂ fibers originate
from slowly adapting mechanoreceptors in
the bladder wall, and excitation of these fibers results in activation of the micturition
reflex. In all likelihood, the A∂ fibers are
the peripheral afferent fibers for this reflex
(De Groat et al., 1982; Mallory et al., 1989),
because the unmyelinated C-fibers in the
pelvic nerve do not respond to distention and
contraction of the urinary bladder (Jänig and
Morrison, 1982).
Spinal cord-brainstem pathways involved in
the micturition reflex
In order to function properly the PMC must
be informed about the amount of bladder
filling, since micturition can take place only
when the bladder contains a certain amount
of urine. The current view is that the micturition reflex is a spinobulbospinal reflex. De
Groat (1975), on the basis of physiological
recording studies in the dorsolateral pons of
the rat and cat, suggested that the lumbosacral neurons receiving bladder afferents relay bladder filling information directly to the
PMC, which project to the preganglionic
bladder motoneurons. This would imply a
micturition circuit in which lumbosacral interneurons convey information concerning
bladder filling directly to the pontine micturition centers. In a later paper the same
author found evidence that the PAG receives
bladder information before the PMC (Noto
et al., 1989).
Aim of the thesis
The investigations presented in this thesis
are focussed on two main issues. First,
which neurons are involved in the micturition reflex, and, second, which structures,
although they are not part of the micturition
reflex pathway itself, still influence the re-
15
General Introduction
flex. Clarification of these issues is very important for the understanding of the control
mechanisms in normal individuals, and,
moreover, in incontinent patients.
It is highly probable that dysfunction of certain brain areas cause urinary incontinence
in many elderly (Andrew and Nathan, 1964;
Blaivas, 1982). Urge incontinence occurs
when patients sense the urge to void, but
are unable to delay it long enough to reach
the toilet. In healthy individuals this “urge”
is not immediately followed by micturition
and usually disappears when micturition is
not appropriate at that particular time and
place. Urge incontinence is also frequently
found in patients with stroke (Khan et al.,
1981) or with neurodegenerative diseases,
as multiple sclerosis (Blaivas et al., 1979).
Urge incontinence should not be confused
with genuine stress incontinence, which is
not the result of lesions in the central nervous system and will not be discussed in
this thesis.
Anterograde and retrograde tracers were
used to identify pathways and neurons involved in micturition. Most of the tracer
experiments were done in cats. The tracers
were visualized using histo- and immunocytochemical techniques, and the results
were analyzed with light- and electron microscopy. Electrical stimulation was used in
order to study the effects on the function of
the bladder and EUS. In order to localize
micturition related neurons in humans
positron emission tomography (PET) was
used. PET is a non-invasive technique to
study changes in regional cerebral blood
flow (rCBF) in humans performing specific
tasks (Fox and Mintun, 1989).
The first part of the study is focussed on the
neuronal components of the micturition reflex itself. The micturition reflex consists
of sensory (ascending) and motor (descending) components. Afferents from the bladder to sensory neurons in lamina I and V of
the lumbosacral cord were thoroughly described in the cat (Morgan et al., 1981).
16
Since it was anatomically unclear whether
these lumbosacral cells projected directly to
the PMC, ultrastructural experiments were
done which resulted in the data presented
in the first chapter. It was demonstrated that
the PMC of the cat received only a very
small projection from the lumbosacral cord,
and that none of these afferents contacted
retrogradely labeled cells in the PMC. This
means that the bladder information reaches
the PMC indirectly. The main target of lumbosacral projections to the caudal brainstem
of the cat appeared to be the ventrolateral
PAG.
An important feature of micturition is the
synergic action between the bladder muscle
and the EUS. Stimulation of the PMC results in the same effect (Holstege et al.,
1986). The study presented in chapter two
demonstrates that from the PMC terminals
in the IML more than 75% are directly in
contact with parasympathetic preganglionic
bladder motoneurons, and all are excitatory
in nature. Chapter three investigated ultrastructurally the nature of the PMC fiber terminations in the dorsal gray commissure
(DGC). From the PMC terminals in the
DGC 55% made contact with inhibitory
GABA-ergic interneurons. This investigation provided evidence that the GABA-ergic
interneurons in the DGC play a role in the
relaxation of the EUS during micturition,
which is substantiated by the observation
that electrical stimulation of the sacral DGC
results in a relaxation of the EUS. The sacral DGC of the domestic pig contains the
motoneurons of the external anal sphincter.
Chapter 6 demonstrates that the PMC and
L-regions are not interconnected, which
makes it probable that the PMC controls
micturition only via its sacral projections.
Stimulation of the PAG elicits micturition,
which indicates a possible role in the motor
control of micturition. Since the majority of
the afferents from the lumbosacral cord terminate in the PAG, and not in the PMC, the
existence of PAG projections to the PMC
General Introduction
was investigated in chapter 7. The presence
of this projection completes the concept of
the normal micturition reflex.
The chapters 8, 9 and 10 present the findings of the PET scanning experiments in humans. Since nothing was known about the
central control of micturition in humans the
study was done in male and female volunteers. The results demonstrate that the
brainstem structures, which control micturition and continence, seem to be the same
in cats and humans. Additionally, emotional
related cortical areas play a role in the onset
of micturition, but are not part of the micturition reflex itself. Chapter 9 reports the results of a study on the voluntary motor control of the pelvic floor and abdominal musculature. The general discussion puts the
results of all chapters into perspective, presenting a scheme for the sensory and motor
pathways in the spinal cord and brainstem
involved in micturition and continence.
17