Factors maintaining
urinary continence
Urinary continence depends on many physiological
factors that are not equally important or necessarily operative at the same
time but all act in concert together to preserve the two main functions of the lower urinary
tract that maintain
continence, storage and emptying. These factors are partially local
and partially due to central coordination and include:
1. Functioning
bladder (storage and emptying). This depends mainly on bladder compliance, detrusor
stability and contractility.
2. Intact
urethral sphincters, mucosa and submucosa.
3. Intact
pelvic floor muscles (Peri-urethral muscles).
4. Neural
control (autonomic and somatic innervations)
1- Physiology of the Bladder in relation to
continence
The bladder is composed of mucosa, lamina propria,
smooth muscle cells, nerve endings, fibroblasts and extracellular matrix. It is
this dynamic combination of elements interacting together that makes the bladder
is one of the most compliant organs in the body, enabling its volume to expand
dramatically without significant increases in pressure until a critical point
is reached .
The detrusor (smooth) muscles of bladder consist of sheets
containing many small spindle-shaped cells that contain actin (thin filament),
myosin (thick filament) and cytoskeletal intermediate filaments that assist in
transmission of the force generated during contraction. The actin and myosin
filaments are arranged as myofibrils that cross obliquely in a lattice-like
arrangement. The contraction of smooth muscle is slow, sustained, and resistant
to fatigue maintaining a steady level of contraction and tone.
Tone is important in maintaining the
capacity of the bladder and depends on many factors, some intrinsic and some
extrinsic. Extrinsic factors include activity in the autonomic nerves
and circulating hormones; Intrinsic factors include the response to stretch,
local metabolites, locally secreted agents such as nitric oxide, and
temperature .
2- Physiology of the urethra in relation to
continence
The urethra is
composed of mucosa, submucosa, smooth and striated muscles.
The urethral mucosa and submucosa function as a filler substance to
effectively close the urethral lumen after narrowing of lumen by urethral
sphincter .
Impaired arterial blood supply to the
urethra decreases the intraluminal urethral pressure due to decreased vascular
filling and hypoxic effect on the urethral smooth muscle .
The proximal urethral sphincter has inner longitudinal and outer
circular smooth muscle layers. Contraction of the inner longitudinal smooth
muscle plays a role in resting continence, stabilizing the urethra and allowing
force generated by the circular muscle elements to occlude the lumen .
The distal urethral sphincter consists of an inner longitudinal thin smooth muscle
layer and an outer circular striated muscle composed predominately of slow-twitch
fibers (type I) which are responsible for continence at rest. Despite the
horseshoe configuration of the DUS (see the anatomy), the urethral pressure
recording at the external sphincter during bladder filling increases uniformly
along the entire circumference like an iris. Hypogastric nerve stimulation
augments this pressure, suggesting a role for adrenergic receptors and
sympathetic nerves in the function of the external urethral sphincter .
3- Physiology of the Striated Muscles of the
Pelvic Floor (Peri-urethral muscles)
The peri-urethral striated muscles of the pelvic floor contain predominatly
fast-twitch fibres (type IIa) and few slow-twitch fibers. They are adapted for
the rapid recruitment of motor units required during increases in abdominal
pressure (active continence).
It has been speculated that the successful treatment
of stress incontinence by pelvic floor muscle training (PFMT) or electro-stimulation
(ES) is caused by the conversion of fast-twitch fibers to slow-twitch fibers
that maintain continence at rest .
Types of
Striated Muscle Fibers
The Striated muscle fibers are classified
into: slow-twitch fibers and fast-twitch fibers.
The slow-twitch fibers (type I): found in greater percentage in
muscles that require sustained tension, such as the DUS and to a lesser degree
in the pelvic floor muscles.
These muscle fibers are recruited slowly,
fatigue slowly and can perform high rates of oxidative metabolism because they
possess less of the myosin ATPase activity and contain an increased expression
of a slow iso-form of the Ca2+-ATPase.
The fast-twitch fibers (type II): are found mainly in pelvic floor
muscles that maintain continence in stress conditions when intra-abdominal
pressure is abruptly increased by adding to sphincter tone rapidly (Such
conditions as cough, abdominal straining or voluntary interruption of the
urinary stream). Fast-twitch fibers can be recruited rapidly, tend to fatigue
rapidly and perform predominantly anaerobic metabolism. Fast-twitch fibers are
rich in myosin ATPase that catalyzes the actin-myosin interaction and fast iso-form of the Ca2+-ATPase.
Fast-twitch fibres can be classified into fatigue-resistant (type IIa)
as pelvic levator and fatiguable (type IIb) which is not related to continence .
Micturition Cycle (Fig. 1)
The micturition cycle is divided into two relatively discrete phases:
Bladder filling phase and Bladder emptying phase.
Figure (1): Mechanism of
storage and voiding phase reflexes.
A, Storage reflexes. distention of bladder
produces low-level bladder afferent firing which stimulates the sympathetic
outflow to bladder outlet (base and urethra) and pudendal outflow to external urethral sphincter. These responses
occur by spinal reflex pathways and represent “guarding reflexes,” which promote
continence. Sympathetic firing also inhibits detrusor muscle.
B, Voiding reflexes. At initiation of micturition,
intense vesical afferent activity activates PMC, which inhibits spinal guarding
reflexes and stimulates parasympathetic outflow to bladder and internal
sphincter smooth muscle. Maintenance of voiding reflex is through ascending
afferent input from the spinal cord, which may pass through the periaqueductal
gray matter (PAG) before reaching the PMC
.
4- The
Neural Control of Micturition & Continence (Fig. 2)
The apparently simple lower urinary
tract function comprising the storage and periodic elimination of urine is
under a complex regulatory control of neural system that involves: central and
peripheral neural control.
A)
Central
(Spinal and Sura-spinal) Control of Micturition (Fig. 3)
The spinal center of micturition is
located in the lumbo-sacral spinal cord and responsible of voiding reflexes. It
is controlled by higher supraspinal micturition centers.
The supra-spinal
centers controlling micturition include the pontine micturition center (PMC) (Fig.
3), the periaqueductal grey (PAG) and supra-pontine centers which include the frontal cortex, the hypothalamus,
the para-central lobule, the limbic system and the cingulate gyrus .
Figure (3):
Pontine micturition center (Horizontal section showing significantly
increased blood flow with PET in the dorsal pons during micturition). L, left
side; R, right side of the brain .
Role of spinal center in control of
micturition
The
spinal micturition center controls a number of involuntary reflexes. With bladder
filling, the sympathetic activity is increased, the parasympathetic activity is
inhibited and the pudendal (somatic) neurons are activated (guarding reflex).
Micturition reflex is also a spinal reflex which is controlled by higher
supra-spinal centers .
Role of supra-spinal centers in control
of micturition
The brain supra-spinal centers allow for
the perception of bladder fullness, determine the “social correctness” of the
micturition act and coordinate the activities of the striated and smooth
muscles involved in the micturition reflex to maintain a reciprocal relationship
between the bladder and the urethral outlet .
The PAG is an integrative brain
center that receives sensory stimuli of bladder fullness via the spinal cord then
sends it to the PMC.
The PMC
is essential in co-ordinating the micturition process and is itself under the
control of the suprapontine area .
B)
Peripheral Control of Micturition (Fig. 4)
The bladder is supplied
by three sets of nerve fibres: the pelvic nerve, the pudendal nerve and the
hypogastric nerve.
Figure (4): The sympathetic, parasympathetic, and
somatic innervation of the urogenital tract of the male.
Sympathetic preganglionic pathways emerge from
lumbar cord to symp. chain ganglia (SCG) then through inferior splanchnic
nerves (ISN) to infer. mesenteric ganglia (IMG). Pre- and post-ganglionic
sympathetic axons travel in hypogastric nerve (HGN) to pelvic plexus and
urogenital organs.
Parasympathetic preganglionic axons originate in
the sacral cord & pass in pelvic nerve to ganglion cells in pelvic plexus
and to distal ganglia in organs.
Sacral somatic pathways are contained in pudendal
nerve, which innervates penis and ischiocavernosus (IC), bulbocavernosus (BC),
and distal urethral sphincter (DUS) muscles. The pudendal and pelvic nerves
also receive postganglionic axons from the caudal sympathetic chain ganglia.
These three sets of nerves contain afferent axons from lumbosacral dorsal root
ganglia. (PG: prostate gland; U: ureter; VD: vas deferens).
1- The pelvic nerves (S2–S4)–(Parasympathetic):
The
principal nerve supply of the bladder is by way of the pelvic nerves, which
connect with the spinal cord through the sacral plexus, mainly connecting with
cord segments S-2 and S-3. The pelvic nerves contains both sensory and motor
nerve fibers. The sensory fibers detect the degree of stretch in the bladder wall
and posterior urethra and responsible for initiating the reflexes that cause
bladder emptying. The motor nerves are parasympathetic fibers that innervate the
detrusor muscle .
2- The pudendal nerve (Somatic):
Motor innervation
to skeletal muscle fibers of the distal urethral sphincter is transmitted
through the pudendal nerve.
3- The hypogastric nerves (T11–L2)-(Sympathetic):
Sympathetic innervation emerges from
the sympathetic chain through the hypogastric nerves, connecting mainly with
the L-2 segment of the spinal cord. These sympathetic fibers stimulate mainly the
blood vessels and have little to do with bladder contraction .
