Functional: Neural Control of Storage and Voiding

• Most of the data in this chapter is based on animal studies

Lower urinary tract anatomy

Bladder

• Layers:
  • Urothelium
    • ≈7 cells thick
    • Functions of the urothelium (4):
      1. Relatively impermeable barrier (primary role) to protect the underlying stroma of the bladder
         • Uroplakin and TJ proteins play key parts in urothelial barrier function
      2. Sentinel defense against uropathogenic bacterial infections
         • Uroplakins have also been shown to act as the primary attachment site of type 1 piliated uropathogenic E. Coli.
      3. Afferent signaling; urothelial cells can release and respond to neurotransmitters
         • Myofibroblasts mediate interaction between urothelial cells and afferent nerves
      4. Modulation of detrusor smooth muscle contractility
    • The glycosaminoglycan (GAG) layer may have importance in bacterial anti-adherence, but there is no definite evidence that the GAG layer serves an impermeability function
  • Lamina propria
    • Contains a diffuse plexus of unmyelinated nerve fibers, interstitial cells (myofibroblasts) and microvasculature
  • Detrusor
    • In the bladder body, myofibrils are arranged into fascicles (bundles) in random directions
    • It is unlikely that every smooth muscle cell receives direct synaptic contact; the presence of gap junctions allows excitation to propagate throughout the smooth muscle syncytium
    • Detrusor smooth muscle has afferent innervation that could mediate afferent signals related to smooth muscle activity
    • The detrusor contractile ability rapidly declines if it is deprived of oxygen or a metabolic substrate, as would occur in ischemia

Internal (smooth) sphincter

• A complete and competent ring of smooth muscle and at the bladder neck in males; not found in females
• A physiologic but not an anatomic sphincter
• Not under voluntary control

External urinary (striated) sphincter (EUS)

• Striated muscles are characterized as slow vs. fast-twitch
  • Slow-twitch fibers: suitable for maintaining sphincter tone for prolonged periods
  • Fast-twitch fibers: may be needed to add to sphincter tone rapidly (i.e. to maintain continence when intra-abdominal pressure is abruptly increased)
• Composed of 2 parts:
  1. Periurethral striated muscle of the pelvic floor
     • Closely surrounds the urethra at the level of the membranous portion in males and primarily the middle segment in females
     • Contains both fast-twitch and slow-twitch fibers
       • Hypothesized that pelvic floor exercises or electrostimulation improve continence in SUI by causing by the conversion of fast-twitch to slow-twitch striated muscle fibers
  2. Striated muscle that is a part of the outer wall of the proximal urethra in males and females (also known as rhabdosphincter)
     • Contains predominantly slow-twitch fibers
• In males,
  • Covers the ventral surface of the prostate as a crescent shape proximal to the verumontanum
  • Then assumes a horseshoe shape distal to the verumontanum
  • Then as a crescent shape at the bulbar urethra
• In females,
  • Covers the ventral surface of the urethra in a horseshoe shape
• Under voluntary control
• Innervation: pudendal nerve (S2-S4)
• In addition to striated muscle, the EUS contains smooth muscle which receives sympathetic (noradrenergic) innervation§

Smooth muscle mechanics

• Parasympathetic nerve terminals release acetylcholine (Ach)
  • Which acts on muscarinic receptors
    • To induce detrusor contraction
      • By allowing calcium entry through Ca2+ channels
  • Detrusor smooth muscle contraction sequence
    1. Ca2+ binds to calmodulin, resulting in activation of calmodulin
    2. Activated calmodulin activates the kinase enzyme (myosin light-chain kinase)
    3. The kinase enzyme catalyzes phosphate transfer from adenosine triphosphate (ATP) to myosin, allowing myosin to interact with actin of the thin filaments.
    4. Smooth muscle relaxes when intracellular Ca2+ levels decrease
• Although calcium serves the same triggering role in all muscle types, the contractile response in smooth muscle is slower and longer lasting than that of skeletal and cardiac muscle due to different mechanism of activation
• Interstitial cells or myofibroblasts have been proposed for a pacemaking role in spontaneous activity of the bladder

Bladder mechanics

Urinary storage (filling)

• Definition of bladder compliance: change in volume divided by change in intravesical pressure (C = ΔV/ΔP)
• Decreased compliance of the bladder (steep filling curve) may be the result of impaired (alone or combination) (2):
  1. Viscoelastic properties (3):
     1. Change in composition of the bladder wall (e.g. more collagen, less elastin)
        • Injury, obstruction, or denervation increase collagen content resulting in decreased compliance
        • Types I, III, and IV are the most common types of collagen in the bladder
          • When the collagen component of the bladder wall increases, compliance decreases; the poor storage function of poorly compliant bladders is secondary to an alteration in the connective tissue content of the bladder wall, especially increased collagen type III
            • Can occur with chronic inflammation, bladder outlet obstruction, neurologic decentralization (usually at sacral or infrasacral level), and various other types of injury
            • Once decreased compliance occurs because of a replacement by collagen of other components of the stroma, it is generally unresponsive to pharmacologic manipulation, hydraulic distention, or nerve section. Most often, under those circumstances, augmentation cystoplasty is required to achieve satisfactory reservoir function.
        • Bladder muscle hypertrophy, which can result from outlet obstruction, can also result in decreased compliance because hypertrophic muscle is said to be less elastic than normal detrusor.
        • Once decreased compliance has occurred because of a replacement by collagen of other components of the stroma, it is generally unresponsive to pharmacologic manipulation, hydraulic distention, or nerve section.
     2. Fast filling rate
        • Filling the bladder at a slow physiologic rate maintains the intravesical pressure <10 cm H2O
     3. Hyperactivity of the detrusor smooth muscle
  2. Central neural input (inhibition of parasympathetic and activation of sympathetic innervation)
• Ureteral pressures during bladder filling
  • As the bladder fills, resting pressure within the intravesical ureter increases. This results in an increase in intraluminal (ureteral) pressure and an increase in the frequency of ureteral contractions. The end result is continued excretion of urine into the filling bladder.

Voiding mechanics (emptying)

• Intravesical pressure (Pves) reflects the combined factors of abdominal (Pabd) and detrusor (Pdet) pressures. Pdet = Pves – Pabd
• Low voiding pressure in a females does not necessarily imply impaired detrusor contractility; this may occur in females that are able to open their urethra widely.

Neural control of the Lower Urinary Tract

• Nature Reviews Neuroscience article on the neural control of micturition
  • See Figure 1 for efferent innervation of urogenital tract
• Micturition involves innervation from (3):
  1. Parasympathetic nervous system via the pelvic (S2-S4) nerve
  2. Sympathetic nervous system via the hypogastric (T11-L2) nerve
  3. Somatic nervous system via the pudendal (S2-S4) nerve
  • The pelvic, hypogastric, and pudendal nerve are mixed nerves and carry efferent and afferent innervation

Efferent innervation

Autonomic nervous system

• Parasympathetic
  • Parasympathetic preganglionic neurons innervating the LUT are located in the lateral part of the sacral intermediate gray matter of the S2-S4 spinal cord in a region called the sacral parasympathetic nucleus
  • Parasympathetic preganglionic neurons send axons through the ventral roots to peripheral ganglia, where they release the excitatory transmitter Ach.
  • Parasympathetic postganglionic neurons are located in the detrusor wall layer as well as in the pelvic plexus
  • Patients with cauda equina or pelvic plexus injury are neurologically decentralized but may not be completely denervated. Cauda equina injury allows possible afferent and efferent neuron interconnection at the level of the intramural ganglia.
  • Activation contributes to voiding by (2):
    1. Excitation (contraction) of the bladder
    2. Inhibition (relaxation) of the urethra [not the bladder base]
• Sympathetic
  • Arises from the T11-L2 level of the spinal cord
    • Sympathetic outflow from the rostral (anterior) lumbar spinal cord provides a noradrenergic excitatory and inhibitory input to the bladder and urethra
    • The peripheral sympathetic pathways follow a complex route that pass through the sympathetic chain ganglia to the inferior mesenteric ganglia and then through the hypogastric nerves to the pelvic ganglia
  • Activation contributes to urine storage by (3):
    1. Inhibition (relaxation) of the bladder
    2. Excitation (contraction) of the bladder base
    3. Excitation (contraction) of the urethra
  • Animal studies have shown that sympathetic postganglionic fibers release noradrenaline (NA) and contribute to bladder relaxation during storage (via stimulation of β-adrenergic receptors expressed in detrusor)

Somatic nervous system

• The pudendal nerve arises from the S2-S4 level of the spinal cord
  • The motorneurons of the EUS are located along the lateral border of the ventral horn, commonly referred to as the Onuf nucleus
• Activation contributes to urine storage by (1):
  1. Excitation (contraction) of the EUS

Afferent innervation

• Afferent input reaches the spinal cord from (3):
  1. Pelvic nerve (S2-S4)
  2. Hypogastric nerve (T11-L2)
  3. Pudendal nerve (S2-S4)
• The primary afferent neurons of the pelvic and pudendal nerves are contained in sacral dorsal root ganglia, whereas afferent innervation in the hypogastric nerves arises in the rostral lumbar dorsal root ganglia.
  • The central axons of the dorsal root ganglia neurons carry the sensory information from the LUT to second-order neurons in the spinal cord. These second-order neurons provide the basis for spinal reflexes and ascending pathways to higher brain regions involved in micturition, continence, and mediation of sensation.
• Pelvic nerve afferents
  • Most important afferent for initiating and maintaining normal micturition
  • Consist of myelinated (Aδ) and unmyelinated (C) axons
  • Monitor the volume of the bladder and the amplitude of the bladder contraction
  • Provides most of the afferent input from the bladder and urethra
• Hypogastric nerve afferents
  • Provides some of afferent input from the bladder and urethra
• Pudendal nerve afferents
  • Provides afferent input from the striated muscle of the sphincter and pelvic floor
• Bladder Afferent Properties

Fiber type	Location	Normal function	Inflammation effect
Aδ (finely myelinated axons)	Smooth muscle	Sense bladder fullness (wall tension)	Increase discharge at lower pressure threshold
C fiber (unmyelinated axons)	Mucosa	Respond to stretch (bladder volume sensors)	Increase discharge at lower threshold
C fiber (unmyelinated axons)	Mucosa muscle	Nociception to overdistention; silent afferent	Sensitive to irritants; become mechanosensitive and unmask new afferent pathway during inflammation

• Decreased afferent sensitivity or excitability due to a number of conditions in addition to normal aging may be an important factor leading to impaired voiding; diabetes mellitus has been linked with impaired sensory function and increased residual urine

• Modulators of afferent sensitivity
  • Nitric oxide (NO)
    • Major transmitter mediating relaxation/inhibition of the urethral smooth muscle during micturition
    • PDE5 terminates the action of NO, and PDE inhibitors can be used therapeutically to prolong the action of NO at a number of sites including the bladder, prostate, and blood vessels
    • Influences bladder afferent nerve activity
      • Data suggest that these agents may represent a target for treatment of hypersensitivity disorders of the bladder such as BPS/IC and OAB.
  • Urothelium releases ATP in response to stretch and this acts in a paracrine fashion to influence the function of myofibroblasts and bladder afferent nerves.
  • A number of different members of the transient receptor potential channel family are expressed in the bladder, mostly in association with sensory nerve fibers involved in mechanotransduction and in nociception.
  • Unmyelinated C fibers signal inflammatory or noxious events in the bladder; during inflammation and possibly other pathologic conditions, there is recruitment of mechanosensitive C fibers that form a new functional afferent pathway. This is the rationale for intravesical C-fiber neurotoxin capsaicin and RTX therapy
  • Cannabinoid receptors in the bladder may have a modulatory role in sensory afferent signaling.

Supraspinal pathways

• Midbrain–pontine–spinal cord circuits and reflexes control filling, storing and emptying of the bladder
• Pontine micturition center (PMC)
  • Also known as Barrington nucleus, Dorsal pontine tegmentum, M region
  • Located in the pons
  • The essential control center for micturition under normal conditions, integrating afferent input and ultimately responsible for initiating and coordinating the act of bladder emptying (inhibition of the urethral sphincter coordinated with detrusor contraction)
  • Within the PMC, glutamate is thought to be the primary neurotransmitter involved in stimulating preganglionic parasympathetic neurons responsible for detrusor contraction in the micturition pathway.
  • PMC neurons express corticotropin-releasing factor, which has an inhibitory influence micturition pathway.
• The striated urethral sphincter (rhabdosphincter) is controlled by the interaction between upper motor neurons and the lower motoneurons of the Onuf nucleus
  • PMC neurons do not project to Onuf nucleus. Rather, it has been proposed that the PMC indirectly inhibits Onuf nucleus neurons though excitatory projections to GABA premotor interneurons in the dorsal gray commissure.

Reflex circuitry controlling continence and micturition

Storage reflexes

• See Figure 5a for storage reflex
• Also known as “guarding reflex”, “spinal reflex”, “spinal reflex pathway”, “bladder sympathetic reflex”
• Promotes continence
  • During the storage of urine, distention of the bladder produces low-level bladder afferent firing via Aδ-myelinated nerves
  • There is a gradual increase in proximal urethral pressure during bladder filling, contributed to at least by the striated sphincteric element and perhaps by the smooth sphincteric element as well.
  • Afferent stimulation, in turn, results in efferent(2):
    1. Sympathetic outflow resulting in
       1. Excitation (contraction) of bladder base and urethra
       2. Inhibition (relaxation) of bladder
    2. Pudendal outflow to the EUS
       • The increase in urethral pressure seen during the filling/storage phase of micturition can be correlated with an increase in efferent pudendal nerve impulse frequency and gradual increase in striated sphincter activity.
    3. Note that the parasympathetic outflow is inactive

Voiding reflexes

• See Figure 5b for voiding reflex
• Also known as “micturition reflex”, “spinobulbospinal reflex”
• Upon initiation of micturition, there is high-level afferent activity signaling wall tension, which activates the PMC
• In response, the PMC:
  1. Inhibits the guarding reflex, thereby reducing sympathetic (resulting in inhibition (relaxation) of bladder base and urethra and excitation (contraction) of the detrusor muscle) and pudendal (resulting in inhibition (relaxation) of EUS) nerve outflow
  2. Stimulates parasympathetic outflow resulting in (2):
     1. Excitation (contraction) of bladder
     2. Inhibition (relaxation) of [urethra]/internal sphincter smooth muscle
• Maintenance of the voiding reflex is through ascending afferent input from the spinal cord, which may pass through the periaqueductal gray matter (PAG) before reaching the PMC
• Described as a spinobulbospinal reflex because of the ascending signal from afferent pelvic nerve stimulation, which passes through the periaqueductal gray matter in the bulbar region of the brain before reaching the pontine micturition center and descending to elicit parasympathetic contraction of the detrusor, and somatic relaxation via the pudendal nerve

Sphincter to bladder reflexes

• Afferent activity in the somatic pudendal nerve (projecting to the caudal lumbosacral spinal cord) can inhibit voiding. This stimulation can occur by activation of afferent input from various sites, including the urethral sphincter, penis, vagina, rectum, perineum, and anal sphincter
  • Afferent input from the EUS can inhibit parasympathetic outflow to the detrusor through a spinal reflex circuit
• Contractions of the EUS, and possibly other pelvic floor striated muscles, are likely to stimulate firing in muscle proprioceptive afferents, which then activate central inhibitory mechanisms to suppress the micturition reflex.

Urethra to bladder reflexes

• Stimulation of afferent activity from the urethra can facilitate parasympathetic efferent outflow to the detrusor by a supraspinal pathway passing through the PMC, as well as a spinal reflex pathway.
  • Desensitization of the urethral afferent with intraurethral capsaicin can dramatically alter the micturition reflex

Cerebral control of voiding

• See Figure 6 for brain areas involved in the regulation of urine storage
• Although many factors are involved in the initiation of micturition in adults, increased intravesical pressure producing the sensation of distention is primarily responsible for the initiation of normal voluntarily induced emptying of the LUT.
  • As the bladder fills, increasingly strong bladder afferents travel via synapses in the sacral cord to the brainstem and midbrain, where they synapse in the central periaqueductal gray and possibly PMC.
  • Although the origin of the parasympathetic neural outflow to the bladder, the pelvic nerve, is in the sacral spinal cord, the actual coordinating center for the micturition reflex in an intact neural axis is the pontine micturition center in the rostral brainstem; efferent parasympathetic pelvic nerve activity is ultimately what is responsible for a highly coordinated contraction of the bulk of the bladder smooth musculature.
• If the trigger level is exceeded, efferent signals from the PMC descend to the sacral cord, where they excite an indirect inhibitory pathway via Onuf's nucleus that leads to sphincter relaxation and an excitatory pathway to the bladder that leads to detrusor contraction; thus voiding occurs. Therefore the spinobulbospinal voiding-reflex pathway functions as a switch, either “off” (storage) or “on” (voiding). In the absence of higher control this switching behavior would lead to involuntary bladder emptying (i.e., incontinence) whenever the bladder volume reached a critical level sufficient to trigger the brainstem switch
  • Involuntary reflexive switching occurs in infants or in patients with neuropathic bladder when the bladder wall tension caused by increased volume of urine exceeds the micturition threshold. At this point, increased afferent firing from tension receptors in the bladder reverses the pattern of efferent outflow, producing firing in the sacral parasympathetic pathways and inhibition of sympathetic and somatic pathways, resulting in micturition. .
• White-matter damage that causes permanent incontinence appears to do so by disrupting a pathway (from medial frontal cortex to brainstem, either direct or via the thalamus) carrying the signal that maintains continence by tonically inhibiting the voiding reflex during storage

Storage phase

• The accommodation of the bladder to increasing volumes of urine is dependent on the intrinsic properties of the vesical smooth muscle and stroma, as well as the quiescence of the parasympathetic efferent pathway
• The bladder sympathetic reflex (guarding reflex) also contributes as a negative feedback or urine storage mechanism that promotes closure of the urethral outlet and inhibits neurally mediated contractions of the bladder during bladder filling
• However, the guarding reflex during urine storage may be weak in humans; in patients undergoing bilateral RPLND, in which the sympathetic chains are destroyed, there is no discernible alteration of filling or storage function

Emptying Phase of the Bladder

• The storage phase of the bladder can be “switched” to the voiding phase either involuntarily reflexively or voluntarily
• The expulsion phase consists of:
  1. Initial relaxation of the urethral sphincter
     • A decrease in outlet resistance occurs with adaptive shaping or funneling of the relaxed bladder outlet
  2. Followed in a few seconds by a contraction of the bladder, an increase in bladder pressure, and the flow of urine
• Relaxation of the urethral smooth muscle during micturition is mediated by activation of a parasympathetic pathway to the urethra that triggers the release of NO, an inhibitory transmitter, and by removal of excitatory [sympathetic] inputs to the urethra.
• Secondary reflexes elicited by flow of urine through the urethra facilitate bladder emptying

Spinal ascending and descending influences

• Glutamate plays an important role in the spinal efferent circuitry supporting micturition.
• The spinal noradrenergic system, mediated by α1 adrenoceptors, has a modulatory role in controlling micturition by inhibiting afferent inputs from the bladder and facilitating the descending limb of the spinal micturition reflex to increase bladder contractility
• Glutamate appears to be involved as an excitatory transmitter in the supraspinal circuitry controlling micturition.
• Glutamate may also be a mediator of DO after neural injury
• Glycinergic and GABAergic projections to the lumbosacral cord inhibit the micturition reflex and also inhibit glutamatergic neurons. Clinically, DO can be inhibited by GABA receptor activation.
  • GABA is the most widely distributed inhibitory neurotransmitter in the mammalian central nervous system.
• Activation of central serotonergic system can suppress voiding by inhibiting parasympathetic excitatory input to the urinary bladder
• Duloxetine
  • MOA: combined norepinephrine and serotonin reuptake inhibitor
  • Effects:
    • Increases neural activity of both the urethral sphincter and the bladder
  • Uses:
    • Treatment of both stress incontinence and urgency incontinence.

Spinal cord injury and neurogenic detrusor overactivity

• Normal micturition is associated with a spinobulbospinal reflex mediated by myelinated Aδ afferents.
  • Unmyelinated C fibers are normally relatively insensitive to gradual distention of the urinary bladder.
• After spinal cord injury, a capsaicin-sensitive C fiber–mediated spinal reflex develops and may play a role in the development of DO.
  • Unlike Aδ afferents which are normally unresponsive to low intravesical pressures, C fibers are more mechanosensitive, leading to the development of DO.
  • Capsaicin sensitive C fibers have also been implicated in DO after upper motoneuron diseases, such MS.
  • Treatment of neurogenic DO patients with intravesical capsaicin or another C-fiber neurotoxin, RTX, produces symptomatic improvement in a subpopulation of these patients.
• The bladder ice-water urodynamic test has been suggested as a research method to assess the C fiber–mediated micturition reflex but is not practiced clinically.
  • In a healthy individual, ice-water in the bladder would elicit no response.
  • In a bladder responsive to C fiber-mediated spinal reflexes, patients cannot retain the ice-water.

Questions

1. What are functions of the urothelium?
2. How to fast vs. slow-twitch fibers contribute to urinary contininence?
3. Which nerve and nerve roots inervate the external urinary sphincter?
4. Describe the autonomic innervation of the urinary tract and how they contribute to voiding/storage.
5. What is the primary neurotransmitter of the pontine micturition center involved in stimulating the micturition pathway?
6. Describe the storage and voiding reflexes.
7. What is the first step of the emptying phase of the bladder?
8. Which type of affarent fibers are responsible for sensing bladder fullness? Nociception?
9. What is the primary neurotransmitter involved in mediationg urethral relaxation?
10. What is the mechnism of action of duloxetine and what conditions is it used to treat?
11. List aging-related changed to micturition.

Answers

1. What are functions of the urothelium?
2. How to fast vs. slow-twitch fibers contribute to urinary contininence?
3. Which nerve and nerve roots inervate the external urinary sphincter?
4. Describe the autonomic innervation of the urinary tract and how they contribute to voiding/storage.
5. What is the primary neurotransmitter of the pontine micturition center involved in stimulating the micturition pathway?
6. Describe the storage and voiding reflexes.
7. What is the first step of the emptying phase of the bladder?
8. Which type of affarent fibers are responsible for sensing bladder fullness? Nociception?
9. What is the primary neurotransmitter involved in mediationg urethral relaxation?
10. What is the mechnism of action of duloxetine and what conditions is it used to treat?
11. List aging-related changed to micturition.

References

• Wein AJ, Kavoussi LR, Partin AW, Peters CA (eds): CAMPBELL-WALSH UROLOGY, ed 11. Philadelphia, Elsevier, 2015, chap 69
• Fowler, Clare J., Derek Griffiths, and William C. De Groat. "The neural control of micturition." Nature Reviews Neuroscience 9.6 (2008): 453-466.