Neurophysiology and Neuroanatomy of the Genitourinary Organs




Abstract


The urogenital system has two major subdivisions: (1) the urinary tract, which is responsible for the production, storage, and elimination of urine, and (2) the genital tract, which is involved in reproductive activity. Many functions of the urogenital system are controlled by complex neural pathways in the brain and spinal cord which in turn regulate the activity of peripheral autonomic (sympathetic and parasympathetic) and somatic nerves that innervate the smooth muscle, striated muscle, epithelial cells, and exocrine glands in the urogenital organs. Some urogenital functions (penile erection) are purely involuntary and mediated by reflex pathways in the spinal cord or brainstem; whereas others (micturition) are more complex involving voluntary control by the cerebral cortex. Multiple neurotransmitters, including glutamic acid, dopamine, opioid peptides gamma-aminobutyric acid, and serotonin have been implicated in the central control of the urogenital organs.




Keywords

Ejaculation, Interneurons, Micturition, Parasympathetic, Penile erection, Pontine micturition center, Spinal cord, Sympathetic, Urethral sphincter, Urinary bladder

 






  • Outline



  • Introduction 1437



  • Peripheral Innervation of the Urogenital Organs 1438




    • Anatomy 1438



    • Physiology and Pharmacology of Efferent Pathways 1440





    • Physiology and Pharmacology of Afferent Pathways 1441






  • Anatomy of Central Nervous Pathways Controlling the Urogenital System 1442




    • Pathways in the Spinal Cord 1442




      • Afferent Projections in the Spinal Cord 1442



      • Efferent Neurons 1442



      • Spinal Interneurons 1442




    • Pathways in the Brain 1442




  • Central Neural Control of the Lower Urinary Tract 1443




    • Organization of Urine Storage Reflexes 1443




      • Sympathetic Storage Reflex 1443



      • Urethral Sphincter Storage Reflex 1443




    • Organization of Voiding Reflexes 1443




      • Spinobulbospinal Micturition Reflex Pathway 1443



      • Suprapontine Control of Micturition 1444



      • Spinal Micturition Reflex Pathway 1445




    • Neurotransmitters in Central Micturition Reflex Pathways 1446




      • Excitatory Neurotransmitters 1446



      • Inhibitory Neurotransmitters 1446



      • Neurotransmitters With Mixed Excitatory and Inhibitory Actions 1446





  • Central Neural Control of the Genital Organs 1446




    • Penile Erection 1446



    • Glandular Secretion 1447



    • Emission-Ejaculation 1447



    • Neurotransmitters in Central Sexual Reflex Pathways 1447




  • References 1448




Introduction


The urogenital system has two major subdivisions: (1) the urinary tract, which is responsible for the production, storage, and elimination of urine, and (2) the genital tract, which is involved in reproductive activity. Many functions of the urogenital system are controlled by complex neural pathways in the brain and spinal cord. These central pathways in turn regulate the activity of peripheral autonomic (sympathetic and parasympathetic) and somatic nerves that innervate the smooth muscle, striated muscle, epithelial cells, and exocrine glands in the urogenital organs. Some urogenital functions (penile erection) are purely involuntary and mediated by reflex pathways in the spinal cord or brainstem; whereas others (micturition) are more complex involving voluntary control by the cerebral cortex. This article will review the innervation, central neural circuitry, and neurotransmitters controlling urogenital functions.




Peripheral Innervation of the Urogenital Organs


Anatomy


The lower urinary tract and sex organs are innervated by three sets of peripheral nerves that arise at the level of the lumbosacral spinal cord ( Fig. 121.1 ). Visceral structures (urinary bladder, urethra, cavernous tissue of the penis and clitoris, vas deferens, and exocrine glands) receive an innervation from both divisions of the autonomic nervous system. Parasympathetic efferent axons that originate in preganglionic neurons in the sacral spinal cord pass through the pelvic nerves to synapse with peripheral autonomic ganglion cells that in turn innervate the organs. Sympathetic efferent axons that originate in preganglionic neurons in the rostral lumbar segments of the spinal cord project to autonomic ganglion cells in the prevertebral and paravertebral ganglia which then send axons through various nerves to the target organs. Somatic efferent axons originate in motor neurons in the sacral spinal cord and pass through the pudendal nerves to striated muscles of the urethral sphincter ( Fig. 121.1B ) and periurethral striated muscles (bulbocavernous and ischiocavernosus) ( Table 121.1 ).




Figure 121.1


Efferent pathways of the lower urinary tract.

(A) Innervation of the female lower urinary tract. Sympathetic fibers (shown in blue) originate in the T11–L2 segments in the spinal cord and run through the inferior mesenteric ganglia (inferior mesenteric plexus, IMP) and the hypogastric nerve (HGN) or through the paravertebral chain to enter the pelvic nerves at the base of the bladder and the urethra. Parasympathetic preganglionic fibers (shown in green) arise from the S2–S4 spinal segments and travel in sacral roots and pelvic nerves (PEL) to ganglia in the pelvic plexus (PP) and in the bladder wall. This is where the postganglionic nerves that supply parasympathetic innervation to the bladder arise. Somatic motor nerves (shown in yellow) that supply the striated muscles of the external urethral sphincter arise from S2–S4 motor neurons and pass through the pudendal nerves. (B) Efferent pathways and neurotransmitter mechanisms that regulate the lower urinary tract. Parasympathetic postganglionic axons in the pelvic nerve release acetylcholine (ACh), which produces a bladder contraction by stimulating M 3 muscarinic receptors in the bladder smooth muscle. Sympathetic postganglionic neurons release noradrenaline (NA), which activates β 3 adrenergic receptors to relax bladder smooth muscle and activates α 1 adrenergic receptors to contract urethral smooth muscle. Somatic axons in the pudendal nerve also release ACh, which produces a contraction of the external sphincter striated muscle by activating nicotinic cholinergic receptors. Parasympathetic postganglionic nerves also release ATP, which excites bladder smooth muscle, and NO, which relaxes urethral smooth muscle (not shown). L 1 , first lumbar root; S 1 , first sacral root; SHP , superior hypogastric plexus; SN , sciatic nerve; T 9 , ninth thoracic root.

Reproduced with permission from Fowler, C.J., Griffiths, D., de Groat, W.C., 2008. The neural control of micturition. Nat. Rev. Neurosci. 9 (6), 453–466.


Table 121.1

Male Sexual Reflexes














































Response Afferent Nerves Efferent Nerves Central Pathway Effector Organ
Penile erection
Reflexogenic Pudendal nerve Sacral parasympathetic Sacral spinal reflex Dilatation of arterial supply to corpus cavernosum and corpus spongiosum
Psychogenic Auditory, imaginative, visual, olfactory Sacral parasympathetic, lumbar sympathetic Supraspinal origin
Glandular secretion Pudendal nerve Sacral parasympathetic, lumbar sympathetic Sacral spinal reflex Seminal vesicles and prostate
Seminal emission Pudendal nerve Lumbar sympathetic Intersegmental spinal reflex (sacrolumbar) Contraction of vas deferens, ampulla, seminal vesicles, prostate, and closure of bladder neck
Ejaculation Pudendal nerve Somatic efferents in pudendal nerve Sacral spinal reflex Rhythmic contractions of bulbocavernosus and ischiocavernosus muscles


Afferent axons innervating the urogenital organs arise in the lumbosacral dorsal root ganglia and are contained in the three sets of peripheral nerves. The most important afferents for initiating micturition are those, which travel in the pelvic nerve to the sacral spinal cord ( Fig. 121.2 ). These afferents are small myelinated (A–δ) and unmyelinated (C) axons that convey impulses from tension receptors and nociceptors in the bladder wall ( ). Afferent activity arising from mechanoreceptors in the penis and clitoris is carried in the pudendal nerves ( Table 121.1 ).




Figure 121.2


Neural circuits that control continence and micturition.

(A) Spinal storage reflexes. During the storage of urine, distention of the bladder produces low-level vesical afferent firing. This in turn stimulates the sympathetic outflow in the hypogastric nerve to the bladder outlet (the bladder base and the urethra) and the pudendal outflow to the external urethral sphincter. These responses occur by spinal reflex pathways and represent guarding reflexes, which promote continence. Sympathetic firing also inhibits contraction of the detrusor muscle and modulates neurotransmission in bladder ganglia. A region in the rostral pons (the pontine storage center) might increase striated urethral sphincter activity. (B) Supraspinal voiding reflexes. During the elimination of urine, intense bladder-afferent firing in the pelvic nerve activates spinobulbospinal reflex pathways (shown in blue) that pass through the pontine micturition center (PMC). This stimulates the parasympathetic outflow to the bladder and to the urethral smooth muscle (shown in green) and inhibits the sympathetic and pudendal outflow to the urethral outlet (shown in red). Ascending afferent input from the spinal cord might pass through relay neurons in the periaqueductal gray (PAG) before reaching the PMC. Note that these diagrams do not address the generation of conscious bladder sensations, nor the mechanisms that underlie the switch from storage to voluntary voiding, both of which presumably involve cerebral circuits above the PAG. R represents receptors on afferent nerve terminals.

Reproduced with permission from Fowler, C.J., Griffiths, D., de Groat, W.C., 2008. The neural control of micturition. Nat. Rev. Neurosci. 9 (6), 453–466.


Physiology and Pharmacology of Efferent Pathways


Parasympathetic pathways induce a bladder contraction, urethral relaxation, penile erection, clitoral engorgement, and glandular secretions. Sympathetic pathways relax the bladder, contract the bladder neck and urethra ( Fig. 121.1 ), produce seminal emission, and can elicit penile erection or detumescence ( Table 121.1 ). Motor axons in the pudendal nerves activate bulbocavernous and ischiocavernosus muscles and elicit ejaculation ( Table 121.1 ).


Lower Urinary Tract


Parasympathetic neuroeffector transmission in the bladder is mediated by acetylcholine (Ach) acting on postjunctional muscarinic receptors ( Fig. 121.1B ). Both M 2 and M 3 muscarinic receptor subtypes are expressed in bladder smooth muscle; however, studies with subtype selective muscarinic receptor antagonists and muscarinic receptor knockout mice have revealed that the M 3 subtype is the principal receptor involved in excitatory transmission.


In bladders of various animals, stimulation of parasympathetic nerves also produces a noncholinergic contraction that is resistant to muscarinic receptor blocking agents. Adenosine triphosphate (ATP) elicits the noncholinergic contraction by acting on P2X 1 receptors. Although purinergic excitatory transmission is not important in the normal human bladder it has been identified in bladders from patients with pathological conditions such as idiopathic detrusor instability or interstitial cystitis ( ).


Smooth muscle contractions are initiated by an increase in intracellular Ca 2+ concentration, which can occur by intracellular release of Ca 2+ from the sarcoplasmic reticulum or by influx of Ca 2+ from the extracellular fluid ( ). The former mechanism is an essential step in the cholinergic activation of the detrusor muscle; while activation of P2X purinergic receptors causes the influx of extracellular Ca 2+ as well as depolarization of the cells leading to an opening of voltage gated Ca 2+ channels. This triggers intracellular Ca 2+ -induced Ca 2+ release from the sarcoplasmic reticulum. Activation of M 2 muscarinic receptors also appears to enhance contractions by suppressing β adrenergic inhibitory mechanisms by blocking adenylyl cyclase or K + channels.


Parasympathetic pathways to the urethra induce relaxation during voiding by releasing nitric oxide (NO). NO increases the levels of cyclic guanosine monophosphate (GMP) by stimulating guanylyl cyclase. Cyclic GMP in turn activates protein kinase G that produces smooth muscle relaxation by several mechanisms including activation of potassium channels and desensitization of the contractile machinery to Ca 2+ ( ).


Sympathetic postganglionic nerves that release norepinephrine provide an excitatory input to smooth muscle of the urethra and bladder base, an inhibitory input to smooth muscle in the body of the bladder ( Fig. 121.1B ), as well as inhibitory and facilitatory input to bladder parasympathetic ganglia ( ). The smooth muscle of the bladder base is richly innervated by adrenergic terminals, but the bladder body has a considerably weaker adrenergic innervation. α-adrenergic receptors are concentrated in the bladder base and proximal urethra, whereas β-adrenergic receptors are most prominent in the bladder body ( Fig. 121.1B ). These observations are consistent with pharmacological studies showing that sympathetic nerve stimulation or exogenous catecholamines produce β 3 -adrenergic receptor-mediated inhibition of the body and strong α 1 -adrenergic receptor-mediated contractions of the base and urethra as well as weaker contractions of the bladder body ( ). The α 1A -adrenergic receptor subtype is most prominent in the normal bladders but the α 1D -subtype is upregulated in bladders from patients with outlet obstruction due to benign prostatic hyperplasia. This finding raised the possibility that enhanced α 1 -adrenergic receptor excitatory mechanisms in the bladder body might contribute to irritative lower urinary tract symptoms in patients with prostate disease.


Activation of β-adrenergic receptors in bladder smooth muscle stimulates adenylyl cyclase and increases cyclic AMP which in turn activates protein kinase A. Protein kinase A is thought to act in part by inducing a hyperpolarization of the cells either by opening of K + channels or by stimulating an electrogenic ion pump. Excitatory responses in the urethra and bladder neck mediated by α 1 -adrenergic receptors are attributed to an increased release of Ca 2+ from intracellular stores.


Motor axons innervating the periurethral striated muscles, pelvic floor muscle, and the striated muscles mediating ejaculation release acetylcholine which activates nicotinic cholinergic receptors.


Genital Organs


Postganglionic axons which control erectile tissue in the penis and secretion from seminal vesicles, prostate, and urethral glands synthesize and release several transmitters including NO, ACh, vasoactive intestinal polypeptide (VIP), and ATP. NO is the major transmitter mediating neurally induced erections ( Table 121.1 ); whereas ACh appears to be involved in stimulating glandular secretion. The functions of VIP and ATP are uncertain.


Penile erection is a vascular phenomenon resulting from neurally mediated increase in blood flow to the penile erectile tissue (corpora cavernosa and corpus spongiosum). The erectile tissue consists of large venous sinuses that contain very little blood when the penis is flaccid, but distend considerably when blood flow is increased. Dilation in the arterial supply to the cavernous tissue coupled with a relaxation of the sinusoidal smooth muscle in the trabecular tissue is responsible for erection.


Activity in both sacral (parasympathetic) and the thoracolumbar (sympathetic) preganglionic axons can activate postganglionic neurons that express neuronal nitric oxide synthase (nNOS) and release NO which elicits erections ( ). The endothelial cells in the penis also express endothelial nitric oxide synthase (eNOS) and can release NO in response to mechanical stimuli (shear stress) associated with changes in blood flow ( ). NO directly activates soluble guanylyl cyclase in the penile smooth muscle to increase the formation of cyclic GMP which in turn induces smooth muscle relaxation via activation of cyclic GMP-dependent protein kinases. The effects of NO are terminated by the enzymatic breakdown of cyclic GMP by phosphodiesterase. Pharmacologic studies in animals have shown that erections elicited by stimulation of autonomic nerves are reduced by NOS inhibitors and enhanced by phosphodiesterase (PDE) inhibitors ( ).


eNOS has also been implicated in neurally mediated erections. While erections are initiated by NO synthesized in nerves by nNOS, this mechanism triggers a transient increase in blood flow and expansion of the penile vasculature and sinusoidal spaces. However, the resulting shear force on the endothelium activates a phosphatidylinositol 3-kinase pathway that in turn stimulates a serine/threonine protein kinase, causing direct phosphorylation of eNOS and synthesis of NO ( ).


The sympathetic noradrenergic innervation of the penis that provides an excitatory input to penile blood vessels is thought to be involved primarily in detumescence ( Table 121.1 ). Electrical stimulation of sympathetic axons in either the hypogastric or pudendal nerves in various species produces a substantial reduction in penile blood flow. The effect is blocked by α-adrenergic receptor blocking agents. Several mechanisms have been implicated in the noradrenergic vasoconstrictor effect including: (1) prejunctional inhibition of parasympathetic nitrergic nerve terminals mediated by α 2 adrenergic receptors; (2) activation of postjunctional α 2 adrenergic receptors which then inhibits adenylate cyclase and formation of cyclic AMP; (3) activation of postjunctional α 1 adrenergic receptors stimulates phospholipase C activity followed by formation of IP 3 and diacylglycerol, leading to release of intracellular Ca +2 as well as sensitization of contractile mechanisms to Ca +2 ( ).


Physiology and Pharmacology of Afferent Pathways


Lower Urinary Tract


Afferent pathways that initiate the sensation of bladder fullness, the desire to void, and pain are small myelinated (Aδ) and unmyelinated (C) fibers that pass through the pelvic nerve to the sacral spinal cord ( ). Aδ bladder afferents in the cat respond in a graded manner to passive distension as well as active contraction of the bladder and exhibit pressure thresholds in the range of 5–15 mmHg, which are similar to those pressures at which humans report the first sensation of bladder filling. These fibers also code for noxious stimuli in the bladder. On the other hand, C-fiber bladder afferents in the cat have very high thresholds and commonly do not respond to even high levels of intravesical pressure. However, mechanosensitivity in some of these afferents is unmasked or enhanced by chemical irritation of the bladder mucosa. These findings indicate that C-fiber afferents in the cat have specialized functions, such as the signaling of inflammatory or noxious events in the lower urinary tract. C-fiber afferents are responsive to the neurotoxins, capsaicin, and resiniferatoxin, which have been used to desensitize the afferents and to treat bladder pain and afferent-evoked bladder overactivity. In the rat, A-fiber and C-fiber bladder afferents are not distinguishable on the basis of stimulus modality; thus both types of afferents consist of mechanosensitive and chemosensitive populations ( ). . Nociceptive and mechanoceptive information is also carried in the hypogastric nerves to the thoracolumbar segments of the spinal cord. Recent anatomical tracing studies in rats have also identified an additional afferent innervation of the bladder passing through the vagus nerves from sensory neurons in the nodose ganglion ( ). The function of this afferent pathway is not known.


Mechanosensitive afferents with receptors in the wall of the urethra and which detect urine flow pass through the pudendal nerves to the sacral spinal cord. Nociceptive afferents innervating the urethra pass through pudendal, pelvic, and hypogastric nerves to the lumbosacral spinal cord.


Afferents release rapidly acting excitatory transmitters (glutamic acid and ATP) which are involved in synaptic transmission in the spinal cord and also slowly acting neuropeptide transmitters (CGRP, VIP, pituitary-adenyl cyclase activating peptide [PACAP], tachykinins, and opioid peptides) which can be released at central as well as peripheral afferent terminals ( ). Many of these substances as well as others such as NO, prostaglandins, and neurotrophic factors released in the bladder by urothelial cells and inflammatory cells can modulate afferent sensitivity and change the response to mechanical stimuli.


The urothelium not only functions as a passive barrier at the bladder luminal surface but also appears to have specialized sensory and signaling properties that allows it to respond to chemical and mechanical stimuli and to engage in reciprocal chemical communication with nerves in the bladder wall ( ). These properties include: (1) expression of nicotinic, muscarinic, tachykinin, adrenergic, and capsaicin (TRPV1) receptors; (2) close physical association with afferent nerves and responsiveness to transmitters released from these nerves; (3) ability to release chemical mediators that can regulate the activity of adjacent nerves ( ) and thereby trigger local vascular changes and/or reflex bladder contractions.


The role of ATP in urothelial-afferent communication has attracted considerable attention because bladder distension releases ATP from the urothelium and intravesical administration of ATP induces bladder hyperactivity, an effect blocked by administration of P2X purinergic receptor antagonists that suppress the excitatory action of ATP on bladder-afferent neurons. Mice in which the P2X 3 receptor was knocked out exhibited hypoactive bladder activity and inefficient voiding, suggesting that activation of P2X 3 receptors on bladder-afferent nerves by ATP released from the urothelium is essential for normal bladder function ( ). It has also been reported that urothelial cells obtained from patients or cats with a chronic painful bladder condition (interstitial cystitis) released significantly larger amounts of ATP in response to mechanical stretching than urothelial cells from normal patients ( ). This raises the possibility that ATP-mediated signaling between the urothelium and afferent nerves is involved in the triggering of painful bladder sensations.


Genital Organs


The mechanosensitive afferent receptors in the glans penis are primarily free nerve endings with a density higher than in any other area of the body ( ). In humans, most of the afferents innervating the penis are of the A-fiber and C-fiber type.

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Sep 9, 2018 | Posted by in NEUROLOGY | Comments Off on Neurophysiology and Neuroanatomy of the Genitourinary Organs

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