Reflex Control Of The Lower Urinary Tract

Supraspinal Micturition Reflex - Figure 1 shows the "normal" supraspinal, parasympathetic micturition reflex responsible for determining when micturition occurs under normal conditions (without underlying pathology). This reflex is initiated by stretch receptors in the detrusor muscle (the smooth muscle of the bladder body) which passively stretches during filling and actively contracts during micturition. The stretch receptors are terminal specializations of thinly myelinated AS afferent fibers that traverse the pelvic nerve to reach the spinal cord. The cell bodies of these fibers are medium sized primary afferent neurons located in the sacral dorsal root ganglia (DRG). The central branches of these primary afferent neurons project along Lissauer's tract and the lateral edge of the dorsal horn (the "lateral projection") to contact second-order neurons in the dorsal horn of the sacral spinal cord (5). Via a spinobulbospinal pathway, second-order neurons in the sacral spinal cord project to the periaqueductal grey matter of the brain, which in turn activate neurons in the pontine micturition center (PMC).

The PMC (also called "Barrington's nucleus or "M" region) is located in the medial portion of the "dorsolateral tegmentum", medial to the locus coeruleus. Neurons in the PMC project directly to bladder preganglionic neurons located in the lateral band region of the sacral parasympathetic nucleus and to interneurons in the sacral dorsal gray commissure. The axons of cholinergic sacral parasympathetic preganglionic neurons traverse the pelvic nerve to activate pelvic parasympathetic post-ganglionic neurons in the pelvic plexus via nicotinic cholinergic receptors. The cholinergic postganglionic neurons, in turn, release acetylcholine resulting in detrusor smooth muscle contraction via stimulation of muscarinic M2 or M3 receptors.

Suprapontine (e.g. cortical or hypothalamic) control of the micturition reflex is crucial for ensuring that micturition occurs within the proper behavioral and environmental conditions. Compromise of these suprapontine controls is thought to play a role in the etiology of overactive bladder that accompanies, for example, cerebrovascular stroke.

C-fiber Spinal Micturition Reflex - Under normal conditions, primary afferent C-fibers do not respond to bladder distension (i.e. are not mechanosensitive) at normal bladder volumes. However, under pathological conditions, C-fibers become responsive to bladder distension at volume thresholds below those of AS fibers. In

Suprapontine

Figure 1. Schematic diagram depicting supraspinal micturition reflex pathway. Neuronal pathways between the bladder, spinal cord and brain are shown. The periaqueductal grey and pontine micturition center are supraspinal centers located in the brainstem. Numbers refer to the regions listed on Table 1. (DRG, dorsal root ganglion; DH, dorsal horn; VH, ventral horn)

Figure 1. Schematic diagram depicting supraspinal micturition reflex pathway. Neuronal pathways between the bladder, spinal cord and brain are shown. The periaqueductal grey and pontine micturition center are supraspinal centers located in the brainstem. Numbers refer to the regions listed on Table 1. (DRG, dorsal root ganglion; DH, dorsal horn; VH, ventral horn)

contrast to the supraspinal organization of the bladder reflex initiated by myelinated AS fibers, the bladder reflex that is initiated by unmyelinated C-fibers is organized within the sacral spinal cord (not shown in Figure 1.) The bladder C-fibers also have their cell bodies in the sacral DRG and probably also project along Lissauer's tract and the lateral projection to activate second order neurons. However, unlike the supraspinal micturition reflex pathway, the C-fiber reflex does not depend on supraspinal communication to activate sacral parasympathetic preganglionic neurons. The parasympathetic efferent pathway of this C-fiber spinal reflex and the A8 fiber supraspinal reflex are the same. The C-fiber spinal reflex, however, is not consistently demonstrable in animals with an intact spinal cord, unless the bladder outlet is obstructed for weeks or under conditions of bladder inflammation or irritation. Importantly, loss of descending control following spinal cord injury also allows for the C-fiber reflex to be revealed. This latter observation suggests a tonic descending inhibition of a C-fiber contribution to normal micturition and that interruption of this supraspinal inhibition reveals C-fiber reflex activity. For these reasons and others, sensitization of, and/or removal of inhibition from, the C-fiber reflex pathways are thought to contribute to the etiology of overactive bladder and urge incontinence that accompanies benign prostatic hyperplasia, spinal cord trauma or disease, and urinary tract infections.

Sympathetic Storage Reflex - Figure 2 shows the sympathetic storage reflex (pelvic-to-hypogastric reflex) pathway. This reflex is also initiated by bladder distension and activation of A5 fibers of the pelvic nerve that, in turn, activate sacral dorsal horn interneurons. An intersegmental, polysynaptic pathway projects rostrally to activate efferent sympathetic preganglionic neurons, situated at L1-L3 levels. Their efferent axons travel along the inferior splanchnic nerve to the inferior mesenteric ganglion (IMG) where they either synapse (minor component) or continue along the hypogastric nerve to synapse in the pelvic plexus (major component).

Postganglionic sympathetic neurons release norepinephrine, which facilitates urine storage by stimulating P3 adrenergic receptors that relax bladder smooth muscle, by stimulating ai adrenergic receptors that contract urethral smooth muscle, and by stimulating a adrenergic Fjgure 2 Schematic diagram depicting sympathetic receptors that inhibit storage pathways. Dotted lines represent afferent and ganglionic transmission (6). efferent pathways previously shown in Figure 1. Solid During micturition, this reflex lines represent novel sympathetic storage pathways, pathway is markedly Numbers refer to the regions listed on Tables 1 and 2. inhibited via supraspinal (DRG. dorsal root ganglion; DH, dorsal horn; VH, mechanisms to allow the ventral horn: SPN' sacral Parasympathetic nucleus; bladder to contract and the MG. inferior mesenteric aanalion)

urethra to relax.

Somatic Storage Reflex - Figure 3 shows the somatic storage reflex (pelvic-to-pudendal reflex) pathway. This reflex is also initiated by activation of A8 fibers of the pelvic nerve that, in turn, activate sacral dorsal horn interneurons. An

Onufs Nucleus

intrasegmental path travels from the sacral dorsal hom to the sacral ventral horn via polysynaptic connections. Efferent somatic urethral sphincter motor neurons are located in the lateral subdivision of "Onufs nucleus" (7). The axons from these motor neurons traverse the pudendal nerve and release acetylcholine which, in turn, activates nicotinic cholinergic receptors on striated muscle fibers of the urethra causing them to contract. During urine storage, this pathway is tonically active, and during micturition this reflex is strongly inhibited via spinal and supraspinal mechanisms to allow the urethral sphincter to relax and permit passage of urine through the urethra.

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