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Pontine Center

Dorsal spinal cord

(intermediolateral column)

Sacral spinal cord

Detrusor

Uretral sphincter

Fig. 1. Anatomy and physiology of the lower urinary tract lateral column of the S2 to S4 spinal cord and terminate in the postganglionic neurons in the bladder wall and in the pelvic plexus [98]. The main neurotransmitter released by the parasympathetic postganglionic nerve terminals is acetylcholine. Acetylcholine can act on different subtypes of detrusor muscarinic receptors, among which the M3 are most important for mediating evoked smooth-muscle contractions neurally in the bladder [40].

The sympathetic preganglionic neurons are located within the inter-mediolateral cell column of the T11 to L2 spinal cord. They make synaptic connections with postganglionic neurons in the inferior mesenteric gan-glionic neurons in the paravertebral ganglia and pelvic ganglia. Sympathetic postganglionic terminals release norepinephrine which acts on alpha-1 vesical and urethral receptors and beta-2 adrenergic detrusor receptors. The effect of norepinephrine on the former is a contraction of the bladder base and urethral smooth muscle. Norepinephrine, via an action of the Beta 2 receptors, can also relax the bladder body.

Somatic afferent pathways that originate from the motoneurons in the Onuf nucleus of the anterior horn of the S2 to S4 spinal cord innervate the external striated urethral sphincter muscle and the pelvic floor musculature. Somatic nerve terminals release acetylcholine, which acts on nicotinic receptors to induce a muscle contraction. The striated urethral sphincter also receives noradrenergic input from the sympathetic nerves. The combined activation of the sympathetic and somatic pathways elevates bladder outlet resistance and contributes to urinary continence. The striated sphincter (via the pudendal nerve) is the unique element of voluntary continence and micturition.

Afferent Pathway

Sensory information regarding bladder fullness is conveyed to the spinal cord via afferent axons in the pelvic and hypogastric nerves, which possess neuronal somata in the dorsal root ganglia at the S2 to S4 and T11 to L2 spinal segmental levels. Afferent fibers passing in the pelvic nerve carry impulses from tension receptors in the bladder wall to neurons in the dorsal horn of the spinal cord. These are mainly small myelinated (Ad fibers) [49, 110] and unmyelinated (C fibers) axons [28]. In several mammalian species including the human, the normal micturition reflex is mainly mediated by Ad fibers afferents that respond to bladder distension [91]. The C fibers, which have a high mechanical threshold, are usually unresponsive to bladder distension and are thus called silent C-fibers, but many of them do respond to chemical, noxious or cold stimuli [58, 74].

Spinal Centers

The sacral micturition center involves laminae VI, VII and X. The innter-neurones participate in local control of elementary programs via para-

sympathetic and somatic pathways [73]. The C fibers project to the dorsal horn and via a polysynaptic reflex [48] with medullar interneurones [140] to form the «C reflex» of Bradley [28].

Pontine Centers

Among the sub-encephalic centers involved in micturitional control (Fig. 1), the most important are localized at the pontine level [10, 16]. This part of the tegmentum receives afferent pathways from collateral spinothalamic (from dorsal horn, laminae I and IV) to form the spino-ponto-spinal reflex or the «A reflex» of Bradley [28]. Two pontine centers have been characterized in mammalians [98]. The first is localized in the medial part of the dorsolateral pontine tegmentum, and is thus called the M-region or Pontine Micturitional Center (PMC) [121]. The PMC projects to the sacral intermediolateral cell column, in which are localized the para-sympathetic center connected to the bladder motoneurons and to the sacral intermedioventral cell column. The PMC is involved in the voiding phase via both these projections. The excitatory PMC projection to bladder motoneurons is responsible for an increase in bladder pressure during micturition. The relaxation of the striated uretral sphincter during micturition is due to excitatory projection to inhibitory interneurones in the spinal dorsal gray commissure.

The second pontine center, located more ventrally and more laterally in the pontine tegmentum than the PMC, is involved in the storage of urine during continence. During the storage phase, this L-center or Pontine Storage Center (PSC) acts by direct excitatory projection to the urethral sphincter in the nucleus of Onuf [85].

Suprapontine Controls

Several other central structures located in the forebrain and the cerebral cortex have been thought to be involved in lower urinary tract control. At the mesencephalic level, the periaqueductal gray (PAG) is considered as the main center involved in micturitional control. The PAG is thought to act as a central sensorimotor integrative relay of the micturition reflex, via the reception of sensory information concerning bladder fullness and the direct projection to the PMC [15].

In the forebrain, the most documented structure is the pre-optic area of the hypothalamus, which is thought to play a role in the initiation of the voiding phase via direct projection to the PMC. In addition, the anterior cingulated gyrus, amygdala, bed nucleus of the stria terminalis and septal nuclei are susceptible, when excited, to elicit bladder contraction [16]. The superomedial part of the precentral gyrus and the superolateral part of the precentral gyrus seem to be involved in voluntary control on the pelvic floors and in abdominal straining, respectively. Finally, the exact role of the cerebellum is not fully understood, but both afferent and efferent contributions to the micturitional reflex have been proposed [122].

Reflex Mechanisms Controlling Micturition

Storage Reflexes

The bladder functions as a low pressure reservoir during urine storage due to the combined effect of the visco-elasticity of the bladder wall and the quiescence of the parasympathetic pathway to the bladder. Continence during bladder filling is reinforced by the activation of a sacral-to-thoracolumbar intersegmental spinal reflex pathway, initiated by afferent fibers linked to a bladder tension receptor, which triggers firing in sympathetic pathways to the bladder, thus mediating an inhibition of bladder activity and a contraction of the bladder neck and proximal urethra. Simultaneously, the activation of pudendal motoneurons during bladder filling induces a contraction of the striated sphincter muscle, which in turn contributes to urinary continence.

In addition to these spinal continence reflexes, a supraspinal urine storage center located in the dorsolateral pons is involved in continence via descending inputs activating the pudendal motoneurons to increase ure-thral resistance (Fig. 1).

Voiding Reflexes

When bladder volumes reach the micturition threshold, intense afferent activity originating in the bladder mechanoceptors triggers the micturition reflex, which consists of spino-bulbo-spinal reflex pathways passing through the pontine micturition center. Activation of the pontine micturition center induces both a firing in the sacral parasympathetic pathways leading to bladder contraction and secondarily to inhibition of the sympathetic and somatic pathways relaxing urethral and bladder outflow. Before reaching the pontine micturition center, afferent inputs from the spinal cord pass through an integrative relay center in the periacqueductal gray. This center functions as an "on-off" switch activated by afferent activity derived from bladder mechanoceptors, and it also receives inhibitory and excitatory inputs from the brain regions (Fig. 1).

Voiding is also facilitated by an urethrovesical reflex initiated by the stimulation of urethral afferents triggered by urine flow in the urethra, thereby enhancing bladder contractions.

The suppression of the striated urethral sphincter activity during micturition is mainly due to a direct pontine micturition center projection to sacral inhibitory interneurons in the dorsal gray commissure, also known as the intermediomedial cell column. These inhibitory dorsal gray commissure interneurons in turn inhibit sphincter motoneurons in Onuf's nucleus during micturition.

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