Effects of Analgesic and Anesthetic Medications on Lower Urinary Tract Function

Sammy E. Elsamra, MD; Pamela Ellsworth, MD, FAAP, FACS

Disclosures

Urol Nurs. 2012;32(2):60-67. 

In This Article

Physiology of Micturition

Storage and voiding involves complex interactions between the bladder, urethra, urethral sphincter, and nervous system. The urinary bladder and urinary sphincter are the principle components of the LUT responsible for urinary storage and voiding. The urinary bladder, with a typical adult capacity of 400 to 500 ml, serves to store or expel urine by way of relaxation or contraction of the detrusor muscle, respectively. The urinary sphincter, composed of an internal component, a continuation of detrusor smooth muscle that converges to form a thickened bladder neck controlled by the autonomic nervous system, and a somatically controlled external component (striated muscle), must relax to allow for the contracting bladder to expel its load. Storage of urine is achieved by bladder relaxation and contraction of both the bladder neck (internal urinary sphincter) and the external urinary sphincter. Micturition occurs when the bladder neck and the external urinary sphincter relax and the bladder contracts, allowing for the unobstructed expulsion of urine.

Bladder storage and emptying, as well as coordinated contraction or relaxation of the urinary sphincter, are under the control of the sympathetic, parasympathetic, and somatic nervous systems (Ouslander, 2004). In general, urinary storage is a function of the sympathetic nervous system, whereas micturition is a function of the parasympathetic nervous system. While both are autonomic functions in nature, the somatic nervous system is responsible for the control of the external urinary sphincter, allowing for volitional continence. As seen in Figure 1, storage of urine (bladder relaxation and internal sphincter contraction) is under sympathetic control via impulses transmitted through the hypogastric nerve. The pelvic nerve is the principle conduit of the parasympathetic input for the LUT and allows for coordinated voiding by stimulating bladder contraction with sphincter relaxation. The somatic nervous system, through the pudendal nerve (and to a small degree the pelvic nerve), allows for the contraction or relaxation of the external urinary sphincter (striated pelvic diaphragm muscle under voluntary control). These nerves are lower motor neurons and are under the control of spinal reflexes and upper motor neuron input from the central nervous system (Ouslander, 2004).

Figure 1.

Neurologic Pathways Involved in Lower Urinary Tract Function
Description: The function of the lower urinary tract is under the control of several neurologic pathways. The sympathetic nervous system allows for bladder relaxation and internal sphincter contraction. This is mediated through the hypogastric nerve, and these signals originate from the spinal cord at levels T10-L2. The parasympathetic system allows for bladder contraction and internal sphincter relaxation. This is mediated through the pelvic nerve, and these signals originate from the spinal cord levels at S2–S4. The somatic (voluntary) system allows for the control of the external sphincter. All three of these systems are part of reflex pathways (not depicted in this illustration) and are under the influence of upper neurologic control (cerebrum and pons micturition center in the cerebellum).

Storage of urine is primarily a sympathetic and somatic function. Sympathetic input to the LUT is mediated through stimulation of adrenergic receptors. The stimulation of alpha-1 adrenergic receptors at the bladder neck by post-ganglionic norepinephrine results in bladder neck contraction. The sympathetic nervous system also inhibits parasympathetic input into the bladder, thus inhibiting stimulatory signals from reaching the detrusor. Further, stimulation of beta-3-adrenergic receptors with norepinephrine, as shown in animal models, allows for relaxation of the detrusor (Verhamme, Sturkenboom, Stricker, & Bosch, 2008).

External sphincter motor neurons originate from Onuf's nucleus, located on the anterior horns of the sacral spinal cord at levels S2–S4, and send their axons into the pudendal nerve (and to a lesser degree, the pelvic nerve) that stimulate the striated muscle to contract via the release of acetylcholine (Darrah, Griebling, & Silverstein, 2009; deGroat, 2006). This acetylcholine then binds to post-junctional nicotinic receptors, resulting in contraction of the external sphincter. Both alpha-receptors and serotonin 5-HT2 receptors are located in Onuf's nucleus and facilitate the storage reflex (Verhamme et al., 2008).

Bladder Filling/Storage

Bladder filling/storage is regulated by two separate storage reflexes – the sympathetic (autonomic) reflex and the somatic reflex (Thor & Donatucci, 2004). The sympathetic-mediated storage reflex is involved with bladder filling and is mediated by myelinated A-delta fibers. Afferent activity travels in the pelvic nerves to the spinal cord. At the L1-L3 level, sympathetic activity is initiated, which leads to a decrease in excitatory parasympathetic stimulation of the bladder. Postganglionic neurons release noradrenaline, which binds to beta-adrenoreceptors in the detrusor, leading to detrusor relaxation (Andersson, 2007).

The somatic storage reflex, ofter-end referred to as the "guarding reflex," occurs in response to sudden increases in intra-abdominal pressure. In this reflex, afferent activity travels along the myelinated A-delta fibers in the pelvic nerve to the sacral spinal cord, where efferent somatic urethral motor neurons in Onuf's nucleus are located. Afferent activity is also relayed to the periaqueductal gray (PAG) region and then on to the pontine micturition center (PMC). The PMC sends impulses to motor neurons in Onuf's nucleus, and axons from these neurons travel in the pudendal nerve and stimulate the rhabdosphincter to contract (Andersson, 2007).

Bladder Emptying

Studies in cats and rats indicate that the voiding reflex involves the PMC as well as other regions in the brain, including the hypothalamus and the cerebral cortex (Griffiths, 2004; Griffiths, Derbyshire, Stenger, & Resnick, 2005; Holstege, 2005). The PAG receives afferent activity from the bladder as well as from the cerebral cortex and hypothalamus. This activity is integrated in the PAG and PMC. The PMC controls the descending pathways involved in the micturition reflex, activating or inhibiting the parasympathetic pathways depending on the level of activity in the afferent fibers (Andersson, 2007).

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