Micturition
The process of bladder emptying is known as micturition or urination. The wall of the mammalian urinary bladder is made up of smooth muscle layers of the detrusor muscle. The smooth muscle fibres extend in all directions and fuse so that low-resistant electrical pathways exist from one muscle to another.
The epithelial lining of the bladder is an impermeable transitional epithelium that can accommodate the changes in the bladder size. The highly folded bladder wall can flatten out to increase bladder storage capacity. In animals, the small triangular area on the dorsal aspect of the urinary bladder (posterior aspect in human beings) lying immediately above the bladder neck is trigone. The ureters enter the bladder at an oblique angle in the trigone to form a functional valve to prevent the backflow of urine at the time of bladder filling (Fig. 9.10).The urethra is a tubular structure conveying urine from the neck of the bladder to the exterior. It has two sphincters, an internal urethral sphincter and an external urethral sphincter. Since the internal sphincter is a continuation of the smooth muscle fibres of the detrusor muscle, it is involuntary in action. Its peculiar anatomical arrangement keeps it closed until the pressure inside the bladder exceeds a critical threshold. The external sphincter is lying beyond the bladder and is composed of skeletal muscle, which is under cortical control by a voluntary motor neurone.
Micturition reflex is an autonomic spinal cord reflex (involving the spinal cord’s sacral segment) initiated by the stretch of the receptors in the bladder wall during filling. Afferent impulses are sent through the sensory fibres of pelvic nerves and efferent impulses through the parasympathetic fibres of the same nerve. Thus, contraction of the detrusor muscle begins, and no special mechanism is needed to open the internal sphincter; changes in the shape of the bladder during contraction mechanically pull the internal sphincter open.
When urine reaches the neck of the bladder and the external sphincter, afferent impulses are sent to another reflex centre located in the pons. Efferent impulses from this centre (through the pudendal nerve) prevent contraction of the bladder and relaxation of the external sphincter allowing the urine to flow through the urethra. Once the micturition reflex begins, it is self-regenerative, and as the bladder continues to fill, the generation of reflex becomes more frequent and powerful. The external sphincter relaxes when the pressure exceeds the critical threshold, and urine gets voided out. Voluntary inhibition of micturition is possible by the tonic contraction of the external sphincter until a convenient time comes with the involvement of cortical centres of the brain.Fig. 9.10 Structure of urinary bladder. (a) Gross morphology of urinary bladder. (b) Cross section of the bladder wall (haematoxylin and eosin staining). (c) Scanning electron microscopic view of the inner lining of urinary bladder showing corrugated appearance in the non-distended state
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