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RS 15 Renal System - 5, Filling of Urinary Bladder, Micturition, Filling…

- - - - It stretches when the bladder distends.
      - It forms a complete barrier to the passage of fluid and electrolytes.
        Therefore, urine stored in the bladder remains unchanged in chemical composition.
      - It secretes a glycosaminoglycan barrier that prevents bacterial adherence.
- - - - is about 3.8 cm long.
        
        It extends from the neck of the
        bladder to the external meatus.
        
        It traverses the external sphincter and
        lies immediately in front of the vagina.
  - - - Beyond the bladder neck, as the urethra passes through the urogenital diaphragm, it is encircled by a ring of voluntary (skeletal type) muscle known as external sphincter of the bladder. The external sphincter provides voluntary control over micturition. In males, its stronger action can stop micturition midstream even after it has been reflexly initiated.
- - - - The parasympathetic efferent fibres (nervi erigentes) are derived from
        the second, third and fourth sacral segments (mainly S2 and S3).
        
        These fibres carry motor impulses to the urinary bladder causing contraction of detrusor muscle and emptying of the bladder.
        
        These fibres are inhibitory to the internal sphincter.
        
        If these fibres are destroyed, normal micturition is not possible.
    - - These nerves arise in the 11th thoracic to the second lumbar segments (T11–L2).
        
        These fibres are said to be inhibitory to detrusor muscle and motor to the sphincter vesicae.
        
        Many workers regard them to be chiefly vasomotor.
        
        Sympathetic activity is not involved in micturition.
        
        Increased sympathetic discharge to the bladder occurs during ejaculation and helps to prevent the reflux of sperms from the prostatic urethra into the bladder.
    - - The somatic pudendal nerve (S2, S3 and S4) supplies the external sphincter which is voluntary.
  - - - Afferents from the detrusor stretch receptors travel to the spinal cord via the pelvic splanchnic nerve (nervierigentes).
        From the region of the bladder neck and trigone, the afferents travel via the hypogastric plexus
        to spinal cord segments T11–L2.
        
        In the spinal cord, the fibres of awareness of bladder distension run in the posterior column (fasciculus gracilis) to reach the spinal, pontine and suprapontine micturition centres.
    - - The pain fibres are stimulated by excessive distension or spasm of the bladder wall, or by stone, inflammation or malignant disease irritating the bladder.
        The pain fibres run predominantly in the hypogastric plexus but are also present in the nervi erigentes.
        
        In the spinal cord, the fibres carrying pain sensation run in the lateral spinothalamic tract. Bilateral anterolateral cordotomy, therefore,
        selectively abolishes pain without affecting the awareness of bladder distension and the desire to micturate
        (which runs in the posterior column).
    - - including sensation of imminent voiding associated with maximal bladder filling, reach the spinal cord via the pudendal nerve.
        
        In the spinal cord, fibres carrying urethral sensations travel in the dorsal column.
- - - - This is possible because of phenomenon of adaptation.
        
        The adaptation occurs because of the inherent property of plasticity, the
        smooth muscles of detrusor and because of law of Laplace.
- - - - In the bladder, as volume of fluid rises, the radius increases due to
        relaxation of detrusor and therefore the pressure rise is almost nil.
        
        It is important to note that adaptation is independent of nervous mechanism
        and it, therefore, presents in the denervated bladder, but not after birth.
- - - - It remains purely a reflex act until approximately 2–2.5 years of age, at
        which time it begins to come under voluntary control.
        
        The main physiological events in the process of micturition are:
         Filling of urinary bladder, and
         Emptying of urinary bladder.
- - - - The peristaltic wave can move urine against an obstruction with a
        pressure as high as 25–50 mmHg
- - - - Micturition reflex is a spinal reflex.
      - The afferents from the stretch
        receptors in the detrusor muscle and
        urethra travel along the pelvic
        splanchnic nerves and enter the spinal
        cord through dorsal roots to S2, S3 and
        S4 segments to reach the sacral
        micturition centre.
      - Sacral micturition centre.
        
        is formed by the sacral detrusor nucleus and sacral pudendal nucleus.
        
        Sacral detrusor nucleus
        
        is a cluster of preganglionic parasympathetic
        neurons located in the intermediolateral grey matter of S2, S3 and S4
        segments. Afferents of micturition reflex excite the neurons of this
        nucleus.
        
        Sacral pudendal nucleus
        
        refers to a cluster of neurons found in the
        ventral horn primarily of the S2 and to a lesser degree of S3 and S4
        segments. The afferents from the stretch receptors in the bladder wall
        inhibit this nucleus thereby exciting the external sphincter of urethra.
        
        Efferents.
        
        Efferents arising from the sacral
        detrusor nucleus are the preganglionic
        parasympathetic fibres which relay in
        the ganglia near or within bladder and
        urethra.
        
        The postganglionic parasympathetic
        fibres are excitatory to the detrusor
        muscle and inhibitory to the internal
        sphincter.
        
        Response.
        
        Once micturition reflex is initiated, it is self-regenerative, i.e. initial
        contraction of the bladder wall further activates the receptors to
        increase sensory impulses (afferents) from the bladder and urethra which
        cause further increase in reflex contraction of detrusor muscle of the
        bladder.
        
        The cycle thus keeps on repeating itself again and again until the
        bladder has reached a strong degree of contraction.
        
        After a few seconds, reflex fatigues and regenerative cycle cease.
        
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