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RS 16, Voluntary Control of Micturition, Role of Perineal and Abdominal…

- - - - Filling of bladder by 300–400 ml of urine in adults constitutes the adequate stimulus for the micturition reflex to occur.
      - Though, under natural circumstances, the first urge to empty the bladder occurs at approximately 150 ml of urinary volume but it can be easily suppressed.
      - Afferents.
        
        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.
        
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- - - - Voluntary control is gradually acquired as a learned ability of the toilet training.
        
        Once voluntary control is acquired, the supraspinal control centres exert
        final control of micturition by the following means:
        
        The higher centres keep the micturition reflex partially inhibited all
        the time except when it is desired to micturate.
        
        Even if the micturition reflex occurs, still the higher centres can
        prevent micturition by continual tonic contraction of external urinary
        sphincter until a convenient time presents.
        
        When the convenient time to urinate presents, the higher centres
        facilitate the sacral micturition centre to initiate a micturition reflex
        and inhibit the external urinary sphincter so that urination can occur.
        
        Supraspinal control centres which control the micturition reflex
        (a completely automatic cord reflex)
        include the pontine micturition centre
        (PMC) and suprapontine centres.
- - - - Functions of PMC are:
        
        Co-ordination of detrusor contraction and sphincter relaxation, which is important for proper micturition. Therefore, in the absence of control from PMC, there occurs loss of co-ordination between the bladder and external sphincter mechanism
        (detrusor–sphincter dyssynergia).
        
        To relay inputs from the suprapontine centres, therefore, in the lesions of PMC due to disruption of suprapontine influences, there
        occurs decreased storage and incomplete voiding.
- - - - The cortical detrusor motor area is located in the medial frontal lobe (superior frontal gyrus).
      - Basal ganglion
        
        inhibits the sacral micturition centre, and thereby the detrusor activity.
        Therefore, in Parkinson’s disease, there may be poor detrusor function.
        
        Limbic system
        (anterior cingulate gyrus and anterior genu of the corpus callosum)
        
        excites the sacral micturition centre. It forms the anatomical basis for the effect of emotions on voiding.