QUESTIONS

1. What is the function of the detrusor muscle and describe the detrusor reflex.
    
2. In order to urinate, the muscles of the urinary sphincter must relax.  Neurologically, how is this accomplished?
    
3. How is the sympathetic nervous system connected to the urination story?
    
4. Why is it very important to express an upper motor neuron bladder at least TID?
    
5. What is reflex dyssynergia?
    
6. What is the normal amount of residual urine that is left in an "empty" bladder?
    
7. Describe the mechanism of action for:
   
   a)  Bethanecol (urecholine)
   b)  Phenylpropanolamine
   c)  Phenoxybenzamine
   
   
   
   
    

ANSWERS

1. The detrusor muscle empties the urinary bladder.  As the bladder reaches capacity, tension receptors in the bladder wall are stimulated.  The fibers attached to these receptors are found in the pelvic nerve (part of the parasympathetic nervous system).  Cell bodies are found in S123.  The "your bladder is full" message is sent up to ARAS in the brainstem.  A motor discharge comes down the cord to the preganglionic parasympathetic neurons of the intermediate horn of S123.  These fibers terminate in the pelvic ganglion & synapse with post-ganglionics there.  The post-ganglionic fibers synapse on the detrusor muscle fibers & cause contraction & peeing.
   
   The afferent pathway also sends collaterals to the cerbral cortex allowing for voluntary overriding of the reflex or voluntary initiation.
   
   
   
   
    
2. Sensory receptors in the urinary sphincter perceive stretching (when the bladder is full).  This message is carried in the pudendal nerve to S123.  Motor instructions are carried back down the pudendal nerve keeping the sphincter tight. In addition to receiving input from the receptors of the sphincter, the afferent information from the detrusor muscle (brought to the cord by the pelvic nerve) also can activate the sphincter via the pudendal nerve.
   
   When one wishes to urinate voluntarily, cortical nerves activate the detrusor reflex. The afferent fibers telling the cortex & ARAS that "the bladder is full" also send collaterals to inhibitory interneurons in the sacral cord which serve to send an inhibitory message to the pudendal nerve.  In this way detrusor contraction & sphincter relaxation can be coordinated.  Separate cortical fibers send a stimulatory message to the pudendal nerve which can block urination if the time & place are not right.
   
   
   
   
    
3. Preganglionic sympathetic nerurons to the bladder are located in the lumbar cord (L2-5 in the cat & L1-4 in the dog).  These fibers go to the caudal mesenteric ganglion to synapse with the hypogastric nerve.  The hypogastric nerve goes to the bladder & pelvic plexus.  Alpha adrenergic receptors are located primarily in the trigone & prox urethra causing contraction (important in the prevention of retrograde ejaculation).  Beta adrenergic receptors are found all over the bladder & lead to relaxation. 
   
   Sympathetic lesions to do not really change your life much here  BUT there are sensory fibers running from the bladder  via the hypogastric nerve.  In the cat, the cortex receives "my bladder is full" info from both the hypogastric nerve and the pelvic nerve but the dog's cortex only receives info from the hypogastric (hypogastric input is perceived as pain).  ( I guess this means that in the detrusor reflex of the dog there are no collaterals leading to the cortex - the message that the bladder is full comes from the hypogastric nerve instead.)  THIS MEANS THAT A SACRAL LESION COULD ABOLISH THE ABILITY TO EMPTY THE BLADDER WHEN IT FILLS BUT THE PAIN OF ITS BEING FULL IS STILL PERCEIVED IN THE BRAIN (hypogastric fibers enter the cord in the lumbar area).
   
   
   
   
    
4. Detrusor muscle cells are connected by tight junctions which are essential for the spread of excitation.  When a bladder is overdistended, these tight junctions are disupted.  If scar tissue fills in the gaps between the tight junctions then normal excitation is prevented forever, even if nerve function recovers.  TID expression ought to prevent overdistention.
   
   
   
   
    
5. To pee properly, the detrusor contraction must be coordinated with sphincter relaxation.  So we have the detrusor reflex activated either on its own or by cortical instructions.  The afferent fibers also tell the pudendal nerve to relax the sphincter via inhibitory interneurons (unless cortical instructions override this).  Now let's say there's some kind of long tract lesion.  This could prevent integration of this reflex (normal input cannot get to ARAS etc).  A steady stream cannot be maintained once it is initiated.  You see short  bursts & then non-productive straining after a normal start.
   
   
   
   
    
6. 0.2-0.4 ml/kg = a typical residual urine volume.
   
   
   
   
    
7. BETHANOCHOL - This is a cholinergic drug.  It increases detrusor contractility by increasing parasympathetic tone (turning up the volume on the detrusor reflex)
   
   PHENYLPROPANOLAMINE - This is a sympathomimetic drug.  It acts by increasing urethral resistance by acting on the alpha adrenergic receptor of the sphincter & encouraging contraction.
   
   PHENOXYBENZAMINE - This is a sympatholytic drug.  It decreases urethral resistance by doing the opposite of the above.