Neurology Networks tries to offer broad exposure to various topics that may be presented on the veterinary neurology board exam.

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To understand the pathophysiology of urination, it is critical to understand the relevant anatomy. 



Local reflex arcs:

The urinary bladder is a hollow vesicle lined by layers of smooth muscle that together form the detruser muscle.  This muscle tissue has both sympathetic and parasympathetic autonomic receptors that help with bladder filling and with bladder emptying (afferent function).

  • Stimulation of the sympathetic (β-adrenergic) receptors cause bladder relaxation which allows for bladder filling.  This function is mediated by the hypogastric nerve which originates in the intermediolateral grey matter of the L1-4 spinal cord segments in the dog (L2-5 in the cat).
  • Stimulation of the parasympathetic (muscarinic cholinergic) receptors causes detruser muscle contraction to help with bladder emptying.  This function is mediated by the pelvic nerve which originates from the intermediolateral grey matter of the S1-3 spinal cord segments.

Afferent receptors primarily sense pain and stretch.  Pain sensation is mediated by both hypogastric and pelvic nerves.  The sensation of stretch is mediated by the pelvic nerve.


The bladder empties into the urethra.

  • The proximal aspect near the neck of the bladder has smooth muscle with sympathetic (α-adrenergic) receptors that are innervated by the hypogastric nerve.  Stimulation of these receptors cause urethral sphincter constriction.
  • The more distal aspect of the urethra has striated muscle with somatic (nicotinic cholinergic) receptors that are innervated by the pudendal nerve to cause sphincter constriction.  The pudendal nerve originates in the ventral horn grey matter of the sacral spinal cord segments.

Afferent receptors sense pain and stretch and are also mediated by the pudendal nerve.


Brainstem micturition center

The reticular formation in the dorsolateral pons has two groups of neurons.  One of these groups is referred to as the M group (Barrington’s nucleus) which is located more medially.  The M neurons project excitatory neurons to the parasympathetic efferent neurons of the pelvic nerve and also innervate inhibitory (GABA-ergic) interneurons that synapse on the nicotinic neurons of the pudendal nerve.  Stimulate the M region to evacuate the bladder.


The second group is referred to as the L group which is located more laterally.  The L neurons have excitatory connections with nicotinic motor neurons in the pudendal nerve.  Stimulate the L region to facilitate bladder filling.


*The brain stem micturition center is the UMN for normal urination


Detruser reflex (brainstem reflex)

In addition to communicating with the segmental reflexes, stretch signals from the detruser travel up the spinothalamic pathways to the brainstem.  Responses are sent through the reticulospinal and tectospinal tracts to the relevant spinal cord segments to coordinate urination.


Conscious influence

Afferent information including stretch and pain reaches the cerebral cortex via the hypogastric nerve (and pelvic nerve in the cat) and ascending spinal cord tracts.  The detruser reflex can be consciously inhibited or initiated via the cerebral cortex.  Animals with forebrain disease will still be able to urinate but will do so at inappropriate places and times.

Basal nuclei and preoptic area of the hypothalamus might help with initiation of bladder evacuation.


Cerebellar influence

Inhibitory influence on the detruser reflex.  Cerebellar disease might sometimes cause increased frequency of urination.



Bladder filling

The L region of the pons registers signals from the spinal cord that describe lack of significant bladder stretch.  This area will facilitate the sympathetic system to cause detruser relaxation and internal urethral sphincter constriction.  The somatic efferent function of the pudendal nerve is also facilitated to further promote urethral sphincter constriction.  The cholinergic efferent neurons of the pelvic nerve are inhibited to prevent detruser contraction.

*Image taken from BSAVA Manual of Canine and Feline Neurology


Bladder emptying

Urination is coordinated by both brainstem and local reflexes:


From the brainstem, the M region of the pons responds to a threshold stimulus triggered by critical detruser stretch.  Signals from the pons then triggers inhibition of the somatic efferent neurons from the pudendal nerve as well as the adrenergic neurons of the hypogastric nerve.  The pons will also facilitate the cholinergic afferent neurons of the pelvic nerve.  This ultimately results in bladder contraction and urethral relaxation for bladder emptying.


Locally, as the bladder contracts, further signals are sent to the sacral spinal cord to trigger two efferent reflexes.  One of these results in inhibition of the efferent neurons in the pudendal nerve which will cause external urethral sphincter relaxation.  The other results in signals traveling up to the proximal lumbar spinal cord segments to inhibit the sympathetic cell bodies of the hypogastric nerve.  This results in the inhibition of detruser relaxation and also causes internal urethral sphincter relaxation.  All of these assist in bladder contraction and urethral relaxation.

*Image taken from BSAVA Manual of Canine and Feline Neurology


UMN vs LMN bladder dysfunction

UMN injury proximal to L7 generally leads to disinhibition of the bladder filling system.  This results in excess bladder filling and increased urethral sphincter tone.  The bladder is large, turgid, and difficult to express.


LMN injury to the sacral segments or associated nerves (pudendal and pelvic nerves) generally leads to a large, flaccid bladder that is very easy to express.  Decreased rectal tone and perineal reflexes can also be seen.  Rarely there is enough tone from the hypogastric influence to make it difficult to express the bladder.



Pharmacologic interventions

There are numerous drugs used for pharmacologic intervention of micturition disorders.  The basic categories include:


  • Drugs to promote urination usually involve drugs to stimulate detruser contraction (cholinergics) or decreased urethral tone (α-antagonists and skeletal muscle relaxants).  Sometimes a β-antagonist may be used to help promote urination.


  • Drugs to limit incontinence/ urine leakage can involve drugs that decrease detruser hyperreflexia (anticholinergic or antispasmodics) or increase urethral tone like α-agonists or drugs that increase sensitivity to norepinephrine like diethylstilbesterol.





A Practical Guide to Canine and Feline Neurology, 2nd edition.  Dewey.  Wiley-Blackwell 2008:  p.419-425.


BSAVA Manual of Canine and Feline Neurology, 3rd edition.  Platt and Olby.  BSAVA 2004:  p.312-318.