Page 183 - Problem-Based Feline Medicine

11 – THE CAT STRAINING TO URINATE 175

TRAUMA
● Urethral trauma* (p 189)
May result from pelvic fractures, surgery and catheterization especially when associated with ure-
thral obstruction. Urethral rupture may result in subcutaneous accumulation of urine in the perineal
region leading to swelling, discoloration and necrosis of the skin.
TOXIC
● Cyclophosphamide toxicity (p 191)
The use of cyclophosphamide as a therapeutic agent can lead to hematuria and dysuria.

Stretch receptors in the detrusor muscle are gradually
INTRODUCTION
stretched during bladder fill. Afferent pelvic nerve
fibers (parasympathetic) detect changes in the
MECHANISM? amount of stretch during filling and when the thresh-
old of bladder capacity is reached, they discharge
Straining in cats during micturition is due to
impulses to the sacral spinal cord.
increased urethral outflow resistance and/or lower
● These impulses are relayed through the spinoretic-
urinary tract inflammation or pain.
ular pathways to the pons (pontine mesencephalic
Straining cat exhibits clinical signs such as dysuria reticular activating system) in the brain stem where
(difficult or painful urination), stranguria (slow and a detrusor response is integrated.
painful urination with a narrow or dribbling urine ● Efferent motor impulses descend the spinal cord via
stream), pollakiuria (abnormally frequent urination) the reticulospinal pathway to the sacral parasympa-
with or without hematuria. thetic nucleus, and then on to the detrusor muscle
via the pelvic nerve.
Hematuria associated with dysuria indicates lower uri-
nary tract bleeding.
Each muscle fiber in the detrusor muscle does not
Owners may complain of inappropriate urination or receive direct innervation; rather “pacemaker” fibers
excessive time spent in the litter box, so these need to scattered through the detrusor are depolarized with sub-
be differentiated from behavioral causes of inappropri- sequent spread of excitation to adjoining muscle fibers
ate urination and constipation. through “tight junctions”. Tight junctions are areas of
fusion of the outer components of the cell membrane.
Urinary bladder innervation is primarily autonomic.
The storage phase is controlled by sympathetic The resulting wave of excitation causes contraction of
innervation. the detrusor, which pulls the neck of the bladder
● During the filling phase, sympathetic autonomic open like a funnel and squeezes out the urine.
activity (hypogastric nerve) dominates and
Simultaneously with detrusor muscle contraction,
causes the body of the bladder to relax (β-adren-
inhibitory interneurons are activated in the sacral
ergic stimulation) and the internal sphincter to
spinal cord, which synapse on pudendal motor neu-
contract (α-adrenergic stimulation).
rons. The pudendal neurons are inhibited, resulting
● There is also an α-receptor-mediated inhibition of
in relaxation of the external urethral sphincter.
parasympathetic ganglion transmission.
Once the bladder is empty, pelvic nerve activity ceases
Pudendal nerves (somatic innervation) stimulate con-
and the hypogastric and pudendal nerves are no longer
traction of the external urethal sphincter (and anal
inhibited. Detrusor muscle relaxation and urethral
sphincter).
sphincters contraction returns and the filling phase
Bladder emptying is under parasympathetic control. begins again.