PROCESS DIAGRAM
T h e m i c t u r i t i o n r e f l e x • F i g u r e 1 5 . 8
The micturition reflex involves the urinary bladder and parasympathetic
nervous system. It is also influenced by the cerebral cortex.
When the volume of urine in the
urinary bladder exceeds
200-400 mL, pressure inside
increases and stretches the •>
.
bladder walls.
\
CN
Stretch receptors in the
urinary bladder send nerve
impulses to the spinal cord.
Parasympathetic nerve
activity causes contraction of
p " . -i
^ detrusor muscle and
relaxation of internal urethral
sphincter muscle.
Spinal cord inhibits motor
neurons and causes the
external urethral sphincter
muscle to relax.
Brain -
Spinal cord
C1
C
2
n
C3
C4
C5
C
6
<
i
C7
C
8
T1 <
T2
T3
T4
T5
T
6
T7
T
8
T9
T10
T11
T12
L1
L2
L3
L4
T 5 -
S1'j
S2-
S3'
54
55
Coccygeal
Nerve impulses from the
cerebral cortex can
voluntarily contract the
external urinary sphincter
muscle, thereby delaying
the passage of urine
through the urethra for a
limited time.
5
Urination (micturition) occurs.
Urination Is a Spinal Reflex
As urine accumulates in the blad-
der, it stimulates a reflex that ini-
tiates the process of urination, or
m ic tu r itio n
. The
m i c t u r i t i o n r e -
f le x
involves the urinary bladder’s
smooth muscles and sphincter muscles and is regulated
by the parasympathetic nervous system (
F ig u r e 1 5 . 8
).
The cerebral cortex can tempo-
rarily suppress the reflex; young
children learn how to control this
reflex when toilet training.
Urinary
in c o n tin e n c e
(in-
KON-ti-nens) occurs when the uri-
nary bladder’s sphincter muscles weaken. Causes of urinary
incontinence include frequent urinary tract infections, side
effects of medications, age, trauma, neuromuscular disease,
in c o n tin e n c e
(in-
KON-ti-nens) Inability
to retain urine, semen,
or feces through loss
of sphincter control.
m ic tu r itio n
(mik'-
choo-RISH-un) The act
of expelling urine from
the urinary bladder.
constipation, and enlarged prostate (in males). Childbirth
is a common cause of incontinence in females due to trau-
ma to nerves and muscles in the perineal and pelvic areas
as the baby passes through the birth canal. Treatment for
urinary incontinence depends on the cause of the condition
and may include medication, muscle strengthening exer-
cises, or behavior modifications. In some situations, surgery
may be part of the treatment regimen.
CONCEPT CHECK
1.
Which
tube transports urine into the urinary
bladder?
2.
Which
muscle group generates the force neces-
sary to expel urine from the urinary bladder?
3.
What
types of materials enter the male pros-
tatic urethra?
452 CHAPTER 15
The Urinary System and Fluid, Electrolyte, and Acid-Base Balance
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