Page 1047 - Fundamentals of anatomy physiology

1034 Unit 5 Environmental Exchange
Figure 26–20 The Micturition Reflex.

Brain C2 Projection fibers Clinical Note
from thalamus
If convenient, the Urinary Obstruction Local blockages of the collecting
C3 individual voluntarily deliver sensation ducts or ureters can result from casts—small blood clots,
epithelial cells, lipids, or other materials that form in the
relaxes the external to the cerebral collecting ducts. Casts are commonly eliminated in urine.
urethral sphincter. They are visible in microscopic analyses of urine samples.
cortex. Renal calculi (KAL-kū-lī), or kidney stones, form within the
The afferent fibers urinary tract from calcium deposits, magnesium salts, or
stimulate neurons C1 An interneuron crystals of uric acid. The condition is called nephrolithiasis
involved with: relays sensation (nef-rō-li-THĪ-uh-sis; nephros, kidney; lithos, stone). The
blockage of the ureter by a stone or by other means (such
L a local pathway, to the thalamus. as external compression) creates urinary obstruction. This
problem is serious. In addition to causing pain, it decreases
and or prevents filtration in the affected kidney by increasing
the capsular hydrostatic pressure.
C a central pathway
Calculi are generally visible on an x-ray. If peristalsis
L2 Senory L3 Parasympathetic and fluid pressures cannot dislodge them, they must be
fibers in preganglionic either surgically removed or destroyed. One nonsurgical
procedure involves disintegrating the stones with a litho-
pelvic motor fibers in tripter, a device originally developed from machines used
to de-ice airplane wings. Lithotripters focus sound waves
nerves. pelvic nerves. on the stones, breaking them into smaller fragments that
can be passed in the urine. Another nonsurgical approach
L1 Distortion Urinary L4 Postganglionic is the insertion of a catheter armed with a laser that can
bladder neurons in shatter calculi with intense light beams.
of stretch
receptors. intramural potential inconvenience. At the end of a typical micturition, less
Start than 10 mL of urine remains in the bladder.
ganglia stimulate
Infants lack voluntary control over urination, because the
detrusor muscle necessary corticospinal connections have yet to be established.
Accordingly, “toilet training” before age 2 often involves train-
contraction. ing the parent to anticipate the timing of the reflex rather than
training the child to exert conscious control.
C4 Voluntary relaxation of the
Incontinence (in-KON-ti-nens) is the inability to control
external urethral sphincter urination voluntarily. Trauma to the internal or external ure-
causes relaxation of the thral sphincter can contribute to incontinence in otherwise
internal urethral sphincter. healthy adults. For example, some mothers develop stress urinary
incontinence (SUI) if childbirth overstretches and damages the
Urination occurs sphincter muscles. In this condition, increased intra-abdominal
pressures—caused, for example, by a cough or sneeze—can
26 the sacral spinal cord and (2) stimulates interneurons that relay overwhelm the sphincter muscles, causing urine to leak out.
sensations to the thalamus and then, through projection fibers, Incontinence can also develop in older people due to a general
to the cerebral cortex. As a result, you become aware of the fluid loss of muscle tone.
pressure in your urinary bladder.
Damage to the central nervous system, the spinal cord, or the
The urge to urinate generally appears when your bladder nerve supply to the urinary bladder or external urethral sphincter
contains about 200 mL of urine. As Figure 26–20 shows, the mic- can also produce incontinence. For example, incontinence com-
turition reflex begins when the stretch receptors provide adequate monly accompanies Alzheimer’s disease or spinal cord damage.
stimulation to parasympathetic preganglionic motor neurons. A In most cases, the affected individual develops an automatic
further increase in bladder volume begins the cycle again, usu- bladder. The micturition reflex remains intact, but voluntary
ally within an hour. Each increase in urinary volume leads to
an increase in stretch receptor stimulation that makes the sensa-
tion more acute. Once the volume exceeds 500 mL, the blad-
der contractions triggered by the micturition reflex may generate
enough pressure to force open the internal urethral sphincter.
This opening leads to a reflexive relaxation of the external urethral
sphincter. Urination takes place despite voluntary opposition or