MICTURITION REFLEX
Micturition reflex is the
reflex by which micturition occurs. This reflex is elicited by the stimulation
of stretch receptors
situated on the wall of urinary bladder and urethra. When about
300 to 400 mL of urine is collected in the bladder, intravesical
pressure increases. This stretches
the wall of bladder resulting in stimulation of stretch receptors and
generation of sensory impulses.
Pathway for Micturition Reflex
Sensory (afferent) impulses from the
receptors reach the sacral segments of spinal cord via the
sensory fibers
of pelvic (parasympathetic) nerve. Motor (efferent) impulses produced in spinal
cord, travel through motor fibers of pelvic nerve towards bladder
and internal
sphincter. Motor impulses cause contraction of detrusor muscle
and relaxation of internal sphincter so that, urine enters the urethra
from the bladder.
Once urine enters urethra, the stretch receptors in the urethra are
stimulated and send afferent impulses
to spinal cord via
pelvic nerve fibers. Now the impulses generated from spinal centers inhibit
pudendal nerve. So, the external sphincter relaxes and
micturition occurs. Once
a micturition reflex begins, it is self-regenerative, i.e. the initial
contraction of bladder further activates the receptors to cause still
further increase in sensory impulses from the bladder and urethra. These
impulses, in turn cause further increase in reflex contraction of bladder. The
cycle continues repeatedly until the force of contraction of bladder reaches the maximum
and the urine is voided out completely. During micturition, the flow of urine
is facilitated by
the increase in the abdominal pressure due to the voluntary contraction
of abdominal muscles.
Higher Centers for
Micturition
Spinal centers for micturition are
present in sacral and lumbar segments. But, these spinal centers
are
regulated by higher
centers. The higher centers, which control micturition are of two types,
inhibitory centers and facilitatory centers.
Inhibitory centers
for micturition
Centers in midbrain and
cerebral cortex inhibit the micturition by suppressing spinal micturition centers.
Facilitatory centers
for micturition
Centers in pons facilitate micturition
via spinal centers. Some centers in cerebral cortex also facilitate
micturition.
FILLING OF URINARY BLADDER
PROCESS OF FILLING
Urine is continuously formed
by nephrons and it flows into
urinary bladder drop by drop through ureters. When urine collects in the
pelvis of ureter, the contraction sets up in pelvis. This contraction
is transmitted through rest of the ureter in the form of peristaltic wave up to
trigone of
the urinary bladder. Peristaltic
wave usually
travels at a
velocity of 3 cm/second. It develops at a frequency of 1 to
5 per minute. The peristaltic wave moves the urine into the bladder. After leaving the
kidney, the direction of the ureter is
initially downward and
outward. Then, it turns horizontally before entering the bladder. At the entrance
of ureters into
urinary bladder, a valvular arrangement is present. When peristaltic wave
pushes the urine towards bladder, this valve opens towards the
bladder. The position of ureter
and the valvular arrangement at the end of ureter prevent the back flow
of urine from bladder into the ureter when the detrusor muscle contracts. Thus,
urine is
collected in bladder drop by drop.
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