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1. The reflex & mechanism of micturition; indicate the spinal cord
segments and the nerves involved.
Urination, also known as micturition, voiding, and, more rarely, emiction, is the
process of disposing of urine from the urinary bladder through the urethra to the
outside of the body.
The Urinary Bladder is a Hollow, muscular organ (with rugae except in trigone)
which is distensible for storage for urine. Its Superior surface covered by layer of
peritoneum – anterior, posterior, inferior surfaces lie outside peritoneal cavity. The
Trigone is a triangular area formed by ureteric openings and the urethral opening,
which acts as a funnel during contraction. Ureters penetrate the bladder via their
orifices without penetrating the peritoneal cavity; they enter posterioinferiorly,
obliquely forming a valvular flap
Innervation of Bladder
Sympathetic outflow is from first and second lumbar segments (hypogastric plexus),
and these nerves inhibit contraction of the detrusor muscle and stimulate closure of
the sphincter vesicae (ie the internal sphincter)
Parasympathetic outflow is from 2nd, 3rd, and 4th sacral segments of the spinal cord
(pelvic splanchnic nerves), and in this way the PSNS stimulates contraction of the
detrusor muscle and inhibit the action of the sphincter vesicae
It should also be noted that males have a prostate gland between the internal and
external sphincters (ie sphincter urethrae)
Sensory nerve fibres enter the spinal cord at the aforementioned levels
Pontine micturition centre and suprapontine centre regulates inhibition of micturition
reflex (inhibition of PSNS)
Summary of Reflex
1. Bladder fills with urine
2. Excitation of sensory stretch receptors → signals travel through pelvic nerves to
sacral segments of spinal cord then back to bladder through PSNS fibres of pelvic
nerves
3. Release of Ach onto muscarinic Rc → contraction of detrusor muscle →↑ Pressure
4. Transient contractions followed by relaxation [Note – contraction of bladder pulls
apart the internal sphincter]
5. ↑filling of bladder → ↑ frequency and strength of micturition reflexes
(contractions/syncytium)
6. Centres in pons and cerebral cortex have final control over micturition (keep
micturition reflex partially inhibited; cause continual tonic contraction of external
able to defer micturition until a suitable time
7. Once micturition reflex powerful enough →triggers reflex through pudendal nerves
→inhibition motor control of external sphincter →sphincter opens
8. If CNS inhibition of external sphincter greater than voluntary control →urination
2. The glandular arrangement and ‘capsules’ of the prostate.
In a normal healthy human adult, the prostate gland weighs around 20g and is roughly
the size of a walnut. It surrounds the urethra just below the urinary bladder and can be
felt during a rectal exam. Within the prostate, the urethra coming from the bladder is
called the prostatic urethra and merges with the two ejaculatory ducts. This therefore
means that the male urethra has two functions; to carry urine from the bladder during
urination to carry semen during ejaculation.
The prostate parenchyma can be divided in two different ways: by zone, or by lobe.
Zone classification is mostly used for pathology where as lobe classification mainly
for anatomical purposes
Zones - The prostate gland has four distinct glandular regions, two of which arise
from different segments of the prostatic urethra:
Name
Percent
Composes up to 70%
of the normal
prostate gland in
young men
Description
The sub-capsular portion of the posterior aspect
Peripheral
of the prostate gland which surrounds the distal
zone (PZ)
urethra. It is from this portion of the gland that
more than 64% of prostatic cancers originate.
This zone surrounds the ejaculatory ducts. The
Constitutes
central zone accounts for roughly 2.5% of
Central zone approximately 25%
prostate cancers although these cancers tend to
(CZ)
of the normal
be more aggressive and more likely to invade
prostate gland
the seminal vesicles.
Prostate cancer originates in this zone in
roughly 34% of patients. The transition zone
Responsible for 5%
Transition
surrounds the proximal urethra and is the region
of the prostate
zone (TZ)
of the prostate gland which grows throughout
volume at puberty.
life and is responsible for the disease of benign
prostatic enlargement.
Anterior fibro- Accounts for
This zone is usually devoid of glandular
muscular zone approximately 5% of components, and composed only, as its name
(or stroma)
the prostatic weight suggests, of muscle and fibrous tissue.
NB. The parenchyma are the functional parts of an organ in the body. This is in
contrast to the stroma, which refers to the structural tissue of organs, being exactly,
connective tissues.
3. Prostatic enlargement and its effects (noting the significance of
enlargement of the median lobe).
As mentioned above, lobe classification is more often used for anatomy. The lobes of
the prostate include;
Anterior lobe (or isthmus)
roughly corresponds to part of transitional zone
Posterior lobe
roughly corresponds to peripheral zone
Lateral lobes
spans all zones
Median lobe (or middle lobe) roughly corresponds to part of central zone
Most carcinomas arise form peripheral areas of the gland, whereas Nodular
Hyperplasia commonly arises from more centrally situated glands. The transitional
zone surrounds the proximal urethra and is the region of the prostate gland which
grows throughout life and is responsible for the disease of benign prostatic
Hyperplasia (BPH). BPH is likely to cause urinary obstruction via constriction of the
urethra, making urination difficult and painful, leading to symptoms such as difficulty
commencing urination, increased frequency of urination, pain, reduced volume etc.
4. The significance of: (i) the prostatic venous plexus communication
with the internal vertebral venous plexus; (ii) the recto-vesical
septum (Denonvilliers fascia).
(i) – the prostatic venous plexus communicates with the internal vertebral venous
plexus. The vertebral system of veins is valveless meaning that flow may occur in
either direction. This therefore means that cancer of the prostate can easily and quite
often does spread to the vertebral column via these communicating networks
(ii) – the recto-vesical septum is dense connective tissue that separates the prostate
and the urinary bladder from the rectum. This therefore provides a limited barrier to
the local spread to cancer between the prostate and the rectum (and vice versa).