URETHRAL SPHINCTERS
There are two urethral
sphincters in urinary tract:
1. Internal urethral sphincter
2. External urethral
sphincter.
1. Internal
Urethral sphincter
This sphincter is
situated between neck of the bladder and upper end of urethra. It is made up of
smooth muscle fibers and formed by thickening of detrusor muscle.
It is innervated by autonomic nerve fibers. This sphincter closes the urethra
when bladder is emptied.
2. External
Urethral sphincter
External sphincter is
located in the urogenital diaphragm. This sphincter is made up of circular
skeletal muscle fibers, which are innervated by somatic nerve fibers.
NERVE SUPPLY TO URINARY BLADDER AND
SPHINCTERS
Urinary bladder and the internal
sphincter are supplied by sympathetic and parasympathetic divisions of autonomic nervous
system where as, the external sphincter is supplied by the somatic
nerve fibers.
SYMPATHETIC NERVE
SUPPLY
Preganglionic fibers of
sympathetic nerve arise from first two lumbar segments (L1 and L2) of spinal
cord.
After leaving spinal
cord, the fibers pass through lateral sympathetic chain without any synapse in the
sympathetic ganglia and
finally terminate in hypogastric
ganglion. The postganglionic fibers arising from this ganglion form the hypogastric nerve, which supplies the detrusor muscle
and internal sphincter.
Function of
Sympathetic Nerve
The stimulation of
sympathetic (hypogastric) nerve causes relaxation of detrusor muscle and
constriction
of the internal
sphincter. It results in filling of urinary bladder and so, the sympathetic nerve is
called nerve
of
filling.
PARASYMPATHETIC NERVE
SUPPLY
Preganglionic fibers
of parasympathetic nerve form the pelvic nerve or nervus erigens. Pelvic
nerve fibers arise
from second, third and fourth sacral segments (S1, S2 and S3) of
spinal cord. These fibers run through hypogastric ganglion and synapse with
postganglionic neurons situated in close relation to urinary bladder and
internal sphincter.
Function of
Parasympathetic Nerve
Stimulation of parasympathetic
(pelvic) nerve causes contraction of detrusor muscle and relaxation of
the internal sphincter leading to
emptying of urinary bladder. So, parasympathetic nerve is called the nerve of emptying or nerve of
micturition. Pelvic nerve has also the sensory fibers, which
carry
impulses from stretch receptors
present on the wall of the urinary bladder and urethra to the central nervous system.
SOMATIC NERVE SUPPLY
External sphincter is innervated
by the somatic nerve called
pudendal
nerve. It
arises from second, third and fourth sacral segments of the spinal cord.
Function of Pudendal
Nerve
Pudendal nerve
maintains the tonic
contraction of
the skeletal muscle fibers of the external sphincter and
keeps the external
sphincter constricted always. During micturition, this nerve is inhibited. It causes
relaxation of external
sphincter leading to voiding of urine. Thus, the pudendal nerve is responsible
for
voluntary control of micturition.
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