URETHRAL SPHINCTERS

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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