Micturition - Anatomy & Physiology

Micturition is the normal process of the passive stoage and active voiding of urine.

Introduction

After entering the renal pelvis the modification of the urine is over in all domestic species other than the horse where mucin is added. The urine passes along the ureters and enters the bladder. It is here that the urine is stored until it is to be voided. Urine is not constantly excreted and it is only when there is a significant amount present in the bladder that the process of voiding occurs. Both the Bladder and the Urethra have smooth muscle and thus receive autonomic influence with regard to their activity. However the urethra also has an element of skeletal muscle giving the animal some degree of concsious control over the voiding of urine.

Innervation Involved in the Reflex

Sensory

The fullness of the bladder is detected by stretch sensitive nerve endings in the bladder wall

Motor

The motor components of the reflex are the Detrusor Muscle, Internal Urethral Sphincter and the External Urethral Sphincter. They former two are supplied by the autonomic nervous system with the latter being of somatic innvervation.

The general aim for the sympathetic and somatic systems is to retain urine
The general aim for the parasympathetic system is to void urine

Somatic Motor Supply

S1-S2
Pudendal Nerve
No synapse
Innervates the urethral skeletal muscle - external urethral sphincter
Function is to retain urine

Parasympathetic Supply - Bladder Detrusor Muscle

S1-S3
Synapse in pelvice plexus or bladder wall
Innvervate the detrusor muscle
Action - excitatory
Function - empty bladder

Sympathetic Supply - Detrusor Muscle

L1-L4
Syanpse in caudal mesenteric ganglion - bladder wall
Receptor - beta
Inhibitory action
Allows bladder filling

Sympathetic Supply - Internal Urethral Sphincter

L1-L4
Synapse in caudal mesenteric ganglion
Receptor - alpha
Excitatory action
Function - retain urine through increased urethral tone

Central Connections

There is aspects of control by the Pons and the Cerebral Cortex with possible influences from the cerebellum.