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The properties and innervation of the muscle layers of the membranous urethra of the male dog

Van der Werf, Barbara Alexandra (1999) The properties and innervation of the muscle layers of the membranous urethra of the male dog. PhD thesis, Murdoch University.

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The membranous urethra of the male dog contains mainly striated muscle oriented in a circular layer and an inner layer of longitudinal smooth muscle. The aim of this project was to characterise the properties and identify the innervation of these muscles in the urethra of the greyhound in order to determine their importance in urination and ejaculation.

To determine the properties of striated muscle, stereological, histochemical and microelectrode studies were performed. Stereological analysis of serial transverse sections of the membranous urethra indicated that it contained 74.8±3.7 % striated muscle, 9.1±2.6 % smooth muscle and 16.0±2.8 % connective tissue in greyhounds. The percentage of striated muscle decreased after 6 years of age. The thickness of the ventral and dorsal walls decreased from the proximal to distal region of the urethra. Histochemical staining for ATPase at pH 9.5, 4.6 and 4.3 indicated that the predominant fibre type in the urethra of all dogs was type Ha (69 %). Greyhounds had a significantly smaller percentage of type I fibres (5.3±2.3 %) than other dogs (14.1±5.1 %) and a significantly larger percentage of type Tic fibres (25.3±3.6 %). The diameter of type I fibres (21.1±5.4 pm) was significantly smaller than type II fibres (24.2±6.2 pm) in greyhounds. The electrical properties of striated muscle fibres, recorded with microelectrodes, showed a continuous variation of values for RMP (75.4±6.1 mV) and the evoked action potential. It was not possible to determine differences in fibre types from electrophysiological properties.

In order to determine the innervation of striated muscle detailed anatomic dissection of the pudendal, pelvic and hypogastric nerves was carried out. The urethral pressure and contractile responses to stimulation of these nerves were recorded in vivo and in vitro. Pudendal nerve branches entered the striated muscle from the caudal end and stimulation of these produced rapid, visible contraction that was completely abolished by D-tubocurarine in anaesthetised greyhounds and in the isolated urethra. Field stimulation of strips resulted in rapid D-tubocurarine-sensitive contractions that were well maintained with continuous or intermittent stimulation unless non physiological frequencies greater than 20 Hz were used.

Small branches of the pelvic nerve, labelled with Dil, entered the membranous urethra on the lateral side but passed through the striated muscle to the inner smooth muscle. Stimulation of the pelvic nerve in anaesthetised dogs produced a slow contraction and pressure increase of the intact urethra, which was unaffected by Dtubocurarine but reduced to 74.7±53.3 % of the control value by atropine. Field stimulation of strips of longitudinal smooth muscle of the membranous urethra produced contraction at basal tone and relaxation with phenylephrine-induced tone. Acetylcholine and sometimes noradrenaline were involved in smooth muscle contraction, whereas nitric oxide produced phasic relaxation in all strips. VIP and the beta action of noradrenaline caused tonic relaxation of some strips. There was some evidence that contraction of the striated muscle was influenced by the autonomic nervous system.

Overall, the results confirm that the pudendal nerve exclusively produces contraction of the striated muscle component of the membranous urethra, although autonomic agents affect the contraction of this muscle. Cholinergic (excitatory) and nitrergic (inhibitory) nerves innervate the longitudinal smooth muscle with some input from adrenergic and other nerves. There are two possible scenarios when longitudinal smooth muscle contracts and shortens the urethra. Firstly, an increased number of lumenal folds may increase the resistance to urine flow and close the urethra within the contracted circular striated muscle. Secondly, if striated muscle is relaxed, the urethra may widen and allow the passage of urine. The longitudinal smooth muscle may also contract during ejaculation, while contractions of the circular smooth muscle constricts the bladder neck by adrenergic nerves, to prevent reflux of sperm into the bladder.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Veterinary and Biomedical Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Creed, Kate
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