Urinary System: Physiology of Micturition - PDF

Urinary System module Session 8: Lecture 1: The Physiology of Micturition 1 Objectives: ❑ the anatomy of the bladder ❑ in general the innervation of the bladder ❑ the normal voiding reflex 2 ...

Urinary System module Session 8: Lecture 1: The Physiology of Micturition 1 Objectives: ❑ the anatomy of the bladder ❑ in general the innervation of the bladder ❑ the normal voiding reflex 2  Objective 1 The anatomy of the bladder 4 Detrusor Urinae muscle Detrusor – Latin – detrudere –to push down Made from a meshwork of muscle fibres in roughly 3 layers oInner longitudinal oMiddle circular oThis arrangement of muscle fibres gives the bladder strength, irrespective of which direction it is being stretched in. Supplied by the autonomic nervous system, not under voluntary control Spinal nerve supply is bilateral. oOuter longitudinal 5 6 Internal Urethral Sphincter o Continuation of the Detrusor muscle and made of smooth muscle. o Physiological sphincter at the bladder neck ▪ Physiological Sphincter – no muscle thickening, action due to structure o Primary Muscle of continence External Urethral Sphincter o Anatomical sphincter ▪ Localised circular muscle thickening to facilitate action o Derived from pelvic floor muscles o Skeletal muscle, under somatic, voluntary control o Contracts to constrict urethra and “hold in” urine 7  Objective 2 Describe in general the innervation of the bladder  The functions of the nervous system in relation to the lower urinary tract are (a) to provide sensations of bladder filling and pain, (b)to allow the bladder to relax and accommodate to increasing volumes of urine, (c)to initiate and maintain voiding so that the bladder empties completely, with minimal residual volume and (d)to provide an integrated regulation of the smooth muscle and skeletal muscle sphincters of the urethra. 9 The functional activity of the lower urinary tract is commonly divided into two phases: filling, in which the bladder relaxes and accommodates increasing volumes of urine while the urethral sphincters increase their tone to maintain continence; voiding of urine during which urethral sphincters relax and the bladder contracts. The extent of the bladder contraction is greater in men than women. In men the prostatic urethra, a region sometimes called the urethral sphincter, functions to prevent retrograde ejaculation into the bladder. 10 11 12 Detrusor Parasympathetic Pelvic Nerve (S2-S4) Ach M3 Receptors Contraction Sympathetic Hypogastric Nerve (T10-L2) NA β3 Receptors Relaxation Internal Urethral Sphincter Sympathetic Hypogastric Nerve (T10-L2) NA α1 Receptors Contraction 13  External Urethral Sphincter Somatic o Pudendal Nerve (S2-S4) o Spinal motor outflow from Onof’s Nucleus of the ventral horn of the cord oAch Nicotinic Receptor Contraction 14  Objective 3 Describe the normal voiding reflex The threshold for feelings suggestive of a full bladder is ~400ml. When the bladder is full, an urge to urinate arises. Brain Micturition Centres Spinal Micturition Centres Parasympathetic Neurones The increase in parasympathetic stimulation to the bladder via the Pelvic nerve causes the Detrusor to contract and increase intravesicular pressure. The Cerebral Cortex then makes a conscious, executive decision to urinate, reducing somatic stimulation to the External Urethral Sphincter. The contraction of the Detrusor coupled with the relaxation of the External Urethral Sphincter results in the bladder emptying through the urethra. 16 17 Storage of Urine in the Bladder The ureters, urinary bladder, internal and external urethral sphincters work together to pass urine into the urinary bladder and store it over many hours (e.g. at night). The walls of the bladder have many folds, which distend when filling with urine. Because of this, as the bladder fills intravesicular pressure hardly changes. At around 400ml of filling, afferent nerves from the bladder wall (possible stretch receptors) start to signal the need to void the bladder (Pain/temperature sensation). Brain Continence Centres Spinal Continence Centres Sympathetic Neurones 18 The increase in sympathetic stimulation to the bladder via the hypogastric nerve causes the Detrusor to relax and the Internal Urethral Sphincter to contract. The Cerebral Cortex then makes a conscious, executive decision not to urinate, increasing somatic stimulation to the External Urethral Sphincter. This causes it to contract, constricting the urethra. The relaxation of the Detrusor, coupled with the contraction of the Internal and External Urethral sphincters reduces intravesicular pressure and constricts the urethra, preventing micturition. 20