What is Continence?

What is Continence?

Before learn about urinary incontinence, it is important to understand what urinary continence is and what mechanisms allow women to have urinary continence.

The word “continence” refers to self-control. It’s the ability to hold it all in. The word “continence” comes from the Latin word “continentia” which means “a holding back.” “Continence” means holding back bodily functions – in the case of “urinary continence” it means the body’s ability to hold back urine.

In order to understand the mechanism of continence, it’s important that we understand the anatomy of the bladder and the urinary tract in women.

The Bladder

The bladder consists of three layers of smooth muscle, known collectively as the ‘detrusor’. The outer smooth muscle layer is oriented longitudinally, the middle layer is oriented circularly and the inner layer is oriented longitudinally. The detrusor muscle has been shown to be rich in enzymes and nerve supply. The trigone only comprises two layers of smooth muscle. The inner layer is similar to the rest of the detrusor, but the outer layer consists of smooth muscle bundles with a sparse nerve supply. The outer layer extends to, and is continuous with, the proximal urethra and the distal portion of the ureter, where it may have a role in preventing ureteric reflux. The smooth muscle that forms the bladder neck is separate from the detrusor, with little or no sphincteric effect. Two or three layers of transitional epithelia cover the detrusor and secrete a protein on to their luminal surface that forms a watertight blood–bladder barrier. When empty, the urothelium relaxes into numerous folds or rugae.

The Urethra

The normal female urethra is 30–50 mm in length. It comprises smooth and striated muscle. The smooth muscle is continuous with that of the detrusor, but has minimal innervation and little sphincteric effect. The external urethral sphincter is one of two striated muscle components surrounding the urethra. Its fibres are bulked anteriorly at the mid-urethral level, are slow twitch in nature and are involved in maintaining continence at rest. The second striated muscle component is the peri-urethral portion of the levator ani, which is separated from the external urethral sphincter by a connective tissue septum. The fibres are bulked anterolaterally at a lower level than the external urethral sphincter, are fast twitch in nature and are involved in maintaining continence under stress.

The mucosa of the urethra is lined by pseudostratified transitional epithelia proximally, changing to non-keratinized stratified squamous epithelia distally. The junction between the two cell types alters with age and oestrogenic status, which may affect urinary symptoms. In young females, the submucosa has a rich venous supply which engorges the tissues, helping to close the urethra. This ceases after the menopause, and may be involved in the development of stress incontinence later in life due to poor urethral closure.

The intrinsic closure mechanism is controlled essentially by the 3 layers of the urethra:

1. Tunica mucosa (responsible for coaption of the urethra IE that is the urethral lining lying in such a way that there is closure of the urethral lumen at rest)

2. Tunica spongiosa (submucosa with vascular plexus)

3. Tunica muscularis (inner longitudinal and outer circular muscles)

Normal urinary continence relies on the functional coordination of the anatomy by the nervous system. When an element is abnormal, the other mechanisms may be able to compensate and maintain continence but, 1 component alone may not be able to keep a patient dry. Continence is maintained by an equal contribution from the muscular and submucosal vascular components of the urethra.

You will often hear “De Lancey’s Consolidated Theory of Stress Urinary Incontinence” quoted by specialists in the field. De Lancey notes 3 components of the continence mechanism:

1. internal sphincter activity

2. external sphincter function / proximal urethral support

3. non-muscular component - vascular plexus (submucosal vascular cushions)and mucosal coaptation

The role of the submucosal vascular plexus still poorly understood whilst it occupies a large space within the urethra (vascular cushion). Urethral support may not be as important as previously thought.

The Pelvic Floor

The pelvic floor structures include: the levator ani muscles, the endopelvic fascia and the ligaments (condensations of fascia). The pelvic floor plays a very important role in the mechanism of continence.

The Function of the Bladder

The bladder largely serves two functions:

• Temporary store of urine – The bladder is a hollow organ. The walls are very distensible, with a folded internal lining (known as rugae), this allows it to hold up to 600ml.

• Assists in the expulsion of urine – During voiding, the musculature of the bladder contracts, and the sphincters relax.

How your bladder works

Your bladder is controlled by a complex system of nerves from your autonomic nervous system (parasympathetic and sympathetic), as well as by your spinal nerves. Some of these allow you conscious control, but others cause unconscious actions (things you cannot control).

• Sympathetic nerves from the T11-L2 levels of the spinal cord cause the internal urethral sphincter to tighten, helping to hold stored urine in the bladder.

Which nerves control what?

Bladder nerves

The nerves that control your bladder can be described as follows:

• Parasympathetic nerves from the S2, S3 and S4 levels of your spinal cord cause the upper part of your bladder to contract and your bladder neck to relax, assisting in the process of micturition (urination).

• Sympathetic nerves from the T11-L2 levels of your spinal cord do the opposite, causing the upper section of the bladder to relax and the bladder neck to contract, ensuring you can store urine.

Internal urethral sphincter nerves

The nerves that control your urethral sphincter can be described as follows:

• Parasympathetic nerves from the S2, S3 and S4 levels of the spinal cord control the internal sphincter, causing it to relax to allow urine to pass out of the bladder.

• Both of these functions are involuntary. This means that they operate in an automatic or reflex way, beyond your control.

External urethral sphincter nerves

• Nerves from the S2-S4 levels of your spinal cord control your external urethral sphincter. This sphincter is able to be voluntary or consciously controlled.

Filling and emptying your bladder

When the amount of urine in your bladder reaches around 250ml, sensors in your bladder muscle are stimulated. Your bladder signals your brain, and you will feel a slight urge to pass urine. Once you have around 400-500 ml in your bladder, this urge grows in intensity and you need to empty your bladder.

When full, the stretch receptors in your bladder stimulate nerves to initiate the subconscious reflex called the micturition reflex. The final stage of urination remains in your conscious control, until you can access an appropriate place and relax the external sphincter.

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