A Comprehensive Report on the Innervation of the

Bladder, Micturition Reflex and How Urine is formed

Kenneth Pierre M. LopezSources : Clinical Anatomy By System (Richard Snell)

Medical Physiology 11th Edition(Guyton and Hall)

Dictionary of Medical Terms 2nd Edition(Mikel Rothenberg and Charles Chapman)

Videos from McGraw Hill and DNAtube

Innervation of the BladderImportant notes

• Has a Max Capacity of 500ml• Pyramidal in shape • It’s muscle coat is called the Detrusor Muscle

• Pressure in the bladder when Detrusor Contracts is 40 – 60 mm Hg

• It’s thickened circular component of the muscle coat found on its’ neck is called the Sphincter Vesicae (Involuntary)• Synonyms: annulus urethralis, internal urethral sphincter, musculus

sphincter vesicae, preprostate urethral sphincter,proximal, sphincter muscle of urinary bladder.

• External Sphincter of the Bladder (Voluntary)

Nerve Supply is formed from the Hypogastric Plexuses

Sympathetic (Sensation of Fullness of the bladder and Pain) : Originates from the 1st and 2nd Lumbar Ganglia to form the Hypogastric Plexuses

Parasympathetic (Motor; Internal Sphincter; Detrusor Muscle; Involuntary) : Originates from the 2nd, 3rd, and 4th Sacral Nerves to form the Pelvic Splanchnic Nerves

• Most afferent sensory fibers from the bladder reach the CNS via the Pelvic Splanchnic Nerves

• Some afferent fibers travel with the sympathetic nerves via the Hypogastric Plexuses

The Detrusor Muscle fuse with one another so that low-resistance electrical pathways exist from one muscle cell to the other making an action potential spread throughout it, causing the entire bladder to contract at once

Somatic Nerve Fibers (Motor; External Sphincter; Voluntary): Skeletal motor fibers that innervate and control the voluntary skeletal muscle of the sphincter are transmitted through the Pudendal Nerve

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Sympathetic (Sensation of Fullness of the bladder and Pain) : Originates from the 1 st and 2nd Lumbar Ganglia to form the Hypogastric Plexuses

Parasympathetic (Motor; Internal Sphincter; Detrusor Muscle; Involuntary) : Originates from the 2 nd, 3rd, and 4th Sacral Nerves to form the Pelvic Splanchnic Nerves

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US Sympathetic (Sensation of Fullness of the bladder and Pain) : Originates from the 1st and 2nd Lumbar Ganglia to form the Hypogastric

Plexuses

Parasympathetic (Motor; Internal Sphincter; Detrusor Muscle; Involuntary) : Originates from the 2nd, 3rd, and 4th Sacral Nerves to form the Pelvic Splanchnic Nerves

Definition of Terms Plexus : A network of intersecting nerves, blood vessels or

lymph vessels Ganglia / Ganglion: Collection of nerve cells forming a

knot like shape and usually lying outside the brain and spinal cord (Outside CNS)

Afferent : Carrying inward or towards the Center (Brain), as a nerve carrying a sensory impulse to the brain

Efferent : Carrying outward or away from the center (Brain), as a nerve carrying impulses from the brain to a muscle, gland or other effector organ

Action Potential : Electrical charge developed in a muscle or nerve cell that leads to its discharge or contraction

Distended / Distention: State of being stretched out or enlarged

Function of the Sympathetic and Parasympathetic Nerves Before

› Sympathetic Nerves inhibit contraction of the detrusor muscle of the bladder wall and stimulate closure of the sphincter vesicae

› Parasympathetic Nerves stimulate contraction of the detrusor muscle of the bladder wall and inhibit the action of the sphincter vesicae

Today’s Theory

› Sympathetic Nerves have little or no action on the smooth muscle of the bladder wall and are distributed mainly to the blood vessels and plays a minor role in the contraction of the sphincter vesicae

› In males the Sympathetic Innervation of the sphincter causes active contraction of the bladder neck during ejaculation, thus preventing the semen to enter the bladder

Micturation Reflex

Clinical Notes

• Urinary Calculus : Stone formed in any part of the urinary system also known as Kidney stone and/or Renal Calculus• Urinary Hesitancy : Difficulty in beginning the flow of urine and decrease in the force of the urine stream. In men it is associated with prostate gland enlargement; in women with narrowing of the opening of the urethra or obstruction between the bladder and urethral it may also be caused in either sex by emotional stress and other factors

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How The Micturition Reflex Works

The reflex is initiated when the volume of the urine reaches 300 ml

Stretch receptors in the bladder wall are stimulated an transmit impulses to the CNS, this gives the individual the conscious desire to micturate (urinate)

Sensory signals from the bladder are conducted to the sacral segments of the cord through the pelvic nerves and then reflexively back again to the bladder through the parasympathetic nerve fibers by way of these same nerves.

These impulses from the parasympathetic nerve fibers causes the Detrusor Muscle to Contract and at the same time make the Urethral Sphincter Relax

Efferent impulses also pass to the urethral sphincter via the pudendal nerve (S2, S3, and S4), and this undergoes relaxation.

Once urine enters the urethra, addition afferent impulses pass to the spinal cord from the urethra and reinforce the reflex action› Note: Micturition can be assisted by contraction

of the abdominal muscles to raise the intra-abdominal and pelvic pressure and exert external pressure on the bladder

› Muscles that compress abdominal Contents : External Oblique, Internal Oblique, Transversus,

Rectus Abdominis

Micturition in Adults and In Children (before 2nd or 3rd year of life)

Children› A simple reflex

that acts and takes place whenever the bladder becomes distended.

Adults› The same simple

reflex is inhibited by the Cerebral Cortex until the time and place for micturition are favorable

› Voluntary Movement is Controlled by Brodmann’s Area 4,6,8

Facilitation or Inhibition of Micturition by the BrainClinical Notes

•Overflow Incontinence : When sensory fiber nerves from the bladder to the spinal cord are destroyed preventing transmission of stretch signals from the bladder a person loses bladder control. Instead of emptying periodically, the bladder fills to capacity and overflows a few drops at a time through the urethra.•Automatic Bladder caused by spinal cord damage above the Sacral Region : Since the damage is above the sacral region normal micturition reflex can occur. However they are no longer controlled by the brain. Hence periodic but unannounced bladder emptying occurs.

The micturition reflex is the basic cause of micturition, but the higher centers of the brain (cortex) normally exert final control of micturition as follows: 1. the micturition reflex partially inhibited, except

when micturition is desired2. Prevent micturition, even if the micturition

reflex occurs, by continual tonic contraction of the external bladder sphincter until a convenient time presents itself

3. When it is time to urinate, the cortical centers can facilitate the sacral micturition centers to help initiate a micturition reflex and at the same time inhibit the external urinary sphincter so that urination can occur.

Voluntary Urination Usually initiated in the following way:

1. A person voluntarily contracts his or her abdominal muscles, which increases the pressure in the bladder and allows extra urine to enter the bladder neck and posterior urethra under pressure, thus stretching their walls

2. This stimulates the stretch receptors, which excites the micturition reflex

3. Simultaneously inhibits the external urethral sphincter allowing urine to flow

Urine Formation

Important Notes

• The function of the kidney is to excrete most of the waste products of metabolism. They play a major role in controlling the water and electrolyte balance within the body• The urine is propelled along the ureter by peristaltic contractions of the muscle coat, assisted by the filtration pressure of the glomeruli

Urine formation begins when a large amount of fluid that is virtually free of protein is filtered from the glomerular capillaries into Bowman’s Capsule.

As the filtered fluid leaves Bowman’s capsule and passes through the tubules, it is modified by reabsorption of water and specific solutes back into the blood.

The rate at which the substance is excreted in the urine depends on the relative rate of filtration, reabsorption, and secretion.

Urinary excretion rate = Filtration rate – Reabsorption rate + Secretion rate

How Urine is Formed

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