Urology - Physiology of bladder function Flashcards
Describe the afferent innervation of the bladder
The bladder receives both afferent and efferent innervation. Afferent innervation (i.e. coming from the bladder) is carried by visceral afferent fibres of the parasympathetic and sympathetic systems. The afferent pathways are responsible for the sensation of bladder fullness.
Fibres carried by sympathetic fibres pass to the hypogastric plexus. From here, they enter the spinal cord through the posterior rami and terminate in the anteromediolateral column T9-L2. Sympathetic fibres transmit sensation of painful distension conveyed from the bladder wall.
Parasympathetic fibres sense pain and distension conveyed from the bladder wall and internal sphincter. They are carried by the pudendal nerve and enter the cord through the posterior rami and terminate in the anterolateral column S2, S3 and S4.
Do males and females have the same number of sphincters?
No. Males have an internal sphincter which is a muscular band of tissue present around the neck of the bladder and early part of the prostatic urethra. They also have an external sphincter, which is present within the urogenital diaphragm (actually in a muscle called pubococcygeus, part of the levator ani complex). Females only have an external sphincter, which is also located in the urogenital diaphragm.
Describe the efferent innervation of the bladder.
The efferent innervation of the bladder is divided into sympathetic, parasympathetic and somatic.
Sympathetic fibres originate from T9 - T12 and pass via the sympathetic chain into the inferior hypogastric ganglion and then via the hypogastric nerve. They supply the detrusor muscle of the bladder wall and the internal sphincter.
Parasympathetic supply comes from S2-S4 via the pelvic nerves (or nervi erigentes). These pass to the hypogastric plexus to supply the detrusor muscle and internal sphincter like the sympathetic system.
The external sphincter is composed of skeletal muscle and is under voluntary control. It receives somatic efferents via the pudendal nerve which originates in the anterior horn cells of S2, S3 and S4 spinal levels. This is under cortical control from the frontal lobe.
What receptors are present and where are they located for each of the efferent systems controlling the bladder?
Remember that there are 3 efferent inputs into the bladder - sympathetic, parasympathetic and nicotinic.
Sympathetic fibres release NAd and act on beta 3 and alpha 1 adrenoceptors. Beta 3 receptors are located on the bladder detrusor muscle and alpha 1 receptors are present in the internal sphincter. Sympathetic input causes relaxation of the detrusor muscle and contraction of the internal sphincter.
Parasympathetic fibres activate the M3 muscarinic receptor via ACh. These are present on the detrusor muscle. Parasympathetic input causes contraction of the detrusor muscle and internal sphincter relaxation.
Somatic fibres activate nicotinic AChRs on the external sphincter which is under voluntary control.
What is the overall effect of the sympathetic and parasympathetic system on the bladder?
Sympathetic stores (by causing detrusor muscle relaxation and internal sphincter contraction).
Parasympathetic voids (by causing contraction of the detrusor muscle and internal sphincter relaxation).
What is the storage reflex?
If the bladder is empty then there is little distension of the detrusor muscle. This is detected by the stretch receptors within the bladder wall that send low frequency afferent impulses via the pelvic nerves to the lower thoracic or upper lumbar spinal cord. These then synapse with sympathetic pre ganglionic fibres which in turn synapse in the inferior hypogastric ganglion. Post ganglionic sympathetic fibres are carried by the hypogastric nerve and cause detrusor muscle relaxation and internal sphincter contraction.
Afferent impulses from the pelvic nerves also synapse with the pudendal nerve in the sacral spinal cord. This causes contraction of the external sphincter to store urine.
What is the micturition reflex?
Bladder distension is detected by stretch receptors in the wall of the bladder and transmitted to the pons via the pelvic sensory nerves. The activated pontine micturition centre projects fibres to the lower thoracic or upper lumbar cord. Here, they inhibit sympathetic pre ganglionic fibres that synapse in the inferior hypogastric plexus, and in turn prevent firing of post ganglionic fibres carried by the hypogastric nerve. The pontine centre also projects to the sacral cord. Here, it firstly inhibits the somatic efferents carried by the pudendal nerve which relaxes the external sphincter. It secondly activates the parasympathetic efferents in the sacral cord which activates M3 receptors on the detrusor muscle causing contraction and relaxation of the internal sphincter.
What is a cystometrogram? How is it performed?
This is a curve which studies the relationship between intravesical pressure (IVP) and volume.
The bladder is emptied of urine and the pressure is recorded through a transducer connected to a 2 way catheter. The bladder is then filled with 50ml of water and the pressure is recorded for every increase in volume.
What are the 3 phases of the cystometrogram?
Ia - initial slight rise, produced by the first increment of volume by about 50ml.
Ib - long flat segment, produced by a further increase in volume up to 400ml.
II - sharp rapid rise, produced by a further increase in volume above 400ml.
Filling of the bladder is passive, voiding produces pressure waves that are superimposed on the rising phase of the cystometrogram.
Explain the Ib phase of cystometrogram
Segment Ib is a manifestation of the law of Laplace, which states that the pressure in a spherical viscus is equal to twice the wall tension divided by the radius (P = 2T/r). In the case of the bladder, the tension increases as the organ fills but so does the radius. Therefore, the pressure increase is only slight until the bladder is relatively distended.
What is a neuropathic bladder?
Neurogenic bladder is bladder dysfunction that may be either flaccid or spastic (i.e. upper or lower motor neuron). It is caused by neurological damage and the main symptom is overflow incontinence.
What neurological conditions can cause a neurogenic bladder?
These can be divided into CNS, PNS or mixed aetiologies.
CNS - CVA, MND, spinal cord injury, spina bifida
PNS - diabetes, alcohol, vitamin B12 deficiencies, iatrogenic (damage due to pelvic surgery)
Mixed - MS, syphilis, tumours
How are neurogenic bladder conditions classified?
Several systems are used, but a common one uses urodynamic findings.
1) Flaccid (hypotonic) bladder (i.e. LMN bladder) - in this condition the bladder volume is large, pressure is low and detrusor contractions are absent. It may be caused by peripheral neuropathies or damage to the S2-S4 level.
2) Spastic bladder (i.e. UMN) - the bladder volume is normal or small and involuntary contractions occur. It usually results from brain or spinal cord damage above T12. Bladder contraction and external sphincter relaxation are typically uncoordinated (detrussor-sphincter dyssynergia)
3) Mixed patterns
What is the main presentation of neurogenic bladder?
Overflow incontinence is the primary presenting symptom in patients with both spastic and flaccid bladders. Patients retain urine and have constant overflow dribbling. Men typically also have erectile dysfunction.
Patients with a spastic bladder may have frequency, nocturia, and urgency with spastic paralysis with sensory deficits.
