Micturition Flashcards
Describe the difference in function between the internal and external urethral sphincter
Internal urethral sphincter – small function in continence but mostly for preventing retrograde ejaculation, so is much more developed in men.
External urethral sphincter – skeletal muscle part of the pelvic floor major role in continence
How is urine expelled from the bladder?
Detrusor muscle are contained within the walls of the bladder and help expel urine when they contract.
How does pressure change in the bladder as it fills?
Pressure in bladder remains constant until a point at which is begins to rise very quickly. First part is called the storage stage. At about 500ml your bladder becomes unbearably full. Begin to feel that there is something there at about 150ml.
How does the pressure vary as urine is expelled?
As the bladder is voided there are rhythmic contractions of the detrusor muscles causing regular spikes in pressure as the volume decreases.
Which parts of the nervous system are involved in the string and voiding of urine?
Cerebral hemispheres – conscious control
Brainstem – regulate reflexes
Spinal cord – reflex arcs
Describe the main autonomic signals involved in storage of urine?
Stretch receptor in bladder communicates to a sensory neurone that sends a signal to the spinal cord (between S2 and S4 levels – S2, S3, S4 keeps the piss off the floor). These axons then go up through the cord until they reach T10-L2 levels (where sympathetic neurones live). Here it synapses. Then a pre-sympathetic fibre leaves the cord and synapses with a post sympathetic fibres which inhibits the detrusor muscles relaxing them (Beta 3 adrenoreceptor via the hypogastric nerve). It also stimulates the internal sphincter to contract via an Alpha 1 adrenoreceptor again from the hypogastric nerve. This only occurs at low levels of bladder wall stretch.
Which somatic nerves are involved in urine storage?
During storage stage the external sphincter is also closed. Coming from S2, 3 and 4 somatic neurones stimulate the External urethral sphincter to contract. These are stimulated from the L centre in the brain stem via the pudental nerve which releases ACh to a nicotinic receptor.
Describe the autonomic reflex arc that causes voiding
Stretch detected by the stretch receptors send communications to S2,3 and 4 synapsing to a parasympathetic neurone. This then sends a stimulation to the detrusor muscle – reflex arc. This is done by the pelvic nerve which releases ACh acting on M3 receptors.
In babies, they have no control over this micturition.
How does the brain-stem stimulate this reflex arc?
M centre in the brain stem sends communications down to the parasympathetic neurones and stimulates it to allow micturition to occur.
High levels of stretch detected by the stretch receptors will stimulate the M centre in the brain stem.
How do we have control of micturition?
the cortex sends inhibitory stimulation to the M centre to prevent it from stimulating the parasympathetic neurones.
When we decide, we do want to void then the Cortex send excitatory to the M centre all allow the reflex to take place.
Finally, to prevent the L centre from preventing micturition the M centre has a connection to the L centre which inhibits it allowing the motor control of the external sphincter to stop. Also, another communication from the M centres inhibits the sympathetic fibres preventing relaxation of the detrusor muscles and stimulating relaxation of the internal sphincter.
What’s the difference in speed between pre-ganglionic and post ganglionic neurones?
Note pre-ganglionic neurones are myelinated whilst post aren’t
Where do somatic neurones leave the spinal cord?
Via the ventral root
