Lecture 5: Micturition Flashcards
Your body filters how much plasma per minute (GFR) and how much is reabsorbed per minute; how much is filtered per day?
125 mL/min and 124 mL/min is reabsorbed
180 L/day is filtered
What’s the average volume of urine of day; leaves what percentage in nephron to be reabsorbed?
1.5 L/day; leaving 99% of fluid in nephron to be reabsorbed
What are the 3 exchange processes in nephrons that occur before excretion can take place?
1) Filtration - movement from blood into lumen of nephron
2) Reabsorption - from filtrate in lumen back into blood
3) Secretion - removes selected molecules from blood and adds them to the filtrate
Filtration only takes place in; what allows it to happen?
Renal corpuscle; walls of glomerular capillaries and Bowman’s capsule are modified to allow bulk of fluid
2 main jobs of bladder; pressures at each?
1) Storage of urine (low pressure)
2) Emptying (high pressure)
How do stretch receptors get activated in micturition?
- Increased volume of fluid in bladder expands the wall and activates the stretch receptors.
What are the 3 parts of the bladder that are involved with urination?
- Detrusor muscle
- Internal urethral sphincter
- External urethral spinchter
Explain what happens via sympathetic, parasympathetic, and somatic control when stretch receptors are activated for micturition reflex
Decreased sympathetic = relaxation of internal urethral sphincter
Increased parasympathetic = contraction of detrusor muscle
Decreased somatic motor neuron = relaxation of external urethral sphincter.
What kind of muscle is found in the internal and external urethral sphincter?
Internal = smooth muscle
External = skeletal muscle
The sympathetic nerves under normal conditions will excite and inhibit what?
- Excite the external urethral sphincter (constriction)
- Inhibit the detrusor muscle (relaxation)
What is the major step in emptying the bladder?
Contraction of the Detrusor muscle (smooth muscle)
What allows for the entire bladder to contract at once?
AP can spread throughout the muscle, due to the low-resistance electrical pathways created by smooth muscle cells
What allows for a person to hold in their urine even when bladder is getting really full?
The external sphincter muscle is under voluntary control (skeletal muscle)
Micturition is governed by what 2 mechanisms?
1) The micturition reflex (stretch receptors)
2) Voluntary control
Most adults can accommodate how much urine before tension rises enough to activate stretch receptors?
250 to 400 mL
What kind of fibers carry impulses from the stretch receptors into the spinal cord?
Afferent fibers
The micturition reflex is what kind of reflex?
Spinal, meaning you don’t have control over the reflex itself, the detrusor will start contracting and the internal sphincter will relax as soon as the stretch receptors are activated.
*Infants have not learned voluntary control yet, which is why they urinate as soon as the reflex kicks in.
What part of brain is receives sensory input from the bladder; what parts of the brain does it have connection with?
Periaqueductal grey (PAG); connects with the pontine micturition center (PMC), thalamus, prefrontal cortex, and insula
During voiding what happens to the PMC; what state is it usually in?
- Usually is suppressed, but becomes excited by activity in the prefrontal cortex and the hypothalamus,
- Activates descending pathways to the spinal cord and causes urethral relaxation and detrusor contraction
What nerve is involved with the somatic control of the bladder?
Pudendal nerve
What NT is involved in sympathetic stimulation of the bladder and what receptors are found on the detrusor and urethra?
- Norepinephrine
- Detrusor: Beta-3 receptors (relaxation)
- Urethra: Alpha receptors (contraction)
Impulses arising from what spinal segments and from what nerve to the bladder body are used for sympathetic control?
T11 to L2; Hypogastric nerve
What kind of receptor is on the detrusor muscle for parasympathetic innervation and what is the NT?
M3; ACh
Stimulation of M3 receptors on detrusor cause what series of events to occur?
Hydrolysis of PLC leading to release of intracellular Ca2+ and a smooth muscle contraction
Parasympathetic preganglionic fibers originate from what spinal segments and travel in what nerve to ganglia where?
S2-S4; Pelvic nerve; ganglia in pelvic plexus
Stimulation of the pudendal nerve causes what NT to activate what receptor and the affect on the muscle?
Release of ACh and stimulation of nicotinic cholinergic receptors resulting in contraction of the extrinsic sphincter.
When no urine is present in the urinary bladder, the internal pressure is about ______ mmHg; how high is the pressure when there is 100 mL?
- 0 mmHg
- At 100 mL will only rise to about 10 mmHg
At what point will the pressure inside the urinary bladder rise at an exponential rate?
Volumes above 400 mL
What’s occurring in phase Ia of a Cystometrogram?
Initial phase of filling, pressure goes from 0 to 10 cm H2) at about 50 mL
What’s occurring in phase Ib of a Cystometrogram?
Plateau phase at 10 cm H2O until bladder volume reaches 400 mL
What occurs in phase II of a Cystometrogram; difference if there is voiding or no voiding?
Pressure rises exponentially above 400 mL
With voiding: pressure raises by about 20-40 cm H2O
Without voiding: pressure rises slower from 10 cm H2O
At what volume (mL) does the urge to pee become unbearable?
600 mL
What causes Atonic bladder?
- Syphilis
- Crushing injuries to the spinal cord
What is Atonic bladder?
- Destruction of sensory nerve fibers from bladder to spinal cord
- Stretch signals are inhibited, therefore, micturition reflex is inhibited
