Lecture 5: Micturition

Question 1

Your body filters how much plasma per minute (GFR) and how much is reabsorbed per minute; how much is filtered per day?

Answer 1

125 mL/min and 124 mL/min is reabsorbed

180 L/day is filtered

Question 2

What’s the average volume of urine of day; leaves what percentage in nephron to be reabsorbed?

Answer 2

1.5 L/day; leaving 99% of fluid in nephron to be reabsorbed

Question 3

What are the 3 exchange processes in nephrons that occur before excretion can take place?

Answer 3

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

Question 4

Filtration only takes place in; what allows it to happen?

Answer 4

Renal corpuscle; walls of glomerular capillaries and Bowman’s capsule are modified to allow bulk of fluid

Question 5

2 main jobs of bladder; pressures at each?

Answer 5

1) Storage of urine (low pressure)

2) Emptying (high pressure)

Question 6

How do stretch receptors get activated in micturition?

Answer 6

• Increased volume of fluid in bladder expands the wall and activates the stretch receptors.

Question 7

What are the 3 parts of the bladder that are involved with urination?

Answer 7

• Detrusor muscle
• Internal urethral sphincter
• External urethral spinchter

Question 8

Explain what happens via sympathetic, parasympathetic, and somatic control when stretch receptors are activated for micturition reflex

Answer 8

Decreased sympathetic = relaxation of internal urethral sphincter

Increased parasympathetic = contraction of detrusor muscle

Decreased somatic motor neuron = relaxation of external urethral sphincter.

Question 9

What kind of muscle is found in the internal and external urethral sphincter?

Answer 9

Internal = smooth muscle

External = skeletal muscle

Question 10

The sympathetic nerves under normal conditions will excite and inhibit what?

Answer 10

• Excite the external urethral sphincter (constriction)

- Inhibit the detrusor muscle (relaxation)

Question 11

What is the major step in emptying the bladder?

Answer 11

Contraction of the Detrusor muscle (smooth muscle)

Question 12

What allows for the entire bladder to contract at once?

Answer 12

AP can spread throughout the muscle, due to the low-resistance electrical pathways created by smooth muscle cells

Question 13

What allows for a person to hold in their urine even when bladder is getting really full?

Answer 13

The external sphincter muscle is under voluntary control (skeletal muscle)

Question 14

Micturition is governed by what 2 mechanisms?

Answer 14

1) The micturition reflex (stretch receptors)

2) Voluntary control

Question 15

Most adults can accommodate how much urine before tension rises enough to activate stretch receptors?

Answer 15

250 to 400 mL

Question 16

What kind of fibers carry impulses from the stretch receptors into the spinal cord?

Answer 16

Afferent fibers

Question 17

The micturition reflex is what kind of reflex?

Answer 17

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.

Question 18

What part of brain is receives sensory input from the bladder; what parts of the brain does it have connection with?

Answer 18

Periaqueductal grey (PAG); connects with the pontine micturition center (PMC), thalamus, prefrontal cortex, and insula

Question 19

During voiding what happens to the PMC; what state is it usually in?

Answer 19

• 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

Question 20

What nerve is involved with the somatic control of the bladder?

Answer 20

Pudendal nerve

Question 21

What NT is involved in sympathetic stimulation of the bladder and what receptors are found on the detrusor and urethra?

Answer 21

• Norepinephrine
• Detrusor: Beta-3 receptors (relaxation)
• Urethra: Alpha receptors (contraction)

Question 22

Impulses arising from what spinal segments and from what nerve to the bladder body are used for sympathetic control?

Answer 22

T11 to L2; Hypogastric nerve

Question 23

What kind of receptor is on the detrusor muscle for parasympathetic innervation and what is the NT?

Answer 23

M3; ACh

Question 24

Stimulation of M3 receptors on detrusor cause what series of events to occur?

Answer 24

Hydrolysis of PLC leading to release of intracellular Ca2+ and a smooth muscle contraction

Question 25

Parasympathetic preganglionic fibers originate from what spinal segments and travel in what nerve to ganglia where?

Answer 25

S2-S4; Pelvic nerve; ganglia in pelvic plexus

Question 26

Stimulation of the pudendal nerve causes what NT to activate what receptor and the affect on the muscle?

Answer 26

Release of ACh and stimulation of nicotinic cholinergic receptors resulting in contraction of the extrinsic sphincter.

Question 27

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?

Answer 27

• 0 mmHg

- At 100 mL will only rise to about 10 mmHg

Question 28

At what point will the pressure inside the urinary bladder rise at an exponential rate?

Answer 28

Volumes above 400 mL

Question 29

What’s occurring in phase Ia of a Cystometrogram?

Answer 29

Initial phase of filling, pressure goes from 0 to 10 cm H2) at about 50 mL

Question 30

What’s occurring in phase Ib of a Cystometrogram?

Answer 30

Plateau phase at 10 cm H2O until bladder volume reaches 400 mL

Question 31

What occurs in phase II of a Cystometrogram; difference if there is voiding or no voiding?

Answer 31

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

Question 32

At what volume (mL) does the urge to pee become unbearable?

Answer 32

600 mL

Question 33

What causes Atonic bladder?

Answer 33

• Syphilis

- Crushing injuries to the spinal cord

Question 34

What is Atonic bladder?

Answer 34

• Destruction of sensory nerve fibers from bladder to spinal cord
• Stretch signals are inhibited, therefore, micturition reflex is inhibited