Neural control of micturition

Question 1

What shape is the bladder?

Answer 1

Triangular pyramid

Question 2

Describe the flow of urine through the bladder.

Answer 2

Ureters empty urine into the bladder posterior surface.

The bladder empties into the urethra inferiorly.

Question 3

Explain the location and function of the detrusor muscle.

Answer 3

Surrounding the bladder.

Contraction causes urine to pass out of the bladder

Question 4

Explain the location and function of the internal urethral sphincter.

Answer 4

Located at the neck of the urethra.

Involuntary muscle

Question 5

Explain the location and function of the external urethral sphincter.

Answer 5

Inferior to the internal sphincter.

Under voluntary control

Question 6

Explain the location and function of the bulbospongiosus muscle.

Answer 6

Inferior to the urethra.

Only men have this muscle and it helps with emptying urine from the bladder

Question 7

What it the average bladder capacity?

Answer 7

Roughly 500mL

Question 8

At what capacity will the first voiding urge occur?

Answer 8

150mL

Question 9

At what capacity is bladder fullness felt?

Answer 9

400mL

Question 10

What happens to bladder capacity during pregnancy?

Answer 10

It is reduced

Question 11

Explain the sympathetic nervous control of micturition.

Answer 11

Exits spinal cord at T12-L2. Along lumbar splanchnic nerves.
Synapse at the inferior mesentric ganglion.
Post synaptic neuron travels to the bladder.
Innervation relaxes the detrusor muscle and causes the internal urethral sphincter to contract

Question 12

Explain the parasympathetic nervous control of micturition.

What do sensory stretch nerves in the bladder do?

Answer 12

Exit spinal cord at S2-S4.
Preganglionic pelvic nerve
Causes contraction of the detrusor muscle and relaxation of the internal urethral sphincter
Sensory stretch nerves in the bladder pass information to the parasympathetic afferent neuron.

Question 13

What innervates the external urethral sphincter?

Answer 13

Somatic nervous system - Voluntary control

Question 14

What is the location and function of the pontine micturition centre?
How does neuronal imput change based on bladder fullness?

Answer 14

In the pons, in the brainstem.
The relay station for the ascending spinal bladder information and the higher brain areas.
PMC relaxes the external urethral sphincter and contracts the detrusor muscle (the brain can inhibit this if required).
Neurons are silent when the bladder is empty. They are activated by stretch mechanoreceptors, which are triggered when the bladder is full.

Question 15

What is the Periaqueductal grey?

Answer 15

Sensory and motor relay station. Drugs and the brain can have an influence on it.

Question 16

Describe foetal urination.

Answer 16

Amniotic fluid is largely created by urine. It gets more concentrated as full gestation approaches.
Baby swallows and inhales amniotic fluid (Insufficient may cause the lungs to under develop, contractures and club feet.

Question 17

What is renal agenesis?

Why may this occur?

Answer 17

Newborn is missing one or both kidneys.

If the foetus stops urinating into the amnion fluid.

Question 18

What starts the production of amniotic fluid?

Answer 18

Broncho-pulmonary epithelium

Question 19

Explain the automatic reflex bladder in babies.

Answer 19

Information reflex in the spinal cord (Information passed from the sensory to motor neurons, bypassing the brain).
Stretch receptors detect bladder filling and transmit afferent signal to spinal cord.
Signal returned to bladder from spinal cord segments S2 and S3 via parasympathetic fibres in pelvic nerve. Efferent signals excite the detrusor muscle and relax the internal urethral sphincter.

Question 20

What can trigger the automatic micturition reflex in babies?

Answer 20

Suprapubic precussions
Thigh scratching
Touching penile skin
Pulling pubic hair

Question 21

Explain voluntary control of micturition that is developed during potty training.

Answer 21

Micturition centre in the pons receives signals from stretch receptors.
If timely to urinate , pons returns signals to spinal interneurons that excite detrusor and relax the internal urethral sphincter. Also sacral neurons that keep the external sphincter closed are inhibited.
If untimely to urinate, signals from the cerebrum excite spinal interneurons that keep the external urethral sphincter contracted.

Question 22

Why is it difficult to urinate while you have an erection?

Answer 22

Parasympathetic nerves allow both urination and erection to proceed.
Pressure in the corpus spongiosum during an erection compresses the urethra.
During ejaculation the pressure in the urethra is enough to overcome the corpus spongiosum.
Pudendal nerve contracts bulbocavernosus muscle, which prevents urine release and stops semen from entering the bladder.

Question 23

Describe autonomous bladder.

Answer 23

Occurs due to loss of sacral nerves (parasympathetic and pudendal nerves)
Sympathetic nerves remain unopposed.
Urination is abolished
Back fill may cause renal failure
Suprapubic catheterisation can cause relief

Question 24

Describe the causes and effects of an atonic/neurogenic bladder?

Answer 24

Spinal cord is damaged and there is spinal shock.
It lasts several days to a few weeks.
Detrusor relax, internal sphincter contracts, urine retained.
Followed by an automatic reflex bladder (patient passing urine very ferquently)

Question 25

What factors impair micturition in dementia patients?

Answer 25

Difficulty of cerebrum interpreting micturition sensory information.
Loss of visuospatial abilities
Difficulty undressing and using the toilet
Locomotion problems