24 - Neural Control of Motivational Behaviour

Question 1

What controls drinking and thirst?

Answer 1

Osmolarity of the blood

The higher the osmolarity - the stronger the urge to drink

Question 2

What detects osmolarity of the blood?

Answer 2

Osmoreceptors in the subfornical organ

In the wall of the 3rd ventricle

Question 3

How do we regulate thirst?

Answer 3

Osmoreceptors in the subfornical organ increases behavioural thirst
Subfornical organ projects to the medial preoptic nucleus of the hypothalamus
Connects to the LIMBIC system to regulate the sense of thirst

Question 4

What does the subfornical organ activate?

Answer 4

Limbic system
Paraventricular nucleus (around the third ventricle)
Supraoptic nucleus (above the optic chiasm)
These two nuclei release ADH to reduce urine flow and loss of water

Question 5

What are 3 effects of ADH that contributes to a decreased loss of water in urine?

Answer 5

• Aquaporins in the membrane of the collecting duct epithelial cells, allow water to pass out into the renal medulla
• Increases permeability of the collecting duct to urea, increased reabsorption of urea into medullary interstitium
• Stimulates sodium reabsorption, in the thick ascending loops of henle, increases Na+K+2Cl- cotransporter

Question 6

Lesions in what area of the hypothalamus cause a) anorexia b) obesity?

Answer 6

a) Anorexia - lateral hypothalamus

b) Obesity - medial hypothalamus

Question 7

Which nuclei do medial lesions of the hypothalamus affect?

Answer 7

Arcuate and periventricular nuclei

Question 8

What is another name for the medial area of the hypothalamus?

Answer 8

Satiety centre

Question 9

What is another name for the lateral hypothalamic nucleus?

Answer 9

Orexigenic (hunger) centre

Question 10

What is the difference between the paraventricular and periventricular nucleus?

Answer 10

Paraventricular nucleus - releases ADH

Periventricular nucleus - food intake

Question 11

What are the 2 factors which regulate food intake?

Answer 11

Internal stimuli - how hungry we feel, levels of blood chemicals
External stimuli - how attractive the food is, sight and smell of food

Question 12

What is Ghrelin?

Answer 12

Hunger hormone, a protein hormone produced by the oxyntic glands
Stimulates eating by an action on the arcuate nucleus of the hypothalamus

Question 13

What causes ghrelin release?

Answer 13

Stomach contractions, high insulin levels

Question 14

Which 2 neurones in the arcuate nucleus does ghrelin stimulate? Where do they project to? What is their function?

Answer 14

Neuropeptide Y (NPY) - Periventricular nuclei - inhibit neurons - inhibits satiety - releases eating behavior
Agouti related peptide (AGRP)
(Ghrelin increases hunger by indirectly inhibiting the satiety centre)

Question 15

What is leptin?

Answer 15

A protein hormone which reflects the total amount of adipose tissue in the body - gives the brain a reading of total energy store

Question 16

Where is leptin produced?

Answer 16

Adipose tissue

Question 17

What can leptin levels affect?

Answer 17

Fertility in women, if circulating leptin is too low than a woman ceases to menstruate

Question 18

Is leptin administration a cure for obesity?

Answer 18

No - clinical trials failed - people gained back the weight they had lost - leptin resistance

Question 19

What is the role of CCK?

Answer 19

Peptide synthesised by cells in the mucosal epithelium of the small intestine
Causes the release of digestive enzymes from the pancreas and bile from the gallbladder
Acts on the dorsal part of the arcuate nucleus to produce satiety
SATIETY SIGNAL

Question 20

What does CCK stand for?

Answer 20

Cholecystokinin

Question 21

What is the the role of Glucagon like peptide 1 (GLP-1)

Answer 21

Released from endocrine cells in the gut when food is present
Produces rapid satiety
Stimulates insulin secretion

Question 22

What is the role of oxyntomodulin and PYY?

Answer 22

Peptide hormones released from enteroendocrine cells
Released after a meal together
Act on the hypothalamus to inhibit feeding
Oxyntomodulin - delays gastric emptying and decreases gastric acid secretion
PYY - increases illeal absorption, slows gastric emptying, delays bladder and pancreatic secretion

Question 23

Which hormone stimulates eating?

Answer 23

Ghrelin

Question 24

Which hormones inhibit eating?

Answer 24

CCK, GLP-1, oxyntomodulin, PYY

Question 25

What does pro-opiomelanocortin (POMC) do?

Answer 25

CCK, GLP stimulate neurones after feeding in the dorsolateral nucleus that contain pro-opiomelanocortin(POMC)
They stimulate cells in the periventricular satiety centre and stop feeding behavior

Question 26

What is diuresis?

Answer 26

Production of urine by the kidney

Question 27

What is the difference between the urethra and ureter?

Answer 27

Ureter - kidney to bladder

Urethra - bladder to exit

Question 28

What is the urine flow in an adult?

Answer 28

1ml/min

Question 29

What muscles control urine release?

Answer 29

Internal and external urethral sphincters

Detrusor muscle - contracts bladder to expel urine

Question 30

Which receptors mediate inhibitory and excitatory actions on the bladder sphincters?

Answer 30

Inhibitory - adrenergic B receptors

Excitatory - A receptors

Question 31

Which afferents allow tonic contraction of the urethral sphincter muscles during filling and storage?

Answer 31

Sympathetic efferents

Also inhibit contraction of detrusor muscle

Question 32

Which nerves do the sympathetic afferents travel in?

Answer 32

Hypogastric plexus

Question 33

Which drugs can have an effect on bladder control?

Answer 33

Antihypertensives, antiasthmatics (act on sympathetic nervous system)

Question 34

Which tract do the cells synapse on?

Answer 34

Spinothalamic tract

Question 35

What happens when the bladder fills?

Answer 35

• Bladder fills and stretches
• Stretch receptors in the walls detect and activate afferents in the pelvic nerves
• Synapse on cells of the spinothalamic tract
• Relay to genital area of somatosensory cortex (bladder fullness)
• Not appropriate for micturition = external urethral sphincter tightens (skeletal muscle under vol control)
• The lower motor neurones controlling this muscle run in the pudendal nerve

Question 36

Describe the micturition reflex

Answer 36

• If appropriate for micturition = signal sent from forebrain to pons (where a micturition centre is)
• Micturition centre activates the micturition reflex
• Activates decending fibres in the reticulospinal tract
• Inhibit sympathetic and somatic outputs
• Activates parasympathetic efferents
• Stimulates detrusor muscle, bladder contracts, urine out of urethra

Question 37

Which reflex contracts bladder detrusor muscles? Which receptor is this mediated by?

Answer 37

Parasympathetic

Muscarinic (m3) receptors

Question 38

Which drugs block the micturition reflex?

Answer 38

Drugs which block muscarinic receptors e.g atropine

Question 39

What is defecation? Explain the process of defecation?

Answer 39

The sigmoid colon empties out into the rectum and out the anus
Internal anal sphincter and external anal sphincter
Defecation - temporary interruption of continence, which is maintained by the tonic contraction of the internal and external anal sphincters. Internal anal sphincter is controlled by sympathetic efferents
- Rectum is usually empty
- When faecal material enters it stretches and activates stretch receptors
- Info via the spinothalamic tract

Question 40

What is the anorectal reflex?

Answer 40

• Stretch of the rectal afferents activates the anorectal reflex
• Reflex relaxation of the internal anal sphincter
• Caused by and inhibition of the sympathetic nerves