24 - Neural Control of Motivational Behaviour Flashcards
What controls drinking and thirst?
Osmolarity of the blood
The higher the osmolarity - the stronger the urge to drink
What detects osmolarity of the blood?
Osmoreceptors in the subfornical organ
In the wall of the 3rd ventricle
How do we regulate thirst?
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
What does the subfornical organ activate?
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
What are 3 effects of ADH that contributes to a decreased loss of water in urine?
- 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
Lesions in what area of the hypothalamus cause a) anorexia b) obesity?
a) Anorexia - lateral hypothalamus
b) Obesity - medial hypothalamus
Which nuclei do medial lesions of the hypothalamus affect?
Arcuate and periventricular nuclei
What is another name for the medial area of the hypothalamus?
Satiety centre
What is another name for the lateral hypothalamic nucleus?
Orexigenic (hunger) centre
What is the difference between the paraventricular and periventricular nucleus?
Paraventricular nucleus - releases ADH
Periventricular nucleus - food intake
What are the 2 factors which regulate food intake?
Internal stimuli - how hungry we feel, levels of blood chemicals
External stimuli - how attractive the food is, sight and smell of food
What is Ghrelin?
Hunger hormone, a protein hormone produced by the oxyntic glands
Stimulates eating by an action on the arcuate nucleus of the hypothalamus
What causes ghrelin release?
Stomach contractions, high insulin levels
Which 2 neurones in the arcuate nucleus does ghrelin stimulate? Where do they project to? What is their function?
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)
What is leptin?
A protein hormone which reflects the total amount of adipose tissue in the body - gives the brain a reading of total energy store
Where is leptin produced?
Adipose tissue
What can leptin levels affect?
Fertility in women, if circulating leptin is too low than a woman ceases to menstruate
Is leptin administration a cure for obesity?
No - clinical trials failed - people gained back the weight they had lost - leptin resistance
What is the role of CCK?
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
What does CCK stand for?
Cholecystokinin
What is the the role of Glucagon like peptide 1 (GLP-1)
Released from endocrine cells in the gut when food is present
Produces rapid satiety
Stimulates insulin secretion
What is the role of oxyntomodulin and PYY?
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
Which hormone stimulates eating?
Ghrelin
Which hormones inhibit eating?
CCK, GLP-1, oxyntomodulin, PYY
What does pro-opiomelanocortin (POMC) do?
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
What is diuresis?
Production of urine by the kidney
What is the difference between the urethra and ureter?
Ureter - kidney to bladder
Urethra - bladder to exit
What is the urine flow in an adult?
1ml/min
What muscles control urine release?
Internal and external urethral sphincters
Detrusor muscle - contracts bladder to expel urine
Which receptors mediate inhibitory and excitatory actions on the bladder sphincters?
Inhibitory - adrenergic B receptors
Excitatory - A receptors
Which afferents allow tonic contraction of the urethral sphincter muscles during filling and storage?
Sympathetic efferents
Also inhibit contraction of detrusor muscle
Which nerves do the sympathetic afferents travel in?
Hypogastric plexus
Which drugs can have an effect on bladder control?
Antihypertensives, antiasthmatics (act on sympathetic nervous system)
Which tract do the cells synapse on?
Spinothalamic tract
What happens when the bladder fills?
- 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
Describe the micturition reflex
- 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
Which reflex contracts bladder detrusor muscles? Which receptor is this mediated by?
Parasympathetic
Muscarinic (m3) receptors
Which drugs block the micturition reflex?
Drugs which block muscarinic receptors e.g atropine
What is defecation? Explain the process of defecation?
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
What is the anorectal reflex?
- Stretch of the rectal afferents activates the anorectal reflex
- Reflex relaxation of the internal anal sphincter
- Caused by and inhibition of the sympathetic nerves
