Transcript
Motivation and Homeostasis
The Role of the Hypothalamus
Motivation
• Motivation is the driving force in behavior • The stronger the motivation, the more
likely the behavior • Control of behavior involves decisions
among different motivated actions with different outcomes
• Motivation for behaviors basic to survival is controlled by the hypothalamus
The Hypothalamus & Homeostasis• Responds to departures from ideal in
homeostasis • Humoral response
– hypothalamic neurons respond to inputs from sensors by increasing or decreasing the release of pituitary hormones into blood stream
• Visceromotor response – hypothalamic neurons respond to inputs from sensors
by adjusting the balance between the sympathetic and parasympathetic output of the ANS
• Somatic motor response – hypothalamic neurons, especially in the lateral
hypothalamus, respond to inputs from sensors by causing a somatic motor response
• Very often all three happen together or sequentially
Regulation of Feeding Behavior
• Energy balance regulates feeding• Hormonal and hypothalamic
regulation of body fat and feeding
Feeding & Energy Balance • Prandial state
– after eating a meal
• Postabsorptive state – fasting times between meals
• Obesity occurs when there is consistently more intake than usage and adipose cells enlarge and increase in number
• Starvation occurs when there is consistently less intake than usage and loss of fat tissue occurs
• In virtually all mammals except humans and their pets, homeostatic mechanisms act to avoid both obesity and starvation
The Prandial State• Occurs after eating a meal • Glucose, fatty acids, and ketones are
produced for all cells; neurons only use glucose
• Excess energy is stored, either as glycogen in the liver and skeletal muscles, or as triglycerides in adipose tissue = fat cells
• Anabolism is the assembly of glycogen and triglycerides from what you eat
• Because the storage capacity for glycogen is limited, excess energy is stored as fat
The Postabsorptive State
• Fasting times between meals • Energy is produced from glycogen
and triglycerides • Catabolism is the process of
breaking down glycogen and triglycerides and making glucose, fatty acids, and ketones
Hormonal Regulation of Feeding
• Lipostatic hypothesis – The brain monitors the amount of body
fat and acts to keep it constant – Basically true
• Adipocytes release leptin into blood • Neurons in the periventricular zone of
hypothalamus detect a drop in leptin levels
• Neurons in the lateral hypothalamus are then excited and drive feeding behavior
Elevated Leptin & Feeding
• From 'holiday feasting' for example• Leptin receptors on neurons in the
arcuate nucleus are activated • These neurons use two peptide
transmitters:– aMSH and CART
• Response is three-part, triggered by nerve fibers leaving the arcuate nucleus
The Response• Humoral response is through the
paraventricular nucleus – Leads to increased release of TSH and ACTH– both of which serve to raise the metabolic rate
• Visceromotor response is to activate the sympathetic division of ANS – increases the metabolic rate, partly by raising
body temperature. – occurs both directly because of activity of arcuate
nucleus outputs and through the paraventricular nucleus outputs to the spinal cord
• Somatic motor response is to inhibit feeding behavior through the neurons of the lateral hypothalamic area
Lowered Leptin & Feeding• Turns off arcuate nucleus neurons that use
aMSH and CART as transmitters • Turns on other arcuate nucleus neurons that
use NPY and AgRP as transmitters • These neurons have connections with the
paraventricular nucleus and the lateral hypothalamic area
• Inhibit secretion of TSH and ACTH • Activate the parasympathetic division of ANS • Stimulate feeding behavior
Key Neurotransmitters
• aMSH and CART – depress feeding behavior – therefore called anorectic peptides
• NPY and AgRP – stimulate feeding behavior – therefore called orexigenic peptides
Role of the Lateral Hypothalamus
• Some neurons in the lateral hypothalamic area receive direct input from the arcuate nucleus neurons– These use the peptide transmitter, MCH – MCH neurons innervate most of the cerebral
cortex
• Other neurons in the lateral hypothalamic area also receive direct input from the arcuate nucleus neurons – Use the peptide transmitter, orexin. – Orexin neurons also innervate most of the
cerebral cortex
• So, when leptin levels fall, there is a major system to drive feeding behavior!
Regulation of Feeding Behavior• Starting a meal: • Cephalic phase
– sight and smell of food activates the parasympathetic and enteric divisions of the ANS
– saliva is secreted in the mouth and digestive juices, in the stomach
• Gastric phase – the cephalic phase responses are intensified by
chewing, swallowing, and filling the stomach with food
• Intestinal phase (or substrate phase) – As the filling stomach begins to empty in to the
intestines, satiety signals arise to stop feeding
Satiety• Ending a meal• Gastric distention
– mechanoreceptors in the stomach wall connect to the nucleus of the solitary tract via the vagus nerve and inhibit eating
• The gustatory nucleus is part of the nucleus of the solitary tract, which is why great food can override signals from a full stomach and lead to a totally bloated stomach
Cholecystokinin (CCK)
• Released from neurons in the enteric nervous system and other cells lining the intestines
• Triggered in response to stimulation of intestines by fatty foods
• Acts on sensory receptors connected to the vagus nerve and signals to stop eating
Insulin
• Pancreatic cells produce insulin necessary to transport glucose into all cells except neurons
• Directly affects neurons in arcuate nucleus and ventromedial nucleus of the hypothalamus – similar to leptin
• Late in the eating stage, rising insulin is a satiety signal
The Role of Dopamine• Liking food and wanting food are
mediated by two different systems • Wanting food is driven by the
mesocorticolimbic dopamine system • Animals with lesions in this system
still have the hedonic experience (like food), but seem unmotivated to want it
• Cravings that lead to addiction involve the same dopamine system
Drinking
• Water, not alcohol• Two different drives for drinking
– use two different mechanisms • Decrease in blood volume drives
drinking • Increase in the concentration of
solutes in the blood, blood osmolarity, drives drinking
Volumetric Thirst• Decreased blood volume drives
magnocellular neurons in the hypothalamus – release ADH – leads to concentrated urine – humoral response
• Decreased blood volume stimulates the sympathetic division of the ANS– helps to correct low blood pressure– visceromotor response
• Decreased blood volume stimulates neurons in the lateral hypothalamus – drives seeking and consuming water – somatic motor response
Osmometric Thirst• Hypertonicity of the blood is sensed by
vascular organ of the lamina terminalis (OVLT) – outside the blood brain barrier just like the
subfornical organ
• OVLT cells when excited, directly drive magnocellular neurons to release ADH – humoral response
• Hypertonicity drives the OVLT to stimulate the motivation to drink water through the lateral hypothalamic area – somatic motor response
Diabetes Insipidus• One type of diabetes includes failure to
release ADH • Caused by loss of magnocellular
neurosecretory neurons in hypothalamus, or viral infection of hypothalamus
• ADH is not secreted, urine is very dilute • Osmometric thirst drives patients to drink
vast amounts of water • Vast amounts of urine are passed • Patients become sorely sleep deprived -
dangerous • Synthetic ADH can be administered nasally
Temperature Regulation• In addition to the somatosensory thermal
receptors, there are lots of neurons in the brain sensitive to temperature.
• Most important cluster is in the anterior hypothalamus– monitors blood temperature
• Humoral and visceromotor responses are then initiated by the medial preoptic area of the hypothalamus
• Somatic motor responses are initiated by the lateral area of the hypothalamus
Decreasing Temperature
• A fall in temperature in the blood leads to:• Humoral response
– TSH is released by the anterior pituitary causing:– thyroxin release by the thyroid gland– increases cellular metabolism and produces heat
• Visceromotor response – constricted blood vessels near the skin surface
and piloerection (goose bumps)
• Somatic motor response – involuntary shivering – voluntary action to seek warmth
Increasing Temperature
• A rise in temperature in the blood leads to: • Reduction of TSH release
– humoral response
• Blood is shunted toward surface capillary beds– visceromotor response
• Animal seeks shade – voluntary somatic motor response
• Animal pants or humans sweat – involuntary somatic motor response
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