Motivation

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