HOMEOSTASIS! LECTURE#18 ! PSY280: BIOLOGICAL PSYCHOLOGY! DR. OLLIE HULME, SPRING 2011, SFU!

Details!

Office hours:! Wed 1.40pm – 3.30pm! Room RCB 6242!

Exams:! Final 40% ~ Apr 14th 8.30am, in AQ3181! 30 Multiple choice, 2 essays, 110mins! Non-cumulative! Internet: ! hyooom.com! [email protected]! Twitter: @PSY280!

Essay question format! You have to specialise in answering 2 essay questions:! Question1: You need to know a) Motor system part 1 (Upper and Lower) and b)Motor system 2 (Basal ganglia and Cerebellum)! Question2: You need to know a) Homeostasis and b) Sleep ! Question3: You need to know a) Emotion and b) Psychopathology! Question4: You need to know a) Decision making and b) Happiness! To answer each question you need to know both topics since either may come up in the exam! You therefore need to know 2 pairs of topics to answer both questions, thus 4 topics in total!

Course outline! Action Motor system (part 1) 11 Motor system (part 2) 11 Regulation Homeostasis 13 (mon 28th) Sleep 14 (wed 30th) Emotions & Disorders Emotion 15 (mon 4th) Psychopathology 16 (mon 4th) Cognitive Decision making 18 (wed 6th) Special topics (reading beyond book) Happiness (mon 11th) Review (mon 11th)

Numbers = Textbook Chapter

Roadmap!

Introduction Temperature regulation Fluid regulation Food regulation

Claude Bernard!

‘Internal Mileu’ The fixity of the milieu supposes a perfection of the organism such that the external variations are at each instant compensated for and equilibrated.... The stability of the internal environment is the condition for the free and independent life.

19th C French physiologist

Why maintain the stability of the internal environment move?

Walter Cannon!

I coin this regulation process of a stable interior Homeostasis [Homeo = similar, Stasis = standing still]

20th C American physiologist

Three features of homeostasis!

1. Constancy in systems such as our bodies requires active mechanisms that resist change 2. The regulating system that determines the homeostatic state consists of a number of cooperating mechanisms acting simultaneously or successively 3. Homeostasis does not occur by chance, but is the result of organized self-government

Life regulation! To stay alive we need to keep our bodily state within narrow ranges (pH, Temp, energy, hydration, O2, CO2) e.g. ~ normal T = 33 – 38deg If T increases above 40deg, life threatening hyperthermia If T decreases below 30deg, life threatening hypothermia

Homeostasis is..!

…the process of life regulation that maintains our internal state within narrow ranges for survival

Negative Feedback!

Most homeostatic mechanisms rely on negative feedback Deviation away from a set point / zone, results in compensatory mechanisms to bring it back

Redundancy! Most species have multiple parallel mechanisms that all help maintain homeostasis for a particular system (O2, T etc) = redundancy Multiple failsafe mechanisms Damage one mechanism and homeostasis can still be achieved by the operation of the others Redundancy makes evolutionarily sense since the failure of a single mechanism does not cause death

Obligatory losses! Regulation of any internal resources is complicated by the fact that many resources are depleted over time = obligatory losses

Temperature, Water, O2

Energy

Roadmap!

Introduction Temperature regulation Fluid regulation Food regulation

Temperature regulation!

Many of our internal processes depend on chemical reactions Rate of chemical reactions depends on temperature Too low, reactions too slow Too high, proteins denature, we begin to cook

Braving the Cold!

Evolution has equipped some species to tolerate very low temperatures Antifreeze prevents ice molecules forming which would otherwise cause irreparable damage to cell membranes

Cryogenic digression!

Since cell membranes become demolished by ice molecules cryogenically frozen old people (mainly Californians) need to wait until we have the technology to mend every single cell membrane (!)

Heat generation! Mammals = endotherms

If heat is continually lost, it must be generated from somewhere Endotherms can generate own heat through internal processes Ectotherms obtain heat from the environment

Reptiles = ectootherms

Endothermy! Generating your own heat through endothermy + maintaining it within narrow ranges is very costly in energy Most food intake goes toward heat generation and regulation Why bother? Greater independence from environment Aerobic metabolism provides greater endurance, endotherms better at longer distances

Heat generation!

At rest 1/3 of heat produced is by the brain Rate of heat generation can be adjusted to suit conditions by altering the metabolic rate of thermogenic tissues – Brown fat (Adipose tissue) broken down to produce large amount of heat Shivering mechanism causes skeletal muscles to produce extra heat

Thermoregulation for heat gain ! Hypothalamus plays a central role in thermoregulation

Thermoregulation for heat loss ! Hypothalamus also regulates heat loss

Behavioral Control of T ! Ectotherms cannot properly generate own heat, therefore need to control T via behaviour

When too cold, find warm rock

When too hot, minimise exposure or find shade

Behaviour induced fever!

Ectotherms overheat through moving to hot environment to kill off infection Equivalent to an endothermic fever

Thermoregulatory behaviour! Endotherms also achieve homeostasis of T by regulating behaviour

Changing exposure e.g. huddling vs. independence Changing insulation e.g. clothes vs. no clothes Changing environment e.g. shade vs. sun

Hypothalamus! Hot wires placed onto hypothalamus, caused body T to fall Cool wires caused body T to rise Hypothalamus monitors blood T and adjusts T to keep within range, analagous to the thermostat

Homeostatic redundancy! Lesion of lateral hypothalamus abolishes behavioural regulation of T Lesion of preoptic area of hypothalamus abolishes physiological regulation of T Example of homeostatic redundancy, two different mechanisms to regulate same parameter - T

Hypothalamic thermostat!

Hypothalamus is activated more the further away from the set zone

Brainstem thermostat!

Redundant mechanism in brain stem

Spinal cord thermostat!

And in spinal cord

Hierarchy of mechanisms!

Multiple redundant thermostat mechanisms regulate temperature Set zone widens further down the hierarchy

Thermoregulatory system!

Multiple interlinked systems operating together

Roadmap!

Introduction Temperature regulation Fluid regulation Food regulation

Homeostasis of extracellular fluid !

Osmolarity = solute concentration of fluid Osmolarity of extracellular fluid is highly similar across species, and highly regulated by homeostasis Small deviations are lethal

Obligatory losses!

Since we need to use water for processes such as waste removal (urine) we face obligatory losses of water and salts This makes it difficult to maintain osmolarity of fluids

Osmosis! Water can pass through semi-permeable membrane but salt can’t

Water passes through to equalise volume

Osmosis!

Adding salt to one chamber increases the osmolarity of the solution

Water always moves to equalise the concrentration gradient of the solution – osmosis Therefore water flows in

Regulating osmolarity therefore has a strong influence on regulating bodily fluids

Thirst!

Thirst is the motivational drive to regulate bodily fluids It arises from a lack of fluids and/or an increases in osmolarity through too much salt

Hypovolemic thirst!

Blood loss causes loss of blood pressure. Detected by baroreceptors in blood vessels and heart Causes hypovolemic thirst, homeostatic drive to restore blood volume

Hypovolemic thirst!

…is why people who are shot get very thirsty

Hypovolemic hormones! Renin released from kidneys due to reduced blood volume causes cascade of hormones to be released

Circumventricular Organs! These hormones reach the circumventricular organs where the blood brain barrier is weaker Causes motivation for drinking

ventricle

Osmotic thirst!

Osmotic thirst is more frequent and ongoing Caused by obligatory water loss, where more water is lost than salt, causing osmolarity to become too high in extracellular fluid Drinking water restores osmolarity levels

Osmosensory neurons !

Hypothalamus contains osmosensory neurons that respond to changes in osmolarity of extracellular fluid Osmosensory neurons change volume when osmolarity changes causing mechanically gated ion channels to open or close transduce osmolarity into electrical signals

Thirst activity !

Degree of subjective thirst correlates with activity in hypothalamus, cingulate cortex and cerebellum

Activity in all these regions is reduced after drinking

Roadmap!

Introduction Temperature regulation Fluid regulation Food regulation

Food and energy regulation! Hunger is a universal drive for all humans, all species Self-evidently of paramount importance for survival Highly regulated by multiple homeostatic mechanisms Food needed for energy - calories Food needed for nutrients – vitamins, essential amino acids, fatty acids

Food regulation!

Too little food can result in death (really? yes really) But if food reserves are too high (i.e obesity), causes immobility and thus death Thus homeostasis needs to provide tight control over food resources and metabolic reserves for survival

Energy metabolism !

Chemical energy obtained from food by breaking down complex molecules into simple - burning fuel To generate heat body releases chemical bond energy as heat To generate movement we store it biochemically and transduce it into muscle contractions 33% lost during digestion 55% for heat, membrane potentials, basic metabolism 12% for active behavior (more if very active)

Dieting! Multiple homeostatic mechanisms for metabolic regulation make dieting very difficult Homeostasis acts to resist changes in body weight e.g. calorie restriction diet cause body to respond by reducing its metabolic rate, so it burns less calories at rest

Diet! Homeostatic mechanisms dampen changes in body weight

Fuel !

Brain can only use glucose

Body can use either fatty acids or glucose (only in presence of hormone insulin)

Hypothalamus again!

Hypothalamus regulates homeostasis of metabolic rate, food intake and body weight Contains glucodetector neurons that detect blood sugar levels But many other redundant homeostatic mechanisms

Dual center hypothesis! Lateral hypothalamus = hunger center

Ventromedial hypothalamus = satiety center (feeling full)

lesions can cause reduced weight / starvation

lesions cause excess feeding

Lesion-Induced Weight Loss!

Lesion the hunger center (lateral hypothalamus) and new homeostatic set point emerges at lower weight

Lesion-Induced Obesity!

Lesion the satiety center (ventromedial hypothalamus) and new homeostatic set point emerges at higher weight

Inherited Obesity!

Some plausible genetic basis for obesity – ‘obese’ gene

Appetite Controller Hypothalamus! Multiple hormone signals from body influence hypothalamus

Multiple types of neural information from organs also influence hypothalamus

Body Fat Stores! Weight of golden mantle ground squirrel cycles up and down according to time of year

Weight homeostasis!

Homeostasis kicks in to resist change in body fat – moral of story is don’t bother with liposuction (if you are golden mantle ground squirrel)

Obesity! Obesity is a serious health problem – leading cause of death by preventable disease Difficult to treat because of homeostatic mechanisms, which resist change Some drugs to treat: Cannabinoid antagonists – anti-munchies, but mood effects Gut hormones e.g. PYY in clinical trials

Read sleep chapter for wednesday

Sample Questions! Essays: What does homeostasis achieve and how is it regulated by the brain? With reference to both food and temperature, explain what role the brain plays in homeostasitic? Multiple choice: Hypovolemic thirst… Is caused by blood loss Involves hormonal mechanisms of the kidney I caused by extracellular fluids gaining too high an osmolarity A and B A, B and C Redundancy… Is a term which designates multiple homeostatic mechanisms operating silmultaneously Offers failsafe mechanisms for important regulatory functions Means that a mechanism has no role A and B None of the above The dual centre hypothesis states that… The ventral hypothalamus regulates oxygen intake The lateral hypothalamus is a ‘hunger center’ The lateral hypothalamus is a ‘satiety center’ None of the above