Lecture 5: Homeostasis, metabolism, and eating disorders

Homeostasis

The maintenance of a stable internal environment within the body, despite the changing external environment.

• Examples: body temperature, glucose level, oxygen level, and blood pressure.

  • Ex. To maintain a stable internal temperature, your body sweats to cool you down if you get too hot, and shivers generate heat when you get too cold.

Gastrointestinal tract

The part of the digestive tract where food is broken down and absorbed so that the nutrients can be mobilized and used as energy sources.

Saliva

Contains enzymes that facilitate the breakdown and also make the food easy to swallow.

Hydrochloric acid

In the stomach where it breaks food down into smaller particles.

Pepsin

Initiates the breakdown of protein molecules into their component amino acids.

Where does the stomach empty its content to where?

Empties through the pyloric sphincter (pylorus) into the duodenum.

Duodenum

Enzymes from the gallbladder and pancreas are added to the food pulp.

Metabolic breakdown

Of proteins into amino acid, starches and complex sugar molecules into simple sugar molecules.

• The division into smaller particles causes the elements to be small enough to pass through the lining of the duodenum into the bloodstream.

Where are food particles processed?

Once they enter the bloodstream, they are processed by the liver.

Bile

Is made by the liver and stored in the gallbladder, it Breaks down fat (or emulsified).

Do fat molecules pass through the wall of the duodenum?

No, so they are transported through small tubes in the lymphatic system.

• Water and electrocytes (such as sodium and potassium) are separated from the remaining waste material.

  • Remaining waste products are eliminated through the anus.

The general process of digestion provides 3 forms of energy; what are those?

• Lipids: fatty acids or derivatives thereof. They are insoluble in water.

• Amino acids: simple organic compounds that compose proteins when assembled into long chains.

• Glucose (a simple sugar broken down from carbohydrates): a sugar and energy sources.

What is energy stored as?

• Fats (the main storage system, which provides increased body fat).

• Glycogen (which is converted to glucose): a substance deposited in body tissue as a store of carbohydrates.

• Proteins

What are the 3 processes or phases in energy metabolism?

• Cephalic or reflex phase

• Gastric or absorption phase

• Intestinal or fasting phase

Cephalic or Reflex phase

Involves preparatory processes for feeding, in which the sensory stimulus of food activates the digestive system.

• This phase stops when nutrients are absorbed into the bloodstream.

Gastric or Absorption phase

Occurs when the body’s immediate needs are attended to, and nutrients are absorbed into the bloodstream.

• Takes 3-4 hours.

  • Physical relaxation caused by food in the stomach activates stretch receptors that promote the parasympathetic version of acetylcholine (ACh), which increases the secretion of gastric juices.

  • Increasing pH stimulates the release of gastrin by entero-endocrine cells.

ACh

Acetylcholine; it increases the secretion of gastric juices.

• during the gastric or absorption phase.

Entero-endocrine cells

Specialized cells of the endocrine system located in the gastrointestinal tract and pancreas.

Gastrin

A peptide hormone that causes the stimulation of hydrochloric acid (HCL).

• Also responsible for smooth muscle contractions and the movement of food.

HCL

Hydrochloric acid; creates an acidic environment for protein digestion.

Intestinal or Fasting phase

Occurs when nutrients no longer provide immediate energy and the body mobilizes previously stored nutrients, f.e. fats.

• The end of the fasting phase precedes the beginning of the cephalic phase → this is a vicious circle.

Insulin → Function?

Important hormone in the digestive process released by the pancreas for:

• Use of glucose

• Conversion of glucose to glycogen and fat for storage

• Conversion of amino acids into proteins for storage

• Storage of glycogen in the liver and muscles

• Storage of fat in adipose tissue

• Storage of proteins in the muscles

Regulates the number of nutrients transferred by the blood during the different phases.

During the intestinal phase, there are [high/low] levels of glucagon and [high/low] levels of insulin.

High levels of glucagon, Low levels of insulin.

Glucagon

A peptide hormone released by the pancreas when glucose levels become lower.

• It converts stored glycogen into glucose.

Glucagon vs. Insulin

Glucagon has a task opposite to insulin: It mobilizes glucose, while insulin stores glucose.

Glucagon function

Promotes the release of fatty acids from adipose tissue and stimulates the conversion of these fatty acids into ketones.

Ketones

Used by muscles as an energy sources.

What is the purpose of the process of digestion?

To ensure that there is enough energy to survive.

Satiety

The feeling of being full.

Gastric relaxation lead to satiety.

• True or False?

False.

• ?

Biomolecules

Molecules derived from substances that occur naturally in an organism.

• Examples:

  • Amino acids (derived from proteins)

  • Triglycerides (derived from fats)

  • Glucose (derived from carbohydrates)

Catabolism

The breakdown of complex molecules to release energy.

Sucrose

Table sugar = a disaccharide, a molecule made up of two monosaccharides: glucose and fructose.

Anabolism

The creation of complex molecules from simple ones to store energy.

Glycogen

(polysaccharide); a multi-branched polymer of glucose, which serves as energy storage in animals.

Where are fats stored?

In the cytoplasm of body cells.

Adipose tissue

Tissue in which fat is stored.

Where are fats contained in the blood?

In lipoproteins.

Lipoproteins

Transport units for fats secreted by the liver.

• They are in the blood which transport the fat to the fatty tissue.

  • They are a combination of protein, triglycerides and cholesterol in a phospholipid membrane.

Are fatty acids soluble in water?

No, they are fats.

What is an example of a fatty acid?

Phospholipid.

When glycerol and fatty acids are attached together, they form a long chain, called …

Catabolized units.

Anabolized

When phospholipids (membrane) and triglycerides (body fat) are attached together.

Cholesterol

Promotes cell membrane fluidity and can be metabolized into steroid hormones, such as estrogen.

What are the 2 varieties of lipoproteins that transport fats and cholesterol throughout the body?

1. Low-density lipoprotein

2. High-density lipoprotein

The ratio between them is important for cardiovascular health, as LDL promotes plaque formation in cell walls.

What are the 3 things that can heppen with glucose?

1. Glucose can be stored as glycogen.

2. Glucose can be used for energy.

3. Fat tissue (adipose tissue) can convert glucose into fats = lipogenesis.

Lipogenesis

The making of fats.

• Fatty acids are taken up by adipose tissue and combined with a-glycerol phosphate to form triglycerides.

• A large part of proteins can be converted into fats.

  • Amino acids are metabolized into new proteins.

Post-absorptive state

The body is in this phase 4 hours without food, but it still needs energy.

• Stored nutrients, such as fats, must be broken down and absorbed again.

  • Glycogenolysis

  • Gluconeogenesis

Glycogenolysis

Occurs when glucose is released from glycogen.

Gluconeogenesis

New glucose is generated from amico acids or fats.

During the post-absorptive state, there are [high/low] values of glucagon and [high/low] values of insulin.

• High values of glucagon.

  • Low values of insulin.

Cellular resporation

Glucose is converted into ATP in a cell.

• This requires oxygen and water.

• This delivers ATP, heat and CO2.

  • A low level of glucose is unfavorable and will be harmful later on.

    • ex. In stress, glucose is released and insulin helps to lower this value.

Insulin

A peptide hormone produced by cells in the pancreas.

B-cells

In the pancreas, they detect glucose in the blood and release insulin when glucose is high.

When glucose is [high/low], B-cells release insulin.

High

When glucose levels are low, then insulin production is stopped. Insulin stimulates:

• The uptake of glucose by cells

• Conversion of amino acids to proteins

• Conversion of glucose to glycogen and fat

• The storage of glycogen, fat and protein.

Hypoglycemia

When glucose levels in the blood are low.

• Glucagon will be produced.

When glucose levels in the blood are [high/low], glucagon is produced.

Low.

Glucagon

A peptide hormone released by a-cells in the pancreas.

• It stimulates the conversion of glycogen into glucose, the release of fatty acids and gluconeogenesis.

B-cells vs. a-cells

What part of the brain is central when it comes to eating behavior and other homeostatic processes such as thermoregulation?

Hypothalamus

Hypothalamus structure in relation to hunger

• Lateral hypothalamus (LH)

• Ventromedial hypothalamus (VMH)

Lesion of the VMH in rats

Leads to hyperphagia (overeating).

Hyperphagia

Overeating

2 types of hyperphagia

• Dynamic phase: rats gained most of their weight, which occurred immediately after surgery.

• Subsequent static phase: rats ate enough to become obese.

Lesion of the LH

Caused aphagia in rats and cats: they stopped eating.

• Accompanied by adipsia (they stopped drinking).

  • The animal could recover from both conditions with intensive care.

Aphagia

Stop eating.

LH is considered the … center and the VMH the … center.

LH = Hunger, VMH = Satiety

Satiety

The motivational state that ends feeding

LH is stimulated

Feelings of hunger arise, and a person begins to eat.

What controls LH?

LH is under inhibitory control of the VMH.

What are receptors for glucose in the VMH called?

Glucoreceptors.

When glucose is [high/low], the LH is no longer under the control of VMH.

Low.

When glucose concentrations are high, the VMH [stimulates/inhibits] the LH.

Inhibits

What are the behavioral consequences other than altering the regulation of diet of lesions of both the LH and VMH?

Lesions of the LH produce nonspecific deficits, notable impairment of motor behavior.

What other areas of the brain to the neural circuitry that regulates eating behavior are also involved in regulating eating behavior?

2 Areas stand out in motivational behaviors:

• The ventral tegmental area (VTA)

• Nucleus accumbens (NAcc)

Dopamine is released in these areas when primary reinforcers are presented.

• Food, a primary reinforcer and essential for survival, increases DA outflow from the NAcc.

VTA (Ventral tegmental area) receives direct input from the …

Gastrointestinal tract, in which dopamine neurons are activated in the VTA.

• Activation in the VTA increases dopamine input from the NAcc.

Fenfluramine

5-HT agonist.

Anorexia nervosa

Occurs mainly in women → They strive for a perfect body image.

• They limit their food intake, which leads to drastic weight loss.

• Comorbidity is common.

• No specific brain region has been identified as the culprit.

Which brain regions are found to differ in anorexia nervosa?

Insula and Cingulate cortex.

Cingulate cortex (in relation to anorexia nervosa)

Underlies motivation, goal-directed behavior and emotional processes.

Insula (in relation to anorexia nervosa)

Involved in the emotion of disgust.

• Considered the central mechanism underlying anorexia nervosa.

Neurotransmitters in Anorexia nervosa

• Low levels of serotonin.

• Reduced levels of dopamine → Can be linked to changes in motivation and reinforcement.

Main drugs used for anorexia nervosa

Selective serotonin reuptake inhibitors (SSRIs).

Bulimia nervosa drug treatment

Fluoxetine (Prozac), a 5-HT agonist.

Metabolic rate

The speed at which the body uses energy to perform basic functions.

• Average requirement for women = 2000 kcal and men = 2500 kcal.

Basal metabolic rate (BMR)

The amount of energy expressed in calories that a person needs to maintain body functions at rest.

Rest

Defined as existing in a neutral environment during the post-absorptive state.

• Includes breathing, blood circulation, cell growth and brain and nerve functions.

Adrenaline can cause an [increase/decrease] in the metabolic rate in the [short/long] term.

Increase, short-term.

Leptin

A hormone that suppresses the feeling of hunger.

• If the fat cells are empty, then no leptin signal is sent to the hypothalamus and therefore one is hungry.

  • If the fat cells are full, then a leptin signal is sent to the brain and we are hungry.

• People with anorexia nervosa are more sensitive to leptin.

Hypothyroidism

Insufficient thyroid hormone; it is associated with fatigue, depression, libido loss, a decreased metabolic rate and slower heart rate (Bradycardia), etc.

• Anorexia is related to this.

  • When a person has low thyroid hormones, adrenaline can have a diminished function → Epinephrine and thyroid hormones have a stronger effect.

Hyperthyroidism

Too much thyroid hormone; related to hyperactivity, insomnia, libido loss, increased metabolic rate, and a faster heart rate (Tachycardia).

2 Fluid-filled compartments in the body + what do they include?

1. Intracellular fluid (in the cytoplasm)

2. Extracellular fluid

These include:

• intravascular fluid (blood plasma)

• interstitial fluid (between cells)

• cerebrospinal fluid (CSF).

There is movement between the fluid compartments, allowing cells to survive.

Extracellular fluid

Acts in a supportive role for cells and is regulated by brain mechanisms, which control its levels.

• The brain does this by stimulating feelings of thirst.

What are 2 types of thirst?

1. Osmotic thirst

2. Hypovolemic thirst

Osmotic thirst

Occurs when the dissolved concentration of extracellular fluid increases.

• It becomes saltier and absorbs water from the intracellular fluid, which can compromise cells.

• Osmoreceptors, possibly located in the anterior hypothalamus and other areas, detect changes in the concentration of interstitial fluid.

Osmoreceptors

Possibly located in the anterior hypothalamus and other areas, detect changes in the concentration of interstitial fluid.

Hypovolemic thirst

Occurs when intravascular fluid decreases.

• A loss of blood volume is detected by baroreceptors and also by receptors in the kidneys.

• The kidneys secrete renin during times of low blood flow.

• Renin helps the production of angiotensin, a hormone that increases drinking.

Baroreceptors

Detects loss of blood volume.

Renin

Secreted by the kidneys during times of low blood flow.

• It helps the production of angiotensin.