Ingestive Behaviour

Definition Review

  • Ingestive behaviour: Eating or drinking

  • Homeostasis: Process by which the body’s substances and characteristics are maintained at their optimal level (e.g., temperature or glucose level)

    • Systems in place to both start and stop eating

  • Physiological regulatory mechanisms:

    • System variable

      • Variable that is controlled by a regulatory mechanism (e.g., temperature in a heating system)

    • Set point

      • Optimal value of the system variable in a regulatory mechanism

    • Detector

      • If

    • Correctional mechanism

  • Negative feedback

    • Process whereby effect produced by action serves to diminish or terminate that action

      • Characteristic of regulatory mechanisms

  • Satiety mechanism

    • Must be distinct from the error detection mechanism

Note that the hypothalamus is important for all of the behaviours discussed int he second half of the course.

System-Controlled Drinking

Our body loses water (urinate, sweat, natural evaporation). Our body systems/detectors detect our body’s water level is not at its ideal. Drinking occurs (correctional mechanism), stomach sends signal to brain once enough water has reached it (e.g., when full… this is the inhibition of the correctional mechanism), when we stop drinking, satiety mechanism inhibits

Fluid Balance

  • Isotonic

    • no movement of water back and forth, pressure and solution concentration are in equilibrium

  • Hypertonic

  • Hypotonic

Thirst

Most water is lost through evaporation

Also sweating, but that decreases sodium too, producing a need for both.

Two Types of Thirst

Osmometric thirstL produced by increase in osmotic press

Osmometric

Osmometric thirst in humans

  • AV3V is the error detector, which communicates to the ACC (like thermostat triggers heating)

  • The ACC tells us both to drink and to stop drinking (e.g. turn off ACC to stop drinking — the ACC

Volumetric Thirst

  • larger decrease in blood pressure, the more renin is produced

  • Role of angiotensin

    • Renin (renin is released and converted if kidneys detect hypovolemia (i.e., that volumetric pressure has decreases)

    • Angiotension (need to drink if angiotensin levels are high)

Circumventricular Organs

Communication for thirst takes place in the lamina terminalis (located at third ventricle — matters because angiotensin comes from the blood, BBB prevents blood detection signals so instead this process takes place in circumventircular organs that CAN), which houses three key brain areas that promotes the correctional mechanism for drinking.

  • First area: OVLT (organum vasculosum of the lamina terminalis)

    • Where most osmoreceptors are found (main area detecting osmometric thirst)

  • Second area: SFO (subfornical organ)

    • Some osmoreceptors, but mostly a big role detecting angiotension level for volumetric thirst (e.g., all angiotensin receptors)

  • Third area: Median Preoptic nucleus

    • Other areas project to this area

Eating

What is Metabolism?

Eating is important because we need resources for our bodies that come from plants or animals. We ingest these resources to…

  • construct and maintain our own organs

  • to obtain memory for muscular movements and keeping our bodies warm

Short-Term Resevoir

glucose is our short-term energy. anything left over of that glucose is stored as glycogen (converted by insulin), which cannot be used but rather must be reconverted to glucose by a pancreatic hormone known as glucagon.

  • Insulin

    • Pancreatic hormone that facilitates:

      • Entry of glucose and amino acids into cell

      • Conversion of glucose into glycogen

      • Transport of fats into adipose tissue

  • Glycogen

    • Polysaccharide often referred to as animal starch

    • Stored in liver and muscle

    • Constitutes the short-term store of nutrients

  • Glucagon

    • Pancreatic hormone that promotes the conversion of liver glycogen into glucose

Long-Term Reservoir

  • Triglyceride

    • Form of fat storage in adipose cells

    • Consists of molecule of glycerol joined with three fatty acids

  • Glycerol

    • Can be converted by liver into glucose

    • Also called glycerine

  • 3 Fatty Acids

    • Can be metabolized by most cells of body except for brain

Phases

  • Fasting Phase

  • X phase

…

Insulin factors:

  • Decrease in insulin, tells us to eat less

Brain Mechanisms

Brain Stem

Some aspect of ingestion occurs in the brain stem

  • Decerebrated animal is still capable of ingestion and preferences

Hypothalamus

  • Discoveries

Hypothalamus Damage

  • If destroyed, animals stop eating or drinking

NEUR 453: Neurobiology of Motivated Behaviours - Lecture 5: Ingestive Behaviour

Outline of Lecture 5

  • Drinking

  • Eating

    • What is Metabolism?

    • Signals to Start a Meal

    • Signals to Stop a Meal

  • Brain Mechanisms of Ingestive Behaviour

Definition Review

  • Ingestive Behaviour

    • Defined as either eating or drinking.

  • Homeostasis

    • Process by which the body’s substances and characteristics are maintained at their optimal level, such as temperature and glucose levels.

Physiological Regulatory Mechanisms

  • System Variable

    • A variable that is controlled by the regulatory mechanism (e.g., temperature in a heating system).

  • Set Point

    • Optimal value of the system variable in a regulatory mechanism.

  • Detector

    • Mechanism in regulatory processes that signals when a system variable deviates from its set point.

  • Correctional Mechanism

    • A mechanism that can change the value of a system variable.

Negative Feedback

  • Definition

    • A process whereby the effect produced by an action serves to diminish or terminate that action; characteristic of regulatory systems.

  • Satiety Mechanism

    • Brain mechanism that causes cessation of hunger or thirst, produced by adequate and available supplies of nutrients or water.

Example of a Regulatory System

  • Thermostat

    • Air Temperature (Detector)

    • Heat (Correctional Mechanism)

    • Custody of optimal conditions is akin to how a thermostat regulates temperature.

System Controlling Drinking

  • Fluid Absorption

    • Water absorbed; body fluids normalized.

  • Correctional Mechanism

    • Involves detection and response to systems signals as respective fluids are balanced.

  • Satiety Mechanism

    • Signals from the stomach regarding fullness inhibit further drinking.

Fluid Balance

  • Types of Fluid

    • Intracellular Fluid

    • Fluid contained within cells.

    • Extracellular Fluid

    • Any body fluid outside cells, includes:

      1. Interstitial Fluid - Fluid that bathes cells, filling the space in between.

      2. Blood Plasma (Intravascular Fluid) - Fluid found within blood vessels.

      3. Cerebrospinal Fluid (CSF).

  • Osmotic Pressure

    • Isotonic

    • Equal in osmotic pressure to contents of a cell; no net water gain or loss.

    • Hypertonic

    • High solute concentration causing water to exit the cell.

    • Hypotonic

    • Low solute concentration leading cells to swell as water enters through osmosis.

Drinking Background

  • Hypovolemia

    • Reduction in volume of intravascular fluid; the vascular system can contract smaller veins and arteries but has limits in correction abilities.

Thirst

  • Definition

    • Thirst varies under different circumstances; fundamentally refers to the drive to seek and ingest water due to dehydration.

  • Water Loss

    • Primary loss mechanisms through evaporation and sweating, affecting bodily sodium levels.

Two Types of Thirst

  1. Osmometric Thirst

    • Triggered by an increase in osmotic pressure of interstitial fluid, leading to cellular dehydration.

    • Detected by osmoreceptors - neurons that respond to changes in solute concentrations.

  2. Volumetric Thirst

    • Induced by hypovolemia, specifically intravascular volume reduction.

Role of Osmoreceptors

  • Functionality

    • Detects osmotic changes, triggering thirst or satiety based on cell volume impacts.

  • Example Studies

    • Research using micropipettes on osmoreceptors demonstrated membrane potential changes under varying osmotic conditions.

Osmometric Thirst in Humans

  • Egan et al. (2003)

    • Conducted functional imaging studies showing the activation of the AV3V during osmometric thirst conditions.

Volumetric Thirst

  • Involvement of Angiotensin

    • Renin

    • Hormone from kidneys that converts angiotensinogen in blood to angiotensin.

    • Angiotensin

    • Causes blood vessel constriction, water retention, and stimulates thirst.

    • Subfornical Organ (SFO)

    • Detects angiotensin presence and activates drinking.

Neural Mechanisms of Thirst

  • Osmoreceptors

    • Located in OVLT and SFO - they integrate osmometric and volumetric signals that trigger drinking responses.

  • Angiotensin II

    • Acts as a signal for volumetric thirst but does not pass through the blood-brain barrier.

Eating: What is Metabolism?

  • Definition

    • Incorporation of molecules from plant and animal sources for organ maintenance and energy supply.

  • Metabolism phases explained, describing short-term and long-term nutrient utilization, including insulin and glucagon functions.

Signals to Start a Meal

  • Initial signals include environmental cues (sight, smell, social context) and digestive system signals, primarily controlled by hormones like Ghrelin.

  • Ghrelin

    • Peptide hormone secreted by the stomach that stimulates eating.

  • Glucoprivation and Lipoprivation

    • Concepts defining conditions of low glucose and fat availability leading to hunger signals.

Signals to Stop a Meal

  • Two types of satiety signals:

    1. Short-Term from immediate meal effects, such as sensory and gastric signals.

    2. Long-Term signals modulate overall calorie intake regulation.

  • Insulin and the role of receptors in mediating satiety were discussed, emphasizing its regulatory capability over energy reserves.

Brain Mechanisms of Ingestive Behaviour

  • The hypothalamus plays a critical role in hunger and satiety through neurotransmitters and hormonal pathways.

  • Notable substances include:

    • Neuropeptide Y (NPY)

    • Agouti-Related Protein (AGRP)

    • Leptin, a long-term satiety signal activating CART and α-MSH neurons to inhibit eating.

Summary of Brain Mechanisms

  • Hindbrain and Forebrain Circuitry

    • Brain stem influences the control of ingestive behaviors while hypothalamic regions regulate essential hunger and satiety signals through interconnected neurons and hormonal feedback mechanisms, emphasizing a holistic approach to understanding ingestive behaviour.

Next Steps

  • Preparation for Lecture 6 which focuses on Sleep.,

  • The importance of maintaining a comprehensive understanding of ingestive behaviours necessitating detailed study of mechanisms driving hunger and satiety.