Neuroscience: Exploring the Brain - Chapter 16: Motivation

Neuroscience: Exploring the Brain - Chapter 16: Motivation

Introduction

  • Types of Behavior: Two categories of behaviors are identified:

    • Unconscious Reflexes

    • Voluntary Movements

  • Motivation: Understand as the driving force behind behavior.

    • Stronger Motivation: Directly correlates to the likelihood of action.

    • Analogy: likened to ionic driving force; both vary depending on several influencing factors.

    • Probability and direction of behavior can change based on motivation levels.

The Hypothalamus, Homeostasis, and Motivated Behavior

  • Homeostasis: Critical for maintaining the internal environment of the body within a narrow range of physiological conditions.

Role of the Hypothalamus

  • Regulates several bodily functions:

    • Body temperature

    • Fluid balance

    • Energy balance

  • Three Components of Neuronal Response:

    1. Humoral Response: Sensors trigger the release of pituitary hormones.

    2. Visceromotor Response: Sensors adjust the balance of sympathetic and parasympathetic divisions of the autonomic nervous system (ANS).

    3. Somatic Motor Response: Sensors lead to a motor response involving skeletal muscles.

Examples of Motivated Behaviors

  • Responses to Cold:

    • Shivering occurs

    • Blood is redirected from the surface of the body

    • Urine production is inhibited

    • Mobilization of fat reserves

  • Lateral Hypothalamus: Initiates motivated actions such as seeking warmth, drinking, and eating to maintain homeostasis.

Long-term Regulation of Feeding Behavior

  • Energy Balance

  • Prandial State: The body after eating, where:

    • Energy Sources: Glycogen and triglycerides are utilized.

    • Catabolism: Represents breakdown pathways in the absence of food.

    • Obesity: Defined as an excessively positive energy balance (more anabolism than catabolism).

    • Starvation: Defined as a negative energy balance (more catabolism than anabolism).

    • Energy balance in humans and pets can deviate from normal homeostatic regulation.

Energy Balance

  • Key states of energy balance:

    1. Prandial State: After eating, glucose, fatty acids, and ketones are present; neurons require glucose for metabolism.

    2. Postabsorptive State: Between meals, energy is mobilized through catabolism of stored nutrients.

  • Body fat must match energy intake to energy expenditure for proper regulation.

Hormonal and Hypothalamic Regulation of Body Fat and Feeding

  • Leptin and Hypothalamic Control

    • Leptin: A hormone released by adipocytes that modifies energy balance.

    • Lipostatic Hypothesis: Suggests that the brain monitors body fat via leptin levels to maintain energy balance,

    • Adipocytes release leptin in response to increasing fat.

    • Leptin levels tend to lower with reduced food intake, which incites hunger.

Mechanism of Weight Control (Lipostatic Hypothesis)

  1. Body Fat Monitoring: Brain detects leptin release and adjusts appetite accordingly.

  2. Elevated Leptin Effects:

    • Response in the Arcuate Nucleus:

      • Humoral: Increased metabolic rate through release of TSH and ACTH.

      • Visceromotor: Activated sympathetic ANS increases metabolic rate.

      • Motor: Suppress eating behavior through inhibit pathways to the lateral hypothalamus.

  3. Leptin's Biological Implications: High circulating leptin correlates with fat stores,

    • Leptin's Effect on Rodents:

      • Leptin administration decreases food intake and induces weight loss.

      • High levels associated with obesity but fall during food restriction.

Central Role of the Hypothalamus in Feeding Behavior

  • Hypothalamic Centers:

    • Lateral Hypothalamus (LH):

    • Stimulation boosts appetite; lesions lead to anorexia.

    • Ventromedial Hypothalamus (VMH):

    • Activation suppresses appetite; lesions cause overeating and obesity.

Dopamine’s Role in Motivation

  • Motivation connected to two systems:

    • Liking: The hedonic aspect; enjoyment of food.

    • Wanting: The drive to seek food determined by dopamine levels.

  • Experiments show electrical brain stimulation can lead to rapid pressing of levers for reward, overriding needs like hunger.

Effects of Short Sleep and Hormonal Changes on Appetite

  • Professor Cappuccio’s Study: Demonstrates short sleep duration may lead to obesity by altering hormonal levels:

    • Ghrelin stimulates appetite while leptin inhibits it.

Ghrelin

  • Ghrelin: A 28 amino acid hormone produced in the stomach releasing into the bloodstream when empty,

    • Functions:

    • Stimulates appetite via NPY/AgRP pathways.

    • Secreted growth hormone from the anterior pituitary;

    • Ghrelin receptors identified in arcuate nucleus and ventromedial hypothalamus

  • Removal of ghrelin-producing cells leads to appetite loss.

Mechanisms of Drinking Behavior

  1. Two Mechanisms:

    • Volumetric Thirst: Triggered by decreased blood volume.

    • Osmometric Thirst: Triggered by increased blood osmolarity.

  2. Biological Basis:

    • Vasopressin: Hormone promotes water retention and inhibits urine production during dehydration.

Other Motivated Behaviors

  • Temperature Regulation:

    • Regulatory mechanisms maintain core temperature at around 37°C (98.6°F).

    • Neurons manage responses to temperature changes via humoral, visceromotor, and somatic processes.

Eating Disorders

  • Anorexia Nervosa: Characterized by excessive dieting leading to underweight despite feeling fat.

  • Bulimia Nervosa: Involving binge eating followed by compensatory behaviors such as vomiting or excessive exercise.

Summary

  • This chapter elaborates on how various hypothalamic processes and hormonal signals control motivations and behaviors related to eating and drinking, addressing both physiological and psychological facets of motivation.

References

  • Bear, Connors, and Paradiso, Neuroscience: Exploring the Brain, 3rd Ed. Copyright © 2007 by Lippincott Williams & Wilkins.