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Principles of Behavioral Neuroscience: Hunger

Chapter 6: Hunger

Homeostasis

  • Definition: Homeostasis refers to the action of a system to maintain internal stability when faced with a disturbance of its normal condition.

  • Examples of Homeostatic Processes:

    • Hunger

    • Thirst

    • Thermoregulation

  • Function: These systems allow organisms to maintain energy in a safe range and operate primarily automatically.

  • Illustration: Refer to Figure 6-1 for a graphical representation of homeostatic mechanisms.

Digestion of Food

  • Energy Requirement: The human body requires energy to function, which comes from multiple sources:

    • Water

    • Oxygen

    • Minerals

    • Vitamins

    • Carbohydrates

    • Fats

    • Proteins

  • Illustration: Refer to Figure 6-2 for an overview of the energy sources.

Digestive Process

  • In the Stomach:

    • The stomach uses acid to break down food.

  • In the Small Intestine:

    • Enzymes play a critical role in digesting fats, carbohydrates, and proteins.

  • Nutrient Absorption:

    • Nutrients from digested food enter the bloodstream and are transported to cells throughout the body.

    • Water and select minerals are absorbed in the large intestine.

  • Basal Metabolism:

    • Definition: Energy used to keep the body functioning and to maintain body temperature.

    • Illustration: Refer to Figure 6-2 for a visual depiction.

Short-Term Energy Storage

  • Glucose:

    • A simple sugar that primarily originates from carbohydrates.

  • Insulin:

    • A hormone present in the bloodstream that facilitates the entry of glucose into cells.

  • ATP (Adenosine Triphosphate):

    • The energy currency produced when glucose is metabolized inside cells, providing necessary energy for cellular processes.

  • Illustration: Refer to Figure 6-3A for more details on short-term energy mechanisms.

Glucose & Glycogen

  • Glycogen Storage:

    • Excess glucose is stored in the liver as glycogen.

    • Function of Glucagon: When blood glucose levels decline, glucagon is released to convert glycogen back into glucose, providing additional energy.

  • Note: Insulin is not required for glucose to enter neurons.

  • Illustration: Refer to Figure 6-3B for pathways of glucose storage and release.

Long-Term Energy Storage

  • Primary Source: Most long-term energy is stored as fat.

  • Triglycerides:

    • Composed of fatty acids and a glycerol molecule, stored in adipose tissues.

    • Released when energy levels are low, breaking down into fatty acids usable by most cells.

  • Brain's Energy Source: The brain exclusively utilizes glucose and does not utilize fatty acids as an energy source.

  • Illustration: Refer to Figure 6-4 for visual aid on long-term energy storage.

Theories of Hunger and Homeostasis

  • Glucostatic Theory:

    • Suggests that hunger is driven by the need to restore normal blood glucose levels.

  • Lipostatic Theory:

    • Proposes that hunger is related to maintaining fat levels within a stable range.

  • Detection Mechanisms:

    • The body can sense drops in glucose and fat levels, signaling the brain to produce the sensation of hunger and motivate eating.

    • Glucose-sensitive neurons located in the hypothalamus play a key role in energy level detection.

  • Limitations of Homeostasis:

    • Homeostasis cannot fully explain all aspects of eating behaviors, such as:

    • Eating dessert when already satiated

    • Eating driven by pleasure, habit, or social influences.

Signals of Hunger and Satiety

  • Ghrelin:

    • A peptide hormone released to signal hunger when the stomach is empty.

  • Vagus Nerve:

    • Activates in response to stomach stretching when full, contributing to sensations of satiety.

  • Cholecystokinin (CCK):

    • Another signal for satiety, released as food enters the small intestine.

  • Leptin:

    • A long-term satiety hormone, released in response to the accumulation of adipose (fat) cells.

  • Illustration: Refer to Figure 6-5 for a chart outlining hunger and satiety signals.

Genetic Influence on Hunger Signals

  • Genetic Example:

    • Discussion of a mouse genetically modified to lack the leptin gene. This mouse exhibits increased food consumption and fat accumulation due to the absence of the satiety signal.

  • Illustration: Refer to Figure 6-6 for visual representation of this genetic study.

Emotional and Cognitive Factors in Eating

  • Cognitive Control:

    • Individuals often use cognitive control to manage eating habits, particularly in dieting.

  • Behavioral Observations:

    • "Restrained eaters" may consume more food during emotional situations.

  • Cues Triggering Cravings:

    • Certain social contexts can trigger specific food cravings, resulting in impulsive eating.

The Hypothalamus: Control Center for Hunger and Satiety

  • Neuronal Mechanisms:

    • Neurons in the arcuate nucleus of the lateral hypothalamus (LH) detect ghrelin and activate to signal hunger.

    • The vagus nerve detects sensory information related to satiety and activates the solitary nucleus.

  • Neuron Activation:

    • After food intake, a separate set of arcuate neurons activate the paraventricular nucleus (PVN), signaling satiety.

  • Illustration: Refer to Figure 6-7 for a visual schematic of hypothalamic circuits.

Neuronal Interactions in Hunger and Satiety

  • Arcuate Hunger Neurons:

    • Release neuropeptide Y (NPY) to excite LH neurons, which promotes hunger.

    • NPY inhibits the activity of the PVN (satiety).

  • Arcuate Satiety Neurons:

    • Release alpha melanocyte-stimulating hormone (⍺-MSH), stimulating the PVN and inhibiting LH neurons, reducing hunger signals.

  • Orexin:

    • Another appetite-stimulating peptide produced in the lateral hypothalamus.

  • Illustration: Refer to Figure 6-8 for a depiction of neuronal interactions in hunger and satiety control.

Other Influencing Factors on Eating Habits

  • Body Weight Influences:

    • Body weight can fluctuate due to various factors, including:

    • Pregnancy

    • Physical illnesses

    • Sedentary lifestyles (e.g., prolonged sitting)

    • Social and cultural influences

    • Disordered eating patterns

  • Eating Disorders:

    • Overview of three primary eating disorders:

    • Anorexia

    • Bulimia

    • Binge-eating disorder

  • Illustration: Refer to Figure 6-12 for an overview of these disorders and their characteristics.