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Module Overview

  • Title: Physiology of Behavior

  • Themes: Hunger, Thirst, Arousal, Attention, Emotions, Sleep & Circadian Rhythms

Bases of Hunger

  • Definition: Hunger is a drive that compels an organism to seek food.

  • Primary Factors Influencing Hunger:

    • Stomach contractions (hunger pangs)

    • Walter Cannon theorized that these contractions triggered the sensation of hunger.

    • Sensory receptors in the stomach respond to stretching when food is ingested and signal the brain when the stomach is full.

  • Role of Hormones:

    • Insulin and Glucagon:

    • Produced by the pancreas.

    • Insulin lowers blood sugar levels, while glucagon raises them.

    • Insulin release is triggered during eating, creating feelings of satiety.

    • Leptin:

    • Hormone that reduces appetite and increases feelings of fullness by signaling the hypothalamus.

The Neurophysiology of Hunger

  • Hypothalamus Functions:

    • Controls hunger and eating behavior based on the levels of nutrients and hormones (glucose and insulin).

    • Key areas include:

    • Ventromedial Hypothalamus (VMH):

      • Signals when to stop eating.

      • Damage here may lead to selective overeating.

    • Lateral Hypothalamus (LH):

      • Triggers eating when insulin levels rise.

      • Damage can cause starvation due to loss of appetite.

  • Weight Set Point:

    • Hypothalamus maintains a specific weight threshold. Damage can result in significant weight changes, either loss or gain.

Bases of Thirst

  • Importance of Water:

    • Water constitutes approximately 70% of the mammalian body.

    • It regulates biochemical reactions and is crucial for maintaining blood pressure.

  • **Types of Thirst:"

    • Osmotic Thirst:

    • Triggered by the concentration of solutes in the body.

    • Involves water moving across cell membranes due to osmotic pressure from solute concentrations.

    • Mechanism includes specific neurons detecting solute levels and signaling thirst.

    • Hypovolemic Thirst:

    • Occurs when the body loses fluid volume (e.g., bleeding, diarrhea).

    • Hormonal response includes release of vasopressin and angiotensin, which constrict blood vessels and trigger thirst for restoring fluid balance.

Arousal and its Dynamics

  • Definition of Arousal:

    • A physiological and psychological state that enhances responsiveness to stimuli.

  • Neurophysiological Aspects:

    • Activation of the Reticular Activating System (RAS) enhances attention and alertness.

    • Involves neurotransmitters like acetylcholine, norepinephrine, dopamine, and serotonin.

  • Yerkes-Dodson Law:

    • Indicates optimal arousal levels facilitate peak performance while too high or low arousal impairs it.

  • Role in Behavior:

    • Arousal regulates consciousness, attention, and responses to stimuli (e.g., hunger and emotional states).

Attention and its Mechanisms

  • Definition:

    • Attention involves conscious awareness and the selection of specific stimuli from the environment.

  • Neurophysiology of Attention:

    • Physiological changes accompany attention, such as pupil size and heart rate.

    • Neurons in the brain show enhanced responsiveness to focused stimuli due to attentional processes.

  • Attention Models:

    • Selective Attention: Focus on specific sensory modalities at one time could inhibit responses to others.

    • Broadbent's Filter Model (1958): Proposes a filtering process that prevents information overload by only allowing a specific input to advance to cognitive systems.

Understanding Emotions

  • Definition of Emotion:

    • Originates from the Latin word emovere, indicating intensity toward responses to stimuli.

    • Emotions entail physiological arousal, subjective feelings, and behavioral expressions acting toward or away from stimuli.

  • Theoretical Perspectives:

    • James-Lange Theory: Emotions result from perception of bodily changes.

    • Cannon-Bard Theory: Physical and emotional responses occur simultaneously.

    • Cognitive/Appraisal Theory (Lazarus): Emotional responses require conscious evaluation of stimuli before physiological changes occur.

  • Physiological Changes During Emotion:

    • Skin conductance, heart rate, respiration, and muscle tension change in response to emotions, signaling increased arousal.

Sleep and Circadian Rhythms

  • Definition of Sleep:

    • A natural, reversible state characterized by reduced responsiveness to external stimuli.

  • Stages of Sleep:

    • NREM Stages:

    • Stage 1 (NI): Light sleep, presence of alpha waves.

    • Stage 2 (NII): Transitional phase with sleep spindles and K-complexes.

    • Stage 3 (NIII): Deep sleep dominated by delta waves, hard to awaken.

    • REM Sleep: Rapid eye movement sleep characterized by muscle atonia and dreaming. Brain activity resembles wakefulness.

  • Regulation of Sleep:

    • Influenced by homeostatic drive (the body's need for sleep) and the circadian rhythm (the internal clock reset by light exposure).

    • Suprachiasmatic Nucleus (SCN): Located in the hypothalamus, regulates the sleep-wake cycle by responding to light inputs.



The physiology of behavior encompasses various themes such as hunger, thirst, arousal, attention, emotions, and sleep. Hunger is defined as a drive that compels an organism to seek food, with primary factors influencing it including stomach contractions, sensory receptors responding to stretching in the stomach, and hormones. Walter Cannon theorized that stomach contractions trigger the sensation of hunger. Hormones like insulin and glucagon, produced by the pancreas, play vital roles; insulin lowers blood sugar levels while glucagon raises them, creating feelings of satiety when food is ingested. Additionally, leptin is a hormone that reduces appetite by signaling the hypothalamus to indicate fullness.

The neurophysiology of hunger involves the hypothalamus, which controls appetite based on nutrient and hormone levels. The ventromedial hypothalamus signals when to stop eating, while the lateral hypothalamus triggers eating when insulin levels rise. Damage to these areas can lead to overeating or starvation. A weight set point is maintained by the hypothalamus, and any damage can result in significant weight fluctuations.

Thirst, another crucial physiological need, arises from the fact that water constitutes about 70% of the mammalian body. It regulates biochemical reactions and is essential for maintaining blood pressure. There are two types of thirst: osmotic and hypovolemic. Osmotic thirst is triggered by the concentration of solutes in the body, which involves water movement across cell membranes. Specific neurons detect these solute levels and signal thirst. In contrast, hypovolemic thirst occurs when the body loses fluid volume, prompting hormonal responses such as the release of vasopressin and angiotensin, which constrict blood vessels and trigger thirst to restore fluid balance.

Arousal is defined as a physiological and psychological state that enhances responsiveness to stimuli. The activation of the reticular activating system improves attention and alertness, involving neurotransmitters like acetylcholine, norepinephrine, dopamine, and serotonin. The Yerkes-Dodson Law suggests that optimal arousal levels facilitate peak performance, while extremes of arousal can impair performance. Arousal plays a key role in regulating consciousness, attention, and responses to stimuli, including hunger and emotional states.

Attention involves the conscious awareness and selection of specific stimuli from the environment. Physiological changes accompany attention, such as variations in pupil size and heart rate, with neurons in the brain demonstrating enhanced responsiveness to focused stimuli. Selective attention refers to focusing on specific sensory modalities at one time, which can inhibit responses to others. Broadbent's Filter Model proposes a filtering process to prevent information overload by allowing only specific input to advance into cognitive systems.

Emotions, originating from the Latin word "emovere," indicate intense responses to stimuli, comprising physiological arousal, subjective feelings, and behavioral expressions. Various theoretical perspectives on emotions include the James-Lange Theory, which posits that emotions result from the perception of bodily changes; the Cannon-Bard Theory, which claims that physical and emotional responses occur simultaneously; and the Cognitive/Appraisal Theory by Lazarus, which argues that emotional responses require conscious evaluation of stimuli prior to physiological changes. Physiological changes during emotional experiences can include variations in skin conductance, heart rate, respiration, and muscle tension.

Sleep is defined as a natural, reversible state characterized by reduced responsiveness to external stimuli. It comprises different stages, including NREM stages—Stage 1 (NI), which represents light sleep characterized by alpha waves; Stage 2 (NII) as a transitional phase; and Stage 3 (NIII), which reflects deep sleep dominated by delta waves. Additionally, REM sleep, marked by rapid eye movement, features muscle atonia and dreaming, with brain activity resembling that of wakefulness. Sleep is regulated by a homeostatic drive, indicating the body's need for sleep, alongside circadian rhythms, which are regulated by the suprachiasmatic nucleus located in the hypothalamus that responds to light inputs to maintain the sleep-wake cycle.

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