Biological Basis of Behavior

Introduction

The biological basis of behavior explores how the structure and function of the brain and nervous system underlie our thoughts, emotions, and actions. This topic is central to understanding how biology influences psychology.

Phrenology and Early Localization

Phrenology

• Phrenology was an early theory by Franz Gall that proposed different brain areas controlled specific traits, inferred by feeling bumps on the skull.

• Though later discredited, phrenology contributed to the idea of localization of function in the brain.

Case Study: Phineas Gage

• Phineas Gage suffered brain damage that led to personality changes, demonstrating that specific brain regions influence behavior and personality.

Neurons: The Building Blocks of the Nervous System

Structure and Function of Neurons

• Neurons are specialized nerve cells responsible for communication within the brain and nervous system.

• The human brain contains approximately 5 billion neurons, with about 160 billion connections.

• Types of neurons:

  • Sensory neurons: Carry information from sensory receptors to the brain and spinal cord.

  • Motor neurons: Transmit signals from the brain and spinal cord to muscles and glands.

  • Interneurons: Facilitate internal communication within the brain and spinal cord.

Basic Parts of a Neuron

• Dendrites: Branching fibers that receive messages from other neurons.

• Soma (cell body): Contains the nucleus and maintains cell health.

• Axon: Long fiber that transmits impulses away from the cell body.

• Myelin Sheath: Fatty insulation around the axon that increases the speed and efficiency of neural transmission; fully develops around age 25.

• Terminal Buttons: Release neurotransmitters to communicate with other neurons.

• Synaptic Cleft: The gap between neurons where communication occurs.

Neural Communication

Electrical and Chemical Signaling

• Within a neuron, messages travel electrically; between neurons, communication is chemical via neurotransmitters.

• Myelin sheath increases the speed of electrical transmission; its breakdown can lead to disorders like Multiple Sclerosis.

• Nodes of Ranvier: Gaps in the myelin sheath that allow impulses to jump, speeding up transmission.

Action Potential

• Neurons have a resting potential (slightly negative charge inside).

• When excitatory signals minus inhibitory signals exceed a threshold, an action potential (neural firing) occurs, causing depolarization.

Equation:

• Action potentials are "all or nothing"—they do not vary in intensity.

• Stimulus intensity is detected by the number of neurons firing, not the strength of each action potential.

Synaptic Transmission

• The synaptic gap prevents electrical signals from crossing; instead, neurotransmitters carry the message chemically.

• Neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic neuron.

Major Neurotransmitters and Their Functions

Neurotransmitter Regulation

• Reuptake: Excess neurotransmitters are reabsorbed by the presynaptic neuron.

• Reuptake inhibitors: Block reuptake, increasing neurotransmitter availability (e.g., SSRIs for serotonin).

• Agonists: Mimic or enhance neurotransmitter action (e.g., opioids mimic endorphins).

• Antagonists: Block or inhibit neurotransmitter action (e.g., some drugs block reuptake or receptor sites).

Glial Cells

Support for Neurons

• Glial cells provide structural and metabolic support for neurons, making up about 50% of brain volume.

• Functions include supplying nutrients, removing waste, insulating neurons, and preventing signal interference.

Neural Plasticity

Brain Adaptation and Change

• The brain is capable of change throughout life, known as neural plasticity.

• Occurs through development (growth, synaptogenesis, pruning, myelination), learning (new synapses and connections), and adaptation to injury (limited regeneration, stem cell research).

The Nervous System: Organization

• Somatic Nervous System: Controls voluntary movements.

• Autonomic Nervous System: Regulates involuntary functions (internal organs, glands).

• Sympathetic Division: Activates "fight or flight" response.

• Parasympathetic Division: Promotes "rest and digest" functions.

• These divisions work in opposition; when one is active, the other is less active.

Studying the Brain

Methods of Investigation

• Electroencephalogram (EEG): Measures electrical activity in the brain using electrodes.

• Positron Emission Tomography (PET): Tracks radioactive glucose to show active brain areas during tasks.

• Magnetic Resonance Imaging (MRI): Uses magnetic fields and radio waves to produce detailed brain images.

• Functional MRI (fMRI): Compares successive MRIs to reveal brain activity during specific tasks.

Major Brain Structures and Their Functions

Overview

• Spinal Cord: Main communication pathway between brain and body.

• Hindbrain: Controls basic survival functions (e.g., heartbeat, breathing).

• Midbrain: Integrates sensory and motor information; bridge between forebrain and hindbrain.

• Limbic System: Regulates emotions, memory, and survival drives.

• Cerebral Cortex: Responsible for higher-level functions such as thinking, memory, and language.

Key Brain Structures

• Medulla: Controls heartbeat, breathing, and reflexes; crossover point for neural pathways.

• Pons: Involved in sleep and movement coordination.

• Cerebellum: Coordinates voluntary movement, balance, and nonverbal learning.

• Reticular Activating System: Regulates arousal and attention; filters incoming stimuli.

Limbic System Components

• Thalamus: Relay station for sensory information (except smell); important for sleep regulation.

• Hypothalamus: Maintains internal balance; regulates hunger, thirst, body temperature, and sexual behavior.

• Hippocampus: Essential for forming new memories.

• Amygdala: Processes emotions, especially fear and aggression.

Lobes of the Brain and Their Functions

• Frontal Lobe: Involved in reasoning, planning, decision-making, empathy, and sense of self; coordinates the brain's executive functions.

• Parietal Lobe: Processes sensory input; more area is devoted to more sensitive body parts.

• Temporal Lobe: Handles auditory information and memory.

• Occipital Lobe: Processes visual information.

Example: Brain Injury and Behavior

• Damage to the frontal lobe (as in Phineas Gage) can result in changes to personality and decision-making abilities.

Additional info: The notes above have been expanded with definitions, examples, and tables for clarity and completeness, following academic conventions for introductory psychology.