Chapter 2: Behavioral Neuroscience

The Nervous System

The peripheral nervous system (PNS) includes all nerves that extend from the central nervous system (CNS) to the rest of the body, such as the skin, muscles, and organs. Its main role is to carry information to and from the CNS (brain and spinal cord)

The PNS is divided into two main systems:

• Somatic nervous system

• Autonomic nervous system

The somatic nervous system controls voluntary actions. It:

• Sends sensory information (touch, pain, sight, sound, etc.) from the body to the CNS

• Sends motor commands from the CNS to skeletal muscles
  This system allows conscious movement, such as walking or lifting objects.

The autonomic nervous system controls involuntary body functions and works automatically. It connects the CNS to:

• Internal organs

• Smooth muscles

• Glands

It regulates:

• Heart rate and blood pressure

• Digestion

• Body temperature

• Hormone and glucose levels

Although it is automatic, people can learn limited control through techniques like biofeedback and yoga.

The autonomic nervous system has two divisions that work together to maintain balance:

Sympathetic nervous system (“fight or flight”)

This system prepares the body for action, especially during stress.

• Increases heart rate and breathing

• Dilates pupils

• Increases sweating

• Triggers adrenal glands to release epinephrine and norepinephrine
  These changes increase alertness and energy.

Parasympathetic nervous system (“rest and digest”)

This system calms the body once the stress has passed.

• Slows heart rate and breathing

• Constricts pupils

• Conserves energy

• Returns the body to a relaxed stat

Endocrine System

The endocrine system is the body’s second communication system, working closely with the nervous system. It is made up of ductless glands that release hormones, which are chemical messengers that regulate many long-term processes in the body.

Hormones:

• Are released directly into the bloodstream

• Travel to target organs

• Act slower than nerve signals (seconds to days)

• Have long-lasting effects

They influence:

• Growth and development

• Sexual development and reproduction

• Metabolism

• Mood and behavior

Unlike the nervous system, which sends fast electrical signals, the endocrine system works more slowly but steadily. Dozens of different hormones are produced, each with specific effects on the body.

Control of the endocrine system comes from the brain, specifically the hypothalamus.

• The hypothalamus monitors hormone levels

• It controls the pituitary gland

• The pituitary gland is the master gland

  • Pea-sized

  • Located at the base of the brain

  • Releases hormones that tell other glands what to do

There is a feedback loop between the brain and the endocrine glands:

• If hormone levels are too low:

  • Hypothalamus signals pituitary

  • Pituitary tells glands to release more hormones

• If hormone levels are high enough:

  • Hypothalamus signals pituitary to stop release

This works like a thermostat:

• Too cold → heat turns on

• Warm enough → heat turns off

Problems occur when glands malfunction.

Example: Thyroid gland

• Too little hormone → tired, sensitive to cold

• Too much hormone → nervous, irritable, weight loss

The Neuron

The nervous system is made up of two main types of cells:

• Neurons (nerve cells)

• Glial cells

These cells work together to send, process, and support communication throughout the body.

Neurons are the main communication cells of the nervous system. They send and receive information using electrochemical signals.

There are three types of neurons:

• Sensory neurons

  • Carry information from the senses and body to the CNS

  • Examples: sight, pain, taste, touch

• Motor neurons

  • Carry commands from the CNS to muscles and glands

  • Produce movement and physical responses

• Interneurons

  • Found within the CNS

  • Connect sensory neurons to motor neurons

  • Help with processing and decision-making

The human brain contains about 86 billion neurons. Each neuron can connect with thousands of others, creating trillions of connections.

Neurons are organized into neural networks, not randomly scattered.

• Connections get stronger with use and experience

• This allows faster and more efficient communication

Glial Cells

Glial cells support neurons rather than transmit signals.
They:

• Provide structural support

• Supply nutrients

• Insulate neurons

• Help with development, repair, and signal speed

There are nearly 85 billion glial cells in the brain—almost as many as neurons.

Reflexes

A reflex is an automatic, fast response to a stimulus.

Example: knee-jerk reflex

• Sensory neuron sends signal to spinal cord

• Motor neuron sends signal back to muscle

• No brain involvement

• Happens in about 50 milliseconds

Reflexes are protective, such as pulling your hand away from something hot before feeling pain.

Complex behaviors

More complex actions (thinking, talking, driving, reading) require the brain.

Process:

• Sensory input → spinal cord → brain

• Brain processes information and makes a decision

• Signal travels back through spinal cord → muscles

• Behavior occurs

Most behaviors studied in psychology involve this brain-based processing.

Neuron structure

Parts of a neuron

• Soma (cell body)

  • Contains the nucleus

  • Maintains the neuron’s chemical balance

• Dendrites

  • Receive signals from other neurons or sensory receptors

  • More dendrites = more information received

• Axon

  • Sends signals away from the cell body

  • Can be very short or very long

  • Carries the electrical impulse to other cells

• Axon terminals

  • Located at the end of the axon

  • Release chemical messengers to other neurons

Many axons are covered by the myelin sheath:

• A fatty, white insulating layer

• Produced by glial cells

• Speeds up neural signals by preventing signal loss

When myelin is damaged (as in multiple sclerosis), signals slow down, leading to loss of muscle control.

Direction of neural signals

Neural communication follows one direction:

Dendrites → Soma (cell body) → Axon → Axon terminals

Neurons in action

Electrical Nature of Neural Communication (Results / What Happens)

Neural communication occurs through electrical impulses, not just chemical signals. These impulses arise from changes in electrical charge across the neuron’s membrane.

• Every neuron is surrounded by a semipermeable membrane that controls which ions can cross.

• Ions are electrically charged particles dissolved in fluid inside and outside the neuron.

Resting State (Baseline Condition)

When a neuron is not firing, it maintains a stable electrical imbalance.

• Sodium ions (Na⁺)

  • Concentrated outside the neuron

  • Do not easily cross the membrane

• Potassium ions (K⁺)

  • Concentrated inside the neuron

  • Cross the membrane more easily

• Negatively charged ions

  • Trapped permanently inside the cell

As a result:

• The inside of the neuron is negatively charged relative to the outside

• This stored charge represents potential energy

Initiation of the Action Potential

When dendrites are stimulated, the resting balance is disrupted.

• Ion channels in the membrane open

• Sodium ions rush into the neuron

• The inside of the cell becomes less negative

If this change is strong enough:

• An action potential is triggered

• This is a brief, rapid electrical impulse

Propagation of the Action Potential

Once initiated, the impulse travels along the axon.

• The action potential moves like a wave of electrical energy

• Speed ranges from ~2 mph to ~200 mph, depending on the neuron

• Even at top speed, it takes about 1/100 of a second to travel from the spinal cord to a finger or toe

After the impulse passes:

• Positive ions are pumped back out

• The neuron returns to its resting state

• The neuron is ready to fire again

Threshold and All-or-None Law

Neurons constantly receive multiple incoming signals.

• Signals may come from dozens, hundreds, or thousands of other neurons

• The neuron adds these signals together

Outcome:

• If the combined input reaches threshold → an action potential fires

• If it does not reach threshold → no impulse occurs

Key property:

• Action potentials are all-or-none

• They cannot be partial or graded

Neurotransmitters

Neurotransmission at the Synapse

Neural communication between neurons occurs through neurotransmitters, which transmit signals across the synaptic gap separating adjacent cells.

When an electrical impulse reaches the axon terminal:

• Neurotransmitters are released from synaptic vesicles

• They diffuse across the synapse

• They bind to specific receptors on the receiving neuron, muscle, or gland

This process allows neural information to continue from one cell to the next.

Excitatory and Inhibitory Signaling

Neurotransmitters differ in their effects on the receiving cell:

• Excitatory neurotransmitters increase the likelihood of an action potential

• Inhibitory neurotransmitters decrease or prevent action potential firing

Neural firing depends on the net effect of all incoming signals rather than a single neurotransmitter.

Neurotransmitter–Receptor Specificity

Each neurotransmitter binds only to certain receptors.

• Receptors have specific shapes

• Neurotransmitters bind using a lock-and-key mechanism

• This specificity allows precise control of neural communication

Variations in neurotransmitter activity are linked to mood, movement, cognition, pain, and mental disorders.

Overview of Major Neurotransmitters

The nervous system uses many neurotransmitters, but several are especially well studied due to their roles in behavior and disease.

• Acetylcholine (ACh) was the first neurotransmitter identified and is crucial for muscle movement and memory. It links motor neurons to muscles, allowing actions like breathing and walking. Blocking ACh receptors (e.g., with curare) causes paralysis, while excessive ACh release (e.g., black widow spider venom) causes violent muscle contractions. Low ACh levels are associated with Alzheimer’s disease.

• Dopamine regulates movement, motivation, and reward. Loss of dopamine-producing neurons causes Parkinson’s disease, and symptoms can be treated with L-dopa, which restores dopamine levels. Excess dopamine receptor activity is linked to schizophrenia. Dopamine also drives reward-seeking behavior through the mesolimbic (reward) pathway, increasing motivation toward food and drugs via incentive salience.

• Endorphins are the brain’s natural painkillers. Their discovery followed findings that morphine binds to specific brain receptors, implying the brain produces its own morphine-like substances. Endorphins and their receptors are widely distributed throughout the central nervous system.

Neurotransmitters and Behavior

Neurotransmitter activity is associated with:

• Mood and emotional regulation

• Pain and pain relief

• Memory and learning

• Motor control

• Motivation and reward

• Mental and neurological disorders

Disruptions in neurotransmitter balance can result in conditions such as depression, Parkinson’s disease, schizophrenia, paralysis, seizures, and anxiety disorders.

The Brain

Early Ideas About Brain Localization

Before modern neuroscience, Franz Joseph Gall proposed phrenology, the idea that personality traits could be identified by bumps on the skull. Gall correctly believed that different brain regions have different functions (localization), but his method was pseudoscientific because skull shape does not reflect brain structure.

Clinical Case Studies

One major method of studying the brain is the clinical case study, which examines people with brain damage from injury or disease.

• Phineas Gage: Damage to the frontal lobes led to major personality changes but intact intelligence
  → showed frontal lobes are involved in impulse control and behavior regulation

Limitations of case studies:

• Brain areas can compensate for damage (plasticity)

• Injuries are rarely confined to one exact region

• Difficult to establish cause-and-effect

Example: Einstein’s brain

• Overall brain size = average

• A visuospatial/mathematical region was 15% wider

• Cause unclear: innate difference vs. lifelong use
  → Clinical evidence alone cannot give definitive answers

Invasive Brain Research Methods

These methods directly interfere with brain tissue and are mostly used in animal research.

• Lesioning: surgically destroying a brain area to observe behavioral changes

• Drug studies: administering substances (e.g., caffeine, nicotine, hormones) to observe effects on behavior and neurotransmitters

• Electrical stimulation: activating specific brain areas with electrodes

  • Sometimes used during human brain surgery while patients are awake to map functions

Noninvasive Brain Research Methods (Humans)

These techniques allow brain study without surgery and drive modern neuroscience.

EEG (Electroencephalograph)

• Records electrical brain waves via electrodes on the scalp

• Brain waves vary with:

  • alertness

  • sleep

  • relaxation

• Used to diagnose:

  • epilepsy

  • brain damage

  • some psychological disorders (e.g., ADHD, depression)

Limitation:

• Measures overall surface activity → poor spatial precision

Brain Imaging Techniques

These provide structural and functional images of the brain.

CT Scan

• Computer-enhanced X-ray

• Produces horizontal brain slices

• Useful for detecting:

  • tumors

  • strokes

  • structural abnormalities

• Uses X-rays (small radiation risk)

PET Scan

• Measures brain activity using radioactive glucose

• Active regions use more glucose → appear as “hot” colors

• Used to:

  • study mental processes

  • distinguish psychological disorders

  • study hallucinations in schizophrenia

MRI and fMRI

MRI (Magnetic Resonance Imaging)

• Uses magnetic fields (no radiation)

• Produces high-resolution images of brain structure

fMRI (Functional MRI)

• Tracks brain activity over time

• Noninvasive and safe for repeated use

• Widely used to study:

  • memory

  • attention

  • perception

  • cognition

This technique is considered one of the most important advances in cognitive neuroscience.

Brain Anatomy

The Human Brain: Overview

• Humans share some brain abilities with other animals (navigation, senses) but have unique problem-solving, self-reflection, and language abilities.

• The brain can be viewed as three “mini-brains” in one:

  1. Brainstem – primitive, life-support functions

  2. Limbic System – emotions, motivation, memory

  3. Cerebral Cortex – higher cognition, thought, learning, language

Brainstem

Functions: Life-support, motor control, sleep/arousal
Key Structures:

Limbic System

Functions: Emotion, motivation, memory, basic drives
Key Structures:

Cerebral Cortex

Functions: Higher mental processes (thinking, memory, language, perception)
Key Points:

• Outer layer of the brain; highly wrinkled for maximum surface area

• Makes up 80% of human brain volume

• Divided into two hemispheres, each with four lobes:

  • Frontal – planning, decision-making, personality

  • Temporal – hearing, memory, language comprehension

  • Parietal – touch, spatial awareness

  • Occipital – vision

Functional Areas:

Notes on Integration:

• Brain functions are not strictly localized—most activities use multiple interconnected regions.

• Example: Repeating the word “ball” involves visual cortex → angular gyrus → Wernicke’s area → Broca’s area → motor cortex.

Split Brain

Hemispheric Organization

• Each hemisphere is a mirror copy: every brain structure on the left has a corresponding structure on the right (hippocampus, amygdala, hypothalamus, thalamus, sensory/motor areas, association areas, language areas).

• The corpus callosum (4 in × ¼ in, millions of fibers) connects hemispheres, allowing them to share information.

Split-Brain Studies

Purpose: Treat severe epilepsy by severing the corpus callosum to prevent seizures from spreading.

Visual & Auditory Processing:

• Left hemisphere controls right side of body; right hemisphere controls left side.

• Visual crossover: images on the right visual field → left hemisphere, images on the left visual field → right hemisphere.

• Auditory crossover: sounds received in one ear register in the opposite hemisphere first.

• Normally, the corpus callosum shares this information; split-brain patients cannot transfer it.

Key Experiments:

Conclusion:

• Each hemisphere can process and store information independently.

• When disconnected, two separate streams of consciousness exist.

Cerebral Lateralization in Healthy Brains

Neglect Syndrome

• Caused by one sided hemisphere damage (stroke, accident).

• Patients lose awareness of other side of space and body.

• Examples: drawing lines skewed to the right, combing/shaving/eating only the right side.

Brain Reorginization

Neural Plasticity and Experience-Dependent Growth

• Old belief: Adult brains were fixed, incapable of structural change.

• New evidence: Experiences can alter neural circuits, even in adulthood.

Animal Studies

• Rosenzweig’s rat “amusement park” (1984):

  • Rats in enriched environments (ladders, platforms, toys, social interaction) developed heavier, thicker brains with more dendrites and synapses than isolated rats.

  • Visual enrichment: Rats raised with visual stimulation formed 20% more synaptic connections in the visual cortex than those raised in darkness.

  • Motor learning: Rats trained to run acrobatic routes formed new synaptic connections in the cerebellum (balance and motor coordination).

• Implication: Experiences directly sculpt the brain’s neural architecture.

Cross-Modal Plasticity

• Blind humans: Visual cortex activated by sound and touch.

• Deaf humans: Auditory cortex responds to touch.

• Suggests other senses sharpen when one is lost.

Human Studies

• Karni & Ungerleider (1996): Repeated finger-tapping practice led to expansion in the primary motor cortex for the practiced sequence.

• Education and synaptic branching: More educated individuals had more synaptic connections in Wernicke’s area.

• Musicians: Professional musicians had 130% more gray matter in the auditory cortex than nonmusicians.

• Einstein’s brain: Visual-spatial/mathematical region was 15% larger; possibly due to constant use.

Plasticity and Phantom Pain

• Problem: Amputees often experience intense phantom limb pain.

• Mechanism: Brain reorganization—neighboring cortical regions take over the area of the lost limb, generating misleading sensations.

• Merzenich et al. (1983): Severed middle finger nerve in monkeys → other fingers’ neurons activated the dormant cortical region.

• Flor et al. (1995): Greater cortical reorganization correlated with more pain in human amputees.

Neurogenesis (Production of New Neurons)

• Once thought impossible in adults; now known to continue beyond infancy.

• Observed in:

  • Hippocampus (memory formation) in adult mice, guinea pigs, rabbits, and birds.

  • Adult canaries and chickadees: learning new songs or storing food memory creates new neurons.

• Implications: Adult humans may be capable of brain repair and adaptation, aiding recovery from injury or cognitive decline.

Concussion and Brain Injury

• Concussion: Trauma stretches/shears axons, disrupting neuronal communication.

• Symptoms: Confusion, amnesia, dizziness, headaches, blurred speech, temporary unconsciousness (severity varies from Grade 1–3).

• Severe cases: Can damage brainstem, affecting heart rate, breathing, and long-term cognitive/motor function.

Neural Grafting

• Goal: Replace damaged neurons with healthy tissue.

• Animal studies:

  • Sperry (1940s): Eyeball transplants in frogs → new neural pathways, restored vision.

  • Rat Parkinson’s model: Dopamine-producing substantia nigra destroyed → fetal brain tissue grafted → 70% reduction in symptoms after four weeks (Perlow et al., 1979).

• Later studies: Grafting used to reverse cognitive, spatial, and memory deficits in rodents and primates.

• Implication: Neural grafting could offer treatments for Parkinson’s, Alzheimer’s, and other neurodegenerative disorders.