Neuroscience And Behavior

CHAPTER 3—NEUROSCIENCE AND BEHAVIOR

I. Neurons: The Origin of Behavior

A. Discovery of How Neurons Function

1. Neurons – cells in the nervous system that communicate with one another to perform

information-processing tasks

2. Approximately 100 billion neurons in the brain

3. Neurons produce the underlying invisible physical component of visible behavior

a. Neurons came in many shapes and sizes, and communicate without touching

B. Components of the Neuron

1. Cell Body (Soma) – coordinates the information-processing tasks and keeps the cell alive

2. Dendrites – receive information from other neurons and relay it to the cell body

3. Axon – transmits information to other neurons, muscles, or glands

a. Myelin Sheath – an insulating layer of fatty material around the axon that speeds

conduction

b. The myelin sheath is composed of glial cells

a. Glial Cells – support cells found in the nervous system

(a) Clean up dead tissue, provide nutrients to neurons, and provide myelin for axons

4. Synapse – the junction between one neuron’s axon and another neuron’s dendrite or soma

a. Adults have between 100 and 500 trillion synapses

C. Major Types of Neurons

1. Sensory Neurons – receive information from the external world and convey this information to

the brain via the spinal cord

2. Motor Neurons – carry signals from the spinal cord to the muscles to produce movement

3. Interneurons – connect sensory neurons, motor neurons, or other interneurons

D. Neurons Specialized by Location

1. Purkinje cells carry mostly motor information from the cerebellum to the rest of the brain and

spinal cord

2. Pyramidal cells carry all kinds of information from the cerebral cortex

3. Bipolar cells carry visual information into the brain from the retina

II. The Electrochemical Actions of Neurons: Information Processing

A. Electric Signaling: Conducting Information within a Neuron

1. Communication within and between neurons proceeds in two stages – conduction and

transmission, together referred to as electrochemical action

a. First the signal is received and may initiate electrical conduction down the axon

b. Second, the signal travels chemically across the synapse to the next neuron

2. Resting Potential – the difference in electric charge between the inside and outside of a

neuron’s cell membrane

3. Charged molecules, or ions, flow across the cell membrane differentially to set up the resting

potential

 

 

a. At rest there is a higher concentration of potassium (K +) on the inside of the cell and

sodium (Na+) outside of the cell

b. The flow of ions across the cell membrane is controlled by opening and closing channels

that are specific to each ion

4. The resting potential of a neuron is approximately -70 millivolts

5. Action Potential – an electric signal that is conducted along the length of a neuron’s axon to

the synapse

6. Input must pass a threshold to activate an action potential

7. All-or-none, that is, an action potential’s strength remains the same from the beginning to the

end and is not influenced by further changes in input strength

8. Refractory period – the time following an action potential during which a new action potential

cannot be initiated

9. Bare segments of axon between sections of myelin are called the nodes of Ranvier, which

causes action potential to “jump” (saltatory conduction) and speeds conduction

B. Chemical Signaling: Transmission between Neurons

1. Neurotransmitters – chemicals that transmit information across the synapse to a receiving

neuron’s dendrites

2. Receptors – parts of the cell membrane that receive neurotransmitters and initiate or prevent

a new electric signal

a. Act like a lock-and-key system, where only certain neurotransmitters can activate certain

receptors

3. The sending, or presynaptic neuron, releases neurotransmitters into the synapse that are

received by the postsynaptic neuron

4. Neurotransmitters are cleared from the synapse when they are finished binding to receptors

via three different processes

a. Reuptake – neurotransmitters are taken back into the presynaptic neuron through

transporters

b. Enzymatic Degradation – enzymes can destroy the neurotransmitter while still in the

synapse

c. Autoreceptors can also detect if there is too much neurotransmitter being released and

signal the presynaptic neuron to stop the release

C. Types and Functions of Neurotransmitters

1. Acetylcholine – neurotransmitter involved in a number of functions, including voluntary motor

control

2. Dopamine – neurotransmitter that regulates motor behavior, motivation, pleasure, and

emotional arousal

3. Glutamate – major excitatory neurotransmitter involved in information transmission

throughout the brain

4. GABA (gamma-aminobutyric acid) – primary inhibitory neurotransmitter in the brain

5. Norepinephrine – neurotransmitter that influences mood and arousal

6. Serotonin – a neurotransmitter involved in the regulation of sleep and wakefulness, eating,

and aggressive behavior

7. Endorphins – chemicals that act within the pain pathways and emotion centers of the brain

D. How Drugs Mimic Neurotransmitters

 

 

1. Agonists – drugs that increase the action of a neurotransmitter

2. l-dopa increases dopamine and helps treat Parkinson’s disease

3. Prozac increases serotonin by blocking reuptake, which helps treat symptoms of depression

E. Antagonists – drugs that block the function of a neurotransmitter

1. MPTP destroyed dopamine-producing neurons

2. Propanolol blocks the beta receptors for norepinephrine in the heart, which helps with stage fright

III. The Organization of the Nervous System

A. Divisions of the Nervous System

1. Central Nervous System (CNS) – brain and spinal cord

2. Peripheral Nervous System (PNS) – connects the central nervous system to the body’s

organs and muscles

a. Somatic Nervous System – a set of nerves that conveys information into and out of the

central nervous system

b. Autonomic Nervous System – a set of nerves that carries involuntary and automatic

commands that control blood vessels, body organs, and glands

a. Sympathetic Nervous System – a set of nerves that prepares the body for action in a

threatening situation

b. Parasympathetic Nervous System – helps the body return to a normal resting state

B. Components of the Central Nervous System

1. Spinal Cord coordinates breathing, pain, movement, and other functions

a. Spinal Reflexes – simple pathways in the nervous system that rapidly generate muscle

contractions

b. The spinal cord is divided into four regions, each controlling a different part of the body

IV. Structure of the Brain

Generally, simpler tasks are controlled by “lower” regions and complex functions by “higher” regions

A. The Hindbrain – an area of the brain that coordinates information coming into and out of the

spinal cord

a. Medulla – an extension of the spinal cord into the skull that coordinates heart rate,

circulation, and respiration

a. Reticular Formation – cluster of neurons in the medulla that regulates sleep, wakefulness,

and levels of arousal

b. Cerebellum (“little brain”) – large structure of the hindbrain that controls fine motor skills,

coordination, and balance

c. Pons (“bridge”) – structure that relays information from the cerebellum to the rest of the

brain

B. The Midbrain – above the hindbrain, it coordinates orientation and movement in the

environment, and contributes to arousal

1. Tectum – orients an organism in the environment

2. Tegmentum – involved in movement and arousal, including motor behavior (substantia nigra

and dopamine), motivation, and pleasure

 

 

C. The Forebrain – highest level of the brain, controlling complex cognitive, emotional, sensory, and motor functions

1. Tectum – orients an organism in the environment

2. Cerebral Cortex – the outermost layer of the brain, visible to the naked eye, and divided into

two hemispheres

3. Subcortical Structures – areas of the forebrain housed under the cerebral cortex near the

very center of the brain

a. Thalamus – relays and filters information from the senses and transmits the information to the cerebral cortex

b. Hypothalamus (below thalamus) – regulates body temperature, hunger, thirst, and

sexual behavior

i. Four Fs of behavior: fighting, fleeing, feeding, and mating

c. Pituitary Gland – the “master gland” of the body’s hormone-producing system, which

releases hormones that direct the functions of many other glands in the body

d. Limbic System – a group of forebrain structures including the hypothalamus, the

amygdala, and the hippocampus, which are involved in motivation, emotion, learning, and memory

e. Hippocampus – structure critical for creating new memories and integrating them into

a network of knowledge so that they can be stored indefinitely in other parts of the brain

f. Amygdala – located at the tip of each horn of the hippocampus, plays a central role in

many emotional processes, particularly the formation of emotional memories

g. Basal Ganglia – a set of subcortical structures (including the striatum) that directs

intentional movements

4. The Cerebral Cortex

a. Fitting a lot of cortex into small spaces

i. Gyri – smooth, raised surfaces of the cortex

ii. Sulci – indentations or fissures in the cortex

b. Function of the cortex in three levels

i. Separation of cortex into two hemispheres

(a) Each side is roughly symmetrical and controls many functions on the opposite, or contralateral, side of the body

(b) Commissures – bundles of axons that make possible communication between parallel areas of the cortex in each half, the largest being the corpus callosum

ii. Functions of each hemisphere

(a) Each hemisphere has four lobes

(1) Occipital Lobe – a region in the back of the brain that processes visual information

(2) Parietal Lobe – located in front of the occipital lobe and carries out functions such as touch

(3) Temporal Lobe – located laterally and below parietal cortex, is responsible for hearing and language

(4) Frontal Lobe – behind the forehead, has specialized areas for movement,

 

 

abstract thinking, planning, memory, and judgment

(b) Homunculus (“little man”) – rendering of the body in which each part shown is in proportion to the representation in the somatosensory (parietal) or motor (frontal) cortex

(c) Role of specific cortical areas

(1)Association Areas – areas of cortex that are composed of neurons that help provide sense and meaning to information registered in parts of the primary cortex

D. Brain Plasticity

1. The brain is plastic: Functions that were assigned to certain areas of the brain may be

capable of being reassigned to other areas

2. Extensive use of your hands (e.g., concert pianist) can result in larger representations of

hands in the cortex than non-pianists

V. The Development and Evolution of Nervous Systems

A. Prenatal Development of the Central Nervous System

1. The nervous system is the first major bodily system to take form in an embryo

a. After the third week of fertilization the nervous system goes from a sphere with a ridge, to

a groove, to a neural tube

b. Fifth week the forebrain and hindbrain differentiate

c. Seventh week and later, forebrain expands into cerebral hemispheres

2. Ontogeny – how the brain develops within an individual

3. Phylogeny – how the brain developed within a particular species

B. Evolutionary Development of the Central Nervous System

1. Even the simplest animals have sensory and motor neurons

a. Single-celled protozoa have systems for sensing and moving toward food

b. Invertebrates (e.g., jellyfish and flatworms) developed simple nervous systems with

commissures and ganglia

2. Vertebrates developed differently than invertebrates

a. Vertebrates developed separate sensory and motor systems

b. Hierarchy developed in vertebrates

a. Higher parts of the brain developed to deal with more complex behaviors than lower

parts of the brain

c. Different vertebrates have different levels of complexity in the forebrain

a. Birds rely on a highly developed striatum

b. Mammals have a developed striatum and more developed cerebral cortex

d. Primates’ brains, particularly humans, have evolved more rapidly than other mammals,

partially because of gene mutations (changes in a gene’s DNA) that resulted in adaptation

C. Genes and the Environment

1. Nature and Nurture

a. Either genetics or the environment played a major role in producing particular behaviors,

traits, etc.

b. The interaction between nature and nurture determines what humans do

 

 

2. What are Genes?

a. Gene – unit of hereditary transmission, built from DNA (deoxyribonucleic acid)

b. Chromosomes – strands of DNA wound around each other in a double-helix configuration

c. Degree of Relatedness – the probability of sharing genes (e.g., you share 50% of your

genes with each parent)

i. Monozygotic Twins (identical twins) – share 100% of genes because they came from

one fertilized egg

ii. Dizygotic Twins (fraternal twins) – share 50% of genes because they came from 2

fertilized eggs, just like other siblings

iii. Twin studies are often used to help determine the amount of a behavior, trait, or

disorder that can be attributed to genes

d. Heritability – a measure of the variability of behavioral traits among individuals that can be

accounted for by genetic factors

i. Calculated as a proportion and reported as a number from 0 to 1.0

ii. Heritability of .50 for intelligence tells us that 50% of intelligence is accounted for by genes, but not which genes might be controlling that 50%

iii. That heritability score is derived from a population, not one person

iv. Heritability is dependent on the environment

v. Heritability is not fate; circumstances can change the likelihood of behaviors or

pathologies

VI. Investigating the Brain

A. Learning about Brain Organization by Studying the Damaged Brain

1. A lot of research about brain function has come from examining deficits in behavior relative

to specific brain damage (e.g., Broca’s area)

2. The Emotional Functions of the Frontal Lobe

a. Phineas Gage’s accident, essentially separating his frontal lobes from the rest of his brain,

resulted in an understanding that the frontal lobes are critical for maintaining emotional stability

3. The Distinct Roles of the Left and Right Hemispheres

a. Split-brain procedure – surgical severing of the corpus callosum

b. Allowed understanding of how some behaviors are relegated to only one hemisphere (e.g.,

language is usually handled in the left hemisphere)

B. Listening to the Brain: Single Neurons and the EEG

1. Electroencephalograph (EEG) – a device used to record electrical activity in the brain, usually

detected by electrodes on the scalp

2. Patterns of activity from groups of neurons indicated sleep, arousal, and certain perceptions

3. Recording from single neurons has shown us how cells in some parts of the brain respond to

stimuli (e.g., occipital neurons, or feature detectors, respond to dots or lines on a screen)

C. Brain Imaging: From Visualizing Structure to Watching the Brain in Action

1. Neuroimaging Techniques – methods used to produce images of living, healthy brain tissue

and activity

2. Structural Brain Imaging – Computerized Axial Tomography (CAT)

 

 

a. x-rays taken from many angles to produce a composite of the different densities of the

brain

b. Often used to detect structural problems (e.g., tumors)

3. Magnetic Resonance Imaging (MRI) – images that result from brief but powerful magnetic

pulses being applied to the brain and interpreting how cells in the tissue react to the pulses

4. Functional Brain Imaging – allows scientists to watch the brain in action during some

behavior, based on increased blood flow in active regions

a. Positron Emission Tomography (PET)

b. Functional Magnetic Resonance Imaging (fMRI)