Biological Bases of
Behavior
Chapter 3
Biological Explanations of Behavior
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Genes
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Neurons
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DNA or genes that code for physical and
psychological characteristics
Electrical and chemical wiring of the body and
brain
Nervous System
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Combination of genes and neurons that
handles information and communication
Mirror Neurons
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http://www.pbs.org/wgbh/nova/teachers/
viewing/3204_01_nsn.html
Structure of the neuron
Neurons
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Hardware:
 Glia (means glue) – provides structural support and
insulation to promote more efficient signaling
 Neurons – nerve cells that constitute the basic
building blocks of nervous system (100 billion in brain
and spinal cord)
 Soma – cell body
 Dendrites – receive information
 Axon – transmit away
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Myelin sheath covers axons to increase speed of transmission
Terminal buttons – secrete neurotransmitters
Synapse – junction where information is transmitted
Electrical Activity of Neurons
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Neurons do 2 things:
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Generate electricity
Release chemicals
Both inside and outside the neuron are
electrically charged atoms and molecules called
ions
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Positively charged sodium and potassium ions and
negatively charged chloride ions flow back and forth
across cell membrane
Higher concentration of negative ions on inside (-70
millivolts) means neuron is at rest (resting potential)
The Action Potential
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If you sufficiently stimulate an axon, the cell membrane
opens up briefly
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Potassium ions then enter through the cell membrane openings
the interior voltage shifts from -70 to +40 millivolts
This brief shift in a neuron’s electrical charge is called an action
potential
Can occur very quickly
http://faculty.washington.edu/chudler/son.html
http://lessons.harveyproject.org/development/nervous_system/c
ell_neuro/action_potential/propagation.html
All-or-none principle
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Neurons either fire or they don’t
The rate of firing determines strength of stimulus
Thicker neurons transmit info quicker than thinner neurons
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Ex. Local anesthetics attach to sodium ions to prevent passage of
sodium into the cell membrane restricting firing potential
The neural impulse
How Neurons Communicate
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Synaptic space
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Tiny gap between axon terminal and the next neuron
Neurons do not touch one another…then how do they
communicate???
Arrival of action potential at axon’s terminal
button triggers release of neurotransmitters
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Chemical substances that carry messages across the
synapse either to excite other neurons or inhibit their
firing
Neurotransmitters cross synaptic cleft and attach to
receptor sites
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Receptor sites and neurotransmitters fit like a lock-and-key
Overview of synaptic transmission.
How neurons are like toilets
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The all-or-none principle: Toilet either flushes or not
Resting potential: Water in the represents the resting
potential. The toilet is "waiting" to fire, and the water in
the tank represents the overall negative charge inside
the neuron waiting for depolarization
Threshold: Toilet won’t flush properly until you push
past a critical point
Direction of impulse: toilet only flushes one way; the
impulse only travels one direction (you hope!)
Refractory period: after you flush, toilet won't flush
again for a certain period of time, even if you push the
handle repeatedly
Action potential: Represented by the water flowing
down the pipe.
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Peripheral nervous system
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Somatic (voluntary)
Autonomic (involuntary)
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Sympathetic (increased heart rate,
increase in respiration, increase in
blood circulation, pupil dilates,
digestion stops, etc.)
Parasympathetic (heart rate
decreases, respiration decreases,
digestion restarts, pupil constricts,
blood circulation decreases, etc.)
Understanding the Brain
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Neuropsychological tests
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Lesions
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Specific nervous tissue is destroyed with electricity or chemicals
or surgically remove portions of the brain
Electrical Recording
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Measure verbal and nonverbal behaviors of victims of brain
damage
Electroencephalograph (EEG)
Brain Imaging
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Computerized axial tomography (CAT scans): X-rays
Position Emission Tomography (PET scans): measures levels of
glucose in brain
Magnetic Resonance Imaging (MRI): structures and functions
CAT scan
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Brain of stroke victim
Lesioned
(damaged)
areas
PET scans
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Normal brain
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Alzheimer’s brain
MRI and fMRI scans
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Standard MRI
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functional MRI
Structures and functions of the brain
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Brain is separated
into two cerebral
hemispheres
connected by the
corpus callosum
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Neural bridge that acts
as a major
communication link
between two
hemispheres
The left and right brains
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Lateralization: relatively greater
localization of a function in one
hemisphere or the other
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Left hemisphere (verbal abilities and speech,
mathematical and logical abilities, more active
with positive emotions)
Right hemisphere (spatial relationships,
mental imagery, musical and artistic abilities,
more active with positive emotions
 Four
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lobes
Occipital lobe
(vision)
Parietal Lobe
(touch)
Temporal
lobe
(hearing)
Frontal lobe
(movement
of the body)
The Endocrine System
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Consists of glands that secrete chemicals
into the bloodstream that help control
bodily functioning
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Mostly controlled by hypothalamus
Uses hormones (chemical substances released
into the bloodstream by endocrine glands)
Pituitary gland: master gland
Gonads: sexual glands that release sex
hormones
Evolution and Behavior
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Based off of Darwin’s work on natural selection
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Characteristics that increase likelihood of survival and
ability to reproduce will be more likely preserved in
population and become more common in species over
time
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THUS…Natural selection acts as a filter
Adaptation is product of natural selection
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Changes that allow organisms to meet recurring
environmental challenges so their survival and
increases reproductive ability
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Can be physical or behavioral
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Woodpeckers long, strong beak allows to puncture hole in trees
and eat insects
Chameleon’s blend into their environment to avoid predators
Rats won’t eat 2 unfamiliar foods at a time
Tips on evolutionary theory
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Acknowledge the role of remote and
proximate causes that influence behavior
There is only adaptation to environmental
demands
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Example
Male violence is a good adaptation in a time of war
 Male violence is not such an adaptive trait in a
time of peace
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Genetic Influences on Behavior
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Genotype
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Specific genetic makeup of person
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Phenotype
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Ex. Commands of a computer program
Individual’s observable characteristics
Chromosomes: double-stranded molecule of
DNA carrying genetic information
Every cell has 46 chromosomes in pairs (23 from
each parent)
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All genes encoded in various combinations of 4
chemical bases
Genetic relatedness and familial
relatedness
Dominant, Recessive, or Polygenic
Effects
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Some genes are dominant, while other are
recessive
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Brown eyes dominant over blue eyes
Child has blue eyes only if both contribute a recessive
gene for eye color
Polygenic
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A # of gene pairs combine their influences to create a
single phenotypic trait
Between 2 parents there are over 70 million
chromosomal combinations
Is it Heredity or Environment?
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Family Studies
Twin Studies
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Compare trait similarities in identical and fraternal
twins
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Monozygotic and dizygotic
Adoption Studies
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People who were adopted are compared on some
characteristic with both their biological (share genetic
info) and adoptive parents (share no genes)
Ex. schizophrenia
Twin studies of intelligence and personality