Chapter 2: Neuroscience, Genetics, and Behavior
Phrenology : A theory developed by Franz Gall suggesting that bumps on the head
dictate personality and intelligence. The theory was proven false but it did direct our
attention to the learning about regions of the brain and brain function.
Biological Psychologists: Psychologists that study connections between biology and
behavior.
Neural Communication
The Neuron
Our Nervous System is basically made up of billions of nerve cells called neurons. Each
neuron is composed of the following:
1.Dendrites: bushy ends that receive messages.
2. Cell Body: the cell’s control center.
3. Axons: a branching fiber that sends messages away from the cell body to the
next neuron, muscle, or gland.
4.Myelin Sheath: layer of fatty glial cells that help speed up impulses by
insulating the axon and providing Nodes of Ranvier (gaps between the glial cells)
that allow for quick node to node hopping. Glial cells guide neural connections
and provide nutrients to nerve cells in the brain. Myelinated axons form the white
matter of the nervous system while the non-myelinated axons, dendrites and cell
bodies form the gray matter.
*diagram
An impulse or an Action Potential is a brief electrical charge that is received by the
dendrites, simulates the cell body and travels the length of the axon by Depolarizing the
normally polarized axon. A neuron at rest has a positive-inside/negative-inside charge
differential. This is called the cell’s Resting Potential. Because the cell’s membrane is
selectively permeable, a neuron is stimulated and positive ions are able to rush in and
depolarize the axon. During the refractory period, the neuron will pump the positive ions
back outside to restore the cell’s resting potential.
*Note
Neural messages usually travel in one direction only: from the dendrites to the
cell body and down the length of the axon
*diagram or link
The intensity of a stimulus is called the Threshold. A stimulus must exceed the threshold
in order for a transmission to occur. Much like a gun, the neuron either fires or it doesn’t,
there are no half-fires. This is called the all-or-none-response. To differentiate between a
really strong stimulus and a weak one, only the number of neurons firing will increase
and not their speed.
How Neurons Communicate
The axon terminal of the sending neuron is separated from the receiving neuron by a tiny
gap called the Synapse (or Synaptic Cleft). Once the action potential reaches the
synapse, neurotransmitters, or chemical messengers, are released into the gap where
they will bind onto specific receptor sites on the receiving neuron. These chemicals can
either induce or inhibit neural firing.
Scientists have isolated more than sixty neurotransmitters and a few of the more wellknown will be described throughout this course (Acetylcholine, Norepinephrine,
Serotonin, Dopamine, Endorphins). Note: You will need to know their function and to
with which disorders their under-abundance or over-abundance has been associated.
For example:
Acetylcholine (ACh): This is the most well know neurotransmitter and causes muscles to
contract in movement.
Endorphins: a neurotransmitter similar to morphine and is a natural opiate that are
released in response to pain and heavy exercise and induce pleasure (explaining runner’s
high, etc.).
The following types of drugs can alter neurotransmission:
1. Agonists: molecules that mimic the shape of natural neurotransmitters and thereby
mimic its effects. An example would be morphine binding to the same receptor sites to
which endorphins bind.
2. Antagonists: molecules that block neurotransmitters from binding to receptor sites. It
mimics a natural neurotransmitter but is not similar enough to stimulate the receptor and
mimic the effect. An example would be curare, a poison used by South American
hunters, mimic and block Ach sites thereby paralyzing prey.
3. Others: There are a variety of drugs that inhibit the natural breakdown or reabsorption of the neurotransmitter. An example would be Prozac, an anti-depressant drug
that prevents the re-absorption of serotonin from the synapse allowing it to linger longer
in the synapse thereby enhancing serotonin’s mood-lifting effect.
The brain has a Blood-brain barrier that filters out unwanted chemicals in blood stream.
This has its down side as well- When trying to treat chemical disorders of the brain
researchers must develop a drug that must pass through the blood-brain barrier. For
example, Parkinson’s patients show reduced levels of dopamine due to death of the
neurons that produce it. Dopamine as a drug cannot pass through the barrier but given as
L-dopa it can sneak through and be converted to dopamine in the brain.
Neural and Hormonal Systems
The Nervous System is composed two main systems:
1. Central Nervous System (CNS) – the brain and spinal cord.
2. Peripheral Nervous System (PNS) – This system connects the CNS to
body’s muscles and glands by means of nerves which are bundles of sensory
and Motor Neurons (they carry incoming and outgoing information
respectively) via Interneurons which enable internal communication. It ‘s
two main divisions are:
a. The skeletal (somatic) system: This system controls voluntary
movement of our skeletal muscles.
b. The autonomic nervous system (ANS): This system controls the
muscles of our internal organs and our glands. It is further subdivided
into the
Sympathetic Nervous System (arouses the body for defense
(increase heartbeat, dilating pupils, inhibit digestion etc.) and the
Parasympathetic Nervous System (calms the body after stress).
*diagram
A simple Reflex is coordinated by the nervous system and is an automatic response to
stimuli (like knee-jerk) involving messages from Sensory to Interneuron (Spinal Cord) to
Motor Neuron.
*diagram or link
The Endocrine System: It is the comparatively slow response system and it
communicates by releasing Hormones (chemical messengers) into the bloodstream.
In times of stress the autonomic nervous system will signal the Adrenal Glands (found
above kidney) to release epinephrine and norepinephrine hormones (also called
adrenaline and noradrenaline) which cause blood to be diverted to the body’s skeletal
system to prepare for fight or flight.
The Pituitary gland is the most powerful endocrine gland, and under the influence of
hypothalamus in brain, pituitary releases hormones that regulate glands and growth. It
also stimulates the adrenal gland to release the stress hormone cortisol. This stress
response system is a good example of the interdependence between the nervous system
and the endocrine system.
The Brain
The following is a list of the tools and techniques used to help gather information on
the brain:
1. Lesions – The surgical destruction or removal of brain tissue.
2. Electroencephalogram (EEG) – A machine that measures brain electric activity.
3. Computed Tomograph (CT or CAT Scan) – This apparatus takes x-ray photographs
of brain and can reveal brain damage.
4. Positron emission tomograph (PET Scan) – It detects radioactive glucose
consumption in brain. Allows us to see active metabolic areas.
5. Magnetic Resonance imaging (MRI) – It generates detailed pictures of the brain’s
soft tissues by making use of magnetic activity. Makes use of magnetic fields which
appear to be less harmful than x-rays.
Brain Structure
The Brainstem– the oldest portion (the central core) in brain is made up of three main
structures:
1. The Medulla Oblongata – regulates involuntary processes like heartbeat and
breathing.
2. The Pons (“bridge”)- connects the two halves of the cerebellum lying above it
and a portion of the reticular formation. It relays information about body
movements that it receives from both higher brain centers and the spinal cord.
It also appears to be involved in circuits that control sleep.
3. The Reticular formation- (looks like a finger-shaped net) controls arousal,
when you wake or sleep. Damage to this area may cause a person to lapse into
a coma.
The Cerebellum (“little brain”)- found to the rear of the pons, looks like a miniature
version of the forebrain with its two hemispheres. One area is involved in maintaining a
sense of balance, another is involved in coordinating muscular movements while another
part is involved in learning simple motor tasks.
The Thalamus lies above brainstem and is shaped like two eggs. Its function is to act as a
sensory switchboard (visual and auditory information as well as information about touch
pressure temperature and pain). relaying incoming signals to appropriate brain regions. It
does not relay sensory signals dealing with smell.
The brain has two Cerebral Hemispheres, one on the right side and one on the left side.
These two large structures that sit above the central core are the most recent development
in the brain’s evolution. It consists of the limbic system and the cortex and is involved in
the processes of learning, language, memory and reasoning. The right side of the brain is
linked to sensations in the left side of the body while the left side of the brain is linked to
sensations in the right side of the body. An exception to this would be visual sensations.
Here, visual info from the left eye for example, does not solely go to the right
hemisphere. In this case, the left half of your field of vision (in both eyes) goes to your
right hemisphere while the right half of your field of vision (in both eyes) goes to your
left hemisphere.
The Corpus Callosum joins the two hemispheres and is separated to cure epileptic
seizures. People with this separation are referred to as Spilt-brain patients. They are
unable to say what they see in their left visual field because speech is in left hemisphere
and the hemispheres regulate opposite sides of body.
When split-brainers are asked to say what they saw, the left hemisphere will say what is
seen in right visual field; when asked to point, get, or write what they saw, the right
hemisphere will dictate what is seen in the left visual field.
Sign language is nevertheless language and is control by left hemisphere, if deaf people
get a stroke in left hemisphere, signing will be disrupted.
Scientists have found hemispheric specialization. For example:
1. The left hemisphere appears to be more involved than the right in mathematics,
language, logic, reasoning and the interpretation of events and behaviors.
2.The right hemisphere appears to be superior to the left at perceptual tasks such
as copying drawings and information, face recognition, musical and artistic
endeavors, and expressing emotion.
* Read section on “Handedness” (right vs. left) in text.
The Limbic System sits directly above the central core and forms the innermost border
of the cerebral hemispheres. The limbic system includes the:
1. Amygdala – influences emotions (fear, anger). Stimulation of the area can
cause an animal to flee.
2. Hippocampus – processes memory. Removal of amygdala results in
emotionless organisms upon arousal.
3. Hypothalamus- maintains body homeostasis (temperature, hunger, growth)
and governs the pituitary gland. For example stimulation of the lateral (side)
hypothalamus will cause an animal to overeat while stimulation of the
ventromedial (lower middle) hypothalamus will cause an animal to stop
eating.
The Cerebral Cortex, the outermost area of the cerebral hemispheres, is a thin layer of
gray matter consisting of about 9 billion neurons covering the hemispheres. There are two
deep fissures that subdivide each hemisphere into principle regions called lobes. The four
main lobes are:
1. Frontal Lobe (behind forehead) – has the Motor Cortex located at the back of
frontal lobe and controls voluntary movement. The case with Phineas Gage
showed researchers that damages in the frontal lobe could result in personality
alterations because their normal "restraints" or inhibitions are erased. This was
due to a tamping rod that shot from his left cheek and out his head, separating his
internal motives and external judgment.
2. Parietal Lobe (top to back of head) – has the Sensory Cortex located in the
beginning of parietal lobe and receives information from the skin senses (touch,
pressure, heat and pain) and for the sense of body position (vestibular sense).
3. Occipital Lobe (back of head) – very important in the analysis of visual
information.
4.Temporal Lobe (above ears, below parietal lobes) – integrates sensory data
with special attention to auditory data.
*diagram or neuroscience for kids link
75% of the brain is not committed to motor or sensory functions. Theses brain regions are
called Association Areas – areas that are involved in thinking, remembering, and
speaking. With regards to evolution, the animal with the larger association area is the
more intelligent the species with respect to anticipating future events.
Language
Language requires the coordination of many brain areas of the cortex. Damage to any
one of these areas may result in aphasia (language impairment). Such damage has
allowed researchers to piece together the stages in which language occurs:
1. Visual Cortex (occipital lobe) – allows us to see the visual stimulation (words).
2. Angular Gyrus (mid-side of parietal lobe) – converts words into auditory code.
3. Wernicke’s Area (left temporal lobe) – enables us to derive meaning from auditory
code.
4. Broca’s Area (left frontal lobe) – controls motor cortex that in turn activates speech
muscles to pronounce words.
If there is damage to #1- one cannot see, #2- one cannot read, #3- one cannot understand,
and #4- one cannot physically speak.
.
A Quick Review of Genetics and Behavior
There are 23 chromosomes in human egg and sperm; they are combined (fertilized) to
make a total of 46 chromosomes in housing the master plan for the organism.
Chromosomes are composed of coils of DNA which in turn contain thousands of genes.
It is these genes (protein recipes) that make each one of us the individuals that we are.
Evolutionary Psychologists study the effects of evolution of and the behavior of
organisms.
Behavior Geneticists explore the ways that we differ from one another. They focus on
how genetics and environment (upbringing) affect behavior.
Molecular Geneticists use Linkage Analysis whereby they identify genes that are
responsible for certain behaviors, disorders and diseases. Some of the most important
information regarding genetic influences is found in studies using Identical Twins (two
babies within one egg) and Fraternal Twins (two babies in 2 separate eggs) and often to
contrast adoption studies. Identical twins have the same genes while fraternal twins do
not. If for example it can be shown that there is a statistically significant larger number of
identical twins (versus the fraternal twins) each having the same particular disorder,
researchers can assume that there is a specific gene responsible for the disorder.
Hertitability tells us what proportion of trait “variation” among people can be attributed
to their genetic make-up (versus environmental factors such as upbringing , culture, etc.).
If heritability of intelligence is 70%, it means that one can attribute 70% of the
differences in intelligence among people to genetic influence. It is referring to the extent
to which variation among individuals is due to each of them having different genes.