UNIT 2: Biological Bases of Behavior
PART I: Biological Bases of Behavior
Learning Targets
9-1 Explain why psychologists are concerned with human biology.
9-2 Describe the parts of a neuron, and explain how neural impulses are generated.
9-3 Describe how nerve cells communicate with other nerve cells.
9-4 Describe how neurotransmitters influence behavior, and explain how drugs and other chemicals affect neurotransmission.
Why are psychologists concerned �with human biology?
Psychologists from the biological perspective study the links between our biology and our behavior and mental processes.
The Human Brain Book
What is a neuron?
A neuron is a nerve cell that is the basic building block of the nervous system.
What are the parts of a neuron?
What is the cell body (soma)?
the part of the neuron that contains the nucleus, the cell’s life-support center
What are the dendrites?
bushy, branching extensions that
receive and integrate messages,
conducting impulses toward the
cell body
What is the axon?
Attached to the soma, the neuron extension that
passes messages through its
branches to other neurons or to
muscles or glands.
What is the myelin sheath?
the
fatty tissue layer segmentally
encasing the axons of some
neurons; increases transmission speed and provides insulation
What is myelin and why is it important?
AP® Exam Tip 1
Questions about the myelin sheath often show up on the AP ® Exam.
Often the question is about the purpose of the myelin sheath (speeds neural transmissions) and other times the question is about the link between the deteriorating myelin sheath and multiple sclerosis.
Make sure to spend time understanding the myelin sheath! (and pronouncing and spelling it, too!)
What are the terminal branches?
The ends of the axon containing terminal buttons which hold synaptic vesicles that store neurotransmitters.
What are glial cells?
cells that support,
nourish, and protect neurons;
they also play a role in learning,
thinking, and memory
Neurons are like queen bees;
on their own they cannot feed or sheathe themselves.
Glial cells are
worker bees;
they provide nutrients and insulating myelin, guide neural connections, and
mop up ions and neurotransmitters.
How is a neural impulse generated?
If the combined received chemical signals exceed a minimum threshold, the neuron fires, transmitting an
electrical impulse (the action potential)
down its axon by means of a
chemistry-to-electricity process.
What is a threshold?
The level of stimulation
required to trigger a neural impulse.
Neurotransmitters received by the dendrites build up to initiate the action potential.
What is the all-or-none response?
More stimulation does not produce a more intense neural transmission.
The neuron’s reaction is an all-or-none response. Neural firing happens at full response or not at all.
Like guns, neurons either fire or they don’t. Squeezing a trigger harder won’t make a bullet go faster.
Neural impulses can be �excitatory or inhibitory.
excitatory signal
Like the gas pedal
on the car.
Excitatory neurotransmitter signals trigger action.
inhibitory signal
Like the brake pedal
on the car.
Inhibitory neurotransmitter signals depress action.
When the excitatory impulses outnumber the inhibitory impulses, the threshold has been reached and an action potential occurs.
In order for a neuron to generate an action potential, which of the following must be true?
1. What Would You Answer?
What is the resting state of an axon?
Prior to beginning the action potential, the outside of an axon’s membrane has mostly positively charged sodium (Na+) ions and the interior contains negatively charged proteins and a small amount of positively charged potassium (K+) ions.
This creates a slightly negative charge and at this point, the neuron is said to be in the
resting state, or polarized.
What does it mean that an axon membrane is selectively permeable?
The membrane contains voltage gated ion channels that either open to
allow ion exchange (depolarization) or
close to prevent ion exchange (polarization).
What are the steps in an action potential?
The first section
of the semipermeable axon opens its gates once the threshold is met.
Na+ ions flood in through the channels.
Why do Na+ ions rush in?
Since the inside of the membrane is slightly more negative, the Na+ ions try to balance the charge.
This causes a slight depolarization.
What is the next step in an action potential?
The depolarization changes the
electrical charge of the next part
of the axon.
Gates in this second area now
open, allowing even more Na+
ions to flow in.
How do K+ ions move out?
At the same time, gates open in the first part of the axon allowing K+ ions to flow out.
This repolarizes that section of the axon.
What happens next?
The sodium/potassium pump continues to depolarize new sections of the axon and repolarize the previous sections.
How does the impulse move?
The influx of the positive ions is the neural impulse.
The impulse moves down the axon like dominos falling one after the other.
Talk through the steps of the action potential with a partner.
Remember to use key terms (like depolarize or Na+ ions) to show you understand.
What happens after the action potential moves to the end of the neuron??
Neurons need short breaks.
During a resting pause called
the refractory period,
subsequent action potentials cannot occur
until the axon returns to
its resting state.
Then the neuron can fire again.
Let’s review three key terms in the process…
Polarization: the resting state of the neuron, charge is more positive outside the membrane and more negative inside.
Depolarization: the action potential; the rushing in and out of positively charged ions.
Repolarization: the refractory period; the closing of the membrane and reestablishing a more negative charge inside
How do neurons communicate �with each other?
The sending neuron releases neurotransmitters across
a synapse to the receiving neuron.
What is a neurotransmitter?
chemical
messengers
that travel
across the
synapse and bind to receptor sites
on the receiving neuron
What is a synapse?
the junction
between the axon tip of the sending
neuron and the dendrite or cell
body of the receiving neuron
The tiny gap at this junction is called the synaptic gap or synaptic cleft.
How does the process work?
When an action potential
reaches an axon’s terminal branch, it stimulates the
release of neurotransmitter
molecules. These molecules
cross the synaptic gap and bind to receptor sites on the
receiving neuron.
This will either excite or inhibit a new action potential.
What is �reuptake?
a neurotransmitter’s
reabsorption by the sending
neuron
When there is a negative charge inside an axon and a positive charge outside of it, the neuron is
B. about to begin the refractory period.
C. said to have a resting potential.
D. said to have an action potential.
2. What Would You Answer?
Neurotransmitters cross the _____ to carry information to the next neuron.
B. axon
C. myelin sheath
D. dendrites
3. What Would You Answer?
How do neurotransmitters �influence our behavior?
AP® Exam Tip 2
As the text indicates, there
are dozens of different neurotransmitters.
Though there’s no way to predict exactly which
ones you’ll most likely see on the
AP® exam, it’s quite possible that
the ones on the previous slide
are ones you’ll be asked about.
Neurotransmitters have different �neural pathways.
serotonin pathway
dopamine pathway
Let’s consider the neurotransmitter acetylcholine (ACh)…
and skeletal muscles.
muscles cannot contract and
we are paralyzed.
Let’s consider the neurotransmitter dopamine…
Muhammad Ali, the late boxing legend, suffered from Parkinson’s disease.
An undersupply of dopamine at the synapse is linked to Parkinson’s.
Dopamine…
What are endorphins?
Endogenous morphine
Meaning “morphine within”—natural,
opiate-like neurotransmitters
linked to pain control and to
pleasure.
How do drugs alter �neurotransmission ?
Drugs and chemicals from outside the body alter our brain chemistry.
Drugs act as agonists (exciting neuron firing) or as antagonists(inhibiting neuron firing).
How does an agonist work?
An agonist is a drug molecule that increases a neurotransmitter’s action.
How does an antagonist work?
An antagonist is a drug molecule that inhibits or blocks a neurotransmitter’s action.
Antagonists can also work by blocking reuptake.
Learning Target 9-1 Review
Explain why psychologists are
concerned with human biology.
Learning Target 9-2 Review
Describe the parts of a neuron and
explain how neural impulses are generated.
Learning Target 9-3 Review
Describe how nerve cells communicate
with other nerve cells.
Learning Target 9-4 Review
Describe how neurotransmitters influence
behavior, and explain how drugs and
other chemicals affect neural transmission.