Chapter 3: Biological Foundations of Behavior
1. Name the three main parts of the neuron and describe their functions.
Each neuron has three main parts: a cell body, dendrites and an axon.
The cell body (soma) contains the biochemical structures needed to keep the
neuron alive, and its nucleus carries the genetic information that determines how
the cell develops and functions.
Emerging from the cell body are branchlike fibers called dendrites. These
specialized receiving units are like antennas that collect messages from neighboring
neurons and send them on to the cell body. There the incoming information is
combined and processed. The many branches of the dendrites can receive input
from 1000 or more neighboring neurons. The surface of the cell body also has
receptor areas that can be directly stimulated by other neurons.
Extending from one side of the cell body is a single axon, which conducts electrical
impulses away from the cell body to other neurons, muscles or glands. The axon
branches out at its end to form a number of axon terminals as many as several
hundred in some cases. Each axon may connect with dendritic branches from
numerous neurons, making it possible for a single neuron to pass messages as many
as 50,000 other neurons.
2. Which structural characteristics permit the many possible interconnections
The structure of the dendrites and axons make it possible to have trillions of
interconnections in the brain.
3. How do glial cells differ from neurons? What three functions do they have in
the nervous system?
Neurons are supported in their functions by glial cells (from the Greek word glue).
Glial cells surround neurons and hold them in place. Glial cells also manufacture
nutrient chemicals that neurons need, form the myelin sheath around some axons,
and absorb toxins and waste materials that might damage neurons.
4. What causes the negative resting potential of neurons? When is a neuron
said to be in a state of polarization?
In the salty fluid outside the neurons are positively charged sodium ions (Na+) and
negatively charged chloride ions (CL-). Inside the neuron are large negatively
charged protein molecules (anions or A-) and positively charged potassium ions
(K+). The high concentration of sodium ions in the outside the cell, together with the
negatively charged protein ions inside, results in an uneven distribution of positive and negative compared to the outside. This internal difference (of around -70
millivolts) is called the neuron’s resting potential. At rest the neuron is said to be in
a state of polarization.
5. What chemical changes cause the process of depolarization that created
graded and action potentials? How do these potentials differ?
An action potential is a sudden reversal in the neuron’s membrane voltage, during
which the membrane voltage momentarily moves from -70 millivolts (inside) to +40
millivolts. This shift from negative to positive voltage is called depolarization.
In a resting state, the neuron’s sodium and potassium channels are closed and the
concentration of Na+ ions is 10 times higher outside the neuron than inside it. But
when a neuron is stimulated sufficiently, nearby sodium channels open up.
Attracted by the negative protein ions inside, positively charged sodium ions flood
into the axon creating a state of depolarization. In an instant the interior now
becomes positive in relation to the outside, creating the action potential.
6. What is the nature and importance of the myelin sheath? Which disorder
results from inadequate myelinization?
Many axons that transmit information throughout the brain and spinal cord are
covered by tube-like myelin sheath, a fatty, whitish insulation layer derived from
glial cells during development.
The tragic effects of damage to the myelin coating can be seen in people who suffer
form multiple sclerosis. This progressive disease occurs when the person’s own
immune system attacks the myelin sheath. Damage to the myelin sheath disrupts
the delicate timing of nerve impulses, resulting in jerky uncoordinated movements
and in the final stages paralysis.
7. How do neurotransmitters achieve the processes of excitation and
inhibition of postsynaptic neurons?
The binding of a transmitter molecule to the receptor site produces a chemical
reaction that can have one of two effects on the postsynaptic neuron. In some cases,
the reaction will depolarize (excite) the postsynaptic cell membrane by stimulating
the inflow of sodium or other positively charged ions. Neurotransmitters that create
depolarization are called excitatory transmitters.
8. Describe two methods by which neurotransmitter molecules are
deactivated at the synapse.
9. Describe the role of (a) acetylcholine, (b) dopamine, (c) serotonin, and (d)
endorphins in psychological functions. 10. What are the three major types of neurons? What are the their functions?
The three major types of neurons are the sensory neurons, motor neurons and
Sensory neurons carry input messages form the sense organs to the spinal cord
Motor neurons transmit output impulses from the brain and spinal cord to the
body’s muscles and organs.
Interneurons, which far outnumber sensory and motor neurons, perform
connective or associative functions within the nervous system.
11. Differentiate between the central nervous system and the peripheral
nervous system. What are the two divisions of the peripheral nervous system?
The nervous system can be broken down into several interrelated subsystems. The
two major divisions are the central nervous system, consisting of all the neurons
in the brain and spinal cord, and the peripheral nervous system, composed of all
the neurons that connect the central nervous system with the muscles, glands, and
12. Describe the two divisions of the autonomic nervous system, as well as
their roles in maintaining homeostasis.
The sympathetic nervous system has an activation or arousal function, and it
tends to act as a total unit.
The parasympathetic nervous system slows down body processes and maintains
or returns you to a state of rest.
While the sympathetic system speeds up the heart rate, the parasympathetic system
slows it down. By working together to maintain equilibrium in our internal organs,
the two divisions can maintain homeostasis, a delicately balance or constant
13. How do spinal reflexes occur?
Spinal reflexes are simple stimulus-response sequences that can be triggered at the
level of the spinal cord without any involvement of the brain.
For example, if you touch something hot, sensory receptors in your skin trigger
nerve impulses in sensory nerves that flash into your spinal cord and synapse inside
with interneurons. The interneurons then excite motor neurons that send impulses
to your hand so that it pulls away from the hot object. Other interneurons simultaneously carry the “Hot!” message up the spinal cord to your brain, but it is a
good thing that you don’t have to wait for the brain to tell you what to do in such
emergencies. Getting message to and from the brain takes slightly longer so the
spinal cord reflex system significantly reduces reaction time, and in this case
potential tissue damage.
14. Describe four methods used to study brain-behavior relations.
Neuropsychological tests are used to measure verbal and non-verbal behaviors
that are known to be affected by particular types of brain damage.
Destruction and stimulation techniques, where researchers can produce brain
damage under carefully controlled conditions in which specific nervous tissue is
destroyed with electricity, with co