PSYB64 Chapter 12
*excluded pages 348 351 + 358 361 + 366 368
The behavior of organisms can be separated into three major categories: reflexes,
instincts, and learned behaviors
Reflexes are involuntary responses to stimuli. These behaviors are produced by
prewired neural connections or reflex arcs.
Reflexes have the advantage of producing rapid, reliable responses, but their
inflexibility can be a disadvantage when the environment changes.
Like reflexes, instincts are automatic, but their resulting behaviors are more complex.
Most instinctive behaviors involve mating or parenting behavior
Ex: courtship display of the male peacock - the identification of an appropriate
female partner initiates a chain of predictable, stereotyped behaviors
Although somewhat modifiable by experience, instinctive behaviors are consistent
enough to be referred to as fixed action patterns.
Learning: a relatively permanent change in behavior (or the capacity for behavior)
due to experience, provides organisms with the most flexible means for responding to
Human beings occupy nearly every niche on the planet, from blazing equatorial
environments to the frigid Arctic. Much of this adaptability stems from the
remarkable human capacity for learned behavior.
only those behavioral changes that result from experience will be considered
learned. This specification excludes changes in behavior that occur due to
maturation or growth.
Types of Learning
Learning occurs in one of two ways.
Associative learning occurs when an organism forms a connection between two
features of its environment.
Ex: Classical conditioning, which allows organisms to learn about signals that
predict important events
Nonassociative learning a type of learning that involves a change in the
magnitude of responses to stimuli, rather than the formation of connections
between elements or events
Ex: habituation and sensitization
Habituation and Sensitization:
Habituation: a type of learning in which the response to a repeated, harmless
stimulus becomes progressively weaker
Sensitization: type of learning in which the experience of one stimulus heightens the
response to subsequent stimulus
Ex: Right after an earthquake, people will be quick to react to any signs of
movement or noise.
Using Invertebrates to Study Learning
Invertebrates are not only capable of learning, but their large-celled, simple, and,
hence, easily observed nervous systems also make them ideal subjects
researchers have relied on the sea slug, Aplysia californica
Touching the animals siphon reliably produces a protective response known as the
gill-withdrawal reflex, in which the gill is retracted The gill-withdrawal reflex will eventually habituate - when the siphon is touched
repeatedly, the gill-withdrawal reflex will gradually diminish.
invertebrates such as Aplysia have neural nets as opposed to brains. Within these
neural nets, ganglia, or collections of cell bodies, serve as major processing centers.
The siphon is served by 24 touch receptors whose cell bodies are located in the
animals abdominal ganglion.
In the Aplysia abdominal ganglion, the touch receptors form synapses with a
number of excitatory and inhibitory interneurons as well as with the six motor
neurons serving the gill.
Habituation in Aplysia:
It was thought that with repeated stimulation, the sensory neurons serving the siphon
might become less responsive. However, this possibility was discarded after recordings
from the sensory neurons demonstrated steady, ongoing activity in response to touch,
even after the gill-withdrawal reflex had become very weak
Another possibility was a reduction in the gill muscles ability to react in response to
input from the motor neurons. This explanation was ruled out when electrical
stimulation of the motor neurons reliably produced muscle contraction, even after
habituation had occurred.
Final alternative: Repeated touching of the siphon might produce changes at synapses
between the sensory neurons of the siphon and motor neurons that serve the gill
Kandel successfully demonstrated that repeated touching of the siphon reduced the
size of excitatory postsynaptic potentials in both the interneurons and the motor
a smaller amount of input to the motor neurons resulted in diminished activity
between the motor neurons and gill muscles, which in turn produced a weak
The repeated stimulation depletes the amount of available neurotransmitter in the
presynaptic sensory neuron, producing short-term habituation lasting from minutes
to several hours
Depletion of available neurotransmitter is unlikely to be the cause of any longer-
lasting habituation. Instead, long-term habituation probably depends on
postsynaptic processes involving the NMDA glutamate receptor
NMDA glutamate receptor has special qualities that allow it to participate in the
structural changes that accompany learning. Chemicals that block glutamate
receptors effectively prevent the development of long-term habituation
Sensitization in Aplysia
Habituation in Aplysia occurs in a single pathway connecting sensory input from the
siphon to neurons controlling the movement of the gill. In sensitization, however, a
stimulus gains the ability to influence more than one neural pathway
After Aplysia is sensitized by the administration of an electric shock to the head or tail,
touching the siphon results in an enhanced gill-withdrawal response:
Shocking the animals tail stimulates sensory neurons, which form excitatory
synapses with a group of interneurons. These interneurons, in turn, form synapses
with the sensory neurons serving the siphon.
The synapses between the interneurons and sensory neurons are axo-axonic in
form. In other words, the axon from the interneuron forms a facilitating synapse
at the axon terminal of the sensory neuron
The interneurons release serotonin at these axo-axonic synapses. When receptors
on the sensory axon terminal bind molecules of serotonin, a metabotropic process
closes potassium channels With the closing of the potassium channels, action potentials reaching the sensory
axon terminal last longer than they would in a typical response to a siphon touch.
(Recall from Chapter 3 that the opening of potassium channels is responsible for
the repolarization of the cell during an action potential. Delaying repolarization
extends the duration of the action potential)
Longer action potentials produce a greater influx of calcium into the sensory
neuron, which in turn results in the release of larger amounts of neurotransmitter
by the sensory axon terminal. The increased release of neurotransmitter produces a
stronger response by the motor neurons and the gill muscles, leading to the
stronger gill-withdrawal reflex that we observe in sensitization.
As a result of repeated exposure to habituation or sensitization, changes occur in the
number of presynaptic terminals of sensory neurons
the animals that had undergone sensitization training showed the highest numbers
of terminals, 2,800, compared with 1,300 for the control animals and only 800 in
the animals that had undergone habituation training.
In sensitized animals, the dendrites of the motor neurons were also modified to
accommodate the increased number of presynaptic elements.
These structural changes appear to involve actin, a protein that makes up the
microfilaments of the cytoskeleton
Sensitization also involves postsynaptic changes as well
Sensitization involves an increase in the numbers of another type of glutamate
receptor, the AMPA receptor
Classical Conditioning (associative learning) in Aplysia
a slight touch of the mantle shelf serves as the conditioned stimulus (CS + ), and an