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Lecture 4

PSY260H1 Lecture Notes - Lecture 4: Interneuron

Course Code
Martin Ralph

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*Continued in from Lecture Four Part 1*
Recognized that organisms that had simple nervous systems (few neurons) have to
accomplish the same sorts of tasks as more complex organisms but
with less. We (humans) may be a little more exact and precise in
our responses but they are ultimately the same as a sea slugs. At
the basic level we do the same things; we need things and avoid
things for survival. Our nervous system is there to get us what we
need and avoid what will harm us. If we look at aplysia we see an
organism with only a few hundred neurons to control everything.
He recognized that there were different behaviours that can be
exploited in aplysia (different types of responses that could be
elicited) that could be subjected to associative learning conditions.
To put very simply, aplysia don't like to be touched and they respond to when they are
touched. This is usually a defensive type of response and there are numerous defensive
responses all of which tend to be withdrawal type responses. These are subserved by
different parts of the nervous system. Kandel wondered whether or not the very basic
changes in neural transition could be modeled in this relatively simple circuit. The simple
association that they exploited was the gill withdrawal reflex. The gill is an important
organ that extracts oxygen from the water. It hides in the mantle of the organism when it
is attacked or disturbed and emerges when the danger has passed. What they found was
that the sensitivity of the gill is very high (which makes logical sense) and that the
withdrawal response is caused by an unusual touch that is not elicited by the organism
itself (aplysia will not withdraw their gills if they are moving along a rocky surface).
Now, if you keep touching and touching and touching this gill, it will eventually not
withdraw. The associative experiment was to pair the slight touch with a major stimulus
to the tail. A touch to the tail (electrical stimulation) causes withdrawal of the tail and if it
is strong enough it will cause a general contraction. But the main experiment was to to
link this light touch to the siphon with the rather large unconditioned stimulus in the tail.
What they found was that as long as the touch to the siphon preceded the large stimulus
to the tail, the touch to the siphon became associated with that major deleterious tail
stimulation. You could
then elicit a large response
to the gill with a light
This diagram was created
very early on to try and
explain what was going on
in the experiment. Here we
have a shock to the tail that
produces sensory input and
a contraction of the gill,
siphon and tail. The touch

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to the siphon initially produces a response in the siphon and a gill withdrawal but not a
withdrawal of the tail. If you pair the touch of the siphon with the shock to the tail,
eventually a very weak touch to the siphon will cause a withdrawal of the tail. It doesn’t
work well if the touch to the siphon is not temporally paired with the shock to the tail.
There is this interneuron (the facilitator one) that produces a change in the response
characteristics to the stimulation of the siphon. This facilitator interneuron produces a
change in the presynaptic terminal of the sensory neurons of the siphon. The sensory
neurons in the siphon produce a change in the motor neuron causing the gill to withdraw.
The tail shock will cause a gill withdrawal if it stimulates the facilitating interneuron. The
key thing to remember is that something changes when these two stimuli are paired
(shock to tail and shock to siphon). If this sensory neuron (#2) is stimulated and then on
the heels of it you get a second input from the interneuron, then from then on there will
be a change in the responsiveness in the siphon. This action here somehow changes the
biochemistry of the input from the siphon and it lasts a long time.
This length of time is another important feature. In general, changes in the synapse (when
there is stimulation) don’t last long. Synapses are built to reset so information can come
through once again. However if you change things by following stimulation with another
stimulation you get facilitation but even this does not last long. In aplysia however, there
is a relatively long term change in responsiveness to the touch of its siphon. The change
appears to occur in the presynaptic cell. The main effect is in the sensory neuron that
synapses with the motor neuron since this is where the effect of the association is seen.
The input from the tail involves the use of this interneuron which uses serotonin as a
neurotransmitter. Serotonin was discovered to be a neurotransmitter in the 1970s and was
used in one of the first demonstrations in the interruption of associative learning.
Serotonin creates a long term change in the biochemistry of the sensory neuron carrying
information from the cycle. The sensory neuron has action potentials coming down.
Neurotransmitters carry the signal across the synapse to the motor neuron. The relative
efficiency of this transmission can be controlled or changed in a number of different
ways. One being through long term potentiation. In aplysia we find presynaptic
facilitation; there are long term effects felt at the terminal where for a length of time there
is extra neurotransmitter that is released upon each stimulation of this neuron.
This occurs due to the interneuron causing a change in the relative conducts of the ion
channels that bring the signal into the terminal. Changes brought about by serotonin
involve reducing potassium channel conducts and increasing the amount of calcium that
can get into the terminal (if you recall, potassium channels open up and bring the axon
back down into a resting state). This leads to an increased ability to release
Now, the sensory neuron fires when siphon is touched. The action potential comes down,
opens calcium channels, neurotransmitter is released and the gill withdraws. A way to
increase the likelihood and amplitude of the gill withdrawal is to release more
neurotransmitter from the sensory interneuron to the motor neuron. The way to do this is
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