Changes in Synapses That Happen During Classical Conditioning

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QUESTION 1

Describe in detail, the simple changes in synapses that happen during classical conditioning.

Discuss the extent to which all forms of learning can be explained by these simple synaptic synaptic changes.

The brain’s ability to learn, to change in response to experience and to store/retrieve learning through memory it is a fascinating process fundamental to one’s existence. The first scientific study of animal learning demonstrated a form of associative learning - classical conditioning; it can be described as a process of learning where a neutral stimulus (e.g. bell) is paired with an unconditional stimulus (e.g. food) and as a consequence, the neutral stimulus becomes conditioned and comes to elicit the same response (e.g. salivation) as the unconditional stimulus even when presented alone (Murphy & Naish, 2006). It has been proposed that “…classical conditioning…is quite easy to explain on the basis of simple changes in synapses.” In order to assess the merit of this claim, it is necessary to describe the simple changes that occur in synapses during classical conditioning. All forms of learning require some synaptic change, however it isn’t clear whether these can always be explained by the same kind of synaptic changes that happen in classical conditioning (Murphy & Naish, 2006). Some forms of learning will be explored in terms of synaptic changes.

At a neurobiological level, learning is “created” by the interconnectedness between neurons (synapses). Hebb proposed that if the postsynaptic neuron fired while the presynaptic terminal was releasing neurotransmitter (NT), the presynaptic neuron would be more likely to influence the postsynaptic neuron on subsequent occasions, i.e. when previously unassociated neurons fire simultaneously on repeated occasions, new links are formed which increase synaptic efficiency (Hebbian learning). Hebbian learning explains Pavlov’s associative learning - classical conditioning. Pavlov carried out experiments with dogs and noted their salivation reflex in response to food presentation (unconditioned response), later he repeatedly paired the presentation of food (unconditioned stimulus) with the ringing of a bell (neutral stimulus) and finally he sounded the bell (conditional stimulus) without presenting the food and that alone triggered salivation (conditioned response) (Murphy & Naish, 2006). Repeated activity in two neurons simultaneously (e.g. bell, food) strengthens the synapses and eventually activity in one of the two neurons alone will produce activity in the other because new effective links are formed by the repeated and simultaneous firing, creating an auto-associated pattern (bell and food repeatedly presented together lead to a conditioned learning response).

The conditioning of an eye-blink to a buzzer is another example that can be supported by Hebb’s proposal of changes in synaptic efficacy. Activation of the neutral (presynaptic) neuron that fires to the sound of a buzzer at the same time that a puff of air occurs (unconditioned neuron) causes an eye-blinking response (blinking neuron). At first, the connection between the neutral neuron and the postsynaptic neuron is very weak, so the release of NT by the neutral neuron is unlikely to trigger firing in the postsynaptic neuron. However, the unconditioned (presynaptic) neuron has an efficient synapse with the postsynaptic neuron i.e. this presynaptic neuron causes the postsynaptic neuron to fire thus producing a blinking response. If both presynaptic neurons are repeatedly activated at the same time that the air-puff occurs, an excitatory postsynaptic potential is elicited in the blinking neuron and the previously weak synapse will become stronger to the point that, this synapse alone can trigger firing in the postsynaptic neuron that responds to the air-puff and causes eye-blinking (Murphy & Naish, 2006). Classical conditioning was originally interpreted as an automatic response linking a new...
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