Non Associative Learning In Aplysia

(iii) Non-Associative Procedural Learning in the Aplysia

(a) Habituation

Invertebrates can be particularly useful for the analysis of the neuronal basis of behaviour. The sea slug, Aplysia californica has a nervous system comprising about 20 000 neurones, has been used by Eric Kandel and his colleagues to study learning and memory. Non-associative learning in Aplypia involves habituation and sensitisation in the gill-withdrawal reflex. A jet of water squirted on the siphon causes the gill to retract ( fig. 32 ).

Figure 32 Anatomy of the gill withdrawal reflex in the Aplysia2.

The gill withdrawal reflex involves neurons within the abdominal ganglion. Sensory information from the siphon skin travels along the siphon nerve until it reaches

(iv) Classical Conditioning in the Aplysia

Associative learning can also occur in Aplysia. Classical conditioning of the Pavlovian type can be applied to the gill-withdrawal reflex. A gentle water jet to the siphon is the CS. A shock to the tail is the UCS. Conditioned Response ( CR ) is withdrawal of the gill. It was discovered that if the UCS was paired with the magnitude of CS, the subsequent CR was much greater than could be accounted for by sensitisation.

Conditioning occurred only if the CS preceded the UCS by about 0.5 sec. Now, let us examine the critical modifications that occur between the sensory and motor neurone that results in increased neurotransmitter release. Firstly, we need to focus on what happens at the synapse during the CS-UCS pairing. At the cellular level the CS is represented by the arrival of an action potential in the sensory axon terminal and the UCS is represented by the release of serotonin by L 29 ( fig. 35 ).

Figure 35 Classical conditioning in the