First Hull (1943) gave a definition of learning. According to it, learning is the strengthening of existing responses or formation of new responses to existing stimuli that occurs because of practice or repetition. Memory is about keeping “knowledge” someplace and then retrieving it when it is needed (Sargent & Stafford, 1965).

Hebb (1949) proposed a model for cellular mechanism of learning long-lasting activity-dependent changes in synaptic efficacy. According to Hebb's rule the neuronal mechanism of learning is the following: “When an axon of cell A excites cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells so that A’s efficiency as one of the cells firing B is increased.”


TYPES OF MEMORY
Declarative/explicit:

• knowledge of facts (places, things and people)
• must be recalled into consciousness
• patients with bilateral medial temporal lobe lesions have an inability to learn and remember items of factual knowledge

Nondeclarative/implicit

• involves information about how to perform something (reflexive motor or perceptual skills)
• recalled unconsciously
• subtypes:

1. non-associative (Aplysia, a sea slug is used to study the neuronal mechanism)
   
   
   • habituation:
     decrease in response to a benign stimulus when the stimulus is presented repeatedly (e.g. a dog will be aroused when a strange tone is played, but if the tone is played over and over, the dog will eventually no longer be aroused by the tone)
     
     
   • sensitization:
     enhanced response to many different stimuli after experiencing an intense or noxious one (e.g. an animal responds more vigorously to a tone of lesser intensity once a painfully loud tone has been played)
     Note: habituated response can be restored by sensitization (dishabituation), e.g. habituated response to a noise can berestored by strongly pinching the skin.
     
     
2. associative (a response is associated with a given event or has a given consequence)
   
   
   • classical conditioning (Pavlov) :
     repeatedly ringing a bell just before (critical interval of time =0.5 sec) presenting the meat to the dog, the animal come to associate the bell (conditioned stimulus) with the presentation of the meat powder, and it would begin to salivate (conditioned response) when the bell is rung.
     Note: After a while, if no meat is presented, the bell will stop predicting the presentation of meat and the dog ceases salivating when it is rung (extinction)
     
     
   • operant conditioning (trial-and-error learning):
     a person or animal learns that it gets a reward if it does something (e.g. a pigeon learns that it gets food if it pecks at a certain key, but not if it pecks at another; a rat learns that it can avoid getting an electric shock if it presses a bar at a certain time)

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APLYSIA
Aplysia, a sea slug is used to study the neuronal mechanism of non-associative learning.

Habituation
If the siphon of the animal is stimulated mechanically the animal withdraws the gill for protection. The stimulus activates receptors in the siphon which activate the motoneuron (directly or through interneuron) that withdraws the gill. With repeated activation, the stimulus leads to a decrease in the number of dopamine containing vesicles that release their contents onto the motoneuron. No change in the sensitivity of postsynaptic NMDA or non-NMDA receptors. It is presumed that habituation in vertebrates occurs by a similar process.

Sensitization
In sensitization, a stimulus to one pathway enhances reflex strength in another. The stimulation of the siphon leads the animal to withdraw the gill by activating sensory neuron 1, which in turn activates a motoneuron. If the tail of the animal is stimulated just before the siphon is, then the withdrawal of the gill is quicker and more forceful. The mechanism of this appears to involve serotoninergic, axo-axonic synapses. The activation of the sensory receptors in the tail activates a facilitating interneuron (through sensory neuron 2) that excites sensory neuron 1 at the cell body or at the terminals on the motoneuron or the interneuron in the pathway leading the gill withdrawal.
Mechanism: Serotonin (5-hydroxytryptamine or 5HT) is released by the presynaptic axon onto the postsynaptic axon where it binds to receptors and activates a G protein that, in turn, activates adenylyl cyclase to produce cAMP. This cAMP activates a cAMP-dependent protein kinase, PKA. Along with another kinase, PKC, PKA phosphorylates and closes K channels (hypopolarizing the cell), mobilizes vesicles for exocytosis and opens Ca channels. The end result is more transmitter substance to be released from sensory neuron and larger EPSP in the motoneuron leading to a larger response by the gill. With only short-term stimulation, the sensitization or the habituation will fairly quickly disappear when the stimulation ceases. The sensitization or habituation can be made relatively permanent by repeated stimulation causing structural changes of the presynaptic terminals (increase or decrease in the number of synaptic terminals in both sensory and motoneurons) requiring altered protein synthesis.

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