 Habituation and sensitization are forms of non-associative learning which lead to short-term memory. Let's try to understand what are they and what are the physiological mechanisms underlying these forms of learning. First of all as I said that they are forms of non-associative learning that is in habituation and sensitization there is no association between two different stimuli which are causing the response. In habituation basically if a harmless or neutral stimulus is given repeatedly there will be a decrease in response to the stimulus. See suppose a touch stimulus initially causes a withdrawal response but if the touch stimulus is given repeatedly then it no longer elicits a response. In data real life you can understand it by one example where when you wear some cloth you feel the cloth on yourself but later on you even don't realize that the cloth is actually touching the skin. So behaviorally if we see this is a required response because somewhere a decision is being made that it is a neutral stimulus and hence irrelevant for survival. So unnecessary our resources should not be directed for responding to irrelevant stimulus and that's why habituation is actually the most common form of learning. Now what's the physiological basis of this learning? Well the studies have been done in a species aplasia. So when its siphon is touched once there is a gill withdrawal. However when the siphon is touched repeatedly that is with a harmless stimulus slowly the gill withdrawal response decreases and finally there is no withdrawal of the gill. So why is this occurring? Well this decrease in response occurs due to inactivation of calcium channels in presynaptic neuron that is the neuron which is carrying the information about the stimulus. And if calcium channels are inactivated what will happen? There will be decreased entry of calcium into the presynaptic neuron causing a decrease in intracellular calcium and you know that calcium is responsible for the release of the neurotransmitter. So decrease intracellular calcium will cause decrease in the release of the neurotransmitters. So less neurotransmitter will act on postsynaptic neuron and hence decrease in voltage change in postsynaptic neuron will occur. So if a voltage changer that is the greater potential is less then the number of action potentials which are generated will also be less or if the voltage change doesn't reach the threshold then there will be no action potential at all. So there will be no response at all. Fine so that was habituation. Coming to next type of non-associative learning that is sensitization. In sensitization if a harmful or noxious stimulus is given it elicits a response. See the difference here in habituation we were talking about harmless and non-noxious stimulus which is irrelevant to the species. In sensitization we are talking about harmful or noxious stimulus and this harmful stimulus will elicit a response. Then for some times after harmful stimulus what happens that even a harmless neutral stimulus which is irrelevant for the individual that will also elicit a strong response getting it. So it's like suppose there is a very loud sound suddenly and you get a startled then even a small sound for some time will elicit a response from you. So that is known as sensitization but just take note here even we are talking that there is a harmless stimuli which is going to elicit a response. Here we are not talking about the pairing of the stimulus right. They are two different stimulus. They are not being given together or after some time interval some limited time interval between them as happens in conditioned learning. So there is no association between the stimuli. Fine in short there is increase in responsiveness to mild or non-noxious stimulus following noxious stimulus. Again what is the physiological basis of this let's try to see. So as we saw in habituation for sensitization also the experiments had been done in applicious species. So what was done is that if a noxious stimulus is given to the tail the neuron from here connects via interneuron to the motor neuron causing gill withdrawal. Now this interneuron also makes contact with the touch sensory neuron presynaptically which we saw in habituation. So this is making contact with the touch sensory neuron as well. So what happens that there is a response that is gill withdrawal due to this pathway but it causes some changes in other pathway the touch sensory pathway as well. Basically it releases a neurotransmitter serotonin which acts on this touch sensory neuron presynaptically and it actually increases CMP in this neuron intracellularly. So increase in CMP what it does is that it blocks potassium channels. Due to this when subsequent touch stimulus is given what will happen there will be action potential it will reach to this area right the presynaptic terminal but you see the potassium channels have been blocked. So the action potential which is there will be delayed repolarization of this action potential because you know that potassium influx is important for repolarization. So if potassium channels are blocked there will be delayed repolarization and there will be increase in the duration of the action potential. So because of this increase action potential duration there is increased opening of voltage gated calcium channels and there is increase in calcium entry and hence increased release of neurotransmitter by this touch sensory neuron. So further you can guess that if there is increased neurotransmitter release there will be more change of voltage on the post synaptic neuron and hence there will be more generation of action potential and more response. Fine so that was about habituation and sensitization. Habituation decrease in response to non-noxious stimulus and sensitization increase in response to a non-noxious stimulus following a noxious stimulus. Fine now before we end just something to remember here that they are non-associative forms of learning as I already told and both of them are causing changes only in presynaptic neuron right we saw the mechanism it is the changes only in presynaptic neurons which are happening not in post synaptic neuron and these changes are only transient thus it is responsible for only short term memory. So remember habituation and sensitization cause the formation of short term memory. 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