Habituation and Sensitization
what characterizes animal models of the neurological basis of habituation?
(most of the research): assumes general processes and contiguity between humans and other animals
what is habituation a tool for?
- A tool for understanding perception in non- verbal organisms (like babies) - Ex: baby looks at a blue screen and shows a response → after a few trials, they respond less and less → when the screen turns to red, the baby starts responding more to the color (because they see it as a new color)
what characterizes action potentials and how they relate to habituation?
- Action potentials work like a spring- faucet, when it reaches the synaptic terminal (the junction between one neuron and another), it allows the release of neurotransmitters. The neurotransmitter works to initiate (or prevent the initiation) of an action potential in the target neuron - Motor neurons that activates muscle has less action potentials (habituation is occurring presynaptically between the sensory neuron and the motor neuron → less calcium uptake in the terminal less neurotransmitters being released to the motor neuron) - The sensitivity of the motor neuron to neurotransmitter is unchanged The action potentials in the sensory neuron are unchanged... found that less neurotransmitter is being released at the synapse
what does serotonin being released from the tail neuron result in?
- Closed potassium channels (prolongs the depolarization) → - Longer depolarization (more calcium gets to enter) → - More calcium entry (more neurotransmitter gets released) → - More neurotransmitter release → - Stronger effect on the motor neuron (to the gill)
how do calcium levels change in habituation vs. sensitization?
- Habituation: less - Sensitization: more
how does calcium relate to the habituation of the aplysia?
- In a normal functioning axonal terminal, less calcium comes in and less neurotransmitter is sent out in habituation - After the calcium comes in, the neurotransmitter is pushed out: in short term habituation, less calcium comes in so less neurotransmitter is released
non- associative vs. associative learning
- Non associative (an examples of this is a sound → orienting response) - Associative learning: sound → appetitive response (a response that is not innate and needs to be learned, whereas the response in non associative learning is innate)
what is the normal steps of the gill withdrawal response vs. what happens after habituation?
- Normal response: touching siphon → excites sensory neuron → sensory neuron releases neurotransmitter glutamate into synapse → if there is enough neurotransmitter released, this excites motor neuron → motor neuron fires and activates the withdrawal response - After habituation: sensory neuron releases less glutamate → reduces the chance that a motor neuron will fire; decrease in neurotransmitter release is associated with a decrease in the number of vesicles containing glutamate → synaptic depression
where are the possible sites of plasticity/ learning in aplysia in sensitization (THAT ARE WRONG)?
- Not the siphon being extra sensitive from the shock, - not the sensory neuron from the siphon not sending the signal, - not the motor neuron not sending signals, -not the gill muscle level getting tired
what are the levels of plasticity and habituation?
- Plasticity: neuron level, - habituation: behavioral level
how is sensitization measured in aplysia?
- Shock the aplysia on the tail → touch the siphon → gill withdrawal response strengthens - The stimulus is irregular (won't know when it's going to be shocked) - Usually short term, after a week the aplysia won't remember being shocked - Any strong or noxious stimuli causes a stronger response of the gill withdrawal reflex: Shock the tail, touch the siphon → gill withdrawal response increases
what happens in the aplysia following habituation?
- The action potentials in the sensory neuron are unchanged - Less neurotransmitter is being released at the synapse - The sensitivity of the motor neuron to neurotransmitter is unchanged - Motor neuron that activates muscle sends less action potentials - Muscle retracts less - With short term habituation: there is a decrease in the amount of calcium influx into the terminals of the sensory neurons
how do habituation and sensitization relate to the sympathetic and parasympathetic nervous systems and how does this relate to survival?
- The interplay of habituation vs. sensitization activates the parasympathetic or the sympathetic systems (this is very important from an evolutionary/ adaptive standpoint) - Sympathetic: fight or flight; important for short term survival (seeing a dangerous stimulus and running away) - Parasympathetic: resting phases: important for long term survival (rest and digestion)
how does a third neuron relate to sensitization in aplysia?
- There is no change in the sensitivity of the motor neuron, or in the sensitivity of the sensory neuron. -However a third neuron synapses on the presynaptic terminal. This causes a change in the K+ (potassium) channels, increasing the length of each action potential and thus the amount of calcium (CA2+) that enters the terminal and the amount of transmitter released
what is the aplysia and why is it good for research?
- a marine snail that oozes along the ocean floor eating seaweed, it has a siphon for expelling seawater and waste → when the siphon is touched, the adjoining gill withdraws under a protective mantle - advantages: 1. Approximately 20,000 neurons (far fewer than what humans have) 2. Large (the neurons are really big, bigger than human neurons) 3. Stereotypic: same across many different aplysia 4. Simple model system
what happens to the number of connections in long term habituation and sensitization?
- habituation: less connections - sensitization: more connections - Long term changes in habituation and sensitization: a change in the number and morphology/ structure of the synapse, which allows more/ less neurotransmitter to be released. - Habituation was specific to the stimulus; sensitization was related to general arousal, in terms of neuronal circuitry
where does plasticity happen?
- happening at the synaptic junction - Happening between the presynaptic neuron (sensory) and postsynaptic neuron (motor) - Not happening in the periphery (skin, muscle, sensory neuron, motor neuron: it's happening in the synapse)
what are the steps of sensitization in aplysia?
- shock to tail, which activates sensory neuron; in addition to activating motor neuron, it also activates interneurons which releases serotonin; this increases the number of glutamate vesicles that are available to release glutamate from the sensory neuron → activates motor neuron → activates gill withdrawal - Sensory neuron releases more neurotransmitter, increasing the chance that motor neurons will fire (stronger response) → stronger gill withdrawal response
what is sensitization?
- the increased responsiveness of an organism to repeated stimulation - When the prior delivery of an aversive stimulus leads to the increased responsiveness of an organism to stimulation by a previously neutral stimulus
what are the other possible mechanisms of habitation in aplysia (THAT ARE WRONG)?
- the siphon is possibly damaged, - the gill muscle gets tired, - the motor neuron is not sending signals, - the sensory neuron is not sending a signal to the motor neuron
dual process theory
- the theory that habituation and sensitization are independent of each other but operate in parallel - Sensitization and habituation processes occur in response to every stimulus, so the summed combination of the 2 independent processes determines the strength of responding
what are the properties of sensitization?
1. Following a stimuli of high intensity 2. When the stimulus is irregular (can't predict it) 3. Is a general arousal of the system (the whole body/ system responds, in opposition to habituation where the arousal is a general area like your right shoulder) 4. Short- term (but can be long term) → ex: PTSD
what are the properties of habituation? (4)
1. Rhythmic (ex: clock) 2. Inconsequential stimuli (not something that is affecting you directly) 3. Both short and long term 4. Specific to stimulus
what are the steps regarding sensitization in aplysia?
1. The tail is shocked, and an action potential runs down the tail's sensory neuron (this is different from the siphon's sensory neuron) 2. Serotonin is released from the tail's sensory neuron, and the neurotransmitter binds to the presynaptic sensory neuron from the siphon 3. This activates the protein kinase A (PKA) signaling cascade 4. Some potassium channels from the presynaptic sensory neuron (siphon) are blocked 5. If you physically touch the siphon, a normal action potential will run down that pre synaptic neuron 6. More neurotransmitter (?) enters the pre- synaptic terminal 7. Due to the tail shock, unlike normal conditions, some potassium (K+) channels are still blocked, which prolongs the positive environment inside the pre- synaptic neuron 8. If the action potential produces a longer period of depolarization in the pre- synaptic neuron, more neurotransmitter will be released to the post- synaptic motor neuron 9. Producing a stronger gill withdrawal response
where is the site of plasticity/ learning in aplysia in sensitization?
Happening at the synapse between the sensory neuron and the motor neuron : because there is extra serotonin, more neurotransmitter is sent to the motor neuron → stronger gill withdrawal response
what are the mechanisms for long term sensitization and habituation?
Long term (hours/ days): change in structure (size/ number of synapses) - Habituation: less - Sensitization: more
what are mechanisms for short term sensitization and habituation?
Short term (seconds/ minutes): pre- synaptic modification of transmitter release (amount of calcium coming in and the amount of neurotransmitter being released) - For habituation: less calcium → less neurotransmitter - Sensitization: more calcium → more neurotransmitter
what is the mechanism of habitation in the aplysia?
The synaptic level between the sensory neuron and the motor neuron; The synapse between the terminal of the sensory neuron and the dendrites of the motor neuron
how do we measure habituation in aplysia?
Touch siphon over and over again and measure gill withdrawal response
skin conductance response (SCR)
a change in the skin's electrical conductivity associated with emotions such as anxiety, fear, or surprise
what happens to calcium in short term habituation?
a decrease in the amount of calcium influx into the terminals of the sensory neurons
habituation
a decrease in the strength or occurrence of a behavior after repeated exposure to the stimulus that produces the behavior
acoustic startle reflex
a defensive response (such as jumping or freezing) to a startling stimulus (such as a loud noise)
sensitization
a phenomenon in which a salient stimulus (such as an electric shock) temporarily increases the strength of responses to other stimuli
synaptic depression
a reduction in synaptic transmission; a possible neural mechanism underlying habituation
Dishabituation
a renewal of a response, previously habituated, that occurs when the organism is presented with a novel stimulus
orienting response
an organism's innate reaction to a novel stimulus
what is the difference between sensitization and habituation?
increase in response to an aversive stimulus, dishabituation is when an animal has been habituated, but then it starts to respond more but not due to sensitization
heterosynaptic
occuring in several nearby synapses simultaneously (sensitization)
homosynaptic
occurring in one synapse without affecting nearby synapses (habituation)
spontaneous recovery
reappearance (or increase in strength) of a previously habituated response after a short period of no stimulus presentation
what is habituation?
the reduced responsiveness of an organism to repeated stimulation (non- associative)