What do synapses exhibit?
Many forms of plasticity
Due to molecular and structural changes
Changes may represent more permanent changes in brain function
Contribute to learning and memory in
How is the nervous system changed by a specific learning task?
- Exploit the relative simplicity of CNS in some invertebrates
What did Eric Kendal et al use Aplysia Californica to study?
- Elemental forms of behavioural and synaptic plasticity
What does Aplysia exhibit?
Several forms of plasticity
E.g. applying a tactile stimulus to siphon of Aplysia results in withdrawal of animal gills
What does repeated siphon stimulation cause?
- Gill withdrawal to weaken
Animal becomes less response to repeated occurrence to a stimulus (e.g. habituation)
What form of plasticity does GWS exhibit?
Sensitisation
Aplysia that have habituated to siphon touching
What elicits a strong gill withdrawal?
A strong electrical stimulus to the tail paired with a light touch of the siphon
What does sensitisation allow an animal to generalise?
- Aversive response elicited by a noxious stimuli to a variety of non-noxious stimuli
Aplysia that receives a single stimulus to tail?
GWR remains enhanced over an hour
How can behaviour be altered for days/weeks?
Repeated pairing of tail and siphon stimuli
Demonstrate simple form of long term memory
What gives prolonged sensitisation of GWR?
Repeated tail shock for several days
How does CNS make it possible to define a synaptic circuit involved in gill withdrawal?
- The small number of neurons in CNS
2. Monitor activities of inidividual neurons and synapses in these circuits
How can the cell bodies of many neurons involved in gill withdrawal be recognised?
- Size
- Shape
- Position within abdominal Ganglia
What are short term sensitisation be mediated by?
- Signal transduction cascade that involve neurotransmitter, second messengers , one or more protein kinase and ion channels
What does the signal transduction cascade enhance?
- Synaptic transmission between sensory and motor neuron within gill-withdrawal circuit
What is thought to underlie long-term sensitisation?
Serotonin induced enhancement of glutamate release that mediate short term sensitisation
During long-term sensitisation how long is the circuit affected by?
- Several weeks
What is prolonged plasticity evidently due to?
- Gene expression
2. Protein synthesis
How is the transcriptional activated CREB activated?
Repeated training (additional tail shocks)
Serotonin activates PKA involved in short term-sensitization
What does CREB binding to CAMP response element CRE increase?
- The rate of transcription downstream genes
What stimulates the degradation of regulatory subunit of PKA?
- Ubiquitous hydroxylase
What is the molecular mechanism of short term sensitization?
- Serotonin released by the facilitatory interneuron bund to G-protein coupled serotonin receptors on pre-synaptic terminal of siphon sensory neuron activating the G protein
- The activated G-protein dissociates from the receptor and binds to and activates other signalling molecule such as - adenylyl cyclase
- Adenylyl Cyclase stimulates the production of second messengers CAMP from ATP
- The CAMP bindings to PKA
- Liberating catalytic subunit of PKA can phosphorylate several proteins which include potassium channels
- When a sensory neuron synapse is depolarised, calcium channel open and calcium ions enter terminals
- Phosphorylation of potassium channels by PKa results in fewer open potassium ion channel - prolongs the duration of de-polarisation and increasing influx of calcium
- The enhanced influx of calcium - more neurotransmitter being released
Eat accounts for gill withdrawal during habituation and sensitisation?
- Only a few different types of neurons
What do the critical neurons include?
- The sensory neuron that innervate siphon
- Motor neuron that in innervate muscles in the gill
- Interneurons that receive input from a variety of sensory neurons
