Lecture 18 - Learning and Memory 2

Question 1

Model systems for studying the neural basis of learning and memory: links to behaviour:

Answer 1

• LTP is a neural correlate for a change in synaptic strength during learning
• What is the cellular basis of associative conditioning – learn relationships in predicted environments (dogs – ringing bell – started salivating)? This makes us increase our chances of survival
• If a pathway was particularly active, then it would become stronger as it was constantly activated and branching would occur

Question 2

Learning in Invertebrates:

Answer 2

• Caenorhabditis elegans (nematode worm (CNS - 302 neurons))
• Drosophila melanogaster (fruit flies)
• Learning mutants
  1. Involvement of cAMP signalling cascade
  2.

e.g. dunce shows deficits in olfactory associative conditioning: cAMP phosphodiesterase mutation.

Question 3

Aplysia californica – the sea hare:

Answer 3

Gill withdrawal reflex

• Shows habituation, sensitisation, classical conditioning
• Highly influential in the nervous system
• Heavily folded gills that has oxygen pass through it

Question 4

Aplysia abdominal ganglion:

Answer 4

• Control movements of the gill and pick up sensory information
• Accessible brains easily detected
• Nerve cells are the brown structure
• Sensory neurons pick up information from the siphon and the motor neurons pick them out at the MN
• Response in the gill which is the effector

Question 5

Stimulus – response relationship:

Answer 5

mild stimulus so contraction of the gill to protect itself

• can measure how big the response is by measuring the total area of the gill before and after the response is given
• the smaller the gill, the greater the response

Question 6

Habituation of gill withdrawal response:

Answer 6

• Non-associative learning
• Habituation is when you get the same stimulus over and over that you stop responding and it becomes the norm
• 100% response to begin with
• gradually response gets smaller

Question 7

Synaptic depression at sensory neuron-motorneuron synapse:

Answer 7

• pre-synaptic neuron is constantly stimulated to produce and action potential (EPSP) in motor neuron
• became smaller and smaller in the post-synaptic leading to habituation responses

Question 8

Habituation of gill withdrawal involves synaptic depression – a form of synaptic plasticity:

Answer 8

• Synaptic efficacy is reduced with repeated use - due to a reduction in Ca2+ influx per action potential and hence reduced neurotransmitter release
• Learning = change in synaptic function (monosynaptic)

Question 9

Sensitisation of gill withdrawal response: (adaptive form of learning)

Answer 9

Non-associative learning

• Can re-stimulate by adding a different stimulus, in this case, an electric shock
• Shows that sensitisation can be short term or long term
• Long term changes require long term stimulation (lots of it) to allow synthesis process to occur

Question 10

Sensitisation of gill withdrawal involves presynaptic facilitation – a form of synaptic plasticity:

Answer 10

• Single shock to the tail creates another pathway by 3 synaptic arrangements onto the sensory neuron
• Connections back to the tail – heterosynpatic – as there’s 2 synapses
• This can modify the activity (green affects brown)

Question 11

Short-term:

Answer 11

5HT which binds to receptors that activates the G-proteins which allows the intracellular cAMP to protein kinases to stimulate a response to the motor neurons by opening and closing sodium channels

• this causes repolarisation
• 5HT applied, longer action potentials in the sensory neuron
• leads to increased neurotransmitter release which leads to greater action by the motor neurons

Question 12

Long Term:

Answer 12

• shock the tail a few times and the synapses becomes sensitised and strengthened
• involves gene expression and protein synthesis
• enzyme – ubiquitin hydrolase is produced which causes protein kinase A that causes the persistent release of the neurotransmitter and the voltage-gated K+ channels remain open – broadening of the action potentials
• cAMP enzymes also led to structural changes
• Using EM, the area of the synaptic transmission was increased
• Number of sites between a sensory and motor neuron was also increased and actually became stronger structurally and functionally
• Doesn’t mirror LTP – the one above is all in the pre-synaptic neuron

Question 13

Sensitisation of the gill withdrawal response involves:

Answer 13

1. Increased neurotransmitter release at sensory neuron motoneuron synapse (functional plasticity)
2. Activation of cAMP signalling cascade, leading over the long term to changes in gene expression
3. Increased number and area of active synaptic zones (structural plasticity)

Question 14

Classical (associative) conditioning of the withdrawal response:

Answer 14

• Involves similar cellular/molecular processes to sensitisation
• Tail shock (US) → gill/siphon withdrawal (R)
• CS is weak tactile stimulation of siphon
• Graph shows response to CS alone
• Underlie what makes animal behaviour so adaptive
• Learn to associate events to occur – time
• Graph shows that in the experimental group (2 stimuli were paired in time), the response to the weak conditioning stimulus was greatly enhanced after training
• Weak stimulus associated with the electric shock which was painful
• Memory would gradually stay for days
• When unconditioned stimulus was given alone then sensitisation occurred

Question 15

Classical conditioning of gill withdrawal response:

Answer 15

¥ When CS and US are paired, there is greater activation of adenylyl cyclase in the presynaptic terminal than with either stimulus by itself
¥ 5HT and normal stimulus arrives at the same time, you get more of a response as cAMP is enhanced in its activity
¥ This is because the CS action potential admits Ca2+ into the presynaptic terminal
¥ The Ca2+ (by interacting with a protein called calmodulin) increases the response of adenylyl cyclase to G-proteins

Question 16

In Aplysia, a learned change in behaviour can be linked directly to changes in neural pathways at cellular and molecular levels:

Answer 16

• The adenylyl cyclase enzyme is the molecular site at which an association between CS and US is made