2.

Mechanisms of Learning and Memory

A. Habituation 1. Involves depression of synaptic transmission. 2. Behavior: mild tactile stimulation of gill or siphon causes withdrawal of the gill. Repeated stimulation leads to habituation.

2.

Mechanisms of Learning and Memory sensory neuron

THE BASIC CIRCUIT:

+ +

interneuron

siphon skin Head

Motor neuron

Mantle shelf

Gill

Siphon Parapodium

Gill

1

2. A.

Mechanisms of Learning and Memory (cont’d) Habituation (cont’d)

What happens? – Sensory neurons in the siphon skin are stimulated →fire action potentials. – EPSPs are generated in the postsynaptic interneurons and motor neurons. – EPSPs summate → motor neurons fire → gill is withdrawn.

2. A.

Mechanisms of Learning and Memory (cont’d) Habituation (cont’d)

What happens? (cont’d) – If stimulus is repeated, the synaptic transmission between sensory neurons and motor neurons, as well as between interneurons and motor neurons, is reduced → the ability of these neurons to dump glutamate (i.e., NT) is changed. – The mechanism is not yet understood (vesicle mobilization?).

2

2.

Mechanisms of Learning and Memory (cont’d)

A.

Habituation (cont’d) Motor neuron

5mV

Sensory neuron

20mV

Control •

Habituated

Also, the number of synaptic connections goes down with habitation (very cool) S M

M

M

S

S

Control

2.

Habituation

Sensitization

Mechanisms of Learning and Memory sensory neuron

+ +

interneuron

siphon skin Head

Motor neuron

Mantle shelf

Gill

Siphon Parapodium

Gill

3

2.

Mechanisms of Learning and Memory

B. Sensitization 1. Involves enhancement of synaptic transmission. 2. Involves facilitating interneurons.

2.

Mechanisms of Learning and Memory B. SENSITZATION S Tail

F

facilitating interneuron

S

interneuron I

siphon M

sensory neurons

Motor neuron

Gill

4

2.

Mechanisms of Learning and Memory

B. Sensitization What happens? – Sensitization of the gill withdrawal response is produced by stimulating the tail. – The sensitizing stimulus at the tail activates a group of facilitating interneurons.

2.

Mechanisms of Learning and Memory

B. Sensitization What happens? (cont’d) – These facilitating interneurons form synapses on sensory neurons innervating the skin and enhance transmitter release (serotonin: 5HT) from the sensory neurons by presynaptic facilitation. – Long-term sensitization training is thought to lead to gene activation, and eventually, to an increase in synapses.

5

2.

Mechanisms of Learning and Memory B. SENSITZATION S Tail

F

facilitating interneuron

S

interneuron I

siphon M

sensory neurons

Motor neuron

Gill

2.

Mechanisms of Learning and Memory

C. Classical Conditioning 1. Involves an associative enhancement of presynaptic facilitation that is activitydependent. 2. The CS must precede the US by 0.5 sec for classical conditioning to work.

6

2.

Mechanisms of Learning and Memory

C. Classical Conditioning What happens?

2.

–

A CS to the mantle is paired with a US to the tail and / or siphon.

–

The gill is retracted.

–

BUT: shocking (or siphon) the tail excites facilitating interneurons.

Mechanisms of Learning and Memory

C. Classical Conditioning What happens? (cont’d) –

These facilitating interneurons synapse on the presynaptic terminals of the sensory neurons innervating the mantle shelf and the siphon (like sensitization).

–

BUT when the mantle is touched first, the mantle pathway gets activated.

7

2.

Mechanisms of Learning and Memory

C. Classical Conditioning What happens? (cont’d)

2.

–

Activity in the mantle sensory neuron PRIMES it so that it is more responsive to the facilitating interneuron.

–

If the US comes before the CS, it does not work.

Mechanisms of Learning and Memory (cont’d)

C.

Classical Conditioning (cont’d) usually no response

mantle

tickle mantle (CS)

S

S

CS+

US

F

M

tail I

Stimulate tail and siphon (US)

gill S

siphon

8

2.

Mechanisms of Learning and Memory (cont’d)

C.

Classical Conditioning (cont’d) mantle

Tickle the mantle (CS)

1

S

S

CS+

US

M

F

tail I

Stimulate tail and siphon (US)

2

gill S

siphon

WHY?

2.

Mechanisms of Learning and Memory (cont’d)

C.

Classical Conditioning (cont’d) PSP

mantle S

S

CS

US+

Tail (US)

F

M

I

gill S

Siphon (US) Before conditioning, the UC alone produces a PSP

9

2.

Mechanisms of Learning and Memory (cont’d)

C.

..but the CS pathway does not produce a psp

Classical Conditioning (cont’d) mantle

tickle mantle (CS)

(no psp)

S

S

CS+

US

M

F

tail I

Stimulate tail and siphon (US)

2.

gill S

siphon

Mechanisms of Learning and Memory (cont’d)

SO PAIR THE STIMULI: CS FIRST: Tickling the mantle turns the circuit on

1

mantle 2

S

S

CS+

US+

F

M

tail I

gill

2 S

siphon US SECOND: Stimulating the tail (and siphon*) activates facilitator interneurons to “turn up the volume” on the CS circuit

(* Note: that siphon FI’s are not shown for clarity)

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2.

Mechanisms of Learning and Memory (cont’d)

C.

Classical Conditioning (cont’d) Conditioned response becomes established

CR

mantle S

S

CS+

US

F

M

tail I

gill S

siphon

2.

Mechanisms of Learning and Memory (cont’d) A.

Classical Conditioning (cont’d) mantle (CS)

0.5 sec

tail (US)

CS must come FIRST because mantle pathway must be activated for facilitation to affect it

RESPONSE: 5mV

Motor neuron

30mV

Sensory neuron

If not within .5 sec there will be no change in the motor neuron’s response.

11