Synaptic plasticity

Synaptic plasticity refers to the changes that occur over time (in response to activity or experience) at the synapse, resulting in either the strengthening or weakening of synaptic connections over time in response to activity or experience.

Synaptic plasticity can occur at two levels:

o Synaptic level changes (long-term potentiation and long-term depression) – the strength of connections between neurons can change

o Structural level changes (sprouting, rerouting and pruning) – the physical structure of neurons can change

SYNAPTIC LEVEL CHANGES are the modifications/change at the synapse that influence the strength of connections between neurons.

Synaptic changes include:

• Long-term potentiation

• Long-term-depression

Long-term potentiation and long-term depression work together to enable learning and memory.

LONG TERM POTENTIATION (LTP) – synaptic changes

the long lasting, experience dependent strengthening of synaptic connections, which occurs due to repeated, high intensity stimulation at the synapse.

• Long-term potentiation occurs due to the frequent and repeated co-activation of the pre-synaptic and post-synaptic neuron during learning and memory.

LTP results in:

• An increase in receptor sites on the postsynaptic neuron

• More neurochemical being released (specifically glutamate)

• Dendrites becoming ‘bushier’ due to sprouting

• More synaptic connections due to sprouting

• Long-term potentiation results in a higher excitation in the post-synaptic neuron.

• Long-term potentiation causes changes on the pre-synaptic and post-synaptic neurons.

“Neurons that fire together wire together”

The more the connection is activated, the more the connection is strengthened.

The more the connection is strengthened, the more the relevant neural pathway is consolidated.

This ultimately increases the efficiency in transferring information along the pathway, and decreases the likelihood that what has been learnt, will be forgotten.

LONG TERM DEPRESSION (LTD) – synaptic changes

to the long-lasting, experience-dependent weakening of synaptic connections, which occurs due to repeated, low-intensity stimulation at the synapse.

• Long-term depression occurs due to the infrequent co-activation of the pre-synaptic and post-synaptic neuron.

LTD results in:

• A decrease in receptor sites on the postsynaptic neuron

• Less neurochemical is released (specifically glutamate)

• Dendrites being pruned

• Fewer synaptic connections

• Long-term depression results in a lower excitation in the post-synaptic neuron.

• Long-term depression causes changes on the pre-synaptic and post-synaptic neurons.

“use it or loose it”

LONG TERM POTENTIATION AND LONG TERM DEPRESSION WORK TOGETHER

LTP and LTD don’t occur independently, they occur at the same time.

During learning and memory:

o LTP will strengthen the connections for the new skill being learned and form memory traces of this skill.

o At the same time, LTD will weaken connections for an old skill or any errors made during the learning process, and any memories that are no longer required

STRUCTURAL LEVEL CHANGES – synaptic plasticity

Adjustments/changes to the overall physical structure of neurons in response to activity and experience, which ultimately influences the synaptic connections between neurons.

Structural changes include:

• Sprouting (via long-term potentiation)

• Rerouting (via long-term potentiation and long-term depression)

• Pruning (via long-term depression)

SPROUTING – structural changes

Dendrites and axon terminals develop new extensions (via long term potentiation) which can form new synapses

• Bushier dendrites

• Expanded axon terminal branching

REROUTING – structural changes

New connections are made between neurons to create alternate neural pathways. This may involve the existing synaptic connections and/or new connections from the sprouts

LTP and LTD contributions to rerouting:

• Long-term potentiation strengthening neurons connections that frequently fire together, reinforcing new pathways.

• Long-term depression weakens connections between neurons that infrequently fire together, prompting dendrites and axon terminals to migrate and form synapses with more active neurons.

Rerouting may involve a neuron abandoning its current synaptic connection and forming a new synaptic connection with another neuron.

PRUINING – structural changes

Weak, ineffective or unused synapses are eliminated (via long term depression) which enhances brain efficiency

LEARNING AND MEMORY

LEARNING

refers to a relatively permanent change in behaviour due to experience.

MEMORY

refers to an active information processing system that encodes, stores and retrieves information when needed.

Learning and memory are closely related concepts as:

o If information is not learnt, then it cannot be remembered.

o If information cannot be remembered, learning cannot occur.