Synaptic Plasticity and Learning

Synaptic plasticity is the ability of synapses to strengthen or weaken over time in response to activity. It is widely regarded as the cellular basis of learning and memory: when patterns of neural activity persistently change the strength of connections, the network stores information. This topic surveys the principal forms of activity-dependent plasticity and their relationship to learning.

Definition

Synaptic plasticity is the activity-dependent, persistent change in the strength of synaptic transmission — including long-term potentiation and long-term depression — proposed as a cellular substrate for learning and memory.

Scope

The topic covers long-term potentiation and long-term depression, the Hebbian and spike-timing-dependent rules that govern them, the molecular mechanisms involving glutamate receptors and signalling cascades, and the link between synaptic change and behavioural learning and memory. It is a reference survey of mechanisms and does not provide clinical guidance.

Core questions

• How do synapses change their strength in response to patterns of activity?
• What molecular mechanisms underlie long-term potentiation and long-term depression?
• How is the timing of pre- and postsynaptic activity translated into synaptic change?
• How does synaptic plasticity relate to behavioural learning and memory?

Key concepts

• Long-term potentiation (LTP)
• Long-term depression (LTD)
• Hebbian learning rule
• Spike-timing-dependent plasticity
• NMDA and AMPA receptors
• Calcium-dependent signalling cascades

Key theories

Hebbian plasticity

Hebb proposed that when one neuron repeatedly takes part in firing another, the connection between them strengthens — often paraphrased as 'cells that fire together wire together' — providing the conceptual rule for associative synaptic change.

Long-term potentiation as a memory model

Persistent, activity-induced strengthening of hippocampal synapses (LTP), together with its counterpart long-term depression (LTD), is proposed as a synaptic model for how memories are stored.

Mechanisms

At many excitatory synapses, the strength of transmission is adjusted by activity. In the canonical hippocampal pathway, coincident presynaptic release and postsynaptic depolarisation relieve the magnesium block of NMDA receptors, allowing calcium entry that triggers signalling cascades and the insertion or removal of AMPA receptors, producing long-term potentiation or depression, as reviewed by Bliss and Collingridge and by Malenka and Bear. The precise relative timing of pre- and postsynaptic spikes can determine the direction of change, a relationship formalised as spike-timing-dependent plasticity. Kandel's work linked such synaptic changes to gene expression and the long-term storage of memory.

Clinical relevance

Synaptic plasticity provides a conceptual framework for understanding learning, memory, and recovery of function after injury, and is relevant to conditions involving memory and cognition. The entry is educational and describes mechanisms; it is not a basis for diagnosis or treatment.

Evidence & guidelines

The topic rests on experimental neurophysiology and molecular neuroscience rather than clinical guidelines, drawing on the discovery and characterisation of LTP and LTD and on syntheses linking synaptic change to memory.

History

The idea that learning reflects changes in synaptic connections was articulated by Donald Hebb in 1949. The experimental demonstration of long-term potentiation in the hippocampus and its later molecular dissection gave the idea a concrete cellular form, and the discovery of long-term depression and spike-timing-dependent plasticity broadened the account of how activity reshapes synaptic strength.

Debates

Is LTP sufficient to explain memory?

Long-term potentiation is a leading candidate mechanism for memory storage, but the extent to which it fully accounts for learning and memory, as opposed to being one of several contributing processes, remains an active question.

Key figures

• Donald Hebb
• Timothy Bliss
• Graham Collingridge
• Eric Kandel
• Robert Malenka

Seminal works

• hebb-1949
• bliss-collingridge-1993
• kandel-2001
• malenka-bear-2004

Frequently asked questions

What is long-term potentiation?

Long-term potentiation (LTP) is a persistent strengthening of synaptic transmission that follows certain patterns of activity; it is one of the most studied candidate mechanisms for how the brain stores information.

How is synaptic plasticity related to learning?

Activity-dependent changes in synaptic strength are thought to encode experience: as patterns of activity strengthen or weaken specific connections, the network's responses change, which is widely regarded as a cellular basis of learning and memory.