Turn up the volume!

Learning can be defined as “an adaptive change in behaviour resulting from experience”, and memory is the retention of learning. The ability of the brain to change in response to an experience is referred to as ‘neuronal plasticity’. So what are the plastic changes that occur in the brain when we learn something? Coming back to our analogy of a computer, where the hippocampus is the processor and the cortex is the hard drive, we need to consider how the hippocampus encodes information that can then be stored in the cortex. We know that our billions of neurons are connected by a million billion synapses, forming complex networks.

If the number of neurons in the adult brain does not change significantly over our lifetime, how is information encoded? It’s the changes in electrical activity of neurons and the way they communicate with each other that encodes patterns of information. At a gross level we know that learning a lot of information can increase the number of synapses. For instance a neuroimaging study showed that London taxi drivers who learn the road map of London, have a bigger hippocampus compared to an average person. In addition to new connections being formed, learning can alter the properties of existing synapses to encode information.

It is known that learning can increase both the amount of neurotransmitter released from the presynaptic cell and increase the number of neurotransmitter receptors in the post-synaptic cell. This means that for the same stimulation the synapse will communicate information more efficiently. You can imagine that following learning, a synapse that was whispering, starts to shout. So learning can ‘turn up the volume’ of our synaptic connections. So what is special about a learning stimulus compared to the normal communication between neurons? Where is the specificity? Are synapses constantly changing and turning up their volume? Well, it turns out that learning stimuli are very strong. For example, many synaptic connections with the same neuron will fire together, and repeatedly.

These learning stimuli trigger molecular mechanisms within neurons to change the synapses. For the past thirty years, molecular neuroscience research has been dominated by the study of these changes. If neuroscientists can better understand how synapses change when we learn, this may help to develop drugs that improve learning in people that have neurological disorders or neurodegeneration.