The human brain is a highly complex organ shaped by hundreds of millions of years of evolution. It has evolved to detect meaningful patterns, to learn, memorize and recall them, and to adapt. Our neural networks can produce and decode communication signals, extract and process useful features from the environment, and produce vital innate behaviours such as eating, fleeing and mating. Amazingly, this specialized structure selfassembles, growing from one cell to tens of billions, and each developing brain incorporates both hidden biases shaped through natural selection, and the means with which to sculpt itself throughout its lifetime as the individual encounters new experiences and sensations.
Our brain contains billions of neurons, which are specialized cells that process and transfer information, and are arranged into complex cellular circuits. These cells communicate via synapses, which are junctions that allow the transfer of chemical or electrical information from one neuron to the next (Fig. 1).
Neurons are the most diverse cell type in the body. They are usually polarized with specialized projections for receiving (dendrites) and relaying (axons) information (Fig. 2). Sensory neurons convert external stimuli, such as light, sound or pressure, into electrical signals, whereas motor neurons use electrical signals to control muscles. A third, more abundant, type of neuron lie between these inputs and outputs.
Non-neuronal cells, called glia, play fundamental roles in the development, support and plasticity of neural circuits; however, neurons and their synapses remain the focus of learning and memory research. Changes in neuronal activity and synaptic strength are thought to underpin learning and memory. Moreover, neuronal loss and synaptic malfunctioning have been implicated in various neurological disorders that involve learning and memory deficits.