How can we identify individual neurons in a crowd?

How to build a brain

Scientists are far from making a complete map of the billions of connections in the brain, let alone understanding their function. Identifying and labelling all these networks is improving our knowledge of how we recognise a loved one’s face, read a book or kick a ball. In theory this will make it easier to repair the brain as we decipher the components and wires responsible for different systems. Thinking of the brain as a (very powerful!) computer is a useful analogy but it has some critical limitations:

Firstly, each individual neuron is not a passive component such as a resistor or a diode but is itself a miniature computer. Neurons may receive hundreds or thousands of inputs through their dendrites, they combine all this information to send out a signal along their axon to in turn contact anything from one to thousands of target cells (see Anatomy of a neuron). Maybe the brain is more like the internet?

Secondly, even if we could identify all the individual components and work out what they do, how would we direct the re-wiring? This problem underlies our area of developmental biology – the field of 'axon guidance'.
The 7 November 2013 issue of Nature (vol. 503 no.7474) has special articles on the brain and the issues relating to understanding such complex circuitry. I particularly like the description of mapping the human brain on page 152: 'it is as difficult as creating a 3D map of a city 10,000 times the size of Tokyo, London or New York, and locating every inhabitant in every building, street, stairwell, lift and subway...the mapping would then have to unearth all lines of communication - personal interactions as well as phone, post and e-mail - between all inhabitants'.

See also: