Many Types of Neurons
Going by the kinds of animations common in science fiction movies, many people apparently think of the inside of the brain as a network of effectively identical cells, randomly placed in a sponge-like layout, connecting simply to their nearest neighbours. Nothing could be further from the truth. We have known since the time of Ramon y Cajal and Golgi that there are many distinct types of neurons, which are distributed in a highly organised fashion in different brain regions and interconnected with exquisite specificity. And when I say many, I mean many hundreds, possibly thousands of types.
The retina alone has over 60 distinct, recognised neuronal cell types and more subtypes are being defined all the time. Those 60 cell types are arranged in four or five distinct layers, with multiple subtypes in each layer. There are at least a dozen parallel pathways across these several layers, processing various aspects of the visual stimulus – colour, form, direction, motion and many others. If you want to understand how the retina works – to reverse engineer it – you need to know what the functions of these cell types are within the context of the circuit in which they are embedded.
The importance of cell types as functional classes is blindingly obvious for the retina, but the same principle applies to any area of the brain. Subsets of cells in any area not only have discrete jobs to do within that area, making unique contributions to the computations carried out there, they also often connect in distinct, cell-type-specific, parallel circuits with other brain areas.
In the cerebral cortex, different excitatory cell types are arranged into six obvious layers, but these often have several sublayers. And within each layer, there are multiple subtypes of excitatory neuron, intermingled. In layer 5, for example, some neurons project across the corpus callosum to the other hemisphere, some within the cortex on their own side and others to subcortical targets. Each of these types contains multiple subclasses carrying information to distinct targets. That cellular complexity is multiplied by the number of cortical areas – subcortically-projecting layer 5 neurons in motor cortex are molecularly distinct from those in visual cortex, for example.
And we haven’t even started on the interneurons. These are smaller, more locally projecting cells, which are inhibitory – they put the brakes on excitation in neural circuits. They not only prevent runaway excitation, but also, crucially, control many aspects of information processing, such as filtering, gain control and temporal and spatial integration. In addition, they orchestrate the synchronous firing of ensembles of excitatory neurons, which in turn is a central mechanism in mediating communication between brain areas. Just in the hippocampus, there are twenty-some subtypes of interneurons already known, and, again, more are being defined all the time. Each of these subtypes is distributed in a particular manner, expresses different kinds of ion channels and neurotransmitter and neuromodulator receptors and makes specific kinds of synapses on specific subcellular locations of specific target cells.
Quoted on Mon Sep 2nd, 2013