By Dillon Lim - Medicine Student @ Brasenose College, Oxford
Glial cells are non-neuronal cells present in the nervous system, once thought to be no more than ‘glue’ – cells that served a purely structural function. The fact that the glia:neuron ratio is probably around 1:1 pushed neuroscientists to re-evaluate the role that they play – you don’t usually use glue in a 1:1 ratio with your substrate!
There are three types of glial cells in the CNS: astrocytes, oligodendrocytes and microglia. In the PNS, astrocytes are replaced by satellite glial cells and oligodendrocytes by Schwann cells – for simplicity, we will just examine the functions of the CNS glia, since the PNS equivalents perform largely similar functions.
Astrocytes account for roughly 60% of central glial cells and mediate diverse functions. They extend processes to neuronal synapses, forming a “tripartite” connection. This connection puts the astrocytes in a position to be able to monitor transmission at synapses, and since they can be plugged into dozens of synapses at a time, potentially compare the relative neuronal activity at different synapses. Astrocytes also are often tightly associated with each other, sometimes forming a continuous network of cells with direct connections between cytoplasm of adjacent cells (a syncytium). This allows them to share chemical signals with each other. Given these physical relationships, astrocytes are well-placed to send neuromodulatory signals, affecting synapses from the timescale of minutes to days and longer.
Typically, when we talk about neurotransmitters at synapses at a Sixth Form level, we talk about their action on postsynaptic neurons, followed by their inactivation or transport back into presynaptic neurons. It is clear, however, that glia have an important role to play in the recycling of neurotransmitter, and actually respond to neurotransmitters themselves. One of the interesting ways this is studied is with the use of Ca2+-sensitive dyes, since many intracellular signals involve the release of Ca2+ within cells. These dyes fluoresce when binding Ca2+, so can be used to visualised the spread of