Sizing the synaptic cleft

Unlike the transistors in a computer chip, the neurons in our brains are not directly connected to each other. A roughly 20-nanometer gap, the synaptic cleft, separates a neuron’s output terminal from the input terminal of the next neuron in line. Signals cross the cleft as bursts of molecular ions called neurotransmitters. Once across, the neurotransmitters attach to receptors and trigger excitatory or inhibitory responses. Neurotransmitters come in several varieties, as do receptors, and the lateral area covered by a cleft varies several-fold from neuron to neuron. Yet the cleft height remains close to 20 nm. To find out why, Dmitri Rusakov of University College London and Leonid Savtchenko of Dnepropetrovsk National University in Ukraine developed a physical model. Narrowing the gap speeds transmission, as one would expect, but it also increases the electrical resistance of the intracleft medium. For a wide range of parameters, such as cleft area, diffusion coefficient, and number of receptors, the model yields an optimum cleft height of 15–20 nm, which matches the natural value. Evolution doesn’t always produce the best designs, but given the importance of transmitting information inside our brains, it’s not surprising that cleft height should be optimized. (L. P. Savtchenko, D. A. Rusakov, Proc. Natl. Acad. Sci. USA 104 , 1823, 2007.)