Synaptic density on non-spiny dendrites in the cerebral cortex of the house mouse. A phosphotungstic acid study.

A modification of the phosphotungstic acid method was used to investigate long segments of non-spiny dendrites in the electron microscope. The number of synapses on these dendrites was counted. The density was 1.9 synapses per micron of dendritic length. Taking into account the synapses not contained in the sections, (which are thinner than the dendrites) one gets a real density of 3.3 synapses per micron. This is more than the average density of synapses along spiny dendrites. It demonstrates that spines are not necessary for large numbers of synaptic contacts.