Diversity of thalamorecipient spine morphology in cat visual cortex and its implication for synaptic plasticity

A feature of spine synapses is the existence of a neck connecting the synapse on the spine head to the dendritic shaft. As with a cable, spine neck resistance (R(neck)) increases with increasing neck length and is inversely proportional to the cross-sectional area of the neck. A synaptic current entering a spine with a high R(neck) will lead to greater local depolarization in the spine head than would a similar input applied to a spine with a lower R(neck). This could make spines with high R(neck) more sensitive to plastic changes since voltage sensitive conductances, such as N-methyl-D-aspartic acid (NMDA) channels can be more easily activated. This hypothesis was tested using serial section electron microscopic reconstructions of thalamocortical spine synapses and spine necks located on spiny stellate cells and corticothalamic cells from area 17 of cats. Thalamic axons and corticothalamic neurons were labeled by injections of the tracer biotinylated dextran amine (BDA) in the dorsal lateral geniculate nucleus (dLGN) of anesthetized cats and spiny stellates were filled intracellularly in vivo with horseradish peroxidase. Twenty-eight labeled spines that formed synapses with dLGN boutons were collected from three spiny stellate and four corticothalamic cells and reconstructed in 3D from serial electron micrographs. Spine length, spine diameter, and the area of the postsynaptic density were measured from the 3D reconstructions and R(neck) of the spine was estimated. No correlation was found between the postsynaptic density size and the estimated spine R(neck). This suggests that forms of plasticity that lead to larger synapses are independent of spine neck resistance.