Dendritic spines of rat cerebellar Purkinje cells: serial electron microscopy with reference to their biophysical characteristics

Abstract

We have used serial electron microscopy and 3-dimensional reconstructions of dendritic spines from Purkinje spiny branchlets of normal adult rats to evaluate 2 questions about the relationship of spine geometry to synaptic efficacy. First, do relationships between spine geometry and other anatomical indicators of synaptic activity suggest that spine size and shape might be associated with synaptic efficacy? Reconstructed spines were graphically edited into head and neck compartments; the area of the postsynaptic density (PSD) was measured; the volume of spine smooth endoplasmic reticulum (SER) was computed; and all of the vesicles in the axonal varicosities were counted. Spine head volume and the volume of SER contained in the head are well correlated with the area of the PSD and the number of vesicles in the presynaptic axonal varicosity. Spine neck diameter does not fluctuate with PSD area, head volume, or the vesicle number. These results suggest that the dimensions of the spine head, but not of the spine neck, are likely to reflect differences in synaptic efficacy. Second, does the geometry of cerebellar spine necks reduce the transfer of synaptic charge to the recipient dendrite from the theoretical maximum that could be transferred if the synapse were on a dendritic shaft? Comparison of volume to surface area showed that the spine heads are approximately spherical and the necks are approximately cylindrical. Application of results from a biophysical model that assumed these geometrical shapes for spines (Wilson, 1984) showed that the cerebellar spine necks are unlikely to reduce transfer of synaptic charge by more than 5-20% even if their SER were to completely block passage of current through the portion of the neck that it occupies. We suggest that the constricted spine neck diameter might serve to isolate metabolic events in the vicinity of activated synapses by reducing diffusion to neighboring synapses, without significantly influencing the transfer of synaptic charge to the postsynaptic dendrite.

Impact of single-site axonal GABAergic synaptic events on cerebellar interneuron activity

“…It has been shown previously that there is a positive correlation between the size of the active zone and that of the postsynaptic density (Schikorski and Stevens, 1997;Harris and Stevens, 1988); likewise, the size of the postsynaptic density determines the quantal size: a larger postsynaptic area allows a higher number of receptors (Auger and Marty, 1997;Nusser et al, 1997). Moreover, our recent evidence suggests that the size of the active zone is positively correlated with the RRP in presynaptic specializations Pulido et al, 2015).…”

Organization of spines on the dendrites of Purkinje cells

“…5G). These estimates are in agreement with the range of spine densities on the distal dendrites of Purkinje cells measured from small regions in electron micrographs: 6-14 per m for the mormyrid fish (16), 7 per m for the mouse (12), and 5-17 per m for the rat (13,14,19). The lower figures in some of our examples most likely reflect the fact that we have selected preferentially the thinner dendrites, which have smaller numbers of spines per unit length.…”

“…3D reconstructions have been made from electron micrographs of small regions of this network (12)(13)(14)(15), but the long-range organization has not been characterized or described in terms of underlying rules. The present study examines the long-range organization of spines on Purkinje dendrites, with a view to understanding better how a specific dendritic architecture is used to facilitate appropriate synaptic interactions.…”

Persistent Structural Modifications in Nucleus Accumbens and Prefrontal Cortex Neurons Produced by Previous Experience with Amphetamine

“…Very little is known about branched spines (Harris and Stevens, 1988;Chicurel and Harris, 1992), but an increase in branched spines has been reported in dentate granule cells after long-term potentiation (Trommald et al, 1990) or kindling produced by medial perforant path stimulation (Geinisman et al, 1989) and in the dorsolateral striatum after rearing in a complex environment (Comery et al, 1996). Trommald et al (1990) reported that after long-term potentiation both branches of a branched spine were always associated with normal-appearing presynaptic active zones, and in all cases each branch was innervated by a different axon.…”