These ideas can be grouped into three different hypotheses: (i) that spines serve to enhance synaptic connectivity, (ii) that spines are electrical compartments that modify synaptic potentials and (iii) that spines are biochemical compartments that implement input-specific synaptic plasticity. Starting with Cajal’s idea that spines increase the surface area of dendrites ( Ramón y Cajal, 1899), there have been many different proposals that have aimed to explain the specific raison d’être of spines ( Shepherd, 1996). In fact, given the prevalence of spines throughout the brain, one might even go so far as to say that their role is likely to be so prominent that one may not be able to understand the function of brain circuits without solving the spine problem first. So why do excitatory axons choose to contact neurons on spines, rather than on dendritic shafts? Why do neurons make tens of thousands of spines to receive excitatory inputs, when they have plenty of available membrane to accommodate them on their dendritic shafts in the first place ( Braitenberg and Schüz, 1998 Schuz and Dortenmann, 1987)? This is what I define as the “spine problem”: what exactly do spines contribute to the neuron? Spines cannot be an accidental design feature: their large numbers and the fact that they mediate essentially all excitation in many brain regions suggests that they must play a key role in the function of the CNS. What is less appreciated is that, while essentially every spine has a synapse ( Arellano et al., 2007b), the dendritic shaft is normally devoid of excitatory input. Spines cover the dendritic tree of most neurons in the forebrain ( Ramón y Cajal, 1888), and it has been known for over five decades that they receive input from excitatory axons ( Gray, 1959). Reproduced with permission from “Herederos de Santiago Ramón y Cajal.”. Note how axons have straight, vertical trajectories and basal dendrites are well positioned to intercept them. (C) Cajal’s drawing of cellular elements of cerebral cortex. Some axons are also drawn, with straight trajectories. Note how spines protrude to cover the neighboring volume. (B) Cajal drawings of different types of spines. Note how the axonal trajectories are straight. In the background there are some stained axons crossing transversally. The image shows a segment of a dendrite from pyramidal neuron with abundant spines. (A) Photomicrograph of an original Golgi preparation from Cajal. Golgi stains reveal spines and straight axon Spines would endow these circuits with non-saturating, linear integration and input-specific learning rules, which would enable them to function as neural networks, with emergent encoding and processing of information. In this essay, I argue that, when viewed from the perspective of the circuit function, these three functions dovetail with one another to achieve a single overarching goal: to implement a distributed circuit with widespread connectivity. A third possibility is that spines have an electrical role, filtering synaptic potentials and electrically isolating inputs from each other. Another hypothesis is that spines are biochemical compartments that enable input-specific synaptic plasticity. But why do neurons use spines, when they could accommodate excitatory contacts directly on their dendritic shafts? One suggestion is that spines serve to connect with passing axons, and increasing the connectivity of the dendrites. Dendritic spines receive most excitatory connections in pyramidal cells and many other principal neurons.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |