Dendritic spines by Eduard Korkotian Weizmann Institute of Science, Israel.
Randy Gallistel and Adam King in their book Memory and the Computational Brain: Why Cognitive Science Will Transform Neuroscience, claim that addressable memory architecture is necessary to explain complex animal behaviour such as food caching by Scrub Jays or even the human capacity to recollect and reconsider prior beliefs.
Their view is contrasted with non-addressable architecture in contemporary neuroscience. Traditional neural networks suppose that computations in neural tissue are implemented by relaying action potentials between neurons. Gallistel and King argue that the implementation must be sought elsewhere. They offer two neurobiological suggestions of where to look, 1) subcellular, e.g. dendritic spines and 2) molecular, something like re-writable DNA & RNA.
I'm not going to give an in-depth analysis of the philosophical issues surrounding their claims, but I thought dendritic spines were worth further consideration.
Recent studies show that dendritic spines are dynamic structures. Their rapid creation, destruction and shape-changing are essential for short- and long-term plasticity at excitatory synapses on pyramidal neurons in the cerebral cortex. The onset of long-term potentiation, spine-volume growth and an increase in receptor trafficking are coincident, enabling a ‘functional readout’ of spine structure that links the age, size, strength and lifetime of a synapse. Spine dynamics are also implicated in long-term memory and cognition: intrinsic fluctuations in volume can explain synapse maintenance over long periods, and rapid, activity-triggered plasticity can relate directly to cognitive processes. Thus, spine dynamics are cellular phenomena with important implications for cognition and memory. Furthermore, impaired spine dynamics can cause psychiatric and neurodevelopmental disorders [e.g. autism, retardation]...
... Discovered in the 19th century and intensely scrutinized in the 20th century, dendritic spines are found in higher animals and some insects. Spines exist only on certain types of neurons, including pyramidal neurons in the cortex, medium spiny neurons in the basal ganglia and Purkinje cells in the cerebellum. Spines are more abundant in higher brain regions and highly variable in shape. Moreover, dendritic spines are the most actin-rich structures in the brain, and their morphology and density are abnormal in several mental disorders...
...Unfortunately, many studies of these correlates reduce every neuron to its action potential. This presents an incomplete picture of cognitive function, and indeed the brain is more than its ions. Recent experiments using optogenetic tools suggest that mechanisms other than spikes can participate in the creation of internal representations. In this section, we attempt to identify connections between the cognitive and synaptic neurosciences to suggest a new synaptic basis for cognitive function. [E.g. attention, computational speed and memory processing. In addition these spines may help solve the binding problem or even possibly the neural correlates of consciousness]...
...The rapid, responsive movement of synapses shares many features with cognition. Dendritic spines can take part directly in cognitive processes to make them more individual, active and stochastic—unlike a computer, in which memory elements obey simple and deterministic rules. Thus, cognitive processes can be easier to understand when we take account of the spine structural dynamics.
Kasai, H., Fukuda, M., Watanabe, S., Hayashi-Takagi, A. & Noguchi, J. (2010) Structural dynamics of dendritic spines in memory and cognition. Trends in Neurosciences. 33(3). 121-129.
I am experiencing my first taste of the computational theory of dendritic spines. I am intrigued. There's quite a nice graphic on this New York Times article
I'm still reading through the book. More thoughts as I prepare my poster for the SPP 2010