November 16, 2007

Dendrites and Memories

USB memory deviceIn my previous post 'Spiny Dendrites', I discussed about dendrites' role in learning and memory as well as its role in evoking consciousness. Exactly how these dendrites learn and more importantly memorize, has not been clearly understood, but they are being discovered at a rapid pace.

On the dendritic cell membrane, nerve endings from various other neurons converge at the synapse, the gap between those neuronal processes and the neurone on which it is converging upon. The synapses are generally chemically coupled (by neurotransmitters), but they may also be electrically coupled (as in the lateral vestibular nucleus) or both electrical and chemical coupling may be found (conjoint). Chemical coupling is the most common. In this mode of signal transmission, whenever a presynaptic nerve gets an electrical signal (in the form of an action potential), it releases a neurotransmitter (the chemical coupler) into the synaptic cleft. The post-synaptic nerve (the nerve it verges upon, downstream) senses these chemicals (acetylcholine, adrenaline, glutamic acid, GABA etc) by virtue of 'receptors', located mostly in the 'post synaptic densities' (thickened portions of post-synaptic dendritic/neuronal cell membranes, adjacent to that part of the presynaptic membranes releasing the neurotransmitters, i.e. the synaptic vesicles).

Recently, it has been discovered that PSD-95 (post synaptic density-95), a key protein constituent (of postsynaptic densities), on which other proteins are assembled, needed to be phosphorylated, for it to act correctly (as a scaffold, for other proteins to act). Researchers suggested that this (phosphorylation) could help improve cognitive function in schizophrenia, autism and depression. I, personally, think that they will be of use as nootropics too.

In another development in neurobiology, it was shown that memories were burned into the neurons (as you burn 'memories' into a CD using Nero or other programs), and these basic units of memories had to be constantly recycled so that the memories remained there. These memory units, in the form of 'receptors' (of the neurotransmitters; which were located in the postsynaptic neuronal membranes), were in constant motion within the 'fluid' cell membranes. This 'recycling' might explain how the dendritic spines maintain their characteristic identity despite constant molecular turnover. This research might help us fight neurological disorders, such as Alzheimer's disease, schizophrenia, or learning disorders like autism.

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