Dendrites and Memories

In 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.

Sleep Tight and Memories Will be Forever

A recent study done by Dr. Mathew Walker (director of the University of California, Berkeley's Sleep and Neuroimaging Laboratory) and colleagues, published in October 22 issue of Current Biology, has found an association between sleep deprivation and emotional instability.

In their study, they divided 26 undergraduates into two groups. While subjects in the test group were kept awake for about 35 hours at a stretch; those in the control group, were allowed to sleep during nighttime during the study. At the end (of 35 hours since the starting of the study), both the groups were shown drawings and figures capable of evoking emotions. The subjects' brains were scanned by fMRI (a real time scan that portrays neural activities on the basis of enhanced metabolism) while they were shown the images. In the sleep deprived, it was found out, that amygdala, a portion of their brain was much more active in comparison to their well slept counterparts. Amygdala, is concerned with the physiological "fight or flight response" (our body's primitive, automatic, inborn response which prepares us for imminent danger) and thus its activation in the sleep starved, made them more emotionally driven and they showed stronger response to the images. The fMRI pattern was more like that of depression and PTSD (post traumatic stress disorder) in the sleep deprived.

Normally, the prefrontal cortex is in charge. This part of the brain is rather new, evolutionarily. This part thinks logically. Amygdala, on the other hand, is primitive and is less logical and more impulse driven. In the sleep deprived state, the amygdala is overdriven and takes the 'steering wheel'. As in the heart, where the SA Node (the normal pacemaker) is taken over by faster pacemakers (ectopic/ abnormal pacemakers which fire at a higher rate than the SA Node); here also the prefrontal cortex is usurped by the amygdala. Amygdala, in turn activates locus coeruleus, a part of the brain that produces Nor-adrenaline, the fight or flight chemical.

The inference: sleep deprivation may lead to psychiatric problems even in healthy subjects, or exacerbate them, in those already suffering from a psychiatric disorder. The study suggests that the brain is re-wired during sleep deprivation. From another angle, it may be construed that while we sleep, the brain goes into a state akin to the computers' restart procedure, where the brain organizes it's own circuitry, when the running programs are cut down to the minimum.

Thus it is necessary to sleep, not only to dream but also to remain sane. The actions of noradrenaline has been beautifully illustrated by Metallica, in their song 'Whiplash' (I don't think he did it for this purpose though!)

"Adrenaline starts to flow
You're thrashing all around
Acting like a maniac
Whiplash"

Lithium: From Here To Eternity

The lightest metal on earth, Lithium, is an alkali metal. It is unique in many aspects. Like all alkali metals, it has a single electron in its outer shell and is highly reactive. It is one of the very few 'primordial elements', that were produced with in minutes after the Big Bang. It has given new life to the patients of manic depressive psychosis (MDP), while giving cosmologists a nightmare.

Australian psychiatrist, Cade has been credited with its pharmacotherapeutic application in MDP (bipolar illness), after he attributed its calming (tranquilizing) effect in his patients, to lithium. (Lithium salts were used for various purposes in the past). Since then, it is used to treat various illnesses including depression, cluster headaches, neutropenia and various other ailments apart from mania. The exact mechanism of its action is yet unknown, though many possible ways through which it may act, has been postulated. It has a narrow therapeutic index, meaning a slight excess of this drug could be fatal, necessitating its frequent blood level estimation in patients.

Cosmologists, on the other hand, were foxed about its discrepancy in its celestial distribution. They seem to find less of it, than they should, based on theoretical predictions. As has already been stated, it is one of the 'primordial elements' which was created in the dawn of creation of the universe. Big Bang Nucleosynthesis (BBN), as this process of synthesis of elements from subatomic particles called, is in practice a lot different from the nucleosynthesis that occurs in our bodies. (Apart from lithium; hydrogen, deuterium and helium were also produced. Almost all other elements we see, or even have in our bodies, were created in the stars.) Recently, Andreas Korn of Uppsala University in Sweden and colleagues have discovered that stars gobble up lithium, which diffuses from its surface into its interior, where it is burnt up. Their discovery also confirmed theoretical cosmologists' predictions about this, long time ago. It also strengthened the BBN. Thus it seems, not only humans, but also stars ingest lithium alike.