Sunday, September 6, 2009

First Image Of Memories Being Made

The ability to learn and to establish new memories is essential to our daily existence and identity; enabling us to navigate through the world. A new study by researchers at the Montreal Neurological Institute and Hospital (The Neuro), McGill University and University of California, Los Angeles has captured an image for the first time of a mechanism, specifically protein translation, which underlies long-term memory formation.The finding provides the first visual evidence that when a new memory is formed new proteins are made locally at the synapse - the connection between nerve cells - increasing the strength of the synaptic connection and reinforcing the memory. The study published in Science, is important for understanding how memory traces are created and the ability to monitor it in real time will allow a detailed understanding of how memories are formed.

The increase in green fluorescence represents the imaging of local translation at synapses during long-term synaptic plasticity. (Credit: Science)

When considering what might be going on in the brain at a molecular level two essential properties of memory need to be taken into account. First, because a lot of information needs to be maintained over a long time there has to be some degree of stability. Second, to allow for learning and adaptation the system also needs to be highly flexible.

For this reason, research has focused on synapses which are the main site of exchange and storage in the brain. They form a vast but also constantly fluctuating network of connections whose ability to change and adapt, called synaptic plasticity, may be the fundamental basis of learning and memory.

"But, if this network is constantly changing, the question is how do memories stay put, how are they formed? It has been known for some time that an important step in long-term memory formation is "translation", or the production, of new proteins locally at the synapse, strengthening the synaptic connection in the reinforcement of a memory, which until now has never been imaged," says Dr. Wayne Sossin, neuroscientist at The Neuro and co-investigator in the study. "Using a translational reporter, a fluorescent protein that can be easily detected and tracked, we directly visualized the increased local translation, or protein synthesis, during memory formation. Importantly, this translation was synapse-specific and it required activation of the post-synaptic cell, showing that this step required cooperation between the pre and post-synaptic compartments, the parts of the two neurons that meet at the synapse. Thus highly regulated local translation occurs at synapses during long-term plasticity and requires trans-synaptic signals."

Long-term memory and synaptic plasticity require changes in gene expression and yet can occur in a synapse-specific manner. This study provides evidence that a mechanism that mediates this gene expression during neuronal plasticity involves regulated translation of localized mRNA at stimulated synapses. These findings are instrumental in establishing the molecular processes involved in long-term memory formation and provide insight into diseases involving memory impairment.

This study was funded by the National Institutes of Health, the WM Keck Foundation and the Canadian Institutes of Health Research.

Short- And Long-term Memories Require Same Gene But In Different Circuits

Why is it that you can instantly recall your own phone number but have to struggle with your mental Rolodex to remember a new number you heard a few moments ago? The two tasks "feel" different because they involve two different types of memory – long-term and short-term, respectively – that are stored very differently in the brain. The same appears to be true across the animal kingdom, even in insects such as the fruit fly.

Two different types of memory -- long-term and short-term -- are stored very differently in the human brain. The same appears to be true across the animal kingdom, even in insects such as the fruit fly. (Credit: iStockphoto/Mads Abildgaard)

Assistant Professor Josh Dubnau, Ph.D., of Cold Spring Harbor Laboratory (CSHL) and his team have uncovered an important molecular and cellular basis of this difference using the fruit fly as a model. The results of their study appear in the August 25 issue of Current Biology.

The CSHL team has found that when fruit flies learn a task, each of two different groups of neurons that are part of the center of learning and memory in the fly brain simultaneously forms its own unique memory signal or trace. Both types of trace, the team discovered, depend on the activity of a gene called rutabaga, of which humans also have a similar version. A rapidly occurring, short-lived trace in a group of neurons that make up a structure called the "gamma" (γ) lobe produces a short-term memory. A slower, long-lived trace in the "alpha-beta" (αβ) lobe fixes a long-term memory.

A tale of two lobes

Neuroscientists call the rutabaga gene a coincidence detector because it codes for an enzyme whose activity levels get a big boost when a fly perceives two stimuli that it has to learn to associate with one another. This enzymatic activity in turn signals to other genes critical for learning and memory.

A classic experiment that teaches flies to associate stimuli – and one that the CSHL team used – is to place them in a training tube attached to an electric grid, and to administer shocks through the grid right after a certain odor is piped into the tube. Flies with normal rutabaga genes learn to associate the odor with the shock and if given a choice, buzz away from the grid. But flies that carry a mutated version of rutabaga in their brains lack both short- and long-term memory, don't learn the association, and so fail to avoid the shocks.

The team has now found, however, that this total memory deficit does not occur when flies carry the mutated version in either the γ or in the αβ lobes. Flies in which normal rutabaga function was restored within the γ lobe alone regained short-term memory but not long-term memory. Restoring the gene's function in the αβ lobe alone restored long-term memory, but not short-term memory.

Long- and short-term memory involve different circuits

"This ability to independently restore either short- or long-term memory depending on where rutabaga is expressed supports the idea that there are different anatomical and circuit requirements for different stages of memory," Dubnau explains. It also challenges a previously held notion that neurons that form short-term memory are also involved in storing long-term memory.

Previous biochemical studies have suggested that rapid, short-lived signals characteristic of short-term memory cause unstable changes in a neuron's connectivity that are then stabilized by slower, long-lasting signals that help establish long-term memory in the same neuron. But anatomy studies have long hinted at different circuits. Surgical lesions that destroy different parts of an animal's brain can separately disrupt the two kinds of memory, suggesting that the two memory types might involve different neuronal populations.

"We've now used genetics as a finer scalpel than surgery to reconcile these findings," Dubnau says. His team's results suggest that biochemical signaling for both types of memory are triggered at the same time, but in different neuron sets. Memory traces form more quickly in one set than the other, but the set that lags behind consolidates the memory and stores it long-term.

Why two mechanisms?

But why might the fly brain divide up the labor of storing different memory phases this way? Dubnau's hunch is that it might be because for every stimulus it receives, the brain creates its own representation of this information. And each time this stimulus – for example, an odor – is perceived again, the brain adds to the representation and modifies it. "Such modifications might eventually disrupt the brain's ability to accurately remember that information," Dubnau speculates. "It might be better to store long-term memories in a different place where there's no such flux."

The team's next mission is to determine how much cross talk, if any, is required between the two lobes for long-term memory to get consolidated. This work will add to the progress that scientists have already made in treating memory deficits in humans with drugs aimed at molecular members of the rutabaga-signaling pathway to enhance its downstream effects.

Tuesday, September 1, 2009

Liquid Propellant Rockets

In liquid propellant based rocket engines, the fuel and the oxidizer are liquids, which have to be stored separately. The fuel and liquid oxygen are stored in separate tanks from which they are pumped through injectors into a combustion chamber. The injectors mix the fuel and oxidizer thoroughly so that the fuel burns properly. The burning of the fuel in the combustion chamber, produces gases at a high temperature and pressure. The gases are then forced out of a narrow nozzle at the bottom of the rocket. This generates the thrust necessary for the rocket to move upward.
In addition to the weight of the propellants themselves, a liquid propellant based rocket must also carry the additional weight of the storage tanks, pumps, valves, injectors and piping. This increases the entire weight of the rocket.
The additional components, i.e., the pumps, tanks and injectors also means that the design of liquid propellant rockets is more complicated than solid propellant based ones.
However the liquid propellant rockets are very important because they are easy to control. The thrust, and therefore the speed, produced by a rocket can be easily controlled by controlling the amount of propellant that is pumped into the combustion chamber.









Though India’s first moon mission “Chandrayaan-1 ’’ was officially “retired’ ’ on Sunday after lost its contact with the deep space network at the Byalalu on Saturday at 1.30 am, the lunar craft will continue to go around the moon for about 1,000 days more before it crashes on its surfaceThis means that it will remain in orbit till the end of 2012 until preparations get under way for the “Chandrayaan-2 ’’ mission slated for lift-off in 2013. The 1,000-day countdown for Chandrayaan-1 ’s crash on the lunar surface began on Sunday. Isro spokesperson S Satish told TOI from Panaji on Monday that in the next 1,000 days “Chandrayaan-1 ’’ will not be operating with its propellants, but will be just going around the moon on its own without doing any workThe 1,000-day countdown for Chandrayaan-1 ’s crash on the lunar surface began on Sunday. Isro spokesperson S Satish told TOI from Panaji on Monday that in the next 1,000 days “Chandrayaan-1 ’’ will not be operating with its propellants, but will be just going around the moon on its own without doing any work.
He said that since it will be in orbit for 1,000 days Isro will explore the possibility of using the high-powered radars of the US and Russia to locate the spacecraft. “Discussions with these two countries have been initiated ,’’ he said.

Space expert Pradeep Mohandas said that “Chandrayaan-1 ’’ will be slowly pulled in by the gravity of the moon until it crashes on the lunar surface after 1,000 days. “After this it will spiral towards the moon’s surface and crash,’’ he said, while adding that it will slowly begin to lose its altitude. He cited the example of a Nasa spacecraft which was abandoned years ago, but was still in orbit.

Nasa and US government agencies have recommended placing retired spacecraft into an orbit at least 300 km above the geosynchronous orbit so that there is no danger of colliding with operating spacecraft.

Even as the space agency is trying to trace the cause of the communication breakdown, space scientists are divided on the issue regarding the lifespan of the “Chandrayaan-1 ’’ mission. There is a strong opinion that right from the beginning it should be have been declared a one-year mission and not a two-year project.