A fluorescent planarian
Photo: http://www.mcb.ucdavis.edu/faculty-labs/scholey
Even the most untrained, layperson neuroscientist has the correct sense to recognize memories are realized by the brain, that the brain resides in the head, and so it follows removing the head displaces the memories ostensibly to wherever the head now resides.
Now introducing the planarian (c. Elegans), a standard organismic model in behavioral and biological research along with rats and undergraduates. The planarian is a flatworm, approximately 1 cm.. Long They are one of the simplest creatures to be endowed with a brain; possessing exactly 302 neurons,it is capable of basic sight, detection of chemical gradients, electrical fields, magnetic fields, and vibration. With these abilities c. elegans is able to be trained, albeit only to perfrom simple tasks.
Also, it can regenerate parts of its body. This is important.
In the ’60s it was discovered- presumably by a scientist with a maniacal laugh- that c. elegans were not only able to survive decapitation and then regenerate its head but, also, that these trained worms possessed memories that, by all convention, should be locked away in the severed head. The head was removed but the memories remained.
Research on this phenomena carried on for some time. However, training, decapitating, then measuring the behavior of thousands of minuscule worms proved extremely difficult. Further, many dissenting findings occurred and so interest and research on this phenomena all but ceased.
So do planarians perform such a remarkable feat? Had they ever been capable of such a task, or had this quirky behavior been a result of statistical anomaly?
The latest research evidences, yes.
Shomrat and Levin out of Tufts universisty devised an automated manner of measuring the behavior of trained/decapitated/ regenerated worms. 20- to 40 worms were trained over 10-11 days in a custom built fully automated training apparatus (ATA). Planarians naturally possess strong aversion to light (negative phototaxis). During the course of training researchers placed pieces of liver, which planarians apparently love, under a “strong blue lED light… thus, no worm would stay in this quadrant {of the ATA} unless its desire for the liver overcame its natural light aversion.”
Worms who had been trained to appreciate the blue LED light as an indicator of liver navigated the ATA quicker and tolerated the light longer than their untrained counterparts. The worms with re-grown heads had undoubtedly retained their memories; and due to a new high-tech means of recording and training the dispute seems all but over. But how does a creature a retain memories belonging to an appendage which had been removed?
In the first place, planarians possess a nervous much unlike us vertebrates. Their nervous system is a much less dynamical and much less complex, rendering the whole system a highly specialized amalgam of distinct neural tissues. So, although the organs pertaining to phototaxis- the eyes- are situated in the head it does not necessarily follow that the memories and instincts related to those organs are stored completely in the ganglion (bundles of nerves) most associated with the eyes.
Delineation of neuron types in the planarian head Photo:http://www.scholarpedia.org/article/Planaria_nervous_system
Where are the neurons that contain memories, then? It isn’t known. And how are memories which are stored in the body coordinated with a newly grown head and pair of eyes? This isn’t known either. The most compelling theory is that the Peripheral Nervous System (nervous tissue of the body) is where memories are stored and the Central Nervous System (nervous tissue of the head) manages behavior. Given that a decapitated worm produces no behavior, other than regrowing a head, this theory is plausible. And if so, it’s a brilliantly simple adaptation for a miniature creature whose very likely to be squished within its lifetime: if the body is damaged, only memories are lost; and if the head is damaged, only behavior is lost briefly.
Wouldn’t it be interesting if humans exhibited such a modular schematic of our Nervous system: “I’m sorry. I seem to have forgotten your name. You see, the other day I cut the tip off the end of my left thumb in a carpentry accident and now I can’t remember the names of my acquaintances whose first names start with the letters j-m.”
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