This month, I discuss four rather diverse papers. The first paper is a recent review about how the structure of neural networks changes spontaneously in vivo, which raises some questions about
our view of memory engrams. The second one is an intriguing study showing that anticipated eye movements have an influence on the eardrums; it questions our view of the senses as separated
modalities. The next two are about neurobiology of unicellular organisms. I use the term neurobiology because they show sensory transduction, produce action potentials (presumably, in the case of
(3)), leading to motor reactions. These are not very well known but in my view very interesting for theoretical neuroscience.
The authors review recent experimental evidence showing that in vivo, in the absence of any particular task (in particular learning task), synapses and functional properties of single neurons are
not stable. For example, spines disappear and reappear; more significant in my view, motor tuning and place fields drift. Synaptic changes are still observed when ion channel activity is blocked.
This might suggest that they are intrinsic as the authors point out,although in factit does not mean that in normal condition these changes are independent of activity; it could well be that the
fluctuations are entrained by activity, in the same way as the response of an intrinsically noisy neuron is entrained by a time-varying current (Mainen and Sejnowski, 1995;
see also Brette & Guigon, 2003for
some theory). The more significant point, I think, is that functional properties of neurons, e.g. tuning properties, seem to drift over time. This raises questions about the idea of a cell
assembly as a memory engram. If a particular assembly encodes a particular memory, then after some time this same assembly should mean something completely different. Imagine, to take a
caricatural example, that a memory of a red car is stored as a network of two connected neurons, the red neuron and the car neuron. After two weeks the red neuron becomes a green neuron. When
cued with a car, I now remember a green car.
In theoretical neuroscience, one question which has been the subject of many studies is how can synaptic structure be stable enough to sustain memories while plastic enough to allow learning.
Maybe this is not the right question; maybe the right question is: how can learning persists over a time scale longer than the functional dynamics of networks?
This is an intriguing paper showing that the eardrums move in conjunction with the eyes. Specifically, when the eyes saccade to the left, the eardrums move to the right (and conversely), and then
oscillate at 30 Hz for a few cycles (possibly more, as the dampening could be the result of averaging). These oscillations are not that small, equivalent to 57 dB. Eardrum movements seem to start
slightly before eye movements, which suggests that it is a result of anticipatory control from the central nervous system (rather than feedback or coupling). Naturally, one wonders what influence
this might have on auditory perception, in particular on spatial perception of sounds. The fact that the oscillation is at the bottom of the audible spectrum might argue for a small role; on the
other hand, one wonders what function this anticipatory control might serve if not perceptual. More generally, it makes me wonder to what extent results obtained on anesthesized animals (which
form the majority of our knowledge on the auditory system), where the efferent system is down, are meaningful for the physiological condition. Intriguing!
The world of unicellular organisms is fascinating. In this paper, the authors show that a unicellular octoflagellate (8 flagellae) of about 17 µm in length displays three different gaits: run,
shock (change of direction) and rest, corresponding to different beating modes of the flagellae. The shock is a very quick reaction that can also be triggered mechanically. This reminds me of the
avoidance reaction of Paramecium (Eckert & Naitoh, 1972),
and I would bet that this occurs by stimulus-induced depolarization followed by an action potential. It would be interesting to stick an electrode in those!
4. Iwatsuki & Naitoh (1988). Behavioural Responses to Light in Paramecium Bursaria in Relation to its Symbiotic Green Alga Chlorella. (Sorry I did not find it on PubPeer!)
To continue on the theme of unicellular neurobiology, this old paper discusses the photosensitive behavior of Paramecium Bursaria. This is a unicellular organism (a ciliate), which lives in
symbiosis with green algae (ie, cultivates plants inside its cytoplasm) (see former issue on
endosymbiosis). As a result, it tends to accumulate in light. The way it works is very interesting. It uses the avoidance reaction, in which an action potential triggers an abrupt change in
direction. This happens in reaction to various stimuli, for example mechanical stimuli. Here the avoidance reaction is triggered when light intensity decreases; thus, it avoids shade and stays in
light. It seems that the algae somehow hijack the avoidance reaction system through products of photosynthesis. It is not clear whether photosynthesis products directly trigger a depolarization,
or whether they modulate an existing photosensitive system in Paramecium – indeed several species of Paramecium have a photophobic reaction to light increase.