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A Barrel of Spikes Reach out and Touch

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A Barrel of Spikes Reach out and Touch HIGHLIGHTS NEUROANATOMY Reach out and touch Dendritic spines are a popular sub- boutons (where synapses are formed ject these days. One theory is that on the shaft of the axon) would occur they exist, at least in part, to allow when dendritic spines were able to axons to synapse with dendrites bridge the gap between dendrite and without deviating from a nice axon. They carried out a detailed straight path through the brain. But, morphological study of spiny neuron as Anderson and Martin point out in axons in cat cortex to test this Nature Neuroscience, axons can also hypothesis. form spine-like structures, known as Perhaps surprisingly, they found terminaux boutons. Might these also that the two types of synaptic bouton be used to maintain economic, did not differentiate between den- straight axonal trajectories? dritic spines and shafts. Axons were Anderson and Martin proposed just as likely to use terminaux boutons that, if this were the case, the two to contact dendritic spines as they types of axonal connection — were to use en passant boutons, even terminaux boutons and en passant in cases where it would seem that the boutons — would each make path of the axon would allow it to use synapses preferentially with different an en passant bouton. So, why do types of dendritic site. Terminaux axons bother to construct these deli- boutons would be used to ‘reach out’ cate structures, if not simply to con- to dendritic shafts, whereas en passant nect to out-of-the-way dendrites? NEURAL CODING exploring an environment, a rat will use its whiskers to collect information by sweeping them backwards and forwards, and it could use a copy of the motor command to predict A barrel of spikes when the stimulus was likely to have occurred. In addition, the sensory system Sensory events, at least in complex nervous available information that was carried by could use the relative times of first spikes systems like ours, are encoded by neuronal each element. They found that most of the between columns, rather than the absolute populations rather than individual neurons. signal — around 85% — was contained in timing in a single column, to encode It has been difficult to determine exactly the timing of the individual spikes, especially location. Barrel neurons respond to their what features of neural firing — spike timing the first spike fired by each neuron after the preferred whisker with a short latency, but or spike counts, independent spikes or stimulus. The rest of the information was they also respond more weakly and with a patterns — are used to carry information. carried by spike patterns within neurons. longer latency to neighbouring whiskers, so A recent study by Petersen et al.concludes The authors tested whether patterns of comparing the times at which neurons in that, in rat somatosensory cortex at least, the spikes in the population activity reflected neighbouring barrels fire could provide the timing of the first spikes from single neurons redundant or synergistic coding by relevant information. The two techniques in response to a stimulus is the crucial comparing the information conveyed by the could even be combined for greater element for coding stimulus location. population as a whole with that conveyed by precision. The ‘barrel’ cortex in the rat lends itself well individual spikes. Within a cortical barrel, As analytical and recording techniques are to analysis because of its relatively simple and coding was redundant. This might seem to be improved, it should be possible to apply this stereotyped organization. The cortex is inefficient, but the authors point out that it kind of analysis to more complex systems and divided into columnar barrels, each of which could provide useful robustness, and allow more natural, dynamic stimulus sets. receives input primarily from a particular the same information to be transmitted Understanding how the nervous system codes whisker. In addition, the neurons in the simultaneously to different targets. Between and decodes its messages is one of the great barrels tend to fire low numbers of spikes, columns, on the other hand, pairs of neurons challenges for neuroscientists today, and this simplifying the task of analysing the code independently. kind of study takes us closer to that goal. population code. One important question is, if first-spike Rachel Jones Petersen et al. recorded from pairs of time is so important, how can the animal use References and links neurons in barrel cortex while they it? After all, the experimenter knows exactly ORIGINAL RESEARCH PAPER Petersen, R. S. et al. stimulated individual whiskers. They then when the stimulus occurred, but the animal Population coding of stimulus location in rat somatosensory quantified the roles of different elements of doesn’t. One possible explanation lies in the cortex. Neuron 32, 503–514 (2001) FURTHER READING Pouget, A. et al. Information the population activity in encoding stimulus active nature of the somatosensory system processing with population codes. Nature Rev. Neurosci. 1, location by evaluating the proportion of in rats under normal circumstances. When 125–132 (2000) 10 | JANUARY 2002 | VOLUME 3 www.nature.com/reviews/neuro HIGHLIGHTS The authors propose that ter- minaux boutons might share some of the physiological properties of dendritic spines. For example, spines are known to compartmen- talize calcium transients. If these axonal structures also prevent cal- cium from diffusing rapidly into the parent axon during an action poten- tial, terminaux boutons might show more presynaptic facilitation than en passant boutons. Of course, there is no evidence yet that terminaux boutons have this kind of influence on synaptic dynamics and plasticity, but the demonstration that they seem not to be simple bridges does compel us to consider alternative functions. Rachel Jones PROCESS OUTGROWTH References and links ORIGINAL RESEARCH PAPER Anderson, J. C. & Martin, K. A. C. Does bouton morphology optimize axon length? Nature Neurosci. 19 To and fro at the axon November 2001 (10.1038/nn772) FURTHER READING Hering, H. & Sheng, M. Cut an axon and it will probably regress. This the contribution of other genes that are present in Dendritic spines: structure, dynamics and regulation. Nature Rev. Neurosci. 2, 880–888 form of axonal death is commonly termed the triplicated region. (2001) Wallerian degeneration, and although it was But not every axon dies after injury. If the axon originally described many years ago, its is not severed but crushed, it can sometimes underlying mechanisms have not been explored regenerate and function normally. The in detail. Part of the reason for this neglect is the regeneration process requires the deployment of simple assumption that retraction of the severed cellular resources, such as the synthesis of new axon is merely a passive process. It was therefore proteins. Owing to the paucity of ribosomes in surprising to discover a mutant mouse — the axons, it was commonly believed that the cell body c57BL/Wld S strain — in which Wallerian orchestrated the recovery response. However, degeneration did not occur. What protected the Zheng et al. now report that regenerating axons axons of these mice from degeneration? Until can locally synthesize proteins, even if they are recently, we did not know much about this excommunicated from the soma. mutation, other than that it was related to a The authors isolated regenerating sensory triplication of the so-called Wld S region of mouse axons and showed that they contained β-actin and chromosome 4. This region harbors a chimeric neurofilament mRNAs bound to ribosomes. gene that is formed by a small fraction of the Moreover, they found evidence for protein coding region of ubiquitination factor E4B and a synthesis in this preparation, and showed that gene termed D4Cole1e, the function of which was blocking translation caused retraction, but only if so far unknown. But now Mack et al. have the axon was separated from the cell body. This obtained evidence that D4Cole1e is the mouse finding indicated that axonal protein synthesis homologue of NMNAT, a human gene that might contribute to regeneration only if the supply encodes a key enzyme in the synthesis of NAD+. of proteins from the cell body is compromised. But They also showed that overexpression of the more importantly, the data of Zheng et al. chimeric gene protects axons from Wallerian constitute some of the best available evidence that degeneration. protein synthesis can occur in axons, and not only Expressing Ube4b/Nmnat in transgenic mice in somata and dendrites, as we have long assumed. had a dose-dependent protective effect against Juan Carlos López Wallerian degeneration. Although the chimeric References and links protein did show NMNAT activity, the ORIGINAL RESEARCH PAPERS Mack, T. G. A. et al. Wallerian degeneration of injured axons and synapses is delayed by a relationship of this enzymatic action to the Ube4b/Nmnat chimeric gene. Nature Neurosci. 4, 1199–1206 (2001) | protective mechanism is not clear; the protein was Zheng, J.-Q. et al. A functional role for intra-axonal protein synthesis during axonal regeneration from adult sensory neurons. J. Neurosci. found largely in the nucleus and the cellular 21, 9291–9303 (2001) + content of NAD did not increase in the FURTHER READING Conforti, L. et al. A Ufd2/D4Cole1e chimeric transgenic animals. However, the results protein and overexpression of Rbp7 in the slow Wallerian degeneration (Wld S) mouse. Proc. Natl Acad. Sci. USA 97, 9985–9990 (2000) | constitute solid evidence that Ube4b/Nmnat is Twiss, J. L. et al. Translational control of ribosomal protein L4 is required responsible for the Wld S phenotype, and rule out for rapid neurite extension. Neurobiol. Dis. 7, 416–428 (2000) NATURE REVIEWS | NEUROSCIENCE VOLUME 3 | JANUARY 2002 | 11.
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