Further GNW simulations showed that ignition could fail to be tri

Further GNW simulations showed that ignition could fail to be triggered under specific conditions, thus leading to simulated nonconscious states. For very brief or low-amplitude stimuli, a feedforward wave was seen in the initial thalamic and cortical stages of the simulation, but it died out without triggering the late global activation, because it was not able to gather sufficient self-sustaining reverberant activation (Dehaene and Changeux, 2005). Even at higher stimulus amplitudes, the second global phase could also be disrupted if another

incoming stimulus had been simultaneously accessed (Dehaene et al., 2003b). Such a disruption occurs because during ignition, the Epacadostat manufacturer GNW is mobilized as

a whole, some GNW neurons being active while the rest is actively inhibited, thus preventing multiple simultaneous OTX015 in vitro ignitions. A strict seriality of conscious access and processing is therefore predicted and has been simulated (Dehaene and Changeux, 2005, Dehaene et al., 2003b and Zylberberg et al., 2010). Overall, these simulations capture the two main types of experimental conditions known to lead to nonconscious processing: subliminal states due to stimulus degradation (e.g., masking), and preconscious states due to distraction by a simultaneous task (e.g., attentional blink). The transition to the ignited state can be described, in theoretical physics terms, as a stochastic phase transition—a sudden change in neuronal dynamics whose occurrence depends in part on stimulus characteristics and in part

on spontaneous fluctuations in activity (Dehaene and Changeux, 2005 and Dehaene et al., 2003b). In GNW simulations, prestimulus fluctuations in neural discharges only have from a small effect on the early sensory stage, which largely reflects objective stimulus amplitude and duration, but they have a large influence on the second slower stage, which is characterized by NMDA-based reverberating integration and ultimately leads to a bimodal “all-or-none” distribution of activity, similar to empirical observations (Quiroga et al., 2008, Sergent et al., 2005 and Sergent and Dehaene, 2004). Due to these fluctuations, across trials, the very same stimulus does or does not lead to global ignition, depending in part on the precise phase of the stimulus relative to ongoing spontaneous activity. This notion that prestimulus baseline fluctuations partially predict conscious perception is now backed up by considerable empirical data (e.g., Boly et al., 2007, Palva et al., 2005, Sadaghiani et al., 2009, Supèr et al., 2003 and Wyart and Tallon-Baudry, 2009).

33 Hz for 30 min), neither TBS nor RFS further elevated the ampli

33 Hz for 30 min), neither TBS nor RFS further elevated the amplitude of e-EPSCs in RGCs (Figure 6C), indicating that RFS-induced synaptic enhancement occludes TBS-induced LTP. These results indicate that repeated visual inputs can induce LTP at BC-RGC synapses Dasatinib via mechanisms that may be shared by TBS-induced LTP. To further determine the physiological consequence of LTP at BC-RGC synapses, we investigated its effect on light-evoked responses in RGCs. In 3–6 dpf zebrafish larvae, whole-field flash (2 s duration) elicited both ON and OFF responses

in 61% of RGCs (259 of 425), and only ON (83 of 425) or OFF (83 of 425) responses in the rest of RGCs. Examples of light response subtypes are shown in Figure S8A. To assay light-evoked excitatory responses, we measured light-evoked EPSCs (l-EPSCs) in RGCs at the reversal potential of light-evoked inhibitory check details postsynaptic currents (ECl−; Figures S8B and S8C), with a low frequency of light stimulation (0.033 Hz). The magnitude of l-EPSCs in RGCs, as determined by the total integrated charge associated with l-EPSCs within a 100 ms window

after the onset of l-EPSCs (Du et al., 2009), remained stable for up to 50 min under control condition (Figure 7A). However, we observed persistent enhancement of light-evoked ON responses in eight out of eight ON-OFF and one out of one ON RGCs (157% ± 17% of the control, n = 9; p = 0.005; Figure 7B) after the induction of LTP at BC-RGC synapses by TBS applied at the INL. In addition, three out of eight ON-OFF RGCs

also showed a persistent increase in OFF responses after LTP induction. Thus, the LTP at BC-RGC synapses can substantially enhance light-evoked excitatory responses in RGCs, implicating its physiological relevance to retinal functions during development. To explore whether natural visual stimulation can potentiate visual responses of RGCs, we first examined the effect of RFS on light-evoked responses of RGCs in zebrafish larvae at 3–6 dpf. In nine out of ten RGCs possessing Histone demethylase ON responses, RFS induced persistent enhancement of light-evoked ON responses (167% ± 13% of the control, n = 10; p = 0.000007; Figure 8A), whereas five out of eight RGCs possessing OFF responses also showed a persistent increase in OFF responses after RFS. In addition, repetitive MBS (width, 6 μm; speed, 0.1 μm/ms; frequency, 0.25 Hz; duration, 5 min) also induced persistent enhancement of moving bar-evoked responses in five out of five RGCs examined (194% ± 24% of the control, n = 5; p = 0.004; Figure 8B). Furthermore, 30 min pre-exposure of repetitive moving bars occluded RFS-induced potentiation of flash-evoked responses of RGCs (91% ± 5% of the control, n = 5; p = 0.14; Figure 8C). Thus, different patterns of natural visual stimulation can enhance visual responses of RGCs, suggesting that visual experience is effective in modifying the function of developing retinal circuits.

, 2009), cell autonomous activation of PDF-R solely in PDF-negati

, 2009), cell autonomous activation of PDF-R solely in PDF-negative pacemaker neurons with a membrane-tethered GW786034 PDF construct promotes strong rhythmicity in Pdf null flies, which would otherwise be poorly rhythmic ( Choi et al., 2012). Flies deficient in PDF or PDF-R display severe deficits in circadian rhythms and alterations in PER molecular rhythms during constant dark (DD) conditions. Among the four small LNvs, rhythms are maintained but become desynchronized (Lear et al., 2005; Lin et al., 2004). Among PDF target pacemaker groups like the LNd, the amplitude and period of the PER rhythm decrease but cells remain synchronized (Lin et al., 2004; Yoshii et al., 2009). Thus PDF neuropeptide acts over

many daily cycles to

promote the amplitude and pace of PER cycling—it has access to the molecular clockworks in diverse pacemakers and affects them differently. Recent observations have begun to shed light on the signaling pathways by which PDF affects synchronization and how these may differ according to cell type. Because PDF modulation system profoundly affects the circadian molecular oscillator within individual pacemaker neurons, the molecular learn more details of the signaling pathway downstream of PDF-R gains in significance. Among the identified neurons in the pacemaker network, the PDF-expressing subset are termed M cells based on their abilities to influence “Morning” activity levels; several non-PDF pacemakers are termed E cells based on their abilities to influence “Evening” time activity levels (Grima et al., 2004; Stoleru et al., 2004; Yoshii et al., 2004; reviewed by Helfrich-Förster, 2009). Duvall and Taghert (2012) recently used an RNAi-mediated genetic approach to report that adenylate cyclase 3 (AC3) underlies PDF signaling in M cells. Surprisingly, disruption of AC3 does not alter PDF-R mediated only responses in non-PDF pacemakers (specifically, in the PDF-R(+) LNd). Moreover, AC3 disruptions in small LNv did not alter GPCR signaling by other ligands that elevate cAMP levels in these neurons (dopamine and the neuropeptide DH31). Hence, within small LNv, PDF-R

signaling occurs via discrete molecular pathways that are distinct from those controlled by other cAMP-elevating ligands. This provides a molecular mechanism underlying the observation that PDF-R activation in small LNv has potent effects on daily allocation of rest and activity, whereas DH31-R activation does not ( Choi et al., 2012). Furthermore, PDF-R association with a different AC(s) supports PDF signaling in the other circadian pacemakers. Thus critical pathways of circadian synchronization are mediated by highly specific second messenger components. These findings support a hypothesis that PDF signaling components within target cells are sequestered into “circadian signalosomes,” whose compositions differ between different pacemaker cell types ( Duvall and Taghert, 2012).

Taken together, these data support a model in which axonal applic

Taken together, these data support a model in which axonal application of BMP4 elicits a retrograde signal that is translocated by dynein, and involves the appearance of both BMP4 and active BMP4 receptors in the cell body. GDC-0941 solubility dmso During the development of the nervous system, intra-axonal mRNA translation is a component of several signaling pathways, notably those involving axon guidance cues (Lin and Holt, 2008 and Martin and Ephrussi, 2009). We therefore asked if local protein synthesis is required for retrograde BMP4 signaling. Coapplication of either of the translation

inhibitors cycloheximide or anisomycin with BMP4 to axons substantially blocked retrograde BMP4 signaling (Figures 2A and 2B). The effect of axonal anisomycin treatment is not due to inhibition of protein synthesis in the cell body, as a labile control protein ODC in cell bodies is not affected (Figure S2A). In addition to inducing Tbx3, retrograde BMP4 signaling also leads to the repression of OC1, OC2, and Hmx1 in the cell body ( Hodge et al., 2007). These effects were also blocked by axonal application of translation inhibitors ( Figures 2C–2E). We considered the possibility

Protease Inhibitor Library that inhibition of local translation could impair the retrograde translocation of BMP4 signaling endosomes. However, application of anisomycin to axons did not prevent the retrograde transport of biotinylated BMP4 (Figure S2B). Under these experimental conditions, the translation inhibitors did not elicit axonal or cell body toxicity compared to vehicle treatment (Figures S2C and S2D). Additionally, the effects of the protein synthesis inhibitors were not due to alterations in the levels of axonal BMPR1a, 1b, and 2 or cell body SMAD1/5/8, or due to diffusion of the inhibitors into the cell body compartment (Figures

Parvulin S2E–S2J, S1F, and S2K). These data indicate that translation of axonal mRNA(s) are required for retrograde BMP4 signaling. To identify axonal mRNAs that may mediate retrograde BMP4 signaling, we considered proteins that are present in axons and may be translated locally. Although transcription factors are typically localized in the nucleus, previous studies have detected prominent labeling of phospho-SMAD1/5/8 (pSMAD1/5/8) in certain axonal branches of the trigeminal ganglia (Hodge et al., 2007). The axonal localization of these transcription factors raises the possibility that they are synthesized locally and that this axonal pool of SMAD1/5/8 has a role in conveying retrograde patterning signals from target tissues. Although pSMAD1/5/8 is selectively localized to ophthalmic and maxillary axons (Hodge et al., 2007), the absence of pSMAD1/5/8 immunoreactivity in mandibular axons could reflect the absence of SMAD phosphorylation in these axons, or the absence of SMAD protein altogether.

In addition to these parameters—to date mainly explored for the m

In addition to these parameters—to date mainly explored for the more homogenous population of glutamatergic Cell Cycle inhibitor synapses—one might wonder if the enormous diversity of inhibitory neurons (Klausberger and Somogyi, 2008), and hence their tuning capability for neuronal networks, is reflected in the molecular composition of their synapses. How strongly is the diversity in firing properties of different interneurons mirrored by postsynaptic parameters?

The determination of the density of scaffolds and anchored receptors can provide important information about these issues. Receptor occupation rates can tune the amplitudes of evoked currents and hence profoundly influence the impact of a given synapse on the network (Barberis et al., 2011). The quantitative imaging approach reported by Specht et al. (2013) when combined with state of the art electrophysiology VX-770 price and appropriate pharmacology

will allow scientists to explore exact numbers of molecules and to monitor stochastic as well as plasticity-related processes at individual synapses of different types. To achieve this, two experimental obstacles need to be overcome: the temporal resolution has to be improved for counting large amounts of molecules in the range of seconds and below, and the application of single-molecule science microscopy needs to become applicable to living brain tissue (i.e., cultured or acute brain slices). Solving these technical issues will enable investigators to quantitatively monitor multiple synapses, both excitatory and inhibitory, in parallel and interdependently and to understand their role in functional networks. Recently designed Human Brain Mapping Initiatives will develop a high demand for this type of

quantitative information. “
“The dorsal anterior cingulate cortex (dACC), spanning the cingulate gyrus and sulcus from the plane of the anterior commissure to the genu of the corpus callosum (Figure 1), is one of the most heavily studied regions of the brain and yet remains one of the least clearly understood. Although there has recently been an explosion of research on the role of dACC in cognition and behavior, this has led to a proliferation of diverging theories concerning its function. The dACC has been proposed to play a key role in pain processing, performance monitoring, value encoding, decision making, emotion, learning, and motivation. A precise and coherent account of dACC function seems as elusive now as it did in the earliest days of theory development. Two opposing tendencies appear to have slowed progress toward an integrated understanding of dACC function.

In contrast, expression of DN-nectin3 or DN-afadin caused electro

In contrast, expression of DN-nectin3 or DN-afadin caused electroporated cells to accumulate

near the IZ (Figures 2E and 2F), indicating that nectin3 and afadin act in neurons, at least in part, to regulate glia-independent somal translocation. We next determined the mechanism by which nectin3 and afadin regulate radial migration. We reasoned that the two proteins might help to anchor the leading processes of neurons in the MZ. We therefore evaluated neuronal morphology following perturbation of nectin3 or afadin function by knockdown and dominant-negative approaches, which selleck gave similar results. Although neurons largely failed to migrate into the CP following perturbation of nectin3 or afadin, they still properly polarized the Golgi apparatus ahead of the nucleus (Figure 3A) and also developed stereotypical polarized morphologies characterized by leading processes (Figures 3B–3D). At 2–3 days after electroporation, leading processes that

extended toward or even into the MZ were http://www.selleckchem.com/products/lonafarnib-sch66336.html observed in both control neurons and neurons expressing shRNAs against nectin3 or afadin (Figures 3B and 3C). A small decrease in the number of branches was observed after 3 days in the case of afadin shRNA electroporation, suggesting onset of leading-process retraction. However, only control neurons had their cell bodies located close to the MZ, indicative of somal translocation. Cell bodies in the knockdown experiments failed to translocate toward the MZ (Figures 3B and 3C) and remained near the IZ and lower CP, as nonelectroporated cells bypassed them to expand the CP. This CP expansion initially caused the leading processes of affected neurons to appear

longer than those of controls neurons below 2–3 days after electroporation (Figures 3B and 3C), but many of these processes were subsequently retracted by 4 days after electroporation (Figures 2C and 2F). Additionally, whereas the leading processes of control neurons extensively branched in the MZ, no such branching was observed after nectin3 or afadin knockdown (Figure 3D). Together, these data indicate that nectin3 and afadin are not required for neuronal polarization or initial process extension, but are important for leading-process anchorage and arborization in the MZ and subsequent somal translocation. To directly determine whether nectin3 and afadin are required for somal translocation, we carried out time-lapse imaging experiments. Neurons from E13.5 animals were electroporated with control, nectin3, or afadin shRNAs, and neocortical slice cultures were prepared at E15.5. As reported (Franco et al., 2011), control neurons translocated their cell bodies along their leading processes toward the MZ (Figure 3E). In contrast, neurons expressing shRNAs for afadin or nectin3 extended leading processes but failed to undergo somal translocation (Figure 3E).

Taken together, our study shows that RIM proteins coordinately re

Taken together, our study shows that RIM proteins coordinately regulate Ca2+ channel targeting, vesicle docking and priming, and Ca2+ channel-vesicle colocalization at the presynaptic active zone. For the generation of calyx-specific conditional KO mice, we have made use of recently generated floxed mouse lines for RIM1αβ (Kaeser et al., 2008) and RIM2αβγ (Kaeser et al., 2011) and of a previously available Cre knockin mouse line in the Krox20 locus (Voiculescu et al., 2000). The presynaptic neuron pool that generates the large calyx of Held nerve terminals onto MNTB neurons is located in the contralateral ventral cochlear nucleus (VCN) and is largely represented

by globular bushy cells (Cant and Benson, Anti-diabetic Compound Library price www.selleckchem.com/screening/epigenetics-compound-library.html 2003). Krox20 is a transcription factor that is highly specifically active in rombomeres 3 and 5 of the developing hindbrain (Voiculescu et al., 2000), which give rise to a majority of neurons in the VCN (Farago et al., 2006 and Maricich et al., 2009). We found homogeneous phenotypes among the recorded

calyx of Held synapses, which indicates, together with morphological analysis (Figure 1B; L. Xiao, N. Michalski, and R.S., unpublished observations; Renier et al., 2010), that the entire population of calyx of Held-generating neurons stems from Krox20Cre-positive neurons. Thus, the Krox20Cre mouse line enables the conditional removal of floxed alleles at the calyx of Held synapse, which will make it a useful tool to advance our understanding also of other proteins of the presynaptic vesicle cycle. Using direct presynaptic recordings at the nerve terminal, we demonstrated that RIM proteins are essential for localizing Ca2+ channels to the active zone, as shown by the clear decrease in the presynaptic Ca2+ current density in RIM1/2

Adenosine triphosphate cDKO calyces. These direct recordings at the nerve terminal show that RIMs determine presynaptic Ca2+ channel density without changing major biophysical parameters of Ca2+ channel gating, although we cannot exclude that RIMs influence Ca2+ channel inactivation with prolonged pulses (>50 ms, Figure 2; Kiyonaka et al., 2007); this, however, will be relevant only for prolonged presynaptic AP trains. Analyzing RIM-dependent presynaptic Ca2+ channel targeting has been limited because previously no KO mice deleting all RIM1/2 isoforms were available (Schoch et al., 2002 and Calakos et al., 2004) and because previously investigated synapses, including the C. elegans neuromuscular synapse ( Koushika et al., 2001), did not allow for measurements of presynaptic Ca2+ currents. Using cultured RIM1/2 cDKO neurons, Kaeser et al. (2011) have observed an ∼2-fold reduction in Ca2+ transients in presynaptic boutons during an AP. This, together with our finding that presynaptic APs are unchanged ( Figure 2), is evidence for a reduced Ca2+ channel density also at small bouton-like synapses.

We can rationally explain the postsynaptic accumulation of α2βγ2

We can rationally explain the postsynaptic accumulation of α2βγ2 and α3βγ2 receptors. By contrast the postsynaptic clustering of α1βγ2 receptors can occur in the absence of gephyrin and therefore probably depends on alternative mechanisms. Disruption of Tyr phosphorylation in γ2Y365/7F knockin Sirolimus concentration mice results in selective upregulation of GABAARs in CA3 pyramidal cells but, so far unexplained, not in CA1 pyramidal

cells (Tretter et al., 2009). Phosphorylation of γ2S327 is established to modulate the diffusional dynamics of postsynaptic GABAARs (Muir et al., 2010), yet the functionally relevant interaction partner(s) for this effect remain unknown. Lastly, gephyrin exists in multiple splice variants (Paarmann et al., 2006) and is phosphorylated at multiple sites (Huttlin et al., 2010 and Tyagarajan et al., 2011), yet the functional relevance of different gephyrin isoforms and their posttranslational modifications remain largely unexplored. One attractive mechanism underlying postsynaptic differentiation involves gephyrin-mediated interaction of GABAARs with NL2, which accumulates at synapses through interaction with presynaptic neurexin. However, the loss of GABAARs and gephyrin from inhibitory

synapses of NL2 knockout mice is incomplete (Hoon et al., 2009 and Jedlicka et al., 2011), suggesting that additional thus-far-unidentified synaptic adhesion complexes Obeticholic Acid molecular weight exist that substitute for NL2 and contribute to accumulation of GABAARs and gephyrin at many synapses. Interestingly, a recently described transsynaptic interaction between presynaptic neurexins and postsynaptic GABAARs appears to inhibit rather than promote the function of GABAergic synapses (Zhang et al., 2010). This negative effect of neurexin on GABAergic transmission was preserved in NL2 KO neurons and also observed in GABAAR expressing heterologous cells exposed to soluble neurexin constructs, indicating that it does not involve competition between GABAARs and NL2 for interaction with neurexin. The relevance of this interaction for native synapses and trafficking of GABAARs remains to be explored. A complete

understanding of the role of GABAAR trafficking in GABAergic synaptic plasticity will require that these knowledge Isotretinoin gaps be filled. In addition, downstream consequences of altered GABAergic transmission on other signaling pathways will need to be explored. Ultimately, the function of these mechanisms will need to be explored at the level of neural network activity, behavior, and cognition, including in appropriate disease models. We thank Victoria Cavener and Pam Mitchell for critical comments on the manuscript. Research in the Luscher laboratory has been supported by Grants MH62391, MH60989, and RC1MH089111 from the National Institutes of Mental Health (NIMH) and grants from the Pennsylvania Department of Health using Tobacco Settlement Funds.

The increasing capacity of DNA sequencing provides an unprecedent

The increasing capacity of DNA sequencing provides an unprecedented opportunity LY294002 for such large-scale studies using patient samples, and our fl-Htt brain interactome may provide

converging information on candidates that may have both a genetic and proteomic link to Htt. Thus, our study lends strong support to a systems biology strategy of vertically integrating large genetic, genomic, and interactome data sets (Geschwind and Konopka, 2009) derived from HD models of different organismal complexity to unravel the conserved mechanism related to Htt biology and HD pathogenesis. Our study supports the view that the majority of Htt-interacting proteins are relatively stable across brain tissue and age (Figures 2A and 2B), while a portion of the Htt interactome is quite dynamic. The latter group of proteins, particularly those that consistently complex with mHtt in a brain-regional-specific or age-specific

manner (Figures 2C–2D), could be interesting candidates to study for their contribution to selective neuronal vulnerability and age-dependent pathogenesis in HD. Our studies also raised the intriguing possibility that age-dependent changes in the normal brain proteome (e.g., Sirt2) may alter Htt interactions, which could in turn contribute to the presently unexplained role of aging in disease pathogenesis (Maxwell et al., 2011). A major advance Dolutegravir supplier in this study is the use of a systems biology approach to construct in vivo protein interaction networks exclusively using first proteomic interactome data sets generated from complex tissue. We applied, for the first time, WGCNA to analyze all the unique peptide count information for an entire group of Htt complexed proteins in our spatiotemporal AP-MS data set. WGCNA provides an unbiased systems-level organization of gene expression modules in both normal and diseased brains (Voineagu

et al., 2011) and has been demonstrated to be among the most powerful methods for global network construction (Allen et al., 2012). Several lines of evidence support the validity and value of WGCNA analyses of our in vivo Htt interactome data set. We were able to show that the pairwise correlation measure leads to a meaningful ranking of Htt-related proteins with respect to the external annotated knowledge of HD-related proteins (Huntington’s Disease Signaling in IPA; Figure 3C). WGCNA identified six significant Htt-correlated modules with distinct tissue- or age-specific overrepresentation and significant enrichment of distinct biological function previously implicated in Htt biology (Figure 6), effectively providing an in silico dissection of the molecular processes related to fl-Htt biology. Several experimental factors were instrumental to the construction of WGCNA networks based on our AP-MS data set. First, the relative level of bait protein (fl-Htt) brought down by IP is markedly, but reproducibly, variable across all samples.

If individual labs become more efficient in managing their data,

If individual labs become more efficient in managing their data, it will lower the practical barriers to data sharing. It is encouraging CCI-779 molecular weight that the need for data sharing is receiving attention from funding agencies and also in the advisory report on the BRAIN Initiative (http://www.nih.gov/science/brain). During the 1990s, many of my neuroanatomy colleagues bemoaned the decline of systems neuroanatomy. It was increasingly hard to get funding and to recruit graduate students to the field. While I shared the concern, my instincts were that a resurgence was essential for the vitality of neuroscience more broadly

and that the pendulum would swing with the advent of more powerful and efficient neuroanatomical methods. However, never in my wildest turn-of-the-century dreams did I countenance the amazing explosion of interest in matters neuroanatomical that now engage GSK-3 signaling pathway a broad spectrum of investigators, including hardcore molecular neuroscientists who now appreciate the importance of delving into the intricacies of neuroanatomy. To paraphrase Mark Twain, reports of the death of neuroanatomy were greatly exaggerated. The field has undergone an amazing resurgence, fueled by advances on many fronts. In animal models, this includes optogenetics and labeling of neuronal subtypes

and their projections, genetically tractable species like mouse zebrafish and Drosophila ( Schnitzer and Deisseroth, 2013). In monkeys and humans, this includes

the powerful neuroimaging methods discussed in this Perspective. My interests in neural nearly development are deeply rooted but have followed a circuitous trajectory that intersects with the cartography and connectomic themes of this essay. While at Caltech, my lab pursued parallel, highly disparate lines of research on synapse elimination at the neuromuscular junction and on the functional organization of primate visual cortex. After moving to Washington University in 1992, my developmental focus shifted to cerebral cortex. We used postmortem anatomical methods to show that connections between macaque areas V1 and V2 initially form around the time that cortical gyrification occurs (Coogan and Van Essen, 1996). This prompted me to think mechanistically about how cortical folding brings the retinotopic maps of areas V1 and V2 into register. In my favorite “light-bulb” moment of an entire scientific career, it occurred to me one evening that cortical folding might arise if axons generated mechanical tension that pulled strongly connected locations closer to one another. This notion quickly generalized into a theory of tension-based morphogenesis that can account for many other key features of nervous system development (Van Essen, 1997).