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The habenular nuclei: a conserved asymmetric relay station in the vertebrate brain.

Bianco IH, Wilson SW - Philos. Trans. R. Soc. Lond., B, Biol. Sci. (2009)

Bottom Line: The habenulae form part of the dorsal diencephalic conduction (DDC) system, a highly conserved pathway found in all vertebrates.In this review, we shall describe the neuroanatomy of the DDC, consider its physiology and behavioural involvement, and discuss examples of neural asymmetries within both habenular circuitry and the pineal complex.We will discuss studies in zebrafish, which have examined the organization and development of this circuit, uncovered how asymmetry is represented at the level of individual neurons and determined how such left-right differences arise during development.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell and Developmental Biology, University College London, London, UK. ibianco@mcb.harvard.edu

ABSTRACT
The dorsal diencephalon, or epithalamus, contains the bilaterally paired habenular nuclei and the pineal complex. The habenulae form part of the dorsal diencephalic conduction (DDC) system, a highly conserved pathway found in all vertebrates. In this review, we shall describe the neuroanatomy of the DDC, consider its physiology and behavioural involvement, and discuss examples of neural asymmetries within both habenular circuitry and the pineal complex. We will discuss studies in zebrafish, which have examined the organization and development of this circuit, uncovered how asymmetry is represented at the level of individual neurons and determined how such left-right differences arise during development.

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Models for lateralization of neural tissue. (a) Equivalent regions on the left and right of the CNS are identical in composition and differ only in overall size. (b) Unique types of neuron, or patterns of connectivity, may be specified on either the left or right or both sides (indicated by unique red neurons on the left in this schematic). (c) Identical circuit components might exist on both sides of the CNS, but in different ratios. Note that these models are in no way mutually exclusive. In fact, it is likely that all three strategies may be involved in the lateralization of DDC circuitry (see the main text).
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fig4: Models for lateralization of neural tissue. (a) Equivalent regions on the left and right of the CNS are identical in composition and differ only in overall size. (b) Unique types of neuron, or patterns of connectivity, may be specified on either the left or right or both sides (indicated by unique red neurons on the left in this schematic). (c) Identical circuit components might exist on both sides of the CNS, but in different ratios. Note that these models are in no way mutually exclusive. In fact, it is likely that all three strategies may be involved in the lateralization of DDC circuitry (see the main text).

Mentions: This study has identified a fundamental strategy by which neural tissue on the left and right sides of the central nervous system (CNS) may become asymmetric. It gives rise to a model where the same or very similar circuitry components are produced on both sides, but in greatly different ratios, resulting in LR asymmetry in circuit microarchitecture that presumably translates into functional asymmetry. Figure 4 contrasts this model with two other models for how neural circuits might be lateralized. In perhaps the simplest model, equivalent regions on the left and right sides would contain the same classes of neuron and patterns of circuitry but differ only in size (figure 4a). As a result of such ‘scaling’, a particular cognitive function might be lateralized simply as a result of more neural substrate existing on one or the other side. In support of this possibility, Rosen (1996) observed that in the rat somatosensory/somatomotor cortex, asymmetry in tissue volume is strongly associated with the LR differences in the numbers of two subtypes of neuron, but there is only a weakly significant difference in cell packing density for one of the neuronal subtypes, suggesting that the left and right sides have similar neural architectures and show a proportional scaling to achieve differences in the quantity of neural tissue. In a third model, certain types of neuron, or patterns of connectivity, might be specific to one side and would not be present on the other side of the CNS (figure 4b). Hence, circuits on the left and right might receive different types of afferent inputs, perform different neural computations and/or connect to different downstream targets to mediate distinct types of cognition or behaviour. This mode of lateralization might be especially applicable to the zebrafish DDC: the parapineal projects exclusively to the left habenula and a subset of pallial neurons exclusively innervate a subdomain of the right habenula (above).


The habenular nuclei: a conserved asymmetric relay station in the vertebrate brain.

Bianco IH, Wilson SW - Philos. Trans. R. Soc. Lond., B, Biol. Sci. (2009)

Models for lateralization of neural tissue. (a) Equivalent regions on the left and right of the CNS are identical in composition and differ only in overall size. (b) Unique types of neuron, or patterns of connectivity, may be specified on either the left or right or both sides (indicated by unique red neurons on the left in this schematic). (c) Identical circuit components might exist on both sides of the CNS, but in different ratios. Note that these models are in no way mutually exclusive. In fact, it is likely that all three strategies may be involved in the lateralization of DDC circuitry (see the main text).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2666075&req=5

fig4: Models for lateralization of neural tissue. (a) Equivalent regions on the left and right of the CNS are identical in composition and differ only in overall size. (b) Unique types of neuron, or patterns of connectivity, may be specified on either the left or right or both sides (indicated by unique red neurons on the left in this schematic). (c) Identical circuit components might exist on both sides of the CNS, but in different ratios. Note that these models are in no way mutually exclusive. In fact, it is likely that all three strategies may be involved in the lateralization of DDC circuitry (see the main text).
Mentions: This study has identified a fundamental strategy by which neural tissue on the left and right sides of the central nervous system (CNS) may become asymmetric. It gives rise to a model where the same or very similar circuitry components are produced on both sides, but in greatly different ratios, resulting in LR asymmetry in circuit microarchitecture that presumably translates into functional asymmetry. Figure 4 contrasts this model with two other models for how neural circuits might be lateralized. In perhaps the simplest model, equivalent regions on the left and right sides would contain the same classes of neuron and patterns of circuitry but differ only in size (figure 4a). As a result of such ‘scaling’, a particular cognitive function might be lateralized simply as a result of more neural substrate existing on one or the other side. In support of this possibility, Rosen (1996) observed that in the rat somatosensory/somatomotor cortex, asymmetry in tissue volume is strongly associated with the LR differences in the numbers of two subtypes of neuron, but there is only a weakly significant difference in cell packing density for one of the neuronal subtypes, suggesting that the left and right sides have similar neural architectures and show a proportional scaling to achieve differences in the quantity of neural tissue. In a third model, certain types of neuron, or patterns of connectivity, might be specific to one side and would not be present on the other side of the CNS (figure 4b). Hence, circuits on the left and right might receive different types of afferent inputs, perform different neural computations and/or connect to different downstream targets to mediate distinct types of cognition or behaviour. This mode of lateralization might be especially applicable to the zebrafish DDC: the parapineal projects exclusively to the left habenula and a subset of pallial neurons exclusively innervate a subdomain of the right habenula (above).

Bottom Line: The habenulae form part of the dorsal diencephalic conduction (DDC) system, a highly conserved pathway found in all vertebrates.In this review, we shall describe the neuroanatomy of the DDC, consider its physiology and behavioural involvement, and discuss examples of neural asymmetries within both habenular circuitry and the pineal complex.We will discuss studies in zebrafish, which have examined the organization and development of this circuit, uncovered how asymmetry is represented at the level of individual neurons and determined how such left-right differences arise during development.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell and Developmental Biology, University College London, London, UK. ibianco@mcb.harvard.edu

ABSTRACT
The dorsal diencephalon, or epithalamus, contains the bilaterally paired habenular nuclei and the pineal complex. The habenulae form part of the dorsal diencephalic conduction (DDC) system, a highly conserved pathway found in all vertebrates. In this review, we shall describe the neuroanatomy of the DDC, consider its physiology and behavioural involvement, and discuss examples of neural asymmetries within both habenular circuitry and the pineal complex. We will discuss studies in zebrafish, which have examined the organization and development of this circuit, uncovered how asymmetry is represented at the level of individual neurons and determined how such left-right differences arise during development.

Show MeSH