Longitudinal fissure

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Longitudinal fissure
Human brain longitudinal fissure.png
The human brain as viewed from above. Median longitudinal fissure visible in red, running top to bottom.
Longitudinal fissure of cerebrum.gif
longitudinal fissure shown in red (animation)
Details
Identifiers
Latinfissura longitudinalis cerebri, fissura cerebri longitudinalis
NeuroNames35
NeuroLex IDbirnlex_4041
TAA14.1.09.007
FMA83727
Anatomical terms of neuroanatomy

The longitudinal fissure (or cerebral fissure, median longitudinal fissure, or interhemispheric fissure) is the deep groove that separates the two hemispheres of the vertebrate brain. The medial surfaces of the two hemispheres are as similarly convoluted by gyri and sulci as is the outer surface of the brain.

Development[edit]

Phylogenetically[edit]

It is thought that a majority of existing animals, including Homo sapiens, have evolved from a common wormlike ancestor that lived around 600 million years ago, called the urbilaterian. A bilaterian animal is one that has symmetrical left and right body halves. While it is still debated whether this species had a complex brain or not, development of similar species support the hypothesis that it had at least a simple anterior collection of nerve cells, called a cephalon.[1] Furthermore, studies have shown that this cephalon was bilateral in nature, consisting of two or more connected sub-collections that are separated by the mid-sagittal plane,[2] suggesting the first example of such a division.

Ontogenetically[edit]

A neural crest appears in the mammalian embryo as soon as the 20th day of development. In the following stages, a nerve tube appears and is folded into a hollow structure, as shown in Figure 1. The bilateral sides of this structure then give rise to the two hemispheres of the Homo sapiens cortex, but do not merge at any point besides the corpus callosum. As a result, the longitudinal fissure is formed.[3]

Figure 1: Early embryonic nerve tube, depicting the separation of two sides

Structure[edit]

Falx Cerebri[edit]

All three meninges of the cortex (dura mater, arachnoid mater, pia mater) fold and descend deep down into the longitudinal fissure, physically separating the two hemispheres. Falx cerebri is the name given to the dura mater in-between the two hemispheres, whose significance rises from the fact that it is the outermost layer of the meninges. These layers prevent any direct connectivity between the bilateral lobes of the cortex, thus requiring any tracts to pass through the corpus callosum. The vasculature of falx cerebri supplies blood to the innermost surfaces of the cortex, neighboring the midsagittal plane.

Cerebral asymmetry[edit]

The two hemispheres of the human cortex are not perfectly symmetrical, both in structure and in function. For example, the planum temporale, roughly corresponding to the Wernicke’s area, was found to be 10 times larger in the left than the right hemisphere.[4] In contrast, the caudate nucleus, within the basal ganglia, was found to be larger in the right hemisphere.[5]

Clinical Significance[edit]

The longitudinal fissure plays a key role in corpus callosotomy, a neurosurgery resulting in split brain, as it provides unobstructed access to the corpus callosum. Corpus callosotomy is one of the procedures used for pharmacologically treating intractable epilepsy cases, and it consists of the division of the nerve fibers running between the two hemispheres through the corpus callosum. A neurosurgeon separates the two hemispheres physically by pulling them apart with special tools, and cuts through either approximately two thirds of the fibers in the case of partial callosotomy, or the entirety in the case of complete callosotomy. Without the presence of longitudinal fissure, the corpus callosotomy procedure would be significantly more challenging and dangerous, as it would require the surgeon to navigate through densely connected cortical areas. Following the procedure, the two hemispheres are no longer able to communicate with each other as before.

While patients’ brains usually adapt and allow for uninterrupted daily life, cognitive tests can easily determine whether a patient has split brain. In an experiment involving a chimeric figure, with a woman’s face on the left half and a man’s face on the right half, a patient with split-brain focusing on the middle point will point to the woman’s face when prompted to point to the face in the picture, and will answer “a man” if asked what the picture is depicting.[6] This is due to the fact that the Fusiform Face Area (FFA) is in the right hemisphere, while language centers are predominantly in the left hemisphere.

Tractography work[edit]

As the corpus callosum is substantially smaller in surface area relative to the longitudinal fissure (Figure 3), fiber bundles passing through are densely packed together, and precision tracking is essential to distinguish between the individual bundles that originate from and lead to the same cortical centers. Understanding such connections allows us to understand the contralateral concurrences and what diseases can result from lesions to them. Diffusion tensor imaging (DTI or dMRI) along with fiber-tracking (FT) algorithms and functional Magnetic Resonance Imaging (fMRI) is used to image these bundles.[7][8] For instance, occipital-callosal fiber tracts were localized with 1–2 mm precision using DTI-TF techniques - which are very important for the cooperation of visual cortices, and any lesion to them can lead to alexia, the inability to read.

Figure 2: Diffusion Tensor Imaging example
Figure 3: Area of the corpus callosum in comparison with the longitudinal fissure surface area.

Additional images[edit]

External links[edit]

References[edit]

  1. ^ Hejnol, A., & Martindale, M. Q. (2008). Acoel development supports a simple planula-like urbilaterian. Philosophical Transactions of the Royal Society B: Biological Sciences,363(1496), 1493-1501. doi:10.1098/rstb.2007.2239
  2. ^ Mayer, G., Whitington, P. M., Sunnucks, P., & Pflüger, H. (2010). A revision of brain composition in onychophora (velvet worms) suggests that the tritocerebrum evolved in arthropods. BMC Evolutionary Biology, 10, 255. doi:http://dx.doi.org.proxy.library.cmu.edu/10.1186/1471-2148-10-255
  3. ^ Purves, D., Augustine, G. J., Fitzpatrick, D., Hall, W. C., LaMantia, A., White, L. E., . . . Platt, M. L. (2018). Neuroscience. New York ; Oxford: Sinauer Associates.
  4. ^ Jill B. Becker (2002). Behavioral Endocrinology 2e. MIT Press. pp. 103–. ISBN 978-0-262-52321-9. Retrieved 4 January 2013.
  5. ^ Watkins, K. (2001). Structural Asymmetries in the Human Brain: A Voxel-based Statistical Analysis of 142 MRI Scans. Cerebral Cortex, 11(9), 868-877. doi:10.1093/cercor/11.9.868
  6. ^ Levy, J., Trevarthen, C., & Sperry, R. W. (1972). Perception Of Bilateral Chimeric Figures Following Hemispheric Deconnexion. Brain, 95(1), 61-78. doi:10.1093/brain/95.1.61
  7. ^ Dougherty, R. F., Ben-Shachar, M., Bammer, R., Brewer, A. A., & Wandell, B. A. (2005). Functional organization of human occipital-callosal fiber tracts. Proceedings of the National Academy of Sciences, 102(20), 7350-7355. doi:10.1073/pnas.0500003102
  8. ^ Rokem, A., Takemura, H., Bock, A. S., Scherf, K. S., Behrmann, M., Wandell, B. A., . . . Pestilli, F. (2017). The visual white matter: The application of diffusion MRI and fiber tractography to vision science. Journal of Vision, 17(2), 4. doi:10.1167/17.2.4