Longitudinal fissure

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 arises 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.[2]

Though this fissure divides the brain, 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.[3] In contrast, the caudate nucleus, within the basal ganglia, was found to be larger in the right hemisphere.[4]

The corpus callosum connects the two halves of the brain at the bottom of its structure and delivers visual, auditory, and somatosensory messages between each half. Here, billions of neurons and glia can be found working together to send messages that form what is known as the cerebral cortex.[5] The corpus callosum is responsible for eye movement and visual perception, maintaining a balance between arousal and attention, and the ability to identify locations of sensory stimulation. In a clinical setting, those with epilepsy may benefit from the division of the corpus callosum.[6][7]

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.[8] Furthermore, studies have shown that this cephalon was bilateral, consisting of two or more connected sub-collections that are separated by the mid-sagittal plane,[9] suggesting the first example of such a division.

A neural crest appears in the mammalian embryo as soon as the 20th day of development.[10] It is during embryonic development that a neural tube appears and is folded into a hollow structure, as shown in Figure 1. This process is also known as neurulation.[11] The neural tube is where the central nervous system forms, which later on in development will be subdivided and differentiated into distinct sections of the brain and spinal cord. These subdivisions occur by signaling molecules that direct differentiated cells to their correct location of the organism.[12] 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.[13] The longitudinal fissure can appear as early as the eighth week of development, and distinctly separates the two hemispheres by around the tenth gestational week.[14]

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

In studies, low-frequency repetitive transcranial magnetic stimulation (rTMS) applications have been tested with various cognitive processes during time perception tasks. Studies have analyzed the effects of the low-frequency rTMS on tests of time perception when the rTMS has been applied to the "parietal medial longitudinal fissure". Findings have shown evidence to support the hypothesis that participants in this study would underestimate their perception of time for short amounts of time and overestimate for longer periods of time. Specifically, the 20 participants underestimated 1 second time intervals and overestimated 4 second/9 second intervals after applying 1-Hz rTMS.[24]

The longitudinal fissure can serve as an effective surgical passage in the frontal bone during central and pterional craniotomies, which is opening into the skull by surgery.[25][26] While there are variations in the head shapes of many species, dogs have been found to have a high variation in terms of head shapes making it difficult to find a brain surgical procedure that will work effectively for them. One goal of the study was to distinguish the longitudinal cerebral fissure anatomy and their possible variations in brachy‐(B), dolicho‐(D) and mesaticephalic‐(M) dogs. Even though the lateral cerebral fissure morphology was uniform in the dog breeds. Mesaticephalic‐(M) dogs were found to have the greatest surgical passage resulting in access to more brain structures, while the dolicho‐(D) dogs had the smallest surgical passage.