Contralaterality of the forebrain

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The contralaterality of the forebrain ( Latin : contra 'against'; latus 'side', lateral 'side') indicates that the sides of the cerebrum and those of the thalamus predominantly represent the contralateral side of the body. This means that the right side of the brain predominantly represents the left half of the body and the left side of the brain the right half of the body. This contralateral representation affects both the motor skills (for example, a left-sided cerebral infarction causes a right-sided hemiplegia ) and the senses . Forebrain contralaterality affects all vertebrates , but not a single invertebrate .

The contralateral representation is by no means complete. Some exceptions are worth mentioning:

  • The sense of smell is connected to the ipsilateral hemisphere of the brain on the same side via the olfactory tract .
  • The visual paths only partially cross in the optic chiasm , so that both hemispheres of the brain “look” through both eyes .
  • In sharks , the visual pathways cross completely, and then again in the midbrain, so that the visual center in the forebrain has an ipsilateral representation.
  • In addition, some functions in the cerebrum are strongly lateralized (for example, most people have the language centers on the left).
  • The vast majority of the connections in the central nervous system are predominantly bilateral, so that some of the connections from and to the forebrain always connect the own side (ipsilateral) (but only a minority of the connections). If a lesion occurs in early childhood, hemiplegia can be completely overcome.

evolution

From the point of view of paleoneurology , the evolution of the junctions in the CNS is difficult, since only the outer shape of the brain and the cranial nerves can be derived from fossil skulls and because the closest relatives of vertebrates, tunicates and lancetfish do not have a real brain.

However, it is known that forebrain contralaterality occurs in all vertebrates. There are also no known invertebrates with contralaterality in parts of the nervous system. For example, neither the arthropods (arthropods) nor the cephalopods (cephalopods) have developed a predominantly contralateral representation.

anatomy

A chiasm is a crossover of a cranial nerve outside the central nervous system (CNS), a decussation is the crossing of a nerve path within the CNS.

The cranial nerves cross the optic nerve (chiasma opticum of the N. II), the trochlear nerve (dorsal chiasm of the N. IV) and part of the nervus oculomotorius (decussation of the N. III).

It is little known that the hearing also has a predominantly contralateral representation in the auditory cortex .

There are two types of decussations: on the one hand, a crossover in the same segment where the nerve cell attaches (origo), and on the other hand, a crossover into another segment. Crossovers of the first kind are extremely common and occur in all animals (including all invertebrates) and in all parts of the CNS. Well-known examples are the corpus callosum in the cerebrum and the Müller and Mauthner cells, which are responsible for the startle reflex in fish .

Crossovers of the second type occur (as far as known) only in vertebrates and only in nerve cells that run from the forebrain to the other CNS or vice versa. The crossovers of this second type cause the forebrain contralaterally.

Anatomy of some tracts

For the following presentation, tracts were selected that connect different motor and sensory systems. Much of the information in the table is based on or derived from animal studies. For the sake of clarity, the "Crossing" column is reduced to the three options ipsi-, bi- or contralateral. For more detailed information, please refer to the underlying sources. The information relates to the location of the brain area in relation to the respective innervated part of the body.

Ipsilateral Contralateral Bilateral
Spinal cord Spinal ganglia , neurons of the spinal cord
Medulla oblongata Nucleus cuneatus / Nucleus gracilis , Nucleus spinalis nervi trigemini , Nucleus nervi hypoglossi , Nuclei vestibulares laterales , Nucleus cochleares Nucleus olivaris inf. Nuclei vestibularis medialis , Formatio reticularis , Nucleus ambiguus
Cerebellum Exclusively ipsilateral
Pons Nucleus nervi abducentis , Nucleus motorius nervi facialis Nuclei pontis , nucleus trapezoideum Formatio reticularis , nucleus motorius nervi trigemini , nucleus faciales superior
Mesencephalon Oculomotor nerve (partial) Nucleus ruber , Colliculi inferiores , Colliculi superiores , Substantia nigra , Nervus trochlearis , Nervus oculomotorius (partly)
Diencephalon Exclusively contralateral
Telencephalon odor visual , motor , somatosensory , auditory cortex

Developmental and inherited disorders

Theories and explanations

For more than a century there have been theoretical approaches that attempt to give a plausible explanation for the fact that only vertebrates - and indeed all of them - have a contralaterally organized forebrain.

Visual map theory

Presentation of Cajal's theory.
O = optic chiasm;
C = visual cortex (and motor cortex);
M, S = crossed motor and sensory pathways to and from the spinal cord;
R, G: Sensory and motor ganglia and nerves.
A so-called retinotopic map shows that the center of the field of view is not on the inside of the left and right visual cortex, but on the outside.

The visual map theory was developed in 1899 by the neuroscientist and Nobel Prize winner Santiago Ramón y Cajal . He imagines the optic chiasm to have an evolutionary advantage because it would enable a holistic visual map of the left and right halves of the visual field of view (see Cajal's illustration).

The pupil causes the image on the retina to be reversed: Both above and below, as well as left and right, are thus swapped (see camera obscura ). Therefore, the center of each half of the field of view is on the outside of the retina. The visual pathways from the left and right eyes cross in the optic chiasm and continue via the corpus geniculatum laterale in the thalamus to the visual cortex on the back of the cerebrum. The crossover in the optic chiasm would bring the middle of the left and right visual fields together again, as Cajal has shown in his diagram (arrow at letter C). As described below, however, Cajal's idea does not work because there is another chiasm in the visual pathway .

Cajal further developed his theory and applied it to animals with frontal eyes (such as humans). He also derived a contralateral motor representation in the forebrain (see Cala's diagram, letters S and M).

resonance

Ramón y Cajal's theory has held up to this day. Recently, attempts have been made to provide Cajal's theory with a basis in developmental biology and evolutionary theory: the neurologist RG Loosemore has described the possible developmental and evolutionary stages from the single median eye to the pair of eyes with optic chiasm . As an important argument for his theory Loosemore cited the occurrence of the developmental disorder cyclopia .

criticism

Cajal's theory has some serious flaws, as described by de Lussanet and Osse. However, it is crucial that there is another chiasm in the visual pathway between the thalamus and the visual cortex (the so-called visual radiation, radiatio optica ) . As a result, the center of the field of view is still not on the median side of the left and right visual cortex, but on the outside (see illustration of the retinotope map). Consequently, the optic chiasm has no evolutionary advantage and the visual map theory is falsified .

Twist theories

Possible evolutionary scenario of the axial torsion theory.
Development scheme of the axial torsion theory

According to a further approach, contralaterality has no direct evolutionary advantage, but is a holdover from the early evolutionary past that was perpetuated in ontogeny . There are two such theories that have been developed independently.

Somatic twist theory

Somatic rotation theory ( ger .: Somatic twist theory) has its origins in the 1970s, but was not published until-2013. According to this theory, the front side of the head of vertebrates (including the forebrain, smell and eyes) is rotated 180 ° around the body axis. The theory was also developed as an alternative to the dorsoventral inversion theory.

Axial torsion theory

The axial rotation theory ( ger .: axial twist theory, rotation around the longitudinal axis) was published in 2012 de MHE Lussanet and WM Osse. This theory combines an evolutionary scenario with development in early embryogenesis . According to the evolutionary scenario (see figure), an ancestor of all vertebrates is turned to the side, which has led to opposite compensations in the body and in the front head area, which has resulted in a rotation about the body axis. Such twists actually take place and have been demonstrated in the early development of chickens and fish . In addition, similar contractions can be found in all animal groups that are most closely related to vertebrates ( tunicates , lancet fish , echinoderms , hemichordata ( wing gills )), so that such an evolutionary scenario is plausible.

Axial torsion theory explains a number of previously inexplicable phenomena, such as the misalignment of the heart and viscera and the asymmetrical aortic arch . On average, the brain would lie slightly crooked in the head, which was actually found (the so-called Petalia and Yakovlevian torsion ). In a common developmental disorder , holoprosencephaly , the cerebrum is extremely crooked or even diagonally across the head (and in such cases it is sometimes very short). Some developmental disorders are explained by the axial twist theory: Siamese twins with only one head form two spinal columns with a spinal cord, but these are formed on the left and right sides of the body. A rare form, the Janiceps twins , have two faces (front and back) and two central nervous systems, but they are on the left and right sides of the body. Such undesirable developments show strong evidence for the theory, because they show the consequences if the twist is not possible.

The differences between the two theories of twisting have been discussed in the scientific literature. Thus, the axial torsion theory is the best supported theory, although independent studies are still pending.

Further theories and theses

There are a number of other explanations, each attempting to explain some phenomena.

Branching theory

The branching theory (English: Parcellation theory) is EO Ebbesson, who starts from the thesis that with the increase in size of the central nervous system more and more nuclei of local specialization have emerged. These can be on one side or the other and chance determines whether the connections are more ipsilateral or crossed. This theory could be examined using statistical methods, but such a test is still pending.

Sensorimotor control

Some studies have looked at the effects of ipsilateral and contralateral connections from the eye to the legs and fins of fish and land animals. Loeb (1918) and Bertin (1994) developed the thesis that contralateral connections are optimal. Braitenberg (1984), however, showed in a study that both ipsi- and contralateral connections are essential.

Functional loop hypothesis

The functional loop hypothesis is a human tailored hypothesis. The idea is that the structure of the CNS is determined by the degree of specialization of each section. Crossings are therefore a sign of advanced differentiation.

When viewed superficially, the arrangement of the individual sections of the CNS tends to correspond to a sequence of increasing complexity: spinal ganglia , spinal cord, brain stem, cerebellum , mesencephalon , diencephalon , basal ganglia and cerebral cortex. The origin of the optic chiasm is explained as follows. According to the incidence of light on the eyes, i.e. in the direction of the stimulus, the axons grow out and partially or completely cross each other depending on the position of the eyes . In addition, the visual cortex is very far away from the actual organ of vision, which, according to the theory, reflects the high degree of specialization and intensive use of the visual apparatus. The sense of smell is less differentiated in humans and cortical areas are therefore closer to the sense organ.

3-D networking hypothesis

The 3-D wiring hypothesis is based on the ordered course of the nerve pathways, similar to the theory of Ramón y Cajal (see above). If you imagine that you are connecting the right side of the body to the brain in a finely structured manner, this will fit better on the left rather than the right cortex. This hypothesis forgets that the cerebellum and nuclei in the brain stem are not contralaterally connected and does not explain the optic chiasm either .

Information hypothesis

In 2010, and three years later, Banihani and Whitehead published two hypotheses about the intersection of the fiber tracts in the CNS, namely whether the information reaches the brain several times or only once. The latter was the case with living things without extremities. These experienced an equilateral flow of sensory information and also had an equilateral motor innervation of the muscles . The development of the extremities leads for the first time to the fact that different information from both sides of the body has to be processed. However, bilateral information will continue to be transmitted bilaterally. There are basically two link options for page-specific information: ipsi- or contralateral. In Banihani's view, the contralateral option is superior. According to this hypothesis, fish, especially jawless ones , do not have to have contralaterality in the forebrain, which, however, should be the case.

Injury Correlation Hypothesis

According to the hypothesis of Whitehead and Banihani, anatomical structures that reduce the likelihood of a possible limitation of mobility should be preferred in the course of evolution. The authors attribute precisely this function to the contralateral course.

The function of motor skills essentially depends on two variables: a functional brain and an equally functional musculoskeletal system. If a variable is damaged, this has far-reaching consequences for the search for food, protection against predators , for mating behavior and much more. From an evolutionary point of view, moderate damage is particularly serious, since the most severe injuries end in death, but minor damage is motor Hardly affect the system. If both the brain and the body are only slightly injured on one side, they can be compensated for by the uninjured side of the body or the uninjured side of the brain.

Individual evidence

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