question: Why do tracts in the central nervous system mostly have a projection on the opposite hemisphere?
comment: There's a few ideas out there about why the decussation happens, most seem to suggest that it facilitates organization. I had the exact same question for my neuro professors, no one really knows for sure if it provides any sort of evolutionary advantage or if it was just an accident.
passages: Section::::Structure.:Posterior grey column.:Laminae III and IV.
  
  These laminae are also known as the nucleus proprius and contain a much smaller density of neurons than lamina II. There are projection neurons scattered throughout these layers. Mechanosensitive A beta fibers terminate in these layers. The layers receive input from lamina II and also control pain, temperature, and crude touch. C fibers that control nociception and temperature and sensory information from mechanoreceptors are relayed here.
  
  Section::::Structure.:Posterior grey column.:Lamina V.
   More recent studies using CSM have shown that the motor cortex is more complex than the arrangement pictured traditional homunculus, and that motor responses occur in the frontal lobe further away from the narrow strip next to the central sulcus.
  
  Areas of the cortex that inhibit movement upon stimulation have been found in some cases to be supplemental and not vital to motor function. These areas have been removed without compromising a patient's ability to move post-operation.
  
  Section::::Somatotopy.:Language Mapping.
   Ascending tracts typically have three levels of neurons that relay information from the physical point of reception to the actual point of interpretation in the brain. The fasciculus gracilis has first-order, second-order, and third-order nerve fibers described below:
  
  First-order fibers: found on the ipsilateral side of the spinal cord; ends at the gracile nucleus in medulla oblongata of the brainstem.
   As is true in other areas of the posteromedial cortex, the posterior cingulate cortex has no apparent connections to primary sensory or motor areas. Thus, it is unlikely to be involved in low-level sensory or motor processing.
  
  Section::::Anatomy.:Structural connections.:Human structure.
   In the 1940s, neurosurgeon Wilder Penfield and his neurologist colleague Herbert Jasper developed a technique of brain mapping to help reduce side effects caused by surgery to treat epilepsy. They stimulated motor and somatosensory cortices of the brain with small electrical currents to activate discrete brain regions. They found that stimulation of one hemisphere's motor cortex produces muscle contraction on the opposite side of the body. Furthermore, the functional map of the motor and sensory cortices is fairly consistent from person to person; Penfield and Jasper's famous pictures of the motor and sensory homunculi were the result.
  
  Section::::History.:Split-brain patients.
   Projections to the zona incerta arise across the cortical mantel from the frontal to the occipital lobes. The heaviest projections are from cingulate cortex, frontal and parietal areas. The head area of the body seems from these areas to have the largest representation in the zona incerta. These projections preferentially go to cortical layer I neurons. There are projections from the zona incerta back to the cerebral cortex.
  
  BULLET::::- Diencephalon
   Dorsal column nuclei
  
  In neuroanatomy, the dorsal column nuclei are a pair of nuclei in the dorsal columns in the brainstem. The name refers collectively to the cuneate nucleus and gracile nucleus, which are present at the junction between the spinal cord and the medulla oblongata. Both nuclei contain second-order neurons of the dorsal column-medial lemniscus pathway, which carries fine touch and proprioceptive information from the body to the brain. Each nucleus has an associated nerve tract, the gracile fasciculus and the cuneate fasciculus.
  
  Section::::Nuclei and tracts.
  
  The dorsal column nuclei also include their tracts.
  
  Section::::Nuclei and tracts.:Gracile nucleus.
   In addition to their distinctive inputs, the superficial and deep zones of the superior colliculus also have distinctive outputs. One of the most important outputs goes to the pulvinar and lateral intermediate areas of the thalamus, which in turn project to areas of the cerebral cortex that are involved in controlling eye movements. There are also projections from the superficial zone to the pretectal nuclei, lateral geniculate nucleus of the thalamus, and the parabigeminal nucleus. The projections from the deeper layers are more extensive. There are two large descending pathways, traveling to the brainstem and spinal cord, and numerous ascending projections to a variety of sensory and motor centers, including several that are involved in generating eye movements.
   The spinal cord is the main pathway for information connecting the brain and peripheral nervous system. The length of the spinal cord is much shorter than the length of the bony spinal column. The human spinal cord extends from the foramen magnum and continues through to the conus medullaris near the second lumbar vertebra, terminating in a fibrous extension known as the filum terminale.
   The posterior cingulate cortex is considered a paralimbic cortical structure, consisting of Brodmann areas 23 and 31. As part of the paralimbic cortex, it has fewer than six layers, placing its cell architecture in between the six-layered neocortex and the more primitive allocortex of core limbic structures. It has also been associated with the hippocampocentric subdivision of the paralimbic zone. The cytoarchitecture of the PCC is not entirely uniform, instead it contains distinct anterior and dorsal subregions, which are increasingly understood as distinct in function, as well as cytoarchitectural structure.
  
  Section::::Anatomy.:Structural connections.
  
  Section::::Anatomy.:Structural connections.:Nonhuman structure.
   There are three known poles of the cerebral hemispheres: the "occipital pole", the "frontal pole", and the "temporal pole".
  
  The central sulcus is a prominent fissure which separates the parietal lobe from the frontal lobe and the primary motor cortex from the primary somatosensory cortex.
   The general function of the tectal system is to direct behavioral responses toward specific points in egocentric ("body-centered") space. Each layer contains a topographic map of the surrounding world in retinotopic coordinates, and activation of neurons at a particular point in the map evokes a response directed toward the corresponding point in space. In primates, the superior colliculus has been studied mainly with respect to its role in directing eye movements. Visual input from the retina, or "command" input from the cerebral cortex, create a "bump" of activity in the tectal map, which, if strong enough, induces a saccadic eye movement. Even in primates, however, the superior colliculus is also involved in generating spatially directed head turns, arm-reaching movements, and shifts in attention that do not involve any overt movements. In other species, the tectum is involved in a wide range of responses, including whole-body turns in walking rats, swimming fish, or flying birds; tongue-strikes toward prey in frogs; fang-strikes in snakes; etc.
   A case study described recently in The Irish Medical Journal discussed the role the posterior commissure plays in the connection between the right occipital cortex and the language centers in the left hemisphere. This study explains how visual information from the left side of the visual field is received by the right visual cortex and then transferred to the word form system in the left hemisphere though the posterior commissure and the splenium. Disruption of the posterior commissure can cause alexia without agraphia. It is evident from this case study of alexia without agraphia that the posterior commissure plays a vital role in transferring information from the right occipital cortex to the language centers of the left hemisphere.
   Each segment of the spinal cord is associated with a pair of ganglia, called dorsal root ganglia, which are situated just outside the spinal cord. These ganglia contain cell bodies of sensory neurons. Axons of these sensory neurons travel into the spinal cord via the dorsal roots.
  
  Ventral roots consist of axons from motor neurons, which bring information to the periphery from cell bodies within the CNS. Dorsal roots and ventral roots come together and exit the intervertebral foramina as they become spinal nerves.
   Section::::Spinal Cord Anatomy.
  
  Spinal segments are the part of the spinal cord, from which ventral and dorsal roots exit to form a specific pair of spinal nerves.
  
  The spinal chord is not segmented itself, it is only made into segments by the spinal nerves as they leave.
  
  31 deferent segments exists in a human spinal cord:
  
  Section::::Spinal Cord Anatomy.:8 cervical segments.
  
  Cervical nerves exit above C1 segment and below C1-C7.
  
  Note that this creates 8 nerves and segments corresponding to only 7 cervical vertebra.
  
  Section::::Spinal Cord Anatomy.:12 thoracic segments.
  
  Nerves exit below T1-T12.
  
  Section::::Spinal Cord Anatomy.:5 lumbar segments.
   There are subpopulations of glial cells that provide guidance cues for axonal growth. The first set of cells, called the "mid-line glial zipper", regulate the midline fusion and guidance of pioneer axons to the septum towards the contralateral hemisphere. The "glial sling" is a second set, located at the corticoseptal boundary, which provide cellular substrates for callosal axon migration across the dorsal midline. The "glial wedge" is made up of radial fibers, secreting repellent cues to prevent axons from entering the septum and positioning them towards the corpus callosum. The last set of glial cells, located in the induseum griseum, control the positioning of pioneer cingulate neurons in the corpus callosum region.
   The best characterized and first widely accepted function of radial glia is their role as scaffolds for neuronal migration in the cerebral and cerebellar cortexes. This role can be easily visualized using the electron microscope or high-resolution time-lapse microscopy, through which neurons can be seen tightly wrapped around radial glia as they travel upwards through the cortex. Additional evidence suggests that many neurons may move between neighboring radial glial fibers during migration.
   Section::::Regulators of the basal ganglia core.:Central region of the thalamus.
  
  The centromedian nucleus is in the central region of the thalamus. In upper primates it has three parts instead of two, with their own types of neuron. Output from here goes to the subthalamic nucleus and the putamen. Its input includes fibers from the cortex and globus pallidus.
  
  Section::::Regulators of the basal ganglia core.:Pedunculopontine complex.
   The posterior cingulate cortex is highly connected and one of the most metabolically active regions in the brain, but there is no consensus as to its cognitive role. Cerebral blood flow and metabolic rate in the PCC are approximately 40% higher than average across the brain. The high functional connectivity of the PCC, signifies extensive intrinsic connectivity networks (networks of brain regions involved in a range of tasks that share common spatio-temporal patterns of activity).
  
  Section::::Function.:Emotion and memory.
   It is about 45 cm (18 in) long in men and around 43 cm (17 in) in women, ovoid-shaped, and is enlarged in the cervical and lumbar regions. The cervical enlargement, located from C3 to T2 spinal segments, is where sensory input comes from and motor output goes to the arms. The lumbar enlargement, located between L1 and S3 spinal segments, handles sensory input and motor output coming from and going to the legs.
   For a short time, the neural tube is open both cranially and caudally. These openings, called neuropores, close during the fourth week in humans. Improper closure of the neuropores can result in neural tube defects such as anencephaly or spina bifida.
  
  The dorsal part of the neural tube contains the alar plate, which is associated primarily with . The ventral part of the neural tube contains the basal plate, which is primarily associated with motor (i.e., muscle) control.
  
  Section::::Dorsal-ventral patterning.
   The gracile nucleus is medial to the cuneate nucleus; its neurons receive afferent input from dorsal root ganglia sensory neurons subserving the lower trunk and limbs. The gracile nucleus and fasciculus carry epicritic, kinesthetic, and conscious proprioceptive information from the lower part of the body (below the level of T6 in the spinal cord). Because of the large population of neurons in the gracile nucleus they give rise to a raised area called the gracile tubercle on the posterior side of the closed medulla at the floor of the fourth ventricle.
  
  Section::::Nuclei and tracts.:Cuneate nucleus.
   More recent fMRI studies have shown that humans have similar functional regions in and around the intraparietal sulcus and parietal-occipital junction. The human "parietal eye fields" and "parietal reach region", equivalent to LIP and MIP in the monkey, also appear to be organized in gaze-centered coordinates so that their goal-related activity is "remapped" when the eyes move. Both the left and right parietal systems play a determining role in self-transcendence, the personality trait measuring predisposition to spirituality.
  
  Section::::Clinical significance.
  
  Features of parietal lobe lesions are as follows:
  
  BULLET::::- Unilateral parietal lobe
  
  BULLET::::- Contralateral hemisensory loss
   The cytoarchitecture (cellular organization) of the cerebellum is highly uniform, with connections organized into a rough, three-dimensional array of perpendicular circuit elements. This organizational uniformity makes the nerve circuitry relatively easy to study. To envision this "perpendicular array", one might imagine a tree-lined street with wires running straight through the branches of one tree to the next.
   Zona incerta afferents terminate within the spinal cord gray matter, particularly the anterior horn, while spinal projections back to the zona incerta arise from cells located across the posterior horn and intermediate gray.
  
  BULLET::::- Other
  
  Zona incerta also has connections to the amygdala, basal forebrain, the osmoreceptors in the subfornical organ, olfactory bulb, posterior pituitary and habenula.
  
  Some of these projections appear in register; the representation of the same body part in cortex and spinal cord connect to the same areas in the zona incerta. This is possibly so with the superior colliculus.
  
  Section::::Function.
  