Complaints & connections · borters.ch
Decussationes
Movement crosses at the entrance to the spinal cord, pain at once within the cord, touch only in the medulla — and the cerebellum crosses twice, and therefore not at all.
Also known as: crossing of the nerve pathways · pyramidal decussation · optic chiasm · why the left hemisphere controls the right side · crossed deficits
Open in the tool — with search and all the links →This compilation is for information and replaces neither medical diagnosis nor treatment. If you have symptoms, see a doctor.
There is not one crossing point but several — and each lies at a different height. It is exactly from this that neurology reads where a lesion sits. Voluntary movement changes sides at the lowest point of the brain, where the medulla passes into the spinal cord: about 85 to 90 per cent of the fibres cross in the pyramidal decussation, the rest stay and cross at the level of their own segment — and because those fibres mainly serve the trunk, it usually remains surprisingly mobile after a stroke. The sensory pathways cross at two entirely different places: pain and temperature change sides at once in the spinal cord, within one or two segments; touch, vibration and position sense first travel up on their own side and cross in the medulla, a little above the motor crossing. Vision follows a different logic: in the optic chiasm only the fibres from the nasal half of the retina cross, so that the left half of the visual field of both eyes ends up on the right. And the cerebellum crosses twice — its outgoing path upwards, the cortical path back again — which is why cerebellar damage affects the same side of the body.
The combination of deficits says at what height the damage lies — cerebrum, brainstem, cord or nerve. Only then comes the question of what caused it.
Because the pathways cross in different places, the information only emerges from comparison: strength, touch, sharp and blunt, warm and cold — left against right.
The best-known explanation comes from Ramón y Cajal: a lens inverts the image, and for a coherent map of the surroundings to form in the brain the visual pathway had to cross — movement and touch crossed too, to stay aligned with it. All that is certain is that the crossing is very old and is found in all vertebrates.
The most everyday example of this principle — and the reason why the forehead is the decisive question there.
That says what to do immediately with sudden one-sided deficits — this only says why they appear on the opposite side.
Why a defect involves both eyes and yet points to only one hemisphere.
The pathways for movement cross in about eighty to ninety per cent in the lower brainstem — which is why a stroke on the left affects the right side of the body. The pathways for touch and position sense also cross there, while those for pain and temperature cross already in the spinal cord, one to two segments above where they enter. From that difference comes the picture in a half-sided spinal cord injury: paralysis on one side, loss of pain on the other.
The pyramidal decussation lies where the medulla becomes the spinal cord, roughly at the level of the foramen magnum. Damage above weakens the opposite side, damage below the same side.
The ten to fifteen per cent of fibres that do not cross there supply mainly the trunk muscles — they are served by both hemispheres at once.
If one half of the cord is damaged, strength and touch fail on one side, pain and temperature on the other. One injury, two sides — because two pathways cross in two places.
In the optic chiasm only the nasal fibres change sides. Damage exactly there removes the outer field on both sides, damage behind it the same half in both eyes.
The cerebellum crosses twice, and two crossings cancel out. Falling to the left or missing to the left usually means the problem is on the left.
A cranial nerve deficit on one side together with weakness on the other can only arise where the pathways are in the act of changing sides.
The forehead is supplied by both hemispheres, the mouth by only one. If someone can still wrinkle the forehead, the problem is in the brain; if not, it is in the nerve itself.
Hearing runs through so many connections to both sides that one-sided brain damage practically never causes one-sided deafness. Smell stays entirely on its own side — it is the exception.
Without B12 the insulating layer of the nerves is not maintained. A lack causes tingling, numbness and unsteadiness in the dark — and it is one of the few reasons for nerve trouble that can be fully undone if found early.
The nerves cover their energy needs through thiamine. It runs short with alcohol, after stomach operations, with long-lasting vomiting and under water tablets.
Here both count: a lack harms the nerves, and too high an intake over months harms them as well. Supplements therefore belong taken in a dose that has a reason.
Works together with B12. Given alone, folate can improve the blood count while the B12 deficiency goes on damaging the nerves — which is why both are looked at together.
Clearly worthwhile and well supported when deficient — vegan, older adults, certain drugs. No energy boost without a deficiency.
In pregnancy clearly established against neural-tube defects, otherwise mainly with deficiency. Can mask a B12 deficiency — watch both.
The most famous crossing is that of the optic nerves — and the pituitary sits directly beneath it. If something grows there, the outer visual field fails on both sides, as if looking through a tube. A prolactin value shows the most common of these growths in a single blood draw.
Not everything in the head crosses: oxytocin and ADH are made in the midbrain and travel down through the pituitary stalk without changing sides. This is why an injury to that stalk affects both hormones at once.
That the pathways cross sides explains why after a left-sided stroke the right side is affected. For treatment that means: the affected side is trained, even though the damage sits on the other.
AWMF 030-046 · S2e · 2022-11 — extended
Akuttherapie des ischämischen Schlaganfalls
checked 2026-09