Why most neural pathways are crossed over?

In Principles of Neural Science, edited by E. Kandel, et al., 4^th Edition, (page 324), it can be reading: “An important, but as yet unexplained, feature of the organization of the central nervous system is that most neural pathways are bilaterally symmetrical and cross over to the opposite (contralateral) side of the brain or spinal cord.”

I think this could be the explanation:

Sensory and motor activity of one side of the body should be handling together and exclusively by one of the two cerebral hemispheres, but not for both hemispheres.

Sensory activity from both eyes, for example, as a consequence of images of objects in the right visual field fall on half of both retina, and then projects their axons exclusively to the left hemisphere. Half of those axons had to cross over the middle line in the optic chiasm.

Continuing with the right side, as sensory and motor activities should be handled together; the motor activity emerging in the left cerebral hemisphere should cross over to reach the muscles of the right side of the body, nearest to the right visual field where objects were seen.

There is some exception to this rule. As the sternocleidomastoid is the only muscle in the body whose action is toward the opposite side of space (it turns the head to the opposite side), its motor innervations does not cross the middle line! (K´s page 880).

As sensory and motor activity from one side of the body are processing exclusively together in only one hemisphere, damages of some selective cortical areas produce amazing results. For example, damage to the right inferior parietal lobe usually produces sensory neglect of the left side of space. It means that the patient may fail to recognize the existence of contralateral space and deny their left limbs as their own!

Some patients will not dress, undress, or wash the effected side (personal neglect syndrome), even deny or disown their left arm or leg, going so far as to say, “Who put this arm in bed with me?

Because the idea of having a left limb is completely foreign to them, patients also appear to deny the existence of any paralysis in this limb and may attempt to leave the hospital prematurely since they believe nothing is wrong with them.

These patients, then, seem to lose a discrete part of self-awareness (K´s page 393).

It doesn’t mean that both hemispheres don’t communicate each other, but only when sensory and motor information of one of the side of the body were completely processed. That is why these patients cannot use their healthy hemisphere to compensate the damaged one.

These kind of impairs were call by Freud, agnosia (Greek, “not knowing”). There are some interesting kinds of agnosia after stroke; manifested by inability to perceive or recognize motion, faces, depth, color, etc. (K´s page 498-99).

http://www.propeller.com/story/2010/03/20/euro-murzi-why-most-neural-pathways-are-crossed-over/

Estupideces de diseño hecho por mi dios, no por el tuyo !!! ja, ja, ja

De los siguientes tres diseños biológicos que siempre me han parecido estupideces de mi dios, sólo al tercero le encuentro una lógica aproximada !!! Ja, ja, ja

1.- ¿Por qué permitir en la mujer la existencia de una comunicación “libre” desde el exterior de la vagina hasta la cavidad del peritoneo? Es decir, un espermatozoide o bacteria puede viajar desde el exterior de los genitales femeninos hacia la vagina, luego a través del cérvix atravesar la cavidad uterina y llegar hasta el extremo interno de las Trompas de Falopio, y caer en la cavidad peritoneal !!!

En el hombre no existe tal tipo de estupidez, digo comunicación “libre” que se le parezca !!! Ja, ja, ja.

2.- ¿Por qué diseñar una complicada maraña de vías nerviosas sensoriales y motoras de manera cruzada? He tratado de dar una explicación lógica a esta estupidez divina, cuando he afirmado que los hemisferios cerebrales no pueden procesar la información sensorial y la motora, separadamente !!!

http://www.propeller.com/story/2010/03/20/euro-murzi-why-most-neural-pathways-are-crossed-over/

Pero sí mi dios me hubiera pedido mi opinión, yo hubiera entrecruzado en el quiasma óptico los axones provenientes de las hemiretinas temporales en vez de las nasales, y hubiese evitado en parte tremendo enmarañamiento y "dolor de cabeza" para los pobres estudiantes de medicina !!! Ja, ja, ja

De esta manera, el hemisferio cerebral derecho comandaría su lado derecho, y el hemisferio izquierdo su contraparte izquierda !!! Ja, ja, ja.

¿No lo creen? !!! ja, ja, ja

3.- ¿Por qué mantener las gónadas masculinas “guindando” y las femeninas no? Tal vez, porque durante la copulación y eyaculación, la presencia de aire en la vagina, bajaría la temperatura de esta por debajo de los 37^oC.

Sí los espermatozoides no se hubieran “aclimatado” a temperaturas por debajo de los 37 ^oC al estar “guindando”, no podrían sobrevivir su pasaje vaginal “hipotérmico”. Por supuesto que después morirán por la “hipertermia” uterina o de las Trompas de Falopio, pero esto les ha permitido sobrevivir algunas horas adicionales (promedio 36 h) para poder alcanzar el ovulo!

¿Qué les parece mi elucubración biológica? Ja, ja, ja

Chao pescao, nos vemos en Chacao !!! Ja, ja, ja

¿Why we see "dark"?

In the Neuroscience book edited by Purves et al. (page 232), it can be reading: “The reason for this unusual arrangement (rods and cones are active in the “dark” and inactive in “light”) compared to other sensory receptor cells is not known”!

I think the reason could be this:

To human beings can “see” places where come from few photons as a cave entrance or not “visible” photons from black painted surfaces, uses the following trick:

1.- When none or few visible photons are reaching the retina, rods and cones are active and releasing their transmitter.

¿Why? Because with their activity are producing the sensation of “Darkness”; black is a positive sensation; the blind eye does not “see black”; it “sees nothing” (W. Ganong in Medical Physiology, 22 edition, page 163).

2.- When some few photons reach retina, rods will inactive and produce the sensation of nocturnal vision (without color sensation). Meanwhile, cones still keep active.

3.- When a lot of photons reach retina, cones also will inactive and produce the sensation of diurnal vision with color sensations. Then, rods will saturate.

In other worlds, “dark” is a “background”, “filling” or “stuffing” in the visual field, otherwise, poor illuminated areas or objects will be, not “dark”, but invisible!

In poetic words: “Photons write on the retina, as a chalk on a blackboard”!

Saying that we are in the “dark” or in the “light” is a physic mistake, because both terms are mental sensation!

We must say, instead of "being in the dark or light", "being in absence or presence of photons" Ja, ja, ja.

http://www.propeller.com/story/2010/03/19/191why-do-we-see-dark191why-do-we-see-dark/