The Chemoaffinity hypothesis states that

neuron A neuron, neurone, or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous ...

s make connections with their targets based on interactions with specific molecular markers and, therefore, that the initial

wiring diagram A wiring diagram is a simplified conventional pictorial representation of an electrical circuit. It shows the components of the circuit as simplified shapes, and the power and signal connections between the devices. A wiring diagram usually gi ...

of an organism is (indirectly) determined by its

genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...

. The markers are generated during cellular differentiation and aid not only with

synaptogenesis Synaptogenesis is the formation of synapses between neurons in the nervous system. Although it occurs throughout a healthy person's lifespan, an explosion of synapse formation occurs during early brain development, known as exuberant synaptogenes ...

, but also act as guidance cues for their respective

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action ...

Sperry's experiments

Roger Wolcott Sperry Roger Wolcott Sperry (August 20, 1913 – April 17, 1994) was an American neuropsychologist, neurobiologist, cognitive neuroscientist, and Nobel laureate who, together with David Hunter Hubel and Torsten Nils Wiesel, won the 1981 Nobel Prize i ...

pioneered the inception of the Chemoaffinity Hypothesis following his 1960s experiments on the

African Clawed Frog The African clawed frog (''Xenopus laevis'', also known as the xenopus, African clawed toad, African claw-toed frog or the ''platanna'') is a species of African aquatic frog of the family Pipidae. Its name is derived from the three short claws ...

. He removed the eye of a frog and rotated it 180°, Sperry then replaced the eye and the visual nervous system repaired itself. However, the frog now had inverted vision. Initial eye orientation gives that the top of the eye is

Dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...

, and the bottom is

Ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

. Post-operation, the "top" of the eye is now Ventral, and the bottom is Dorsal. When a food source was above the frog, it extended its tongue downwards; meaning that the Dorsal-Ventral orientation of the eye still remained. In follow up experiments, the eye was detached and rotated 180° and the optic nerve was also cut to see if this would affect the Dorsal-Ventral orientation. The results were identical. Sperry concluded that each individual

optic nerve In neuroanatomy, the optic nerve, also known as the second cranial nerve, cranial nerve II, or simply CN II, is a paired cranial nerve that transmits visual information from the retina to the brain. In humans, the optic nerve is derived fro ...

and tectal neuron used some form of chemical marker which dictated their connectivity during development. He reasoned that when the eye had been rotated, each optic fiber and each tectal neuron possessed cytochemical labels that uniquely denoted their neuronal type and position and that optic fibers could utilize these labels to selectively navigate to their matching target cell, hence the visuomotor impairment.

References

{{Reflist Neurophysiology

Sperry's experiments

See also

References