Study Traces Brain and Nerve Pathways Across Vertebrate Evolution

A new article written by Dr. Martin Wessendorf and Daniel Wang explores how certain pathways in the brain and nerves have changed over time. The article appears in Frontiers in Neuroanatomy, and it was reported by Phys.org.

The optic nerve carries visual information, which is what the eyes see, to a place called the optic chiasm. The optic chiasm directs fibers from the right visual field to the left half of the brain. The optic tectum is another important part of this system. In mammals, it is called the superior colliculus, and it is present in all vertebrate species, which are animals with a backbone.

Other pathways carry different kinds of messages. According to Phys.org, the dorsal column pathway mediates light touch, vibration and body position. It has been observed in amphibians, birds, mammals and reptiles. The spinothalamic tract carries information about pain, itch and some forms of touch to the thalamus, a part of the brain that passes messages onward. A contralaterally organized spinothalamic tract, meaning one that crosses over to the opposite side of the body, has been found in birds, mammals, reptiles and some cartilaginous fishes, such as sharks.

The tectospinal pathway begins in the deepest layers of the optic tectum. The rubrospinal pathway starts in the red nucleus, which is found in the midbrain. Only mammals have a six-layered cerebral cortex, the outer layer of the brain. In primates and some dexterous species, which are animals skilled at using their limbs, the corticospinal pathway reaches the part of the spinal cord that controls muscles.

The authors also suggest some ideas about the distant past. They propose that Cambrian-era jawless fishes, which lived roughly 538 to 486 million years ago, had an optic chiasm and an optic tectum. They also propose that contralateral organization did not necessarily evolve as an adaptation, meaning it may not have developed because it helped animals survive.