The brain’s left side generally controls the body’s right side, while the right side controls the left. A new hypothesis proposes that this cross-wired arrangement may have originated more than 500 million years ago, when later sensory and motor circuits developed around ancient visual pathways.
Dr. Martin Wessendorf, a neuroscientist and associate professor emeritus at the University of Minnesota, studies how the brainstem controls neurons in the spinal cord. Daniel Wang specializes in cognitive neuroscience and neurodegenerative diseases. Together, they investigated the evolutionary origins of neural connections that cross from one side of the central nervous system to the other.
In a study published in Frontiers of Neuroanatomy, the researchers examined several contralateral neural circuits in tetrapods—animals with four limbs, including mammals, birds, reptiles and amphibians. They propose that these pathways may have developed because evolution built new circuits around ancient visual systems.
How the brain processes visual information
One possible explanation begins with the eyes. When light passes through the lens, the image projected onto the retina is reversed. Objects to the right of an animal are projected onto the left side of the retina, while objects to the left are projected onto the right side.
Visual information then travels through the optic nerve to the optic chiasm. At this junction, nerve fibers are arranged so that information from the right visual field reaches the left side of the brain, while information from the left visual field reaches the right side.
Some of these signals travel to an ancient brain structure called the optic tectum, known in mammals as the superior colliculus. Located in the midbrain, the optic tectum helps generate rapid responses to visual events, such as detecting sudden movement nearby.
Each side of the optic tectum contains a representation of the opposite visual field. Deeper layers also contain maps of bodily sensations, known as somatosensory information.
The optic tectum is present in all vertebrates and is therefore considered one of the oldest structures in the vertebrate nervous system. Its early appearance may help explain why many other neural pathways cross the midline.
Why sensory signals cross the nervous system
The researchers examined two major pathways that carry sensations from the body to the brain. Both pass through the central nervous system, which consists of the brain and spinal cord, and both appear to be widely distributed among tetrapods.
The dorsal column pathway
The dorsal column pathway conveys gentle touch, vibration and information about the position of different body parts. Signals from the limbs and trunk travel upward through the dorsal portion of the spinal cord before reaching neurons in the brainstem, just above the spinal cord. The pathway then crosses to the opposite side.
Fish do not have this pathway, but it has been identified in amphibians, reptiles, birds and mammals. Its presence across these major animal groups suggests that it emerged early in tetrapod evolution and has remained largely conserved.
The spinothalamic tract
The spinothalamic tract transmits sensations including pain, itch and certain types of touch. Unlike the dorsal column pathway, it crosses to the opposite side within the spinal cord itself.
After crossing, the signals travel upward to the thalamus, a brain region above the midbrain that plays a central role in processing sensory and motor information.
This crossed arrangement occurs in birds, mammals and reptiles. It is also found in some cartilaginous fish, including sharks and rays, suggesting that the pathway has deeper evolutionary roots.
How the brain controls movement
The nervous system’s crossed organization is not limited to sensory processing. Researchers have identified three important pathways involved in controlling skeletal muscles that connect one side of the brain with the opposite side of the spinal cord.
The tectospinal pathway
The tectospinal pathway helps the body respond quickly to information from the environment.
The optic tectum processes more than visual information. It also receives signals about sounds and physical sensations, organizing them into a map that corresponds to visual representations on the opposite side of the brain.
For example, someone preparing to cross a road might suddenly notice a car approaching in their peripheral vision and instinctively turn toward it. The tectospinal pathway is one circuit that helps produce this rapid response. Similar reactions can be triggered by unexpected sounds or touches.
Motor signals originate in the deepest layer of the optic tectum and travel through nerve fibers to the opposite side of the spinal cord. This pathway is present in most tetrapods and in certain cartilaginous fish.
The erythrospinal pathway
Another circuit, known as the erythrospinal pathway, is thought to help animals control their limbs. It begins in a midbrain structure called the red nucleus and extends to the opposite side of the spinal cord.
The pathway is particularly prominent in most mammals, but it has also been observed in other tetrapods and in some cartilaginous fish.
The evolution of fine motor control
A third pathway, the corticospinal tract, is especially important in humans because it supports precise movements, including delicate control of the fingers.
Its evolutionary history is more complex than that of the other pathways. All vertebrates have a telencephalon, the anterior part of the brain. In humans, the telencephalon accounts for much of the brain’s total size. However, only mammals have a cerebral cortex organized into six distinct layers.
The six-layered cerebral cortex supports advanced functions including the integration of sensory information and memory, motor coordination and language processing.
The corticospinal tract extends from the cerebral cortex to the spinal cord, but its function varies among animals.
In most mammals, it primarily influences sensory processing. Nerve fibers reach regions of the spinal cord that receive information from the body, allowing the brain to regulate incoming sensations.
In primates, including humans, and in certain other animals capable of highly precise movements, the pathway also reaches regions of the spinal cord responsible for muscle control. These connections enable complex actions, such as the coordinated finger movements required for delicate tasks.
Depending on the species, corticospinal tracts can modulate sensory information or contribute directly to locomotion. In both cases, the fibers cross the nervous system’s midline.
The researchers suggest that humans may have developed advanced fine motor skills by adapting ancient neural circuits that originally helped control sensory input. Rather than creating entirely new systems, evolution may have assigned additional functions to existing pathways.
A possible origin more than 500 million years ago
Taken together, these pathways may help explain why so many neural connections cross between the two sides of the nervous system.
The researchers propose that early visual circuits established a pattern in which sensory and motor pathways were organized across opposite sides of the nervous system. Later circuits, particularly in tetrapods, may have inherited this arrangement.
All modern vertebrates have an optic chiasm and an optic tectum organized around information from opposite visual fields. The researchers therefore suggest that these structures may already have existed in primitive jawless fish during the Cambrian period, about 538 million to 486 million years ago.
If the hypothesis is correct, the basic organization underlying the brain’s cross-connections began developing more than 500 million years before modern humans appeared.
The idea may be testable. Scientists could examine fossil skulls of ancient jawless fish for anatomical evidence indicating whether these animals had an optic chiasm and an optic tectum.
Evolutionary inheritance rather than direct advantage
The hypothesis challenges the assumption that every unusual biological trait evolved because it provides a direct survival advantage.
Evolution often modifies existing structures instead of creating entirely new systems. Once ancient visual pathways established a crossed organization, their arrangement may have influenced the development of additional sensory and motor circuits.
In particular, the organization of the visual system—and especially the optic tectum—may have provided a structural foundation that later pathways followed.
Under this hypothesis, brain cross-connections did not necessarily arise because controlling the opposite side of the body is inherently more effective. Instead, they may reflect the lasting influence of ancient arrangements that evolution repeatedly adapted for new functions.
The pattern may be one of the clearest examples of the nervous system preserving its evolutionary history. The unusual relationship between the brain’s two hemispheres and the opposite sides of the body may ultimately be linked to the visual systems of animals that lived more than 500 million years ago.
Source: www.sciencedaily.com
