Related Experiment Video
Updated: May 7, 2026
12:23
Dissection and Culture of Commissural Neurons from Embryonic Spinal Cord
Published on: May 25, 2010
Somatic twist: a model for the evolution of decussation
1Department of Psychology, New School for Social Research.
Neuropsychology
|September 18, 2013
Summary
Nerve tract decussation in vertebrates evolved as a byproduct of a body plan inversion. This evolutionary event, a 180-degree somatic twist, relocated the neuraxis and caused nerve tracts to cross.
Area of Science:
- Evolutionary biology
- Neuroscience
- Comparative anatomy
Background:
- Sensory and motor nerve tracts cross sides (decussate) in the vertebrate central nervous system, connecting the brain to the opposite body side.
- This contralateral arrangement is absent in invertebrates, and no adaptive benefit for decussation has been identified.
Purpose of the Study:
- To explain the evolutionary origin of nerve tract decussation in chordates and vertebrates.
- To propose a model for the transition from invertebrate to vertebrate body plans.
Main Methods:
- Comparative morphological review of invertebrates, protochordates, and vertebrates.
- Development of a "somatic twist model" to explain the evolution of decussation.
Main Results:
- Decussation is proposed as a byproduct of a significant evolutionary change: the relocation of the neuraxis from the ventral to the dorsal side.
- This neuraxis shift is hypothesized to have occurred via a 180-degree body twist, which rotated nerve tracts and led to decussation.
Conclusions:
- Nerve tract decussation is not an adaptation but an incidental consequence of a genetically determined partial inversion of the body plan.
- This inversion involved a 180-degree rotation posterior to the brain and oropharynx, explaining the contralateral wiring of the nervous system.
Related Concept Videos
01:28Major Somatic Sensory Pathways
Sensory impulses related to touch, pressure, vibration, and proprioception from various body parts, such as the limbs, trunk, neck, and posterior head, travel to the cerebral cortex through the posterior column-medial lemniscus pathway. The pathway’s name derives from the two white-matter tracts that convey the impulses: the spinal cord's posterior column and the brainstem's medial lemniscus. First-order sensory neurons extend their axons into the spinal cord, forming the posterior columns...
01:16Spinal Cord: Cross-sectional Anatomy
The cross-sectional anatomy of the spinal cord offers a detailed view of its complex structure and function within the central nervous system. At the core of the spinal cord lies the gray matter, characterized by its butterfly or "H"-shaped appearance in cross-section. This central region is enveloped by white matter, with the overall structure divided into symmetrical halves by the dorsal median sulcus and the ventral median fissure.
Gray Matter and its Components
Central to the gray matter is...
Gray Matter and its Components
Central to the gray matter is...
01:29Overview of Somatic Sensory Pathways
Somatic sensory or somatosensory pathways refer to the neural pathways that carry information related to touch, pressure, pain, temperature, and proprioception from the skin, muscles, tendons, and joints to the brain. These pathways involve several stages of processing and integration of sensory information.
The somatosensory system is divided into three main pathways: the dorsal (or posterior) column-medial lemniscus, spinothalamic (or anterolateral), and spinocerebellar pathways.
The dorsal...
The somatosensory system is divided into three main pathways: the dorsal (or posterior) column-medial lemniscus, spinothalamic (or anterolateral), and spinocerebellar pathways.
The dorsal...
01:28Lateralization
Brain lateralization refers to the division of mental processes and functions between the two hemispheres of the brain, a phenomenon that optimizes neural efficiency and underpins complex abilities in humans. This specialization allows each hemisphere to perform tasks where it has a comparative advantage, facilitating more refined cognitive capabilities across different domains.
01:22Somatic Spinal Reflexes
Somatic spinal reflexes are rapid, involuntary muscular responses to external stimuli that involve the somatic musculature and the spinal cord.
One of the most well-known somatic spinal reflexes is the stretch reflex, which is activated by the sudden stretching of a muscle. This reflex involves the activation of specialized sensory receptors called muscle spindles, which are located in the muscle tissue and detect changes in the length and speed of muscle contractions. When a muscle is suddenly...
One of the most well-known somatic spinal reflexes is the stretch reflex, which is activated by the sudden stretching of a muscle. This reflex involves the activation of specialized sensory receptors called muscle spindles, which are located in the muscle tissue and detect changes in the length and speed of muscle contractions. When a muscle is suddenly...
01:21Deformations in a Transverse Cross Section
When a material is subjected to uniaxial stress, it elongates or contracts in the direction of the applied force, and also undergoes changes in the perpendicular directions. This behavior is crucial for understanding how materials behave under stress and is governed by mechanical properties such as Poisson's ratio v, which measures the ratio of transverse strain to axial strain.
As the material stretches, it expands or contracts in orthogonal directions to the load. This phenomenon varies...
As the material stretches, it expands or contracts in orthogonal directions to the load. This phenomenon varies...