Decussation describes a crossing over of nerve fibers from one side of the body to the other within the central nervous system. This crossing creates an X-shaped intersection of pathways. Understanding decussation is important for understanding how the nervous system functions.
Where Decussation Occurs
Decussation occurs in various locations throughout the nervous system, impacting both motor and sensory pathways. A key example is the pyramidal decussation of the corticospinal tracts. About 90% of these fibers cross in the lower medulla of the brainstem. These crossed fibers form the lateral corticospinal tract, which descends to control voluntary, skilled movements of the limbs on the opposite side of the body. The remaining 10% form the anterior corticospinal tract, controlling axial muscles for posture and balance, and often decussate at the spinal cord level they innervate.
Sensory pathways also decussate. The dorsal column-medial lemniscus pathway, which transmits fine touch, vibration, two-point discrimination, and proprioception (body position), decussates in the medulla oblongata. After sensory neurons synapse in the medulla’s gracile and cuneate nuclei, their axons cross to the opposite side, forming the medial lemniscus. This crossing is known as the sensory decussation.
The spinothalamic tract, conveying pain and temperature sensations, decussates at a different location. Its second-order neurons cross to the opposite side of the spinal cord, usually one to two segments above their entry point. These fibers then ascend through the anterolateral spinal cord into the brainstem.
Beyond motor and general sensory pathways, a partial decussation also occurs in the visual system at the optic chiasm. Nerve fibers from the nasal (inner) half of each retina cross to the opposite brain side. Fibers from the temporal (outer) half of each retina remain on the same side. This partial crossing ensures the visual cortex receives input from both eyes for the corresponding visual field, supporting binocular vision and depth perception.
Why Decussation Is Important
The crossing of nerve fibers through decussation enables contralateral control in the nervous system. This means one side of the brain primarily controls and processes information from the opposite side of the body. For example, the left cerebral hemisphere directs movements on the right side of the body, while the right hemisphere controls the left side.
Sensory information from one side of the body is processed by the opposite side of the brain. Sensations like touch, pain, and temperature from the right arm or leg are interpreted by the left hemisphere. This organization supports an integrated understanding of our surroundings and coordinated responses.
Understanding decussation has practical implications for diagnosing neurological conditions. For example, a stroke affecting the left side of the brain often results in motor deficits, such as weakness or paralysis, on the right side of the body due to the decussation of motor pathways. Sensory loss can also be localized based on which side of the body is affected relative to the lesion.
The evolutionary reasons for decussation are a subject of ongoing scientific discussion. One theory, the “somatic twist” hypothesis, suggests decussation is a byproduct of a significant evolutionary change in vertebrates. This theory proposes that early vertebrates’ body plan underwent a 180-degree twist relative to the brain and head. This twisting may have caused nerve tracts to cross, leading to observed decussations without an immediate adaptive advantage.