Decussation is a fundamental organizational principle of the nervous system, describing where nerve pathways cross from one side of the body to the other. This crossing-over of nerve fibers occurs at specific, consistent points within the central nervous system. This anatomical arrangement ensures that the brain’s hemispheres control the opposite side of the body. The concept is rooted in the Latin word for ten, decussis, which is shaped like an “X,” visually representing the intersecting pathways. This intricate wiring pattern governs both movement and sensation.
Defining the Concept of Decussation
Decussation refers to the oblique crossing of nerve fibers over the midline of the central nervous system, such as in the brainstem or spinal cord. This is distinct from a commissure, which is a bundle of nerve fibers connecting the two sides of the brain at the same level, like the corpus callosum. A decussation involves fibers changing sides as they travel up or down the neuraxis.
This organizational structure is thought to have deep evolutionary or developmental roots, although the precise reason for its existence remains a subject of scientific debate. Regardless of the underlying cause, this crossing is a highly conserved feature across many species. The decussation effectively reroutes signals, meaning that the origin of a signal is on one side of the body, but its final destination or control center is on the opposite side.
Motor and Sensory Pathways: Major Crossing Points
The nervous system contains two major categories of pathways that undergo decussation: motor signals traveling down from the brain and sensory signals traveling up to the brain. The primary motor pathway for voluntary movement is the corticospinal tract, also known as the pyramidal tract. These fibers originate in the cerebral cortex and descend through the brainstem until they reach the lower part of the medulla oblongata.
It is at this point, called the pyramidal decussation, that approximately 85 to 90% of the corticospinal fibers cross over to the opposite side. These crossed fibers then form the lateral corticospinal tract, which continues down the spinal cord to control muscles in the limbs and digits. The remaining fibers form the anterior corticospinal tract, which controls the trunk muscles, although many of these eventually cross at the level of the spinal nerves.
Sensory pathways also exhibit decussation, though the location depends on the type of sensation. The dorsal column-medial lemniscus pathway, which carries information about fine touch, vibration, and proprioception (body position), crosses high up in the brainstem. Specifically, the second-order neurons in this pathway decussate in the medulla oblongata to form the medial lemniscus, before continuing their ascent to the thalamus.
In contrast, the spinothalamic tract, responsible for transmitting sensations of pain and temperature, crosses much lower down. The second-order neurons in the spinothalamic tract decussate within the spinal cord itself, typically within one or two segments of where the sensory signal entered. This difference in crossing points means that a localized injury to the spinal cord can produce a complex pattern of sensory and motor loss on different sides of the body.
The Functional Outcome: Contralateral Control
The functional consequence of decussation is the establishment of contralateral control, meaning that one side of the brain governs the opposite side of the body. For instance, the motor cortex in the left cerebral hemisphere sends its signals down the corticospinal tract, ultimately controlling the movement of the right arm and leg. A similar pattern holds for sensory input, as the sensations felt on the right side of the body are ultimately processed by the left side of the brain.
This arrangement is most clearly demonstrated in clinical events like a stroke. If a stroke damages the motor areas in the left hemisphere of the brain, the resulting paralysis or weakness, known as hemiparesis, will manifest on the right side of the body. The functional impact is directly opposite the anatomical location of the damage due to the crossing of the pathways.
This contralateral organization is a fundamental principle that medical professionals use to localize damage within the central nervous system. By observing which side of the body has lost motor function or sensation, a doctor can accurately infer the side of the brain or brainstem where the injury has occurred. The consistent location of the decussation points ensures that this left-right functional mapping remains predictable throughout the nervous system.