Zion Harvey earned his place in medical history as the first child to undergo a successful bilateral hand transplant. He is also the first child in whom scientists have detected substantial changes in how sensations from the hands are displayed in the brain.
Zion tragically had to have both of his hands amputated due to a severe infection during infancy. As an amputee, Zion’s brain exhibited cortical reorganisation before his life-changing transplantation. Following the surgery, his brain reverted towards a more neurotypical pattern.
Topographic body representation is one of the of organising principles in the brain. Each area of the body that receives nerve sensations transmits signals to a corresponding site in the cortex. The spatial pattern in which those signals activate the brains neurons is called somatosensory representation. A cortical homunculus is the brains physical representation of the human body- which is inverted. Scientists have already established (from research in primates and fMRI imaging in adult humans) that following amputation the brain remaps itself when it no longer receives input from the hands. They concluded that “the brain area representing sensations from the lip shifts as much as 2 centimetres to the area formerly representing the hands”.
This phenomenon of the brain remapping is called massive cortical reorganisation (MCR). Zion’s brain exhibited remapping reorientation. The researchers used magnetoencephalography (MEG) technology, which measures magnetic activity in the brain in order to detect the location, strength and timing of the patient’s responses to sensory stimuli applied lightly to his lips and fingers. They performed MEGs four times in the year following the bilateral hand transplant, performing similar tests on five healthy children who served as age-matched controls. During the first two visits, the patient’s finger tips did not respond to tactile stimulation, when experimenters touched the patient’s lips, the MEG signal registered in the hand area of the brain’s cortex, but with a delay of 20 milliseconds compared to controls. In later visits, MEGs signals from lip stimulation had returned to the lip region of the brain, with a normal response time; a strong indication that brain remapping was reverting to a more normal pattern.
Following arm amputation, the region that represented the missing hand in primary somatosensory cortex (S1) becomes deprived of its primary input, causing changed boundaries of the S1 body map. This cortical reorganisation has been attributed to multiple mechanisms, including increased expression of previously masked inputs. In a maladaptive plasticity model, such reorganisation has been associated with phantom limb pain (PLP). Brain activity associated with phantom hand movements is also correlated with PLP, suggesting that preserved limb functional representation may serve as a complementary process. Studies point to a close interaction of sensory changes and alterations in brain regions involved in body representation. previous electrophysiology research in monkeys has identified extensive changes to the map features following amputation of a single digit.
There is no simple relationship between somatosensory map reorganisation, PLP and preserved hand representation. Although injury-related plasticity mechanisms may be the original driver of map changes, over time a plethora of additional factors such as use of a prosthesis, intact hand use, alterations in body representations as a consequence of amputation, may interact with injury-related remapping. Future research should aim to study PLP within this broader context, while taking into consideration the role of multiple brain networks and other contextual factors that can alter or stabilise the S1 body map. Findings demonstrate that manipulations to enhance central plasticity can improve sensorimotor recovery.
- sham x
My poem of the week:
You may forget but
Let me tell you
this: someone in
some future time
will think of us
Sapphi Loeb 60:
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