Like a catastropic storm, a stroke leaves a scene of destruction in its wake. As one of the most frequent life-threatening brain disorders, stroke sufferers often face severy consequences. Recovery may be lengthy, but considering the complexity of the wiring that has been disrupted, it is remarkable that recovery can even be achieved. Researchers from ETH Zurich have now discovered that at least in rats, some of this recovery occurs through re-wiring of the brainstem, a finding published in The Journal of Neuroscience on February 26.
As one of the peculiarities of our brain, the wiring of nerves is crossed: nerves from the left side of the brain project to the right side of the body, and vice versa. When the researchers induced a stroke in rats’ right motor cortex, which controls voluntary movements, the rats’ were not able to use their left forepaws and hindpaws properly. Their movements gradually recovered over four weeks, which suggests that the region of the spinal cord which lost its input through the stroke was connected to by new neurons. By putting a dye in the spinal cord, the researchers found where the neurons responsible for this recovery come from. They show that neurons from the left motor cortex, which was not affected by the stroke, grew into the spinal cord. Neurons from some areas of the brainstem, especially the Medulla oblongata and the raphi nuclei, also grow more connections to the spinal cord after a stroke. And in turn, the growth of neurons from the motor cortex to the brainstem is also increased. The new connections to the spinal cord are functional: when the researchers destroy the previously unaffected left motor cortex in a second stroke, the rats’ movements worsen again.
Unlike in the rest of the brain-spinal cord connections, where neuronal wires are crossed, at least some neurons run on the same side of the body after stroke recovery. The researchers speculate that the brainstem could be a major new relay post on the newly formed highway. As neuron sprouting from the cortex to the brainstem and the brainstem to the spinal cord is most increased, this could be the route through which the unaffected motor cortex can gain control over the denervated spinal cord on the same body side.
Original paper: www.jneurosci.org/content/34/9/3378