Brain damage is often described as either a white or grey matter injury, but what is the difference?
White Matter Injuries
White matter injuries occur when white matter tracts (bundles of myelinated axons) are damaged. Damage essentially cuts off or limits communication between areas of grey matter (neuron cell bodies and dendrites, the neuropil, glial cells, and capillaries). Perhaps the easiest example to envision is a spinal cord injury. The outside of the spinal cord is composed of large white matter tracts. Transecting or compressing these tracts can lead to paralysis because information from the brain’s motor cortex (grey matter) can no longer reach the spinal cord and muscles. White matter tracts relaying sensory information from the muscles and skin to the brain can also be lost. In this case, the patient may not be able to feel an ice cube placed on his or her skin.
White matter injuries are very serious, but, depending on the type and extent of the injury, extensive recovery may occur. As long as the neuron cell bodies remain healthy, axons can regrow and slowly repair themselves. Functional recovery may also occur if the information can be transmitted through an alternative route. In many cases, this occurs through the strengthening of weak connections that already exist.
Grey Matter Injuries
Neuronal death is at the heart of grey matter injury. The neuron cell bodies are responsible for keeping the entire neuron alive and healthy and that takes a lot of energy. Grey matter is therefore very susceptible to injury when oxygen levels are low (hypoxia) such as during an ischemic event. Damage also occurs when the local environment changes such as during an intracerebral hemorrhage or when physical damage occurs. Damaged neurons and glia also release factors that can increase the effect of the injury. In many cases, the initial damage causes a series of downstream effects that can initiate apoptosis (programmed cell death). Once damage has occurred, the cell cannot support its axons or dendrites and the entire cell dies. This is especially true in neurons that have large cell bodies and long axons. In contrast, after a white matter injury, a healthy cell body may be able to repair the damaged axon.
Like white matter injuries, the type and extent of the injury largely dictates recovery. Young infants and children often have better outcomes than adults since their neural systems are still developing and more adaptable. Targeted rehabilitation training can also improve functional outcomes. For instance, after selective damage to the motor cortex, fine finger movements can be improved when the rehabilitative training results in an expansion of the finger motor representation within the motor cortex. In other words, nearby neurons can begin to take over the functions of the damaged neurons.
White and Grey Matter Interactions
Although white and grey matter injuries are often presented as separate entities, it is hard to have one without the other. This is because (1) many injuries simply involve both grey and white matter areas, (2) a single neuron can have its cell body and dendrites in the grey matter and an axon in the white matter, and (3) because neurons depend on their interactions with other neurons.
White matter damage can cause a great deal of stress and increase the energy demands on the neuron. In some cases, the neuron will not be able to meet the energy demands and the entire neuron will die (cell body, axon, and dendrites). This often starts with retraction of the injured axon. On the other end, the grey matter targeted by that axon (i.e. the post-synaptic neurons) can shrink and die from inactivity or be taken over by other functions (i.e. innervated by other neurons). Grey matter near the site of white matter injury can also be damaged by factors released from damaged axons and vice versa.