What happens when the brain or spinal cord are damaged?
Damage to the nervous system can cause functional deficits in three main ways:
- Neurons themselves can be killed.
- Nerve fibres, connecting neurons to other neurons can be cut.
- Nerve fibre conduction can be stopped, by removing the insulating myelin (see later).
The symptoms, and loss of neurological functions that a patient will experience depend precisely which neurons or axons are affected. Different neurons in particular parts of the brain and spinal cord control particular functions. Thus damage in motor cortex, mentioned above, leads to loss of the ability to control muscles, and paralysis. A spinal cord injury blocks the flow of information from the brain the the spinal cord, leading to paralysis. Damage to other parts of the brain may lead to loss of vision, loss of speech, or other deficits depending on the function of the part of the nervous system that is damaged.
How are neurons killed?
Several processes, rapid and slow, can kill neurons. Rapid death of neurons is usually due to an acute process such an head injury or stroke. In head injury a region of the brain suffers direct trauma, which kills neurons and cuts axons. At present there is no effective way of preventing this form of death. In stroke an area of the brain loses its blood supply because an artery becomes blocked. Neurons need a rich blood supply to provide them with oxygen and nutrients, and die rapidly when the blood supply stops.
In some types of neurological disease neurons die slowly over many years. For instance, in Alzheimer’s disease neurons throughout the cerebral cortex gradually die, leading eventually to loss of memory and then dementia. In Parkinson’s disease, a small group of neurons in the substantia nigra region of the brain die over many years. When around 80% or these neurons are lost the symptoms of Parkinson’s disease become apparent. The factors that kill neurons slowly in these diseases are not fully understood. However in many chronic neurological diseases the slow accumulation of abnormal proteins in and around the neurons seems to play a central role in the toxic processes.
How are nerve fibres cut?
Cutting of nerve fibres is usually due to trauma, or less commonly due to a growing tumour. In spinal cord injury, trauma to the spine leads to dislocation of the bones of the spine, which crushes the spinal cord, usually at the base of the neck or under the rib cage. This cuts the nerve fibres that carry motor commands from the brain to the motor neurons and therefore to the muscles, and cuts sensory fibres that carry sensory information from the skin to the brain. The result is both paralysis and loss of sensation below the level of the injury.