Once the soma of a neuron has died the entire neuron dies, and can never be replaced. Repair of damaged neurons only occurs in myelinated neurons. Neuroglia are cells of the nervous system that help protect and support it. A spinal tract is a bundle of fibres in the CNS that travel long distances up or down the spinal chord. Ascending tracts carry impulses up the chord to the brain, while descending tracts carry impulses down the chord from the brain.
Spinal meninges are tough tubes of tissue which protect the chord.
The brain is highly complex, it contains about 1000 billion neurons, and weighs about 3 lbs in adults. Neurotransmitters are substances which excite or inhibit the neurons of the brain, facilitating communication between brain cells. These include endorphins, neuropeptides. Cerebrospinal fluid circulates around the brain and spinal tracts to provide protection in addition to that provided by the meninges and protective bones of the spine and skull. A sample of cerebrospinal fluid might be taken to see if cancerous cells have entered the CNS, or chemotherapy might be administrated to prevent or combat CNS involvement.