Although there are a number of different cells in the brain, the cell that has been studied the most is the neuron.

Neurons are highly complex, proactive structures. There are approximately one hundred billion neurons in the human brain with an average of ten thousand connections to other neurons.^* In embryological development, nerve cells are more plastic, meaning they have more potential to change. As development and growth of the nervous system progresses, nerve cells become more specialized as a result of genetic transcription. Each cell performs a highly specialized function. For example, some cells are always flexing the elbow or extending the elbow while others are always crying or laughing. Neural activity is a result of the balance between these opposing functions.

Two major functions of nerve cells are processing (or association) and modulation. Processing is more of a higher power (cortical) function such as memory recall or creative thought. Modulation is more of a lower power (limbic, brain stem, hypothalamic) function. Cells in different parts of the brain are highly differentiated or specialized to serve these different functions.

Some of the systems within a cell are the dendritic receptor sites, the secondary messenger systems, the nucleus, brain-derived nerve factors, the vesicles in the axon and auto receptors.

Cells communicate with each other by releasing and responding to different substances. These substances may induce the specialization of cells in embryological development. They stimulate or inhibit growth or they may stimulate or inhibit a specific function.

Hormones are produced by cells in specialized glandular tissue for this function, but all cells produce substances which effect other cells. A group of regulatory substances called cytokines, immunocytokines or immunotransmitters are recognized to modulate immune functioning. In contrast, neurotransmitters, neuroregulators or neuromodulators are a group of regulatory substances, which communicate information between nerve cells. The intracellular sensitivity or responsiveness of cells to these substances or the production of these substances by the cell is altered by a number of intercellular events.

Receptor sites are the landing pads on which the extracellular substances exert their effect. Receptors are of two types - ion channel receptors and G-protein receptors. When the extracellular substances effect the receptor site, it triggers a sequence of events within the cell.

The secondary messenger system is a sequence of events that occur after the G-protein receptor is stimulated to communicate information to the cell nucleus. The nucleus contains the genetic code, which responds to the secondary messenger system to either stimulate or inhibit the manner in which the genetic code is transcribed. Brain-derived nerve factors (BDNF) are made as a result of genetic code transcription and communication within the cell to alter cell functioning.

Vesicles in the axons contain neurotransmitters. The neurotransmitters are released into the synapse to communicate with other cells. The neurotransmitters undergo re-uptake back into the nerve cell and are then either used again or broken down (metabolized) with the process being facilitated by enzymes.

The auto receptors are impacted by BDNF within the cell and by neurotransmitters from other cells. They alter the sensitivity of nerve cells to neurotransmitters from other cells.

Degenerative disease is a result of cell damage and death without the repair or replacement of these cells, thereby resulting in a loss of cellular function.