The autonomic nervous system is a part of the peripheral nervous system that controls the function of internal organs including the heart, lungs, and gastrointestinal organs. This is done through the use of nerves that activate and/or inactivate glands and muscles.

The somatic nervous system is a part of the peripheral nervous system and controls voluntary body movements. The central nervous system involves only the brain and spinal cord. The autonomic nervous system is not under voluntary control.

The myelin sheath on neurons is a fatty sheath provided by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system that increases the speed of signal transmission through saltatory conduction. Myelin makes matter in the brain appear white. Areas of the axon that aren’t myelinated are called nodes of Ranvier.

Nodes of Ranvier are unmyelinated portions of the axon, located in areas of the myelin sheath. The junctions between neurons are called synaptic clefts, where neurotransmitters from one neuron are released to cause a change in the receiving neuron.

Demyelination of axons is associated with neurodegenerative diseases. The loss of myelination negatively impacts the ability of neurons to transmit signals throughout the body. Examples of demyelination diseases are multiple sclerosis and Guillain–Barre syndrome. HIV is a viral infection that attacks the immune system. Anemia is a blood disorder in which there are not enough functioning red blood cells to provide the body tissues with sufficient oxygen. Epilepsy is a disease that involves frequent seizures.

Saltatory conduction is a type of signal transmission in myelinated axons. Here, the action potential (wave of sodium ions) passes from one node of Ranvier to another, which increases the rate of transmission.

Membrane potential is a term used to describe the difference in electrical charge across a cell membrane. This difference is determined by ion concentration inside and outside of the cell and can change due to ion-gated channels and ion pumps, among others. Membrane potential allows cells to transmit electrical signals. During an action potential, the membrane is said to be depolarized, which means the membrane potential becomes less negative.

The resting potential of a cell is the relatively static membrane potential that serves as the baseline voltage of a cell, which is about  in neurons. The resting potential stands in opposition to the action potential.

The threshold potential of a cell is a specific membrane potential that is needed in order to initiate action potential. Therefore, threshold potential precedes action potential. Note that reaching the threshold potential triggers a positive feedback loop, causing more sodium to rush into the cell, further depolarizing the membrane potential.

The axon hillock is the area of a neuron that connects the soma and axon. This is also the area of the of the neuron that "decides" whether an action potential would be initiated or not, depending on whether the cell has reached threshold or not.