INSTRUCTOR: A description of a neuron typically includes the dendrites, the cell body, and the axon. The dendrites, the input side of the neuron, receives signals from many other neurons. In most neurons, the dendrites are heavily branched. 

The cell body contains the neuron's nucleus and all the elements needed for normal metabolic activities of these cells. The axon finally is the output side of the neuron and sends neural impulses to other neurons. The axon usually extends outward from the cell body, and it may fork into several branches at its end. 

Neurons come in many shapes and sizes. Their cell bodies vary from 5 to about 100 microns in diameter. In comparison, the average human hair has a diameter of about 100 microns. Neurons dendrites are typically pretty short, say a few microns. Axons however, can be much longer. 

When a neuron is at rest, there is an uneven distribution of ions across the cell membrane. When a neuron is sufficiently stimulated, an action potential is generated and travels along the axon to the terminal branches by means of a change in polarity across the membrane of the axon. 

In response to a signal from another neuron, positively charged sodium and potassium gated ion channels open and close. The positively charged sodium ion channels open at the beginning of the action potential, and positively charged sodium ions move into the axon, causing depolarization. 

Repolarization occurs when the positively charged potassium channels open, and the positively charged potassium ions move out of the axon, creating a change in polarity between the outside of the cell and the inside. In other words, during an action potential, positively charged ions flow rapidly into the neuron, and then just as rapidly flow out. This electrical impulse travels down the axon in one direction only, to the axon terminal, where it signals other neurons. 

When an action potential occurs, we can describe it as the neuron firing. Although the flow of ions in or out of the neuron is fast, even so, the full movement of the action potential across the neuron is surprisingly slow. 

[INAUDIBLE] are mostly made of a fatty substance known as myelin. Soon after birth, these glial cells start to wrap themselves around the axons of neurons, especially the longer axons that span greater distances and require greater transmission speed. 

This myelin sheath covers a portion of the axon, and soon, the entire length of the axon is covered by a series of these wrappers. Crucially, though, there are gaps called the nodes of Ranvier between the successive wrappers, and it's this combination of wrappers and gaps that speeds up the nerve impulses traveling along these myelinated axons. 

If an axon is myelinated, ions can move into or out of the axon only at the nodes of Ranvier. At all other locations, the axon is enclosed within its myelin, and this blocks ion flow. Therefore, in essence, the action potential has to skip from node to node, and thanks to these jumps, it moves more quickly.