The Neuron

[Ready for Fall 2026]

Modified: 2026-07-28 2:00 PM CDST

• The basic unit of the nervous system is the neuron or nerve cell.
  • The first thing we will see is that there are a lot of them.
  • Nobody has counted all of the neurons in the human body, but estimates run from 10-12 billion to100 billion or more.
  • For the sake of comparison, a cockroach has about 250,000 neurons, while a chimpanzee has 3.5 billion (with the same warnings about numbers as above).
  • The point to remember is that there are a great many, and that the large number contributes to the complexity of the nervous system.
  • When one starts to consider the connections between nerve cells, then the numbers truly become astronomical because a neuron may connect to as many as 75 other neurons. Most of the neurons in the human body, about 70%, are in the central nervous system.

• Neurons can be divided into three types, depending on their function.
  • Afferent or sensory neurons run from the sense organs to the central nervous system.
  • Efferent or motor neurons run from the central nervous system to the muscles
  • Interneurons or multipolar neurons are found chiefly within the brain and they are multiply connected to other neurons.
• Axons are a special part of a neuron.
  • They are the part that carries the nerve impulse down the cell.
  • Some axons are covered with Schwann Cells, those:
    • Insulate the axon (making precise movements possible)
    • Make the nerve impulse faster because it can jump along the Nodes of Ranvier
  • Nothing in the axon predisposes it to carry the nerve impulse from the cell body to the terminal buttons.
  • If you electrically stimulate an isolated axon in the middle, the nerve impulse will run in both directions.
  • It is the organization of the nerves that ensures that the impulses travel in the proper direction.

GRAPHIC of a neuron