The human brain weighs between 1.3 and 1.4 kilograms and has 100 billion neurons, which are the cells in the brain that transmit electrical signals. But neurons are not the only cells found in the brain. The other groups of cells, the glial cells, are the supportive cells in the brain. Three types of glial cells are found in the brain: astrocytes, microglial and oligodendroglial. The other two types, satellite cells and Schwann cells, are found in the peripheral nervous system, which is outside the brain and spinal cord.
Neurons
Three types of functional neurons exist, sensory neurons, motor neurons and interneurons. Sensory neurons relay information from the body to the brain, while the motor neurons send information from the brain to the body. The interneurons act as the connections between the sensory and motor neurons. All neurons have a basic structure of dendrites, soma, axon and presynaptic terminal, though they can vary in the number of projections. The dendrites receive the signals from another neuron, which travels down the axon to the presynaptic terminal, where that signal is transmitted to the next neuron. The soma, or cell body, contains the genetic material of the cell.
Astrocytes
Astrocytes derive their name from their star shape. Astrocytes have five major functions. This type of glial acts as a cleaner, ridding the brain of debris and breaking down the parts of dead neurons. Besides taking away discarded parts, astrocytes also bring nutrients to the neurons and hold the neurons in place. The last major function of astrocytes is to regulate what is in the extracellular space, which is the space between cells that contain certain substances, like proteins.
Microglia
Microglia are another type of glial cell in the brain, and play an essential supportive role. Like astrocytes, microglial take away and digest the parts of the neuron that have died.
Oligodendroglial
Oligodendroglial are the glial cells that form the myelin on the axon of the neuron. The myelin is the protective white matter on the axon that is separated into pieces by nodes of Ranvier. The function of the myelin is to insulate the axon and protect the signals when they are traveling down the axon. Schwann cells have the same functions for the neurons that are in the peripheral nervous system.
