In Depth: Inner Brain 3
Neurons are cells that enable communication between different parts of the body. Information, in the form of electrical signals, travels along networks of these neurons. It moves either to the brain from the body or to the body from the brain.
The shape of a neuron is long and slender, almost like the shoot of a plant. At one end is a group of branched tendrils called dendrites. They receive the electrical signals, called nerve impulses, from the previous neuron in the network.
The neurons in a network don’t actually touch. There is a gap between them called a synapse. Most neurons communicate across the gap by releasing chemicals called neurotransmitters that reach the next neuron and stimulate it into firing a nerve impulse.
Because of neuron structure, impulses can only travel in one direction. So neurons are classified by whether they send or receive brain messages. Sensory, or afferent, neurons send information to the brain for processing; motor, or efferent, neurons send commands from the brain back to the body.
There is also a third type of neuron. Found in the central nervous system, association neurons, or interneurons, connect the other two types of neuron and enable the central nervous system to sort information as needed.
The other major category of nerve cells is neuroglial cells. These operate as the support system for neurons and are composed of several cell types with specific tasks.
Neuroglial cells include:
- Astrocytes: These cells connect neurons to capillaries, small blood vessels. This link allows neurons to receive nutrients and flush away wastes.
- Microglial cells: Part of the immune defense of the central nervous system, these cells swarm to damaged areas in the brain and spinal cord to kill germs and also remove waste.
- Schwann cells: These are attached to the axon of a neuron — the part that lies between the cell body and the terminal ends — and produce a material called myelin, which insulates and protects the axon.