A neuron is an extremely small, specialized cell that receives and sends information from one part of the body to another. There is an estimated 100 billion neurons in your brain alone, and each one is so small that it cannot be seen by the naked eye.
Neurons don’t do it all on their own, however. Glial cells, much smaller and much more numerous than neurons (averaging ten glial cells for every neuron cell), provide the structural support, as well as provide nutrients and the removal of waste (which can include, dead or damaged neurons). It is now believed that glial cells aid in inter-neuron communication, with the use of gliotransmitters.
There are three primary types of neurons:
- Sensory Neurons;
- Motor Neurons; and
Each type of neuron handles a different kind of information.
Sensory neurons convey information about the senses ( e.g., touch, sound, vision, smell) from sensory organs to the brain.
Motor neurons communicate from the brain to the muscles and glands throughout the body.
Interneurons connect neurons to each other in order for them to “speak” to each other, enabling the body to handle multiple tasks simultaneously.
The Aspects of a Neuron
The cell body, also known as the soma, provides energy to the neuron, by processing nutrients and manufacturing protein. In the center of the cell body is the nucleus which contain chromosomes, genetic strands of DNA.
Dendrites protrude from the cell body, resembling the branches of a tree. These fibers receive information from other neurons. The number of dendrites on a neuron vary based on its functions, some neurons have thousands of dendrites.
The axon is single, long tube extending from the cell body of most neurons. Axons deliver information from the neuron to other cells particularly other neurons, but also to glands or muscles. The length of a neuron’s axon can vary greatly in length, some can be very small, but some can be a couple feet long.
Axon terminals branch-like fibers at the end of the axon. The terminals carry synaptic vesicles, tiny sacs that contain neurotransmitters. Neurotransmitters are chemicals that send messages to neurons.
The myelin sheath is a white covering of the axon, comprised of glial cells. Neurons with the sheath communicate twenty times faster than neurons without it. The myelin sheath is not a straight and continuous covering, in fact, it is broken into segments, separated by tiny gaps. The gaps of unmyelinated axon are called the nodes of ranvier.
Communication Within a Neuron
Ions, small electrically charged particles, move across the axon’s membrane, generating a brief electrical pulse, which transmits information from the cell body, to the axon terminals, this is called the action potential. After the action potential, is a thousandth of a second period, called the refractory period, during which the neuron cannot transmit another message.
Communication Between Neurons
Neurons do not actually touch each other; instead, there is a small gap between the dendrites of a neuron and the axon terminals of another neuron. This space between the dendrites and axon terminals is called the synaptic gap. There are two forms of communication between neurons: electrical and chemical.
The synaptic gap is incredibly narrow and ions form a “bridge” between the neurons and the electrical charge is passed along. Electrical communication is the fastest form, but less than one percent of all the neurons in your nervous system communicate this way.
In a chemical communication system, the presynaptic neuron, the neuron sending the message, releases the neurotransmitters from its synaptic vesicles. The neurotransmitters cross the synaptic gap and to the receptors on the dendrites of the postsynaptic neuron, the message-receiving neuron, which continues the flow of the message from neuron to neuron. This is called a synaptic transmission, and takes place in less than ten-millionths of a second.
- Micheal A. Arbib, The Handbook of Brain Theory and Neural Networks, 2nd edition, Massachusetts Institute of Technology, 2003.