The Neuron – External Structure and Classification
The neuron’s main function is to transmit electrical signals (information) in one direction (dendrite to axon). The neuron is said to be polarized because information can only travel in a set direction (information cannot travel backwards).
External Structure of the Neuron
1. Cell Body
Each soma receives electrical impulses from a number of dendrites.
Dendrites are like antennas that receive information from other neurons and transmit that information to the soma. Each dendrite that connects to the soma contains many different “branches” and this ensemble is called the “dendritic tree.”
Let’s dive into a little more details here:
Each point where one segment of the dendrite branches into two segments is called a bifurcation. (The first time the dendrite branches in two is called the first bifurcation, the second time it branches in two is called the second bifurcation, and so on.)
The dendrites have the amazing ability to receive incoming signals directly on their membrane, or on tiny little protrusions called dendritic spines. My professor liked to say that these spines look like, “forests of tiny lollipops.”
3. The Axon
Each neuron has one axon that will transmit the information to the following cell. The axon connects to the soma at the axon hillock. The axon hillock is a very important structure as it is the final point where all the information from all the dendrites get integrated into one clear signal that will travel through the axon and to the next cell.
In a human body, axons will very often be covered by a myelin sheath that increases the speed of propagation of the electrical information. This myelin sheath has gaps, and these gaps are called Nodes of Ranvier.
The axon ends in small structures called synaptic terminals. The synaptic terminal will connect to another neuron’s dendrite (or dendritic spine) and transmit the information.
1. Multipolar neurons.
- Pyramidal neurons in the cerebral cortex
- Purkinje neurons in the cerebellum
- Motor neurons in the anterior horn of the spinal cord
2. Bipolar Neurons
Bipolar neurons have only two process that connect to the cell body: one dendrite and one axon. (This is easy to remember as, generally speaking, the prefix “bi” refers to the number two, such as in bilingual – two languages)
Bipolar neurons are only found in specific areas of the nervous system:
3. Pseudounipolar neurons
There is only one process (this gives us the “unipolar part”) that branches into two (which is why we add “pseudo” at the beginning… It doesn’t look unipolar). This process is structurally similar to that of an axon, but it can receive information as well.
Minimum Points to remember:
- A cell body-called the soma
- Dendrites that receive information
- An axon that transmits information to another cell.
Neurons are polarized in that the information can only travel in one direction: dendrite to axon.
There are 3 different types of neurons:
- Multipolar neurons: one axon, many dendrites
- Bipolar neurons: one axon, one dendrite
- Pseudounipolar neurons: One process that branches in two.
That’s it for this introduction to the neuron’s structure.
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