What are neurons? Why are neurons important? What do neurons do? What are the different types of neurons? Sounds a little intimidating? No need to be afraid. All it requires to understand neurons (at least basic knowledge) is a bit of patience and focus. In this article on Neuron facts, we are going to present answers to all the aforementioned questions in the simplest possible way. Stay with us for a while and we are pretty sure that for a smart person like you, neurons will no longer pose a serious challenge. So, let us begin…
Neuron Facts: 1-5 | What are Neurons?
1. There are 4 major types of tissues in our body. One of them is the Nervous Tissue. The Nervous Tissue makes up the nerves, the spinal cord and the brain.
2. The Nervous Tissue is made mainly of two types of cells – the Neurons (nerve cells) and the Neuroglia or the Glial Cells. Glial Cells are integral part of the Nervous Tissue because they are support cells and are very necessary. However, we will keep Glial Cells for another article.
3. Neurons on the other hand are the primary structural and functional cells of the Nervous Tissue. They are highly specialized cells.
4. These Neurons are electrically excitable. This means that they get excited by electrical impulses. They are designed for fast transmission of information in form of signals – both chemical and electrical in nature.
5. Neurons are major components of our brain, our central nervous system, our peripheral nervous system, our spinal cord and our autonomic ganglia.
Neuron Facts: 6-10 | Structure of Neurons
6. There are Neurons of various sizes and shapes. However, a typical Neuron found in a vertebrate body such as humans, the structure of a Neuron can be generally broken down into four broad regions.
7. The four broad regions of a Neuron are: Soma, Dendrites, Axon and Synaptic Terminals. The whole of the Neuron remains wrapped up in a cell membrane.
8. The Soma is the cell body. It is enlarged and is pretty much the same as other cells that are found in a body. The Soma is responsible for carrying out all metabolic and protein synthesis activities of the neuron. It is this Soma inside which you can find:
- The nucleus of the cell.
- Other cell organelles like the Mitochondria, Golgi Apparatus, Endoplasmic Reticulum etc.
9. The Dendrites are small Cytoplasmic extensions of the Neurons. These Dendrites extend out in a tree-like fashion. These extensions are designed for receiving signals, which are transmitted towards the Soma.
10. Then comes the Axon. It is also a Cytoplasmic extension but it is very long and not branched out like the Dendrites. The job of the Axon is to send signals away from Soma.
Neuron Facts: 11-15 | Structure of Neurons
11. The Axon is really long and thin. To put in perspective, the length of the Axon can be ten time or hundred times or tens of thousands of times the total diameter of the Soma.
12. The Axon will however usually branch out just like the Dendrites. This branching out of the Axon is necessary for communicating with several target cells.
13. Now, this Axon has to emerge from the Soma. The part where this emergence takes place is known as Axon Hillock. This Axon Hillock is not just a simple anatomical structure. It is this place of the neuron where maximum density of voltage-dependent sodium channels are found, making it the most easily electrically excitable part of the whole Neuron.
14. Synaptic Terminal or the Axon Terminal is the place where Synapses are present. These Synapses are highly specialized structures designed for releasing neurotransmitters (they are chemicals) so that communication with target Neurons can take place.
15. Synaptic Terminals actually connect with Dendrites of other Neurons or with the Soma of other Neurons or with Effector Cells found in glands or muscles.
Neuron Facts: 16-20 | Types of Neurons According to Shape
16. If we try to classify Neurons on the basis of shape, we have three types of Neurons. They are mentioned in the table below:
|Multipolar Neurons||Several||Only One|
|Bipolar Neurons||Only One||Only One|
17. Note that Dendrites and Axon are called ‘Processes’. In case of a Multipolar Neuron, there are several process that include one Axon and several Dendrites.
18. In case of Bipolar Neurons, we have two Processes – one Dendrite and one Axon.
19. In case of Unipolar Neurons, we have just one ‘Process’ that can branch out. This ‘Process’ can actually work as a Dendrite or it can work as an Axon.
20. There are some Unipolar Neurons that actually started out as Bipolar Neurons during the developmental stage. Such Unipolar Neurons are usually called as Pseudounipolar Neurons.
Neuron Facts: 21-25 | Types of Neurons According to Function
21. Neurons can also be divided on the basis of the functions they perform. On this basis, there are THREE types of neurons. They are:
- Sensory Neurons
- Motor Neurons
22. Sensory Neurons are those Neurons that will pick up information from sensory receptors on body, for example – Ears, Tongue, Eyes, Skin, Nose etc. The information picked up by these Neurons will send the information towards the Central Nervous System.
23. Motor Neurons are those Neurons that will take the information processed in the Central Nervous System and send it away to glands or muscles.
24. Sensory Neurons are also known as AFFERENT Neurons and Motor Neurons are also known as EFFERENT Neurons.
25. Finally, we have the Interneurons that connect the Sensory Neurons and the Motor Neurons for facilitating transmission of information or signals. These Interneurons are located entirely inside the Central Nervous System.
Neuron Facts: 26-30 | Functions of Neuron
26. Neurons have one sole purpose – communication. This takes place in four different steps. These steps are mentioned below.
27. Neurons will pick signals. These signals may originate from a source that is external (such us a touching a hot coal where the hot coal is the external source) or can originate from another Neuron (for example a Neuron in our brain can receive signals from hundreds and thousands of other Neurons that are interconnected with Interneurons forming what is known as the neural network).
28. Once a Neuron picks up a signal, it will process or integrate the signal and decide whether or not to pass on the signal as output. This processing usually takes place in CNS. This integration is pretty interesting and takes place both in time and space. What does that mean? It means that processing will take place right at the point in time when the input is taking place and in case of multiple inputs, processing also takes place during the time between two inputs. This is called integration in time. Then the processing also takes place all along the neuron surface. This is called integration in space.
29. Once the integration is done and the information is to be passed on, the signal will be passed on across its length at a very high speed of about 100 meters per second.
30. Finally, the electrical signal is converted into chemical signal by the Neuron and is passed on the next Neuron as output or is delivered directly to the glands or muscles as output. The passage of signal from one Neuron to another takes place through Synapse.
Neuron Facts: 31-35 | Action Potential and Synapse
31. When a Neuron is stimulated by any stimulus, an electrical activity takes place. It is known that the cell membrane of each and every living cell has a lining of electrically charged atoms called ions. These ions sit both inside and outside of the cell membrane. They stay in a balanced form and create what is known as membrane potential.
32. The electrical activity because of the stimulus suddenly changes the balance of ions, disturbing the membrane potential for a brief period of time, generating electrical signals that propagate from the Soma to Axon and finally to Synaptic Terminals. These electrical signals are called ‘Action Potentials’.
33. At the Synaptic Terminal, an Action Potential will caused Vesicles to be released in a small gap called Synaptic Gap. Vesicles are small bubbles of chemicals. Synaptic Gap is the gap that lies between an Axon ending of one Neuron and Dendrite of another Neuron or Effector.
34. The chemicals that are released in the Synaptic Gap are called Neurotransmitters. These Neurotransmitters sail across the gap and will attach with receptor sites present on the next Neuron.
35. When the Neurotransmitters lock on to the receptor sites on next Neuron, a pathway is opened for ions, disturbing the membrane potential of the next Neuron and the whole process of Action Potential propagation starts all over again in the recipient Neuron. This is how electrical and chemical signals are transmitted by the Neurons.
Neuron Facts: 36-40 | Weird Facts and Neuron Trivia
36. There are approximately 100 billion Neurons in our brain. That’s just 10% of all the brain cells we have. That’s more than the number of stars in our Milky Way. There are around 13.5 million Neurons in our spinal cord.
37. Each and every Neuron in our brain has anywhere between 1,000 and 10,000 connections with other Neurons in brain, taking the total number of neural connections in brain to 100 trillion.
38. We said that our nervous system has two primary types of cells – Neurons and Glial cells. Here is a stunner! There are 50x more Glial cells than Neurons in our Central Nervous System alone! The rest of the Nervous Tissue is not taken into account.
39. A Neuron can have a diameter of anywhere between 4 microns and 100 microns (both included). The largest axon in human body is 1 meter long and the largest axon in a giraffe is 15 feet long.
40. Our memory, basic reflexes, abstract thoughts, emotions – all come because of the Neural network in our body. These Neurons are very special and they require more glucose and oxygen than rest of the cells in our body.
Additional Neuron Facts:
1. Mature Neurons do not have the ability of dividing using mitosis.
2. There are increasing number of evidences which show that new Neurons are born even in adults. Previously it was thought that new Neurons are not formed in an adult brain.
3. In our brain and in our spinal cord, there is something called white matter. These are basically the areas where there are a large number of Axons. Gray Matter are the areas where there are large number of cell bodies (Soma).
4. Sensory Neurons make up only 0.9% of all Neurons present in our body. Motor Neurons account for 9% and the remaining are Interneurons.