The Types of neurons Can be classified according to the transmission of the impulse, the function, the direction, by the action in other neurons, by its discharge pattern, by the production of neurotransmitters, by the polarity, according to the distance between axon and soma, according to the morphology Of dendrites and according to location and shape.
There are about 100 billion neurons in our brain. In contrast, if we talk about glial cells (those that serve as a support for neurons), the number increases to about 360 billion.
Neurons resemble other Cells , Among other things, in that they have a membrane that surrounds them, contain genes, cytoplasm, mitochondria and trigger essential cellular processes like synthesizing proteins and producing energy.
But, unlike other cells, neurons have dendrites and axons that communicate with one another by electrochemical processes, Establish synapses And contain neurotransmitters.
These cells are organized as if they were trees in a dense forest, where they interweave their branches and roots. Like trees, each individual neuron has a common structure, but presents variations in its shape and size.
Smaller cells can have a cell body only 4 microns wide, while the cell bodies of the larger neurons can have a width of 100 microns.
In fact, scientists are still investigating brain cells and discovering new structures, functions and ways of classifying them.
The basic form of a neuron is composed of 3 parts:
- The cell body: Contains the nucleus of the neuron, which is where genetic information is stored.
- The axon: Is an extension that functions as a cable, and is responsible for transmitting electrical signals (action potentials) from the cell body to other neurons.
- The dendrites: Are small branches that capture electrical signals emitted by other neurons.
Each neuron can establish connections with up to 1000 more neurons. However, as the researcher claimed Santiago Ramón y Cajal , The neuronal ends do not merge, but there are small spaces (called synaptic clefts). This exchange of information between neurons is called synapses. (Jabr, 2012)
Classification of neuron types
Neurons can be classified in different ways:
By imparting momentum
A main classification that we are going to find very frequently to understand certain neural processes is to distinguish between the presynaptic neuron and the postsynaptic neuron:
- Presynaptic neuron: Is the one that emits the nervous impulse.
- Postsynaptic neuron: The one that receives this impulse.
It is necessary to clarify that this differentiation applies within a specific context and moment.
By its function
Neurons can be classified according to the tasks they perform. According to Jabr (2012), in a very common way we will find a division between:
- Sensory neurons: Are the ones that handle information coming from the sensory organs: the skin, the eyes, the ears, the nose, etc.
- Motor neurons or motor neurons His task is to send signals from the brain and spinal cord to the muscles. They are mainly responsible for controlling movement.
- Interneurons: They act as a bridge between two neurons. They may have longer or shorter axons, depending on how far away these neurons are from each other.
- Neurosecretories (Gould, 2009): Release hormones and other substances, some of these neurons are in the Hypothalamus .
By your address
- Afferent neurons: Also called receptor cells, would be the sensory neurons we have named earlier. In this classification it is emphasized that these neurons receive information from other organs and tissues, so that they transmit information from these areas to the central nervous system.
- Efferent neurons: Is another way of calling motor neurons, pointing out that the direction of information transmission is opposite to afferents (sending the data from the nervous system to the Effector cells ).
By action on other neurons
One neuron influences the other releasing different types of neurotransmitters that bind to specialized chemical receptors. To make this more understandable, we can say that a Neurotransmitter It works as if it were a key and the receiver would be like a door blocking the passage.
Applied to our case is something more complex, since the same type of"key"can open many different types of"locks" . This classification is based on the effect they cause on other neurons:
- Excitatory neurons: Are the ones that free glutamate . They are called so because, when this substance is captured by the receptors, there is an increase in the firing rate of the receiving neuron.
- Inhibitory or GABAergic neurons: They free GABA , A type of neurotransmitter that has inhibitory effects. This is because it reduces the firing rate of the neuron that captures it.
- Modulators: Do not have a direct effect, but change long-term small structural aspects of nerve cells.
Approximately 90% of neurons release glutamate or GABA, so this classification includes the vast majority of neurons. The rest, has specific functions according to the objectives it presents.
For example, some neurons secrete Glycine Exerting an inhibitory effect. In turn, there are motor neurons in the spinal cord that release Acetylcholine And provide an exciting result.
Anyway, it should be noted that this is not so simple. That is, a single neuron releasing a type of neurotransmitter can have both excitatory and inhibitory effects, and even modulators on other neurons. This seems to depend, rather, on the type of receptors activated by postsynaptic neurons.
By its discharge pattern
We can pigeonhole neurons by electrophysiological features.
- Tonic or shooting ( Spiking ) Regular: Refers to neurons that are constantly active.
- Phasic or"bursting"( Bursting in English): Are those that activate in bursts.
- Quick shots ( Fast spiking ): These neurons stand out because of their high firing rates, that is, they shoot very frequently. Pale balloon cells, retinal ganglion cells, or some classes of cortical inhibitory interneurons would be good examples.
For the production of neurotransmitters
- Cholinergic neurons: This type of neurons releases Acetylcholine In the synaptic cleft.
- GABAergic Neurons: Release GABA.
- Glutamatric Neurones: Secrete glutamate, which, together with aspartate, consist of excitatory neurotransmitters par excellence. When the blood flow to the brain is reduced, glutamate can cause Excitotoxicity Causing over-activation
- Dopaminergic neurons Liberate Dopamine , Which is linked to mood and behavior.
- Serotoninergic neurons: Are the ones that release serotonin, which can act both by stimulating and inhibiting. His absence has been Traditionally associated with depression .
By its polarity
The neurons can be cataloged according to the number of processes that are united to the body cell or soma, being able to be (Sincere, 2013):
- Unipolar Or pseudounipolar: Are those that possess a single process protoplasmático (only a prolongation or primary projection). It is structurally observed that the cell body is on one side of the Axon , The pulses being transmitted without the signals passing through the soma. They are characteristic of invertebrates, although we can also find them in the retina .
- The pseudounipolar: Are distinguished from the unipolar in which the axon is divided into two branches, generally one goes towards a peripheral structure and the other is directed towards the central nervous system. They are important in the sense of touch. In fact, they could be considered a variant of bipolar.
- Bipolar In contrast to the previous type, these neurons possess two extensions that depart from the cellular soma. They are common in the sensory pathways of sight, hearing, smell and taste, as well as vestibular function.
- The Multipole Most neurons belong to this type, which is characterized by having only one axon, usually long, and many dendrites. These can originate directly from the soma, assuming an important exchange of information with other neurons. They can be subdivided into two classes:
A) Golgi I: Long axons, typical of pyramidal cells and Purkinje cells.
B) Golgi II : Short axons, typical of granular cells.
This distinction established it Camillo Golgi , Nobel Prize in Medicine, observing through the microscope neurons stained with a procedure that he himself had invented (Golgi staining). Santiago Ramón y Cajal stated that Golgi II type neurons abound in evolutionarily more advanced animals than Type I.
- Anaxonics: In this type the dendrites of the axons can not be differentiated, being also very small.
Depending on the distance between the axon and the soma
- Convergent : In these neurons the axon may be more or less branched, however, it is not too far from the body of the neuron (soma).
- Divergent: Despite the number of branches, the axon extends long distances and distances itself significantly from the neuronal soma.
According to the morphology of the dendrites
- Idiodendritics: Its dendrites depend on the type of neuron it is (if we classify it according to its location in the nervous system and its characteristic form, see below). Good examples are the Purkinje cells And the pyramids.
- Isodendritic: This kind of neuron has dendrites that divide so that the daughter branches outnumber the mother branches.
- Allodendritic: Have features that are not typical of dendrites, such as having very few spines or dendrites without branching.
According to location and form
There exist in our brain a multitude of neurons that have a unique structure and it is not easy task to catalog them with this criterion.
According to the form (Paniagua et al., 2002) we can consider:
- Fusiformes
- Polyhedral
- Started
- Spherical
- Pyramids
If we take into account both the location and the shape of the neurons we can refine and further detail this distinction:
- Pyramidal neurons: Are called so because the sums present a triangular pyramid shape and are found in the prefrontal cortex.
- Betz cells: Are large pyramidal motor neurons that are located in the fifth layer of the gray matter in the primary motor cortex.
- Basket or basket cells : Are cortical interneurons that are located in the cortex and cerebellum.
- Purkinje cells: Tree-shaped neurons found in the cerebellum.
- Granular cells: Suppose most of the neurons in the human brain. They are characterized by very small cell bodies (Golgi II type) and are located in the granular layer of the cerebellum, dentate gyrus hippocampus Y Olfactory bulb , among others.
- Lugaro cells: So called by their discoverer, are inhale sensory interneurons located in the cerebellum (just below the layer of Purkinje cells ).
- Thorny middle neurons: Are considered a special type of GABAergic cell that represents approximately 95% of neurons of the striatum in humans.
- Renshaw cells : These neurons are inhibitory interneurons of the spinal cord Which are connected at their ends with alpha motor neurons, neurons with both ends linked to alpha motor neurons.
- Unipolar brush cells : Consist of a type of glutamatergic interneurons that are located in the granular layer of the cerebellar cortex and in the cochlear nucleus. Its name is because it presents a single dendrite that ends in the form of a brush.
- Cells of the anterior horn: Are thus called motor neurons located in the spinal cord.
- Neurons in spindle: Also called Von Economo neurons, are characterized by being fusiform, that is, their shape looks like an elongated tube that becomes narrow at the ends. They are located in very restricted areas: the insula, the anterior cingulate gyrus and, in humans, the dorsolateral prefrontal cortex.
But, we ask ourselves:
Do these classifications encompass all types of neurons?
We can affirm that almost all the neurons of the nervous system can be pigeonholed in the categories that we offer here, especially the broader ones. However, it is necessary to point out the immense complexity of our nervous system and all the advances that remain to be discovered in this area.
Research is still underway to distinguish the most subtle differences between neurons in order to know more about brain function and associated diseases.
Neurons are distinguished from each other by structural, genetic and functional aspects, as well as their way of interacting with other cells. It is even important to know that there is no agreement among scientists to determine an exact number of types of neurons, but could be more than 200 types.
A very useful resource to know more about the cellular types of the nervous system is Neuro Morpho, a database in which the different neurons are digitally reconstructed and can be explored according to species, cell types, brain regions, etc. (Jabr, 2012)
In summary, the classification of neurons into different classes has been discussed considerably since the beginning of modern neuroscience. However, we can gradually unravel this issue, as the experimental advances are accelerating the pace in the collection of data on neural mechanisms. Thus, each day we are one step closer to knowing the totality of brain functioning.
References
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- Chudler, E.H. Types of Neurons (Nerve Cells ). Retrieved on June 3, 2016.
- Gould, J. (July 16, 2009). Neuron classification by function . Retrieved June 3, 2016, from University of West Florida.
- Jabr, F. (May 16, 2012). Know Your Neurons: How to Classify Different Types of Neurons in the Brain's Forest . Obtained from Scientific American.
- Paniagua, R.; Nistal, M.; Sesma, P.; Álvarez-Uría, M.; Fraile, B.; Anadón, R. and José Sáez, F. (2002). Plant and animal cytology and histology. McGraw-Hill Interamericana de España, S.A.U.
- Neuronal prolongations. Retrieved on June 3, 2016, from the University of Valencia.
- Sincere, M. (April 2, 2013). Types of Neurons . Retrieved on June 3, 2016, from Explorable.
- Wikipedia . (June 3, 2016). Retrieved on June 3, 2016, from Neuron.
- Waymire, J.C. Chapter 8: Organization of Cell Types. Retrieved on June 3, 2016, from Neuroscience Online.