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Why do neurons have long axons but short dendrites?

Why do neurons have long axons but short dendrites?


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Cian O'Donnell, a British neuroscientist, originally asked this question on Twitter: https://twitter.com/cian_neuro/status/1075432086692089857. I am not a biophysicist by training but I wonder whether there might be existing publications that have shed light on this question.

My primitive back-of-the-envelope analysis involves the following:

  1. If we suppose that most neurons are limited to a volume V, give or take epsilon volume, and the function of axons is to carry a signal over a large distance whereas the function of dendrites is to integrate information from different sources then a semi-spherical radiation of dendrites should be the norm.
  2. This becomes an extremal optimisation problem where there are specific constraints on the minimal axon thickness, the minimal dendrite thickness and an upper-bound on the average number of dendrites that fan out of a given axon.
  3. This becomes an extremal optimisation problem where there are specific constraints on the minimal axon thickness, the minimal dendrite thickness and an upper-bound on the average number of dendrites that fan out of a given axon.

This is a very sloppy analysis which ignores biophysical constraints but I think that we have an extremal optimisation problem here.


Axons are used to transfer electrical signals from Point A to Point B. Dendrites are for receiving electrical signals from different neuron cells via their respective Axons.

There are mainly two different types of Axons, insulated and non-insulated.

Insulated Axons allows extremely high velocity of electrical signals to propagate from Point A to Point B. Please note that insulated axons are not entirely insulated, the axons are only insulated at every given interval for a certain length (in measurement of distance). This forces the electrical signal to jump from one gap to another gap between insulation and result in extreme speed of electrical signal propagation down the axon.

Non-insulated axons still allow high velocity of electrical signal propagation but it is very much slower as compared to insulated axons. This is why when you have a stomach ache, its a slow and dull pain due to non-insulated axons which gives you prolonged pain signal. However, if you accidentally knock into a sharp object, you receive an instant sharp pain for a split second for you to react at that instant (Its a biological life saving function) due to insulated axons that gives you extreme speed of pain signal propagation.

On the other hand, dendrites are not built for signal transmitting but for signal receiving. Dendrites comes in different configurations, one such examples are your color sensory cells in your retina.

This is why dendrites are short while axons are long. They are configured for different purpose. Just a bit more information, electrical signal in biofluid are actually ionic current. Ionic current are different from electron current, the charge carrier are different. Lastly, electrical signal are generated due to electrochemical gradient on cellular level. When a certain membrane potential threshold is reached, action potential will be triggered and they resulted in the firing of electrical signal down the axons.

I hope you find these useful.

Best Regards

Price


I'm not sure that statement holds true. Pyramidal cells are one of the primary excitatory cells in the cortex and are characterized by their long apical dendrites.


Neurons are Communicators

The brain is made up of billions of neurons. Neurons are communicators. They have special features that allow them to send messages with rapid speed and efficiency. Neural messages drive everything we do, from moving our arms, to our emotional responses. These messages are actually little bursts of weak electrical current. Through these signaling bursts, neurons in our bodies and brains communicate.

Dendrites form a dense network of extensions or fibers. These branching structures are input fibers, receiving information from other neurons. The weak electrical currents that carry this information travel through the dendrites to the cell body.

The cell body is a collecting pool for all the incoming signals. The cell body processes these inputs, crunching the numbers. If the collective messages are strong enough, the neuron sends a message of its own.

This message, sent in the form of an electrical current, travels down a neuron’s axon, or output fiber. Once the signal reaches the end of the axon, it is transferred to another neuron.

Connections between neurons are synapses. You can think of these connections as tiny information portals. Minuscule chemical messengers fly through the space between neurons. Traversing the tiny space, these messengers carry bursts of information from one neuron to another.


History

Axon regeneration in the form of peripheral nerve repair has been in clinical practice since the 12th Century and possibly before, but spinal cord injury has been recognized as incurable since ancient times [14]. Cajal made several studies of regeneration in the PNS and its failure in the CNS, and was responsible with Tello for the first CNS regeneration experiment, implanting a graft of peripheral nerve into the CNS [106]. Yet we still do not have a solution for a clinically useful axon regeneration treatment. This stimulated a memorable introduction to a spinal injury research meeting by David Allan, director of the Scottish spinal injury unit, in which he challenged the audience,“the first regeneration experiment by Tello and Cajal was over 100 years ago, but we still do not have a treatment for our patients. What have you all been doing?”. Working out how to stimulate useful CNS axon regeneration has been extremely difficult, but recent progress has produced several interventions that enable partial regeneration, and increasing understanding of the biology gives reasonable optimism that a solution will be found soon. For many years research proceeded on the assumption that there might be a single straightforward fix for CNS regeneration. Peripheral nerves regenerate, and regeneration after axotomy in the CNS is generally successful in lower vertebrates and invertebrates, so surely the solution to lack of regeneration in the mammalian CNS should be straightforward. Sadly this is not so. It is now clear that evolution has gone to considerable lengths and developed several mechanisms to turn off regeneration and plasticity in the adult mammalian CNS. Restoring regeneration and plasticity is therefore complex, and several inhibitory mechanisms have to be addressed. Clearly there must be evolutionary advantages in turning off these developmental processes in adulthood. It may be that a hard-wired nervous system is more efficient, or the energetics or potential for mutation of maintaining regeneration and plasticity may be unfavourable, or there may be issues related to control of inflammation. Whatever the cause, we are now at the stage where the biology of regeneration failure is increasingly understood and solutions are being found. This review gives an overview of the current position.


Neuron shapes

Multipolar neurons

This is the most common type of neuron, with one axon and many dendrites.

Bipolar neurons

Bipolar neurons have one axon and one dendrite tree, each extending from opposite ends of the soma. The dendrite connects with cilia which are

Bipolar neurons are usually connected with sensing, detecting environmental effects and transmitting information about these.

Unipolar neurons

Unipolar neurons have a single axon connecting the dendrite tree and terminal buttons. The soma sticks out to the side of this, connected by a small stalk.

Unipolar neurons, like bipolar neurons, transmit sensory information, in particular touch, temperature change and other skin sensation. They also transmit information about events in muscles, joints and organs.

Pyramidal neurons

Pyramidal neurons are large and multipolar, with triangular somas and are largely found in the corticospinal tract, the cerebral cortex, the hippocampus, and in the amygdala. In the cortex they are associated with cognitive ability. in the corticospinal tract, bundles of axons connect the cortex with the spinal cord.


Intrinisic Control of Regeneration

In the years following the experiments from the Aguayo laboratory that demonstrated that the CNS environment is inhibitory for axon regeneration, the focus of research was on inhibitory molecules. Axons were assumed to be enabled by a permissive environment or blocked by an inhibitory one. Thus the reason why axons would regenerate in peripheral nerves and not the spinal cord was thought to be due to the environment rather than the properties of the axons. It became apparent that this was a much too simplistic view, and that different types of axon have very different abilities to grow and regenerate. The first evidence for this came from the study of embryonic CNS tissue grafted into the adult CNS. If the embryonic neurons were grafted at around the stage of maturity when they would normally be growing axons within the embryo, then axons from these grafts could grow for long distances through the supposedly inhibitory adult CNS, overcoming inhibition from the many inhibitory molecules. Thus grafts of spinal cord, striatum, substantia nigra, hippocampus could all send out axons into the adult CNS, and in many cases there was evidence that these axons could synapse with host neurons [108, 124, 146]. If the grafts came from human embryos, where axons continue to grow over a longer period than in the rodent CNS, then the axonal growth was particularly profuse [143]. Thus it appeared that axons growing from embryonic neurons were not subject to the inhibitory influences that could prevent regeneration of cut adult axons. A second example came from studying regeneration of the central and peripheral branches of sensory axons. The peripheral branch regenerates vigorously, restoring function in experimental animals and human patients. The central branch passes through the dorsal root, which is peripheral nerve territory containing Schwann cells and should therefore be permissive. Yet after dorsal root crushes the central branch shows a much less vigorous regenerative response that the peripheral branch [32]. Even within the adult CNS it became clear that some pathways such as the nigrostriatal tract have a relatively high ability to regenerate while others such as the corticospinal tract are very poor regenerators [22]. Studies of regeneration in the PNS revealed that peripheral axotomy initiates a programme of changes of gene expression with upregulation of a set of molecules termed RAGS (regeneration-associated molecules), while cutting CNS axons led to little or no upregulation of RAGS in their neurons [66, 91]. It was also possible to stimulate regeneration of CNS axons to some degree by treatment with an appropriate neurotrophin to stimulate the axon and neuron [51, 130]. Together, these and other observations started to shift the balance of activity of CNS regeneration research towards the intrinsic regenerative properties of neurons and their axons: current research in this area is described below.


Anatomy of a Neuron

The neuron contains the soma (cell body) from which extend the axon (a nerve fiber conducting electrical impulses away from the soma) and dendrites (tree-like structures that receive signals from other neurons). The myelin sheath is an insulating layer that forms around the axon and allows nerve impulses to transmit more rapidly along the axon.

Neurons do not touch each other, and there is a gap, called the synapse, between the axon of one neuron the dendrite of the next.

The unique structure of neurons permits it to receive and carry messages to other neurons and throughout the body.

Dendrites

Dendrites are the tree-root-shaped part of the neuron which are usually shorter and more numerous than axons. Their purpose is to receive information from other neurons and to transmit electrical signals towards the cell body.

Dendrites are covered in synapses, which allows them to receive signals from other neurons. Some neurons have short dendrites, whilst others have longer ones.

In the central nervous system, neurons are long and have complex branches that can allow them to receive signals from many other neurons.

For instance, cells called Purkinje cells which are found in the cerebellum have highly developed dendrites to receive signals from thousands of other cells.

Soma (Cell Body)

The soma, or cell body, is essentially the core of the neuron. The soma’s function is to maintain the cell and to keep the neuron functioning efficiently (Luengo-Sanchez et al., 2015).

The soma is enclosed by a membrane which protects it, but also allows it to interact with its immediate surroundings.

The soma contains a cell nucleus which produces genetic information and directs the synthesis of proteins. These proteins are vital for other parts of the neuron to function.

The axon, also called a nerve fiber, is a tail-like structure of the neuron which joins the cell body at a junction called the axon hillock.

The function of the axon is to carry signals away from the cell body to the terminal buttons, in order to transmit electrical signals to other neurons.

Most neurons just have one axon which can range in size from 0.1 millimeters to over 3 feet (Miller & Zachary, 2017). Some axons are covered in a fatty substance called myelin which insulates the axon and aids in transmitting signals quicker.

Axons are long nerve processes that may branch off to transfer signals to many areas, before ending at junctions called synapses.

Myelin Sheath

The myelin sheath is a layer of fatty material that covers the axons of neurons. Its purpose is to insulate one nerve cell from another and so to prevent the impulse from one neuron from interfering with the impulse from another. The second function of the myelin sheath is to speed up the conduction of nerve impulses along the axon.

The axons which are wrapped in cells known as glial cells (also known as oligodendrocytes and Schwann cells) form the myelin sheath.

The myelin sheath which surrounds these neurons has a purpose to insulate and protect the axon. Due to this protection, the speed of transmission to other neurons is a lot faster than the neurons that are unmyelinated.

The myelin sheath is made up of broken up gaps called nodes of Ranvier. Electrical signals are able to jump between the nodes of Ranvier which helps in speeding up the transmission of signals.

Axon Terminals

Located at the end of the neuron, the axon terminals (terminal buttons) are responsible for transmitting signals to other neurons.

At the end of the terminal button is a gap, which is known as a synapse. Terminal buttons hold vessels which contain neurotransmitters.

Neurotransmitters are released from the terminal buttons into the synapse and are used to carry signals across the synapse to other neurons. The electrical signals convert to chemical signals during this process.

It is then the responsibility of the terminal buttons to reuptake the excess neurotransmitters which did not get passed onto the next neuron.


Neuroscience For Kids

A neuron is a nerve cell. The brain is made up of approximately 86 billion neurons (Source: Frederico Azevedo et al., Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. J. Comp. Neurol., 513: 532-541, 2009.).

Neurons are similar to other cells in the body in some ways such as:

  • Neurons are surrounded by a membrane.
  • Neurons have a nucleus that contains genes.
  • Neurons contain cytoplasm, mitochondria and other "organelles".

However, neurons differ from other cells in the body in some ways such as:

  • Neurons have specialized projections called dendrites and axons. Dendrites bring information to the cell body and axons take information away from thecell body.
  • Neurons communicate with each other through an electrochemical process.
  • Neurons form specialized connections called "synapses" and produce special chemicals called "neurotransmitters" that are released at the synapse.

There are approximately 1 quadrillion synapses in the human brain. That's 1,000,000,000,000,000 synapses! This is equal to about a half-billion synapses per cubic millimeter. (Statistic from Changeux, J-P. and Ricoeur, P., What Makes Us Think?, Princeton: Princeton University Press, 2000, p. 78)

Types of Neurons

What is behind the saying "We use only 10% of our brain?" Is this true?

No. it is not true. We use all of our brain. I have created a special page called "Do we use only 10% of our brain" that discusses this question in more detail.

The Hows

How big is the brain? How much does the brain weigh?

The adult human brain weighs between 1300 g and 1400 g (approximately 3 lbs). A newborn human brain weighs between 350 and 400 g. For comparison:

elephant brain = 6,000 g
chimpanzee brain = 420 g
rhesus monkey brain = 95 g
beagle dog brain = 72 g
cat brain = 30 g
rat brain = 2 g

How many neurons (nerve cells) are in the brain? How big are they?

There are approximately 86 billion (86,000,000,000) neurons in the human brain.

To get an idea of how small a neuron is, let's do some math:

The dot on top of this "i" is approximately 0.5 mm (500 microns or 0.02 in) in diameter. Therefore, if you assume a neuron is 10 microns in diameter, you could squeeze in 50 neurons side-by-side across the dot. However, you could squeeze in only 5 large (100 micron diameter) neurons.

Some neurons are very short. less than a millimeter in length. Some neurons are very long. a meter or more! The axon of a motor neuron in the spinal cord that innervates a muscle in the foot can be about 1 meter (3 feet) in length.

Think about how long the axon of a motor neuron would be if you wanted to make a model of it. The cell body of a motor neuron is approximately 100 microns (0.1 millimeter) in diameter and as you now know, the axon is about 1 meter (1,000 millimeter) in length. So, the axon of a motor neuron is 10,000 times as long as the cell body is wide. If you use a ping-pong ball (diameter =

3.8 cm or 1.5 inch) to model the cell body, your axon would have to be 38,000 cm (380 meters) or 1,247 feet in length. If you use a basketball (diameter =

24 cm or 9.5 inch) as the cell body, then your axon would have to be 240,000 cm (2.4 kilometers) or 7874 ft (1.49 miles) in length!

How big is the brain compared to the rest of the body?

If you assume the average person is 150 pounds and the average brain weighs 3 lbs., then the brain is 2% of the total body weight.

How long is the spinal cord and how much does it weigh?

The average spinal cord is 45 cm long in men and 43 cm long in women. The spinal cord weighs approximately 35 g.

How fast does information travel in the nervous system?

Information travels at different speeds within different types of neurons. Transmission can be as slow as 0.5 meters/sec or as fast as 120 meters/sec. Traveling at 120 meters/sec is the same as going 268 miles/hr. Check the math out yourself. More about the speed of signals in the nervous system.

More Whats and some Whos, Whys and Hows

What do neuroscientists study?

Perhaps, the best way to describe what neuroscientists study is to list the "levels" at which experiments can be done:

  1. Behavioral Level: study of the neural basis of behavior. In other words, what causes people and animals to do the things they do.
  2. System Level: study of the various parts of the nervous system like the visual or auditory system. This could also include investigations of what parts of the brain are connected to other parts.
  3. Local Circuit Level: study the function of groups of neurons (nerve cells).
  4. Single Neuron Level: study what individual neurons do in relation to some "event." Also, could study what is contained within a single neuron (neurotransmitter studies).
  5. Synapse Level: study what happens at the synapse.
  6. Membrane Level: study what happens at ion channels on a neuronal membrane.
  7. Genetic Level: study the genetic basis of neuronal function.

How do you become a neuroscientist? How long do you have to go to school?

  1. First, you have to finish high school. so from 1st to 12th grade is 12 years.
  2. Second, you get a university degree. at least another 4 years of school.
  3. Third, you go to either graduate school for a Ph.D. degree or go to medical school for an M.D. degree. at least another 4 years of education.

Let's add up what we have so far -- 12 yrs + 4 yrs + 4 yrs = 20 yrs

That's 20 yrs. of school. While you are in graduate school or medical school you can call yourself a neuroscientist in training. After you get your Ph.D. or M.D. you can call yourself a "neuroscientist." Some people go back to school and get another degree so they have both a Ph.D. and an M.D. degree. Most people continue their training in a different laboratory after they get their Ph.D. or M.D. degree. This period of time is called Postdoctoral Training and neuroscientists learn new methods and techniques. This usually lasts 2-4 years. It is the hope of most neuroscientists that they can get jobs at a university, hospital or company after their postdoctoral training period. To find out more about becoming a neuroscientist, read Another Day, Another Neuron, a short essay I wrote for the Genentech Access Excellence Web site.

Ok, so after all this school and training, what kind of jobs are available?

Why do neuroscientists do what they do?

Different neuroscientists have different reasons for getting into their careers. However, I am sure that some scientists are motivated by their curiosity to learn more about the brain. Neuroscientists would also like to find treatments and cures for the diseases that affect the nervous system. Neurological illnesses affect more than 50 million Americans each year - this costs billions of dollars each year. Here is more information on some of the major nervous system diseases (from Brain Facts, Society for Neuroscience and other sources including The American Academy of Neurology)

Major Nervous System Diseases

DiseaseNumber of CasesCost per year
Chronic Pain 97,000,000 $100 billion
Hearing Loss 28,000,000 $56 billion
Depression Disorders 20,500,000 $44 billion
Alzheimer's Disease 4,500,000 $100 billion
Stroke4,700,000 $51 billion
Epilepsy 2,500,000 $3.5 billion
Traumatic Head Injury 5,000,000 $56.3 billion
Huntington's Disease 30,000 $2 billion
Schizophrenia 2,000,000 $32.5 billion
Parkinson's Disease 1,000,000 to 2,000,000 $25 billion
Multiple Sclerosis 2,500,000 $9.5 billion
Traumatic Spinal Cord Injury 250,000 $10 billion

Who was the first neuroscientist?

Hmmm. I don't think anyone really knows the answer to this one. Here is my opinion. Some skulls that are at least 10,000 years old have unusual holes in them. Scientists believe that these holes were put there intentionally to "let out the bad spirits." This implies that these people had some belief that the head or brain had some importance for health and well-being. Perhaps these people could be considered the first neuroscientists.

The first recorded use of the word "brain" belongs to the ancient Egyptians. The word for "brain" and other "neuro" words appear in the Edwin Smith Surgical Papyrus which was written by an unknown Egyptian surgeon around 1,700 BC.

Socrates (469-399 B.C.) and Aristotle (384-322 B.C.) were early "thinkers" who wrote about the brain and mind. However, Aristotle believed that the heart, not the brain, was important for intelligence. Galen (129-199) was another early neuroscientist. Leonardo da Vinci (1452-1519), who came along much later, also could be thought of as a neuroscientist. If you are interested in more about the history of the Neurosciences, try Milestones in Neuroscience Research.

How many research papers about the brain are published each year?

For 2015, a PubMed search using the term "brain" shows that 87,294 papers were published.

For 2014, a PubMed search using the term "brain" shows that 85,025 papers were published.

For 2013, a PubMed search using the term "brain" shows that 80,032 papers were published.

For 2012, a PubMed search using the term "brain" shows that 75,168 papers were published.

For 2011, a PubMed search using the term "brain" shows that 70,279 papers were published.

For 2010, a PubMed search using the term "brain" shows that 65,193 papers were published.

For 2009, a PubMed search using the term "brain" shows that 61,270 papers were published.

For 2008, a PubMed search using the term "brain" shows that 55,874 papers were published.

For 2007, a PubMed search using the term "brain" shows that 53,258 papers were published.

For 2006, a PubMed search using the term "brain" shows that 51,163 papers were published.

For 2005, a PubMed search using the term "brain" shows that 47,383 papers were published.

For 2004, a PubMed search using the term "brain" shows that 42,849 papers were published.

For 2003, a PubMed search using the term "brain" shows that 39,964 papers were published.

For 2002, a PubMed search using the term "brain" shows that 37,304 papers were published.

For 2001, a PubMed search using the term "brain" shows that 36,884 papers were published.

For 2000, a PubMed search using the term "brain" shows that 37,000 papers were published.

For 1999, a PubMed search using the term "brain" shows that 34,828 papers were published.

For 1998, a PubMed search using the term "brain" shows that 33,027 papers were published.

For 1997, a PubMed search using the term "brain" shows that 32,112 papers were published.

For 1996, a PubMed search using the term "brain" shows that 31,040 papers were published.


Difference Between Axon and Dendrites

Central nervous system is one of a chief system of our body. It controls our body in different ways. It has nerves that transmit signals from central nervous system to the parts of the body. The basic unit of the central nervous system is neuron. It is defined as a specialized cell that transmits nerve impulses it is also called a nerve cell. Axons and Dendrites are the part of the neuron. Axon is the long threadlike part of a neuron along which the nerve impulse travel from the cell body to other parts. Whereas, a dendrite is a short part extension of a neuron by which impulses are received from the center and are further transmitted to the cell body or axon of a neuron. In simple words, axons are the output of neuron and dendrites are the input of the neuron. Dendrites receive information from the external or internal environment and transmit information to the cell body and axon of a neuron. Dendrites are multiple in number and short while axon is single but varies in length.

Comparison Chart

AxonDendrite
FunctionAxon takes information or impulse away from the cell body.Dendrite brings information or impulse to the cell body of the neuron.
Ribosomes & Myelin SheathAxons do not have ribosomes, although they may have myelin sheath.Dendrites have ribosomes but no myelin sheath around them.
BranchesAxons have branches far away from the cell body, and these branches are present at terminal point or axon terminal of the neuron.Dendrites have branches near the cell body, and these branches are present at the origin of the neuron.
Nissl’s GranulesAxons do not contain Nissl’s granules.Dendrites have Nissl’s granules.
VesiclesAxons have vesicles that contain neurotransmitter in them.Dendrites have no vesicles.

What are Axon?

Axon is derived from a Greek word which means axis. Axon is the output of the neuron. Its function is to transfer information from the body of the neuron to the other part of the body or the another neuron. Axons have a uniform diameter and a smooth surface. Only one axon per cell is present. Axon begins as axon hillock, which is a swelling at the junction between soma and axon of a neuron. It has many sodium (Na) channels in them which help in the generation of action potential throughout neuron. Axons are usually long, and they end as axon terminal on the other neuron or part of the body. Note that axon has branches only at its terminal. Axons also have many vesicles in them in which different neurotransmitter are present. It also has calcium (Ca) channels in its membrane. Axons do not contain Nissl’s granules. It also has no ribosome. Axons are of two types: myelinated axons and unmyelinated axons. Myelinated axons have myelin sheath around them. Myelin sheath act as an insulator and also forms Nodes of Ranvier which help in salutatory conduction. Unmyelinated axons lack myelin sheath around them. Axons end via a synapse, if axon of one neuron is connected to the axon of another neuron, it is called axoaxonal. If axon of one neuron is connected to the dendrite of another neuron, it is called axodendritic. And if axon of one neuron is connected directly to the soma, it is known as axosomatic. Axons also form neuromuscular junctions at muscle by directly ending on them.

What are Dendrites?

Dendrite is derived from a Greek word which means tree. Dendrite is the input of the neuron. Its function is to receive information from the center and transmit it to the cell body of the neuron. Axons have a nonuniform diameter and a rough surface. There are many dendrites per cell. Dendrite receives information from the surroundings and transmits it forward to the cell body and axon of a neuron. Dendrites are numerous in a single neuron and are relatively shorter as compared to axons it also has many branches that are present only at its origin. If dendrite of one neuron is connected to the axon of the other neuron, it is known as axodendritic. And if dendrites are connected to the dendrite of another neuron, it is known as dendrodendritic. Dendrites contain Nissl’s granules and have ribosomes. They have no myelin sheath around them and have branches near the cell body of the neuron.

Axon vs. Dendrite

  • Axon takes information or impulse away from the cell body, whereas Dendrites bring information or impulse to the cell body of the neuron.
  • Axons are long and single per cell while dendrites are short and multiple per cell.
  • Axons do not have ribosomes, although they may have myelin sheath while dendrites have ribosomes but no myelin sheath around them.
  • Axons have branches far away from the cell body, and these branches are present at the terminal point or axon terminal of a neuron and contrary to this dendrites have branches near the cell body, and these branches are present at the origin of the neuron.
  • Axons do not contain Nissl’s granules, on the other hand, dendrites have Nissl’s granules.
  • Axons have vesicles that contain neurotransmitter in them, but dendrite has no vesicles.

Explanatory Video

Janet White

Janet White is a writer and blogger for Difference Wiki since 2015. She has a master's degree in science and medical journalism from Boston University. Apart from work, she enjoys exercising, reading, and spending time with her friends and family. Connect with her on Twitter @Janet__White


Difference Between Axons and Dendrites

Have you ever wondered what sensations and perceptions involve? The sensations we feel are actually dictated by our brain, based on the impulses and stimulation it receives. These impulses are in the form of electrochemical signals that are passed from one nerve cell to the next, until they reach our brain for calculation and response. This is nervous system 101.

The nervous system is such an interesting and broad subject, and one of its disciplines is the understanding of nerve cells, or more simply called, neurons. There are two parts of nerve cells involved in the conduction of these nerve impulses. They are the axons and the dendrites.

Dendrites are branched projections of neurons its name comes from the Greek word ‘Dendron’, which means ‘Tree’, and is based on its evident tree-like appearance. They are protoplasmic extensions of nerve cells, and operate as conductors of electrochemical stimuli received from neighboring cells. The impulses they receive are carried inwards and towards the soma, or cell body.

Impulses are received by dendrites via synapses. They are situated at different points all over the dendritic arbor. Most neurons have many of these protoplasmic protrusions, although they are rather short. They are heavily branched in structure.

Axons are also called nerve fibers, as they appear elongated and slender. Like dendrites, they are also protoplasmic projections of nerve cells, or neurons, and their primary purpose is to conduct electrochemical impulses away from the cell body of neurons. Most nerve cells only have a single axon.

Axons extend from the soma to its terminal endings. Neural signals are transmitted through them after they have entered the soma of the neuron. Larger axons are said to transmit information signals more quickly. Some axons are myelinated (i.e. covered by a fatty substance identified as myelin). The myelin coverings are insulators, and with their presence, axons are said to transmit more quickly.

Basically, axons’ role is to transmit signals, and dendrites to receive such signals. However, these assertions are in a general sense, as there are some exceptions. Other distinguishing physical characteristics of the axons and dendrites, besides the length and branching, are their shapes. Dendrites tube-like shape usually tapers, while the radius of axons remains constant.

1. Dendrites receive electrochemical impulses from other neurons, and carry them inwards and towards the soma, while axons carry the impulses away from the soma.

2. Dendrites are short and heavily branched in appearance, while axons are much longer.


Axons vs Dendrites

Axons and dendrites are important structures found in a neuron. The neuron is the main structural and functional unit of the nervous system. The axons involve in taking nerve impulses away from the cell body. These signals are passed on to effector cells such as muscles and glands. Dendrites are involved in transmitting nerve impulses towards the cell body. The nerve signals received by sensory organs are passed on to the cell body. This is the difference between axons and dendrites.

Image Courtesy:

1.’Blausen 0657 MultipolarNeuron’By BruceBlaus – Own work, (CC BY 3.0) via Commons Wikimedia
2.’Dendrite (PSF)’By Pearson Scott Foresman (Public Domain) via Commons Wikimedia