Motor Neuron Easy Definition Essay

The Hows, Whats and Whos of Neuroscience

The Whats

What is a neuron?

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? 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

More Brain Weights

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.

How long is a neuron?

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 from 1st to 12th grade is 12 years.
  2. Second, you get a university 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. 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?

Jobs in Neuroscience

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 Pain97,000,000$100 billion
Hearing Loss28,000,000$56 billion
Depression Disorders20,500,000$44 billion
Alzheimer's Disease4,500,000$100 billion
Stroke4,700,000$51 billion
Epilepsy2,500,000$3.5 billion
Traumatic Head Injury5,000,000$56.3 billion
Huntington's Disease30,000$2 billion
Schizophrenia2,000,000$32.5 billion
Parkinson's Disease1,000,000 to 2,000,000$25 billion
Multiple Sclerosis2,500,000$9.5 billion
Traumatic Spinal Cord Injury250,000$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.

What are some of the methods and techniques used by neuroscientists?

You want methods? Here they are.

Copyright © 1996-2016, Eric H. Chudler All Rights Reserved.

Motor neuron diseases are a group of conditions that cause the nerves in the spine and brain to progressively lose function. They are a rare but serious and incurable form of progressive neurodegeneration.

Motor neurons are nerve cells that send electrical output signals to the muscles, affecting the muscles' ability to function.

Motor neuron disease (MND) can appear at any age, but most patients are over 40 years old at diagnosis. It affects men more than women.

The most common type, amyotrophic lateral sclerosis (ALS), probably affects around 30,000 Americans at any given time, with over 5,000 diagnoses each year.

The renowned English physicist, Stephen Hawking, and guitar virtuoso Jason Becker have been living with ALS for several years.

Fast facts on motor neuron diseases

Here are some key points about motor neuron diseases. More detail is in the main article.

  • Motor neuron diseases (MND) are a group of conditions that affect the nerve cells that send muscles to the brain.
  • There is a progressive weakening of all the muscles in the body, which eventually affects ability to breathe.
  • Genetic, viral, and environmental issues may play a role in causing MND.
  • There is no cure, but supportive treatment can improve the quality of life.
  • Life expectancy after diagnosis ranges from 3 years to longer than 10 years.
  • British scientist Stephen Hawking has lived with MND for many years.


Stephen Hawking is one of the most well-known people with a motor neuron disease.
Image credit: Doug Wheller, 2008

There are several types of motor neuron disease.

ALS, or Lou Gehrig's disease, is the most common type, affecting muscles of the arms, legs, mouth, and respiratory system. Mean survival time is 3 to 5 years, but some people live 10 years or more beyond diagnosis with supportive care.

Progressive bulbar palsy (PBP) involves the brain stem. People with ALS often have PBP too. The condition causes frequent choking spells, difficulty speaking, eating, and swallowing.

Progressive muscular atrophy (PMA) slowly but progressively causes muscle wasting, especially in the arms, legs, and mouth. It may be a variation of ALS.

Primary lateral sclerosis (PLS) is a rare form of MND that advances more slowly than ALS. It is not fatal, but it can affect the quality of life. In children, it is known as juvenile primary lateral sclerosis.

Spinal muscular atrophy (SMA) is an inherited MND that affects children. There are three types, all caused by an abnormal gene known as SMA1. It tends to affect the trunk, legs, and arms. Long-term outlook varies according to type.

The different types of MND share similar symptoms, but they progress at different speeds and vary in severity.


MND can be divided into three stages, early, middle, and advanced.

Early stage signs and symptoms

Symptoms develop slowly and can be confused with symptoms of some other unrelated neurological conditions.

Early symptoms depend upon which body system is affected first. Typical symptoms begin in one of three areas: the arms and legs, the mouth (bulbar), or the respiratory system.

They include:

  • a weakening grip, making it hard to pick up and hold things
  • fatigue
  • muscle pains, cramps, and twitches
  • slurred and sometimes garbled speech
  • weakness in the arms and legs
  • increased clumsiness and stumbling
  • difficulty swallowing
  • trouble breathing or shortness of breath

Middle stage signs and symptoms

Motor neuron diseases can leave those with the disease severely restricted in mobility.

As the condition progresses, symptoms become more severe.

  • Muscle pain and weakness increase, and spasms and twinges worsen.
  • Limbs become progressively weaker.
  • Limb muscles start to shrink.
  • Movement in affected limbs becomes more difficult.
  • Limb muscles may become abnormally stiff.
  • Joint pain grows.
  • Eating, drinking, and swallowing become harder.
  • Drooling occurs, due to problems controlling saliva.
  • Yawning occurs, sometimes in uncontrollable bouts.
  • Jaw pain may result from excessive yawning.
  • Speech problems worsen, as muscles in the throat and mouth become weaker.

The person may show changes in personality and emotional state, with bouts of uncontrollable crying or laughing.

Previously, it was believed MND did not significantly affect brain function or memory, but studies have now shown that up to 50 percent of people with ALS have some type of brain function involvement.

This includes difficulties with memory, planning, language, behavior, and spatial relationships. Up to 15 percent of people with ALS have a form of dementia known as frontotemporal dementia (FTD).

Breathing problems may occur as the diaphragm, the main breathing muscle, deteriorates. There may be a shortage of breath, even when sleeping or resting. Ultimately, breathing assistance will be necessary.

Secondary symptoms include insomnia, anxiety, and depression.

Advanced stage signs and symptoms

Eventually, the patient will be unable to move, eat, or breathe without assistance. Without supportive care, an individual will pass away. Despite the best of care currently available, complications of the respiratory system are the most common causes of death.


Motor neurons send signals from the brain to the muscles and bones, and this makes the muscles move. They are involved in both conscious movements and automatic movements, such as swallowing and breathing.

Some MNDs are inherited while others happen randomly. The exact causes are unclear, but the National Institute of Neurological Diseases and Stroke (NINDS) notes that genetic, toxic, viral, and other environmental factors likely play a role.

Risk factors

Here are some of the risk factors associated with MND.

Heredity: In the United States (U.S.), around 1 in every 10 cases of ALS is inherited. SMA is also known to be an inherited condition.

Age: After the age of 40 years, the risk rises significantly, although it is still very small. ALS is most likely to appear between the ages of 55 and 75 years.

Sex: Men are more likely to develop an MND.

Some experts have linked military experience to a higher chance of developing the disease.

Studies have found that professional footballers are more likely to die from ALS, Alzheimer's disease, and other neurodegenerative diseases, compared with other people. This implies a possible link with recurrent head trauma and neurological disease.


The treatment of motor neuron diseases is mostly about managing symptoms and enabling comfortable living.

In the early stage, MND can be hard to diagnose, because the signs and symptoms are common to other conditions, such as multiple sclerosis (MS), an inflamed nerve, or Parkinson's disease.

A primary care physician will normally refer the patient to a neurologist, a doctor specialized in the diagnosis and treatment of diseases and conditions of the nervous system.

The neurologist will start with a complete history and physical exam of the neurologic system.

Other tests may be helpful.

Blood and urine tests: These analyses can rule out other conditions and detect any rise in creatinine kinase. This is produced when muscle breaks down, and it is sometimes be found in the blood of patients with MND.

MRI brain scan: This cannot detect an MND, but it can help rule out other conditions, such as stroke, brain tumor, brain circulation problems, or abnormal brain structure.

Electromyography (EMG) and nerve conduction study (NCS): These are often performed together. An EMG tests the amount of electrical activity within muscles, while NCS tests the speed at which electricity moves through muscles.

Spinal tap, or lumbar puncture: This analyzes the cerebrospinal fluid, the fluid that surrounds the brain and spinal cord.

Muscle biopsy: If the doctor thinks the patient may have a muscle disease, rather than MND, a muscle biopsy may be performed.

After tests, a doctor will normally monitor the patient for some time before confirming that they have MND.

Criteria known as El Escorial criteria can help a doctor check for distinctive neurological signs, that may aid in the diagnosis of ALS.

These include:

  • muscle shrinking, weakness or twitching
  • muscle stiffness or abnormal reflexes
  • symptoms spreading into new muscle groups
  • having no other factors that explain the symptoms


Occupational therapy can help to alleviate some of the stiffness and stress of a motor neuron disease.

There is no cure for MND, so treatments focus on slowing the progression and maximizing patient independence and comfort.

This can include the use of breathing, feeding, mobility and communication appliances and devices.

Rehabilitation therapy may include physical, occupational and speech therapy.

Slowing disease progression

Two drugs are currently approved by the U.S. Food and Drug Administration (FDA) for ALS. Riluzole, or Rilutek, lowers the amount of glutamate in the body. It appears most effective in the early stages of ALS and in older individuals. It has been shown to improve survival.

In early 2017, the drug Radicava (Endaravone) was approved by the FDA for the treatment of ALS.

How it works is not well understood, but it may delay disease progression by working against tissue damage.

Scientists are currently exploring a possible role for stem cells in the treatment of ALS.

Muscle cramps and stiffness

Muscle cramps and stiffness can be treated with physical therapy and medications, such botulinum toxin (BTA) injections. BTA blocks the signals from the brain to the stiff muscles for about 3 months.

Baclofen, a muscle relaxer, may reduce muscle stiffness. A small pump is surgically implanted outside the body and connected to the space around the spinal cord. A regular dose of baclofen is delivered into the nervous system.

Baclofen blocks some of the nerve signals that cause spasticity. It may help with extreme yawning.

Treatment for drooling

Scopolamine, a drug for motion sickness, may help control symptoms of drooling. It is worn as a patch behind the ear.

Uncontrolled laughter or crying

Antidepressants, called serotonin reuptake inhibitors (SSRIs), may help with episodes of uncontrollable laughter or crying, known as emotional lability.

Speech, occupational and physical therapy

Patients with speech and communication difficulties may learn some useful techniques with a qualified speech and language therapist. As the disease advances, patients often need some communication aids.

Physical and occupational therapy can help maintain mobility and function, and reduce stress.

Swallowing difficulties (dysphagia)

As eating and drinking become harder, the patient may need a percutaneous endoscopic gastrostomy (PEG), a feeding tube that is placed on the abdomen, a relatively minor procedure.


A non-steroidal anti-inflammatory drug (NSAID), such as ibuprofen, will help with mild to moderate pain from muscle cramping as spasms. Drugs such as morphine can help relieve severe joint and muscle pain in the advanced stages.

Breathing problems

Respiratory muscles usually weaken gradually, but a sudden deterioration is possible.

Mechanical ventilation can help with breathing. A machine takes in air, filters it, and pumps it into the lungs often through a tracheostomy, a surgical hole in the neck that allows for assisted breathing.

Some people use complementary therapies, including special diets that are high in vitamins. These will not cure MND, but following a healthful diet can improve overall health and wellbeing.

Stem cell transplant for ALS treatment

Stem cell research and gene therapy have shown promise for treating ALS in the future, but more studies are needed.


MND is usually fatal. Depending on the type, most people will not survive longer than 5 years after symptoms appear, but some people live 10 years or longer

, depending on the extent and progression of their disease.

The United Kingdom's (U.K.) National Health Service (NHS) notes that living with an MND can be extremely challenging, and diagnosis can seem like a "terrifying possibility."

However, they add that with community and other support, the patient's quality of life is often better than expected, and the advanced stage "isn't usually distressing." For most people, this comes at home, while sleeping.

Patients may wish to prepare an advanced directive, in which they can state their future wishes about treatment while they are still able to express themselves.

Issues may include where they hope to be treated in the later stages, for example, in the hospital, in a hospice center or at home with hospice care, and whether they would like to receive mechanical breathing or other assistive care.

As medical research continues, scientists hope to understand MNDs more fully. They are working towards finding new treatments.

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