The Hows How big is the brain? How much does the brain weigh? The Human Brain (Image courtesy of the Mammalian Brain Collection) 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? It is estimated that there are 100 billion (100,000,000,000) neurons in the human brain. To get an idea of how many 100 billion is, think of this: Assume that you were going to count all 100 billion cells at a rate of 1 cell per second. How long would it take you to count all 100 billion cells? My calculations say it would take about 3,171 years!!!. Do the math yourself. (Here is a hint on the math: there are 60 seconds in a minute; 60 minutes in an hour; 24 hours in a day; 365 days in a year.) By the way, my calculations did NOT take "leap years" into account. Actually, it would probably take a lot longer than 3,171 years because it takes more than 1 second to say the large numbers. Here is another way to think of 100 billion: Assume the cell body of one neuron is 10 microns wide (this is just an assumption because neurons come in many different sizes. However, 10 microns is small; smaller than the period at the end of this sentence). Ok...if you were able to line up all 100 billion neurons in a straight line, how long would your line be? Check my math!! 1 neuron = 10 microns wide 10 neurons = 100 microns wide 100 neurons = 1000 microns wide = 1 mm wide 1,000 neurons = 10 mm wide = 1 cm wide 100,000 neurons = 100 cm wide = 1 m wide 100,000,000 neurons = 1000 m = 1 km 10,000,000,000 neurons = 100 km 100,000,000,000 neurons = 1000 km (approximately 600 miles) Although all the neurons lined up side by side would stretch 1000 km, the line would be only 10 microns wide...invisible to the naked eye!!! To get an idea of how small a neuron is, let's do some more 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:   Behavioral Level: study of the neural basis of behavior. In other words, what causes people and animals to do the things they do. 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. Local Circuit Level: study the function of groups of neurons (nerve cells). 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). Synapse Level: study what happens at the synapse. Membrane Level: study what happens at ion channels on a neuronal membrane. Genetic Level: study the genetic basis of neuronal function. How do you become a neuroscientist? How long do you have to go to school? First, you have to finish high school...so from 1st to 12th grade is 12 years. Second, you get a university degree...at least another 4 years of school. 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? 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, 1997 and other sources including The American Academy of Neurology). Major Nervous System Diseases Disease Number of Cases Cost 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 Stroke 4,700,000 $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 Reference: Brain Facts, Washington D.C.: Society for Neuroscience, 2005. 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 2006, a PubMed search using the term "brain" shows that 46,823 papers were published. For 2005, a PubMed search using the term "brain" shows that 45,820 papers were published. For 2004, a PubMed search using the term "brain" shows that 41,759 papers were published. For 2003, a PubMed search using the term "brain" shows that 39,150 papers were published. For 2002, a PubMed search using the term "brain" shows that 36,529 papers were published. For 2001, a PubMed search using the term "brain" shows that 36,218 papers were published. For 2000, a PubMed search using the term "brain" shows that 36,370 papers were published. For 1999, a PubMed search using the term "brain" shows that 34,331 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.

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