Neurons

How many neurons are there in the brain? This simple question has thrown up wildly dissimilar answers over the years, mostly because of methodological issues. Recently a "brain soup technique discovered by a Brazilian neuroscientist, Dr. Suzanna Herculano-Houzel and her group has answered this question with reasonable confidence. To count the number of cells in a brain, she took a small piece and chemically dissolved the cells. Then, she only counted the number of nuclei (nucleus; nuclei (plural) - is the master centre of the cell) present in the "brain soup. The nucleus can be specifically colored with a fluorescent dye, which then is counted accurately with an automated cell counter, like a cash counter in a bank. As each cell contains only one nucleus, this technique gives an accurate count of how many cells were there in the brain tissue.

Armed with this technique, she counted the number of cells (neurons, glia) in the brains of a number of different animals from the huge elephant to the tiny smoky shrew. This comparative analysis can answer one of the most vexing questions in neuroscience - what is the relationship between neuronal number and the organism's size and behaviour? The brain mass does increase with body mass, but only to a limit. Counting the number of neurons also correlates with an organism's behavioural repertoire, up to an extent. One can go further deeper by counting neurons in only a certain part of the brain since different parts have different functions. We can now count the number of neurons in the part of the brain that is involved with higher cognitive functions separately and the brain part that controls the body movements separately. This helps us to get a better understanding of how the behaviour of the organism is reflected in the distribution and the ratio of neurons in various parts.

To visualise these connections better, we have created an 'interactive comparative digital neuroanatomy' visualisation. We have used data from the research of the Brazilian neuroscientist Dr Suzana Herculano-Houzel's group. Her team has dissected brains of many organisms and extracted cell numbers from many organisms. Our visualisation compares cell numbers (neuronal/non-neuronal) for different brain parts (cortex/cerebellum/rest of the brain) with the organism's body/brain mass. This approach gives many valuable insights into the neuronal composition of different parts of the brains in terms of body/brain mass and comparison with other animals within and between evolutionary clades.
The brain is divided into three parts in these studies. The "cortex" is the part of the brain that processes perception and higher cognitive functions, like decision making, personality etc. The "cerebellum" - is involved in complex movement execution and the "rest of the brain" includes areas of the brain that are responsible for control of bodily functions like breathing, maintaining body temperature, blood volume etc.
The brain has few kinds of cells, chief among them is the "neuron", the fundamental unit of all mental functions. The other non-neuronal cells include the "glia" which also has an important role in the brain's functioning. This cells counted here are in two categories "neurons" and "non-neurons" in different parts of the brain.
Data was collected from animals from different "orders". An order is a group of closely-related animals that have descended from a common evolutionary ancestor. So, we have colour-coded the order each animal belongs so. The assumption is that animals belonging to the same order must have similar brain anatomy and functions than from a different order. However, this is not strictly true, as the brain of animals from different order could have similar features if they are living in a similar environmental niche.
The animals are numbered and arranged according to body weight. The left-most animal is the heaviest (Elephant) studied, and the rightmost is the lightest (Smokey Shrew). When you place your mouse over the first animal, elephant. A pink line appears connecting each of the horizontal bars. It shows the ranking of this animal on all four parameters. We see that the elephant is highest in body mass/brain mass, cerebellum neurons, but only 4th in cortex neurons.
You can also find out which animal is ranked in each parameter - for example, if you want to know which is the animal ranked 25th on brain mass. Place the mouse on the 25th tick on the "brain mass" horizontal line. The pink line will show which animal it is. In this case, it is a dog that is ranked 25th on brain mass. The line also will show other ranks for the dog (19th - body mass, 24th - neurons in cerebellum and 26th - total neurons and 25th- neurons in cortex. The shift of the pink rightwards says that the animal has smaller body/brain mass and numbers and a shift to the right means they have body/brain mass/numbers larger for their body weight (e.g. Humans, Gorillas, Rhesus Macaque)

Comparing the mass of the body or the brain weight to neuronal numbers alone cannot explain the cognitive capacities of an animal. Animals of similar-sized brains have hugely dissimilar cognitive capacities. Just counting the neuronal number is a better estimate, but larger animals have more neurons than smaller ones, but they are not necessarily smarter. Here, looking at the ratio of neurons in different parts of the brain can give further insight into this data. The number of neurons in cortex can reflect how intelligent the animal is and the number in rest of the brain can reflect the needs of the body, so a larger body should have more increase in 'rest of brain' than cortex, which should increase for smarter animal with a small body.

How to read: First, choose two parameters, you want to compare. Then click on the animal you want to compare against the rest of the animals. The illustration of the animal will appear to the left of the circle, and on the horizontal visualisation below the circle, the organism will move to the left-most positions.
Suppose you choose the ratio between brain mass/body mass and choose 'human' as the anchor animal. The three animals with the closest ratios will appear connected with blue lines of thickness proportional to their strength of the relationship (rhesus macaque, mouse lemur and common marmoset). The three animals with the farthest ratios will appear with red lines of varying thickness reflecting their relationship (Pig, Elephant and Hamster). The colour in the inner line shows the order the animal belongs to. Below the wheel are the linear representation of all animals and their relationship to humans in the chosen parameter.

Comparative study of anatomical structures across animals has a long history and helped us understand the structure of the natural world. The evolutionary tree of animals was built by comparing related structures, like placing chimpanzees closer to humans than monkeys because of the similarity in the structure of arms. Similarly, comparing the neuroanatomical details can help us understand the relationship between the brains of different animals. The latest data from comparative neuroanatomy studies has broken a lot of myths about brains and its relationship with number of neurons and body mass. There are many measures used to account for the size of the brain for an organism or the cognitive capacity of the animal. But, there is no clear measure that can explain the entire picture. We hope that our 'interactive comparative digital neuroanatomy' tool will help neuroscientists and others to unearth new patterns and meanings from the data.

 
Numerator:
Body Mass
Brain Mass
Neurons Cortex
Neurons Cerebellum
Neurons Rest of Brain
Non Neurons Cortex
Non Neurons Cerebellum
Non-Neurons Rest of Brain
Neurons Whole Brain
Non Neurons Whole Brain
Total Neurons
Total Non Neurons
Denominator:
Body Mass
Brain Mass
Neurons Cortex
Neurons Cerebellum
Neurons Rest of Brain
Non Neurons Cortex
Non Neurons Cerebellum
Non-Neurons Rest of Brain
Neurons Whole Brain
Non Neurons Whole Brain
Total Neurons
Total Non Neurons



References

  1. Mammalian Brains Are Made of These: A Dataset of the Numbers and Densities of Neuronal and Nonneuronal Cells in the Brain of Glires, Primates, Scandentia, Eulipotyphlans, Afrotherians and Artiodactyls, and Their Relationship with Body Mass. Brain Behav Evol. 2015;86(3-4):145-63.
  2. Dogs Have the Most Neurons, Though Not the Largest Brain: Trade-Off between Body Mass and Number of Neurons in the Cerebral Cortex of Large Carnivoran Species.Front Neuroanat. 2017 Dec 12;11:118.
  3. Cellular Scaling Rules for the Brains of Marsupials: Not as "Primitive" as Expected. Brain Behav Evol. 2017;89(1):48-63.
  4. The remarkable, yet not extraordinary, human brain as a scaled-up primate brain and its associated cost. Suzana Herculano-Houzel. PNAS June 26, 2012 109 (Supplement 1) 10661-10668

Media Article

  1. https://news.vanderbilt.edu/vanderbiltmagazine/brainiac-with-her-innovative-brain-soup-suzana-herculano-houzel-is-changing-neuroscience-one-species-at-a-time/

Contributions and Credits

    Content and Design: Leslee Lazar, The Centre for Cognitive and Brain Sciences, IITGN & Venkatesh Rajamanickam, IITB
    Programming: Arihant Parsoya
    Source Code: Github Repo
    Contact: Information Design Lab IDC IIT Bombay
    Published on: 22 February 2020




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