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Know Your Neurons: How to Classify Different Types of Neurons in the Brain's Forest
By Ferris Jabr | May 16, 2012
How to Classify Different Types of Neurons, or The Dendrology of the Neuron Forest
Scientists have organized the cells that make up the nervous system into two broad groups:
neurons, which are the primary signaling cells, and glia, which support neurons in various ways.
The human brain contains around 100 billion neurons and, by most estimates, somewhere
between 10 to 50 times as many glial cells.
All these cells are packed into a three-pound organ about the size of both your fists stuck
together. You can think of your brain as a dense forest—the neuron forest—in which different
kinds of trees grow near, around and on top of one another, their branches and roots
intertwining. Just as all trees share a basic structure—roots, trunk, branches—but do not look
exactly alike, all neurons are variations on a common structural theme. The diversity of
structures is extraordinary and scientists are still discovering brain cells that do not really look
like any brain cell they have seen before.
Different Types of Neurons (click to enlarge). A.
Purkinje cell B. Granule cell C. Motor neuron D. Tripolar neuron E. Pyramidal Cell F. Chandelier cell G. Spindle
neuron H. Stellate cell
Before exploring the brain's cellular diversity, let's look at a model neuron. A typical neuron has
three main structures: the cell body, the axon and the dendrites. The cell body contains the
nucleus, which stores the cell's genes; the axon is a long slender cable that carries electrical
signals known as action potentials away from the cell body toward other neurons; and the
dendrites are shorter branching fibers that receive signals from other neurons. Near its end, the
axon of one neuron branches and forms connections with as many as 1,000 other neurons—
but, as 19th century neuroanatomist Santiago Ramón y Cajal insisted, the end of one neuron
does not fuse with the beginning of another into a seamless web. Instead, an axon's branching
tips communicate with the dendrites, axons and cell bodies of other neurons across tiny gaps
called synapses.
Scientists have classified neurons into four main groups based on differences in shape.
Multipolar neurons are the most common neuron in the vertebrate nervous system and their
structure most closely matches that of the model neuron: a cell body from which emerges a
single long axon as well as a crown of many shorter branching dendrites. Unipolar neurons, the
most common invertebrate neuron, feature a single primary projection that functions as both
axon and dendrites. Bipolar neurons usually inhabit sensory organs like the eye and nose. Their
dendrites ferry signals from those organs to the cell body and their axons send signals from the
cell body to the brain and spinal cord. Pseudo-unipolar neurons, a variant of bipolar neurons
that sense pressure, touch and pain, have no true dendrites. Instead, a single axon emerges
from the cell body and heads in two opposite directions, one end heading for the skin, joints
and muscle and the other end traveling to the spinal cord.
Researchers also categorize neurons by function. Sensory neurons collect information from
sensory organs—from the eyes, nose, tongue and skin, for example. Motor neurons carry
signals from the brain and spinal cord to muscles. Interneurons connect one neuron to another:
the long axons of projection interneuons link distant brain regions; the shorter axons of local
interneurons form smaller circuits between neighboring cells.
Do these basic classes account for all types of neurons? Well, just about every neuron in the
human nervous system should fall into one these broad categories—but these categories do
not capture the true diversity of the nervous system. Not even close. If you really want to
catalogue neurons in their many forms—somewhat like the way scientists have classed living
things into families and species and subspecies—you're going to need a lot more categories.
Neurons differ from one another structurally, functionally and genetically, as well as in how
they form connections with other cells. In some ways, it's up to you how far you want to take
this. Some people are content with a few broad categories and do not see a need to identify
and categorize every single type of neuron. Others are fascinated by the differences between
cells in the brain and nervous system, even the subtlest distinctions. Some are fascinated for
practical reasons, because some of these differences help explain, for example, why certain
diseases only harm a certain population of neurons. Others are motivated by pure curiosity.
Since at least the 19th century—even before Cajal convinced the leading anatomists of the time
that the nervous system was made of discrete cells—scientists recognized that not all
components of the nervous system looked the same and began differentiating these
components by name. In 1840, Adolphe Hannover discovered what today we call the ganglion
cells of the retina, the light-sensitive tissue at the back of the eye. In 1866, Leopold August
Besser named large, densely branching neurons "Purkinje cells" after their discoverer, Czech
anatomist Jan Purkyně. Vladimir Alekseyevich Betz discovered the largest cells in the central
nervous system, today known as Betz cells. Cajal tried out various names for different kinds of
neurons, as well as their tinier features. He called little bumps along the length of
dendrites espinas, the Spanish word for thorns. Today, we call them dendritic spines.
So how many different types of neurons have scientists named so far? To find out, I contacted
several neuroscientists who specialize in cell biology and what you could call neuron taxonomy.
Perhaps unsurprisingly, no one has an exact number, but if you count all the types and subtypes
in the entire nervous system, the answer is at least in the hundreds. One great resource for
exploring the cellular diversity of the nervous system is NeuroMorpho.org, a database of
digitally reconstructed neurons that you can browse by species, brain region and cell type.
Check out the Cell Types page and you'll encounter descriptive names like cone cell, climbing
fiber, crab-like, medium spiny cell, pyramidal cell, chandelier cell and tripolar cell—each of
which boasts a unique structure. 3D models of these neurons pop into view when you mouse
over the file names of different reconstructions.
Gordon Shepherd of Yale University pointed me to the Neuroscience Lexicon, a database that
he and his colleagues are building. Take a look for yourself at their current list of types of
neurons. Here's what the Lexicon lists for distinct types of neurons in the cerebellum, an
evolutionarily ancient part of the brain that helps coordinate movement:
And that's just one region of the brain. Remember that the human brain contains around 100
billion neurons densely packed into three-pounds of tissue. Consider that the human brain is
one of the most complex structures we have ever tried to understand. All those layers of
fragile, excitable tissue folded upon one another. Within those folds we will surely discover new
types of neurons of which we have no inkling at present.