Neurophilosophy

Neuroscience, like all other branches of science, is fraught with dogmatic ideas about its subject matter. A number of principles have emerged, principles that have been regarded as fundamental to our understanding of brain function. 

But the human brain is an organ of bewildering complexity – it is often referred to as the most complex object in the known universe – which doesn’t give up its secrets easily. After 100 years of scientific investigation, we still know very little about it. 

So it’s no real surprise that in recent years, neuroscientists have made quite a few discoveries that overturn many of our dogmas about the brain, and caused us to reconsider what we thought we knew about how it works.

Below are 6 long-held dogmas about the brain. Each one is followed by a brief description of research which shows that it is either untrue, or is an oversimplification.

Dogma 1: The adult human brain is not plastic. Until relatively recently, the adult human brain was believed to be completely non-malleable. But the pioneering work carried out by Michael Merzenich and his colleagues in the late 1970s and early 1980s showed that this is not the case. We now know that the brain is capable of reorganizing itself extensively, particularly in response to experience and injury. Learning is now thought to occur as a direct result of the modification of synaptic connections in the brain; reorganization of the brain’s wiring is widely believed to take place following injury, and to underly phenomena such as phantom limb syndrome in amputees.

Dogma 2: The adult human brain cannot regenerate. The view that the adult brain cannot generate new nerve cells has been a central dogma of neuroscience. But it is now well established that the adult human brain contains small populations of neural stem cells, which are capable of dividing to generate new neurons throughout adulthood. The function of these new cells is still unclear, and researchers have so far had little success in coaxing them to divide in vivo. Nevertheless, once they do so, stem cells can potentially be used to develop treatments for neurological conditions such as stroke, epilepsy and Alzheimer’s, Parkinson’s, and Huntington’s diseases.

Dogma 3: Neurons are the functional elements of the nervous system. In the 19th century, the discovery of the neuron was quickly followed by the realization that the nervous system contains another cell type: the glial cell. Glial cells were quickly relegated to a secondary role in which they provide neurons with structural and nutritional support. In recent years, however, this view has begun to change. Glial cells are now known to regulate communication between neurons and to control blood flow through the capillaries in the brain. They can also communicate with neurons, with each other, and with blood vessels, and a study published in April of this year shows that glial cells can generate action potentials. Rather than being mere support cells, glia may yet be shown to be the main players in brain function.

Dogma 4: Neurotransmitters are released from the nerve terminal. According to the conventional view, neurons receive inputs from other nerve cells on their dendrites, integrate these signals in the cell body, and generate an action potential which is propagated along the axon. When the action potential reaches the nerve terminal, it triggers the release of neurotransmitters, which diffuse across the synaptic cleft and elicit a response in the postsynaptic membrane. However, several studies published published last year show that neurotransmitters can also be released from axons in the white matter of the corpus callosum.

Dogma 5: Neurons are binary switches. In other words, a nerve cell is either on or off: at any given time, it is either generating an action potential, or it is not. The action potential was regarded as an “all or nothing” response. That is, a minimum amount of stimulation is required before a neuron will produce a nervous impulse, and a sub-threshold stimulus (one that is smaller than the minimum stimulus amplitude) will not produce a response. It has long been known that cells of the invertebrate nervous system produce graded potentials, whereby the amount of transmitter released is proportional to the intensity of the stimulus. We now have evidence that mammalian neurons can generate graded potentials as well – they are not simple on/off switches, and the action potential is not all or nothing.

Dogma 6: Neurons communicate with each other by propagating action potentials. Neurons evolved to communicate with each other, and they do so by generating nervous impulses which are propagated along the nerve fibres. But because this electrical activity cannot cross the synapse, it is converted to a chemical signal which transmits the signal from one cell to the next. Although all neurons communicate in this way, we now know that some cells in the nervous system can convey signals by the propagation of a secondary messenger cascade. These biochemical signalling cascades can travel along the nerve fibre, and can elicit the release of neurotransmitters from the nerve terminal, in the absence of electrical activity.

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Comments

  1. #1 PhysioProf
    June 9, 2008

    Nice post. Just one clarification:

    We now have evidence that mammalian neurons can generate graded potentials as well – they are not simple on/off switches, and the action potential is not all or nothing.

    We have long known that mammalian neurons can use graded potentials instead of all-or-nothing action potentials. Two well-studied examples are the horizontal and amacrine cells of the retina.

  2. #2 Mo
    June 9, 2008

    So is it now thought that many or most neurons generate them all the time?

  3. #3 Alexander
    June 9, 2008

    This is great! All of these dogmas are things I’ve heard in the classroom. Some of them I have known for some time to be false, like the misconception that the human brain cannot regenerate. Others, though, are really revolutionary new concepts.

  4. Great post! I’ll try to translate it to Spanish to post on my own blog when I have some more time.

  5. #5 David Marjanovic, OM
    June 10, 2008

    the invertebrate nervous system

    Please.

    So you’re not talking about vertebrates. Fine. But which animals are you talking about? Some of them, you know, are much more closely related to us than others are.

  6. #6 Mo
    June 10, 2008

    Read it again carefully:

    It has long been known that cells of the invertebrate nervous system produce graded potentials, whereby the amount of transmitter released is proportional to the intensity of the stimulus. We now have evidence that mammalian neurons can generate graded potentials as well…

  7. #7 tona b.
    June 10, 2008

    How about the long held belief that we only use a 10% of our brain power? Man, was I glad to hear this wasn’t true!

  8. #8 74westy
    June 11, 2008

    This is very interesting in light of a recent allegations that Science is an entrenched priesthood that is only interested in expelling all the little Galileos who dare to question its orthodoxy.

    It would seem that, while skeptical of ideas that contradict the mainstream, science does in fact embrace well-supported challenges to the status quo.

  9. #9 Michael Meadon
    June 24, 2008

    Excellent stuff Mo, I’m loving your blog!

  10. #10 sam
    July 1, 2008

    Re: Dogma 4

    It’s worth adding that although the finding you report is new and surprising, we’ve known for some time that neurotransmitters do not work purely across the synaptic cleft.

    For example, GABA (the inhibitory neurotransmitter) can also “spill over” from a particular synaptic cleft and have a transient (“phasic”) inhibitory effect on other nearby neurons. This is important for synchronising the firing of large populations of pyramidal cells. And low concentrations of ambient GABA can persistently activate certain subtypes of GABA(A) receptor, which are often remote from synapses, to generate a ‘tonic’ conductance.

    This is an excellent, well-written review on the subject:
    Farrant, M. and Nusser, Z. (2005). Variations on an inhibitory theme: Phasic and tonic activation of GABA A receptors. Nature Reviews Neuroscience Volume 6, Issue 3, Pages 215-229 DOI: 10.1038/nrn1625

  11. #11 Houston Lawyer
    March 21, 2011

    It has long been known that cells of the invertebrate nervous system produce graded potentials, whereby the amount of transmitter released is proportional to the intensity of the stimulus. We now have evidence that mammalian neurons can generate graded potentials as well…

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