Assemble 86 billion building blocks
At our best estimate, there are about 86 billion neurons in the human brain. There is no reason to believe that this number varies significantly from person to person, even if you can think of politicans/celebrities/colleagues whose behaviour suggests they may be a few billion short. Similarly, there is no truth that Einstein, or any other ‘genius’, had or has more than their fair share. Unlike other organs, the brain is a unique collection of an unknown number of different varieties of neurons and glia which have to be precisely connected to create The Most Complex Object In The Universe (© this and every other essay or talk about the brain). Yet all these neurons, like every other cell in your body, arose from a single, fertilised egg cell. And somehow they reliably form a functional network that not only controls our basic physiology to stay alive but also gives rise to us: to all the thoughts, memories and emotions that make us human and make us who we are as individuals.
The most important thing you need to appreciate about the brain is that we actually know little about how it works and even less about how it develops in the first place.
Given the pace of progress in scientific and medical understanding, not to mention the countless books and websites claiming to explain consciousness and behaviour or how to be smarter, less stressed and more popular, it’s easy to assume we know a lot about how brains work. In the grand scheme of things this is really not the case at all. Unravelling the mysteries of the brain is up there with the greatest challenges facing mankind, like interplanetary travel, the nature of dark matter or how to dissolve dried porridge. The foundations of neuroscience and their risk of subsidence is a topic for another day but in the meantime I strongly recommend reading ‘The Idea of the Brain’ by Matthew Cobb.
Wiring up a worm brain
A common approach to understanding complex systems is to find a simpler version and work up from that. In the case of a brain, an example would be the microscopic nematode worm Caenorhabditis elegans. Biologists have mapped the development of every single cell in the worm’s body all the way from when it starts, like us, as a fertilised egg. We know that it has 302 neurons and we know exactly how these are wired up yet we have little idea how these produce even the simplest behaviour. All of this information is freely available and regularly expanded at the Worm Atlas. Similar ‘connectome’ projects are at various stages for other species including fruit flies, mice and humans. They are producing beautiful data of astounding complexity yet what they will tell us about how brains actually work remains to be seen. The reductionist approach has nevertheless been applied successfully in different areas of neuroscience, it has helped us start to understand how some reflex circuits form, to link genetic data from inherited disorders to underlying developmental processes and may one day feed into the use of stem cells and bio-chip interfaces to regrow and repair damaged or diseased nervous tissue.
Attach neuron A to neuron B
Eventually I’d like to cover the full breadth of brain development: how the first cells are set on a path to become nervous tissue; how these cells know what type of neuron to become and where they sit within the body; how the brain forms wrinkled outer layers and specific structures deep inside. I’m going to begin in the middle, with the question that occupied my research career for 20 years, namely how is the brain wired up? How does a nerve cell know what its target is? Whether that is another neuron, thousands of other neurons or a different cell entirely such as muscle. This field of study is called axon guidance and before going any further we should first have a look at the anatomy of a neuron to explain what this is about.
One of the earliest depictions of the anatomy of a neuron is in the above painting by a follower of Rembrandt, titled ‘The Neuroanatomy Lesson of Dr Nicolaes Tulp’. Various scholars, of art and science, have speculated on which animal the neuron may have originated from. Neither Tulp nor Rembrandt refer to this in their respective writings so it is likely to remain a mystery. The assembled academics gather round and lean in to peer at the exposed internal neuronal structures. Some consult reference atlases of the brain while hangings on the wall show the influence of Da Vinci’s earlier depiction of Vitruvian man.
Closer inspection of the area of the painting containing the neuron and analysis with specialist imaging equipment revealed the preparatory sketches underneath the paint. These show annotations, probably by the artist himself, of the key structural elements common to all neurons. Like nearly all cells, neurons have a nucleus containing DNA, red blood cells being the major exception. Arrayed out from the cell body are branched projections called dendrites, after their tree-like form. These are the major source of input to the neuron, receiving signals from other cells and carrying them to the cell body. The number of dendrites on a neuron can vary enormously from one to thousands as can the degree to which they branch. The strength and frequency of signals received by the dendrites will determine the nature of the output passed on to other cells. This is carried by the axon, the single longest protrusion from the cell body which makes contact with the target cells of the neuron, typically another nerve or a muscle. The average neuron is a a few tens of microns (1 micron = 0.001 millimetre) in diameter, the average axon is many times longer than this. For example, nerves leaving the base of your spine to innervate your foot may be up to 1 metre long – many thousands of times the size of the cell body! This is all one cell, not a chain of smaller cells, so such an extreme shape places particular demands on a nerve cell. All cells contain an internal scaffold of protein filaments called the cytoskeleton. In the axon these are dense bundles of thick filaments called microtubules, wrapped up by rings of a thinner filament called actin. At the end of a growing axon is a specialised structure called a growth cone; this is central to how the brain is wired up. The growth cone crawls through the developing embryo, pulling the axon behind it, until it reaches its target cell. Once there it forms a synapse, this is the connection between one neuron and another or between a neuron and a muscle. The field of axon guidance has grown up around this central question of how do axons know where to go?
There are many reasons why this is such an amazing process. One is that it doesn’t happen for just a few cells but for all the billions of neurons in your brain and so reliably that it is able to form a functioning nervous system. Not only do the axons somehow have a sense of where they should be going but they cannot see ahead to where that target is. The growth cone has to burrow its way through the developing embryo surrounded by a multitude of other growth cones, sometimes competing for the same signals, negotiating what is directly in front of it without the benefit of an atlas or GPS to steer it. As if this was not challenging enough, the landscape around them is not static. The embryo itself is growing, changing, expanding around it. Nerves do not enter the limbs once the arm or leg has finished forming, they do so at the same time as the limb is extending and being shaped, chasing after their target muscle or patch of skin as it forms.
The growth cone can be thought of as being like a semi-independent nose or hand on the end of the axon, sniffing and feeling its way through its surroundings. Target tissues, such as muscles, secrete chemical signals which attract the growth cone towards them. Different neurons respond to different chemoattractant signals. This is one way muscles can entice motor nerves towards them whereas growth cones of other types of nerve, such as sensory nerves, will guide their axons towards a certain patch of skin by following a different scent.
Not all signals are positive. To prevent nerves growing where they shouldn’t, many cells secrete chemorepellents which make growth cones steer away. At low concentrations, the growth cone will bend away from the source; at high concentrations, a repulsive signal will make a growth cone collapse and the axon will retract. In the same way that neurons differ in their preferred attractive signals, so they vary widely in their response to repellent cues. The same protein that will make one growth cone collapse with barely a whiff will be completely ignored by another type of nerve. The combined action of these positive and negative signals is believed to guide the wiring of a nervous system. These actions can be readily demonstrated in a laboratory culture dish with carefully defined growth conditions and specially chosen neurons. There is plenty of evidence too from genetically engineered animals and congenital human disorders that these principles apply in vivo. The major challenge – and it is a huge one – is understanding how all these cues are combined and interpreted by all the different growth cones. The number of discrete chemical signals is probably in the order of hundreds, maybe thousands. The number of neurons in our brain is in the order of billions. Axon guidance seeks to understand how one is translated into the other by growth cones. This is important for a better understanding of how the nervous system develops and why this sometimes goes wrong; it’s also important for devising therapies to repair the brain after acute injury, such as trauma or a stroke, or after chronic insults such as the degeneration seen in motor neuron disease or dementia.
Growth cones were identified by the pioneering neuroanatomist Santiago Ramón y Cajal at the end of the 19th century and in subsequent decades there was much physiological characterisation of growth cones and neuronal circuits at a cellular level. The 1990s saw an explosion in the identification of the actual molecules that function as axon guidance cues which finally allowed precise genetic and biochemical manipulation of developing neurons. Many of these were identified by Herculean efforts of researchers screening thousands of mutated embryos, often of fruit flies, to spot ones with specific defects in axon growth. Noticing that loss of a certain protein causes axon misrouting is only the first step. Understanding how it causes that misrouting is even more demanding and the mechanisms underlying many axon guidance molecules are still being worked out. One family of axon guidance molecules are the Roundabout proteins, abbreviated to ROBO. Many animals, from flies to humans, have a bilateral body plan meaning it is broadly symmetric along one axis. Within this, the nervous system has a certain overall symmetry but there needs to be connections between the two sides to allow coordinated control. Some of the nerves running from your brain down to your body stay on the same side, many crossover within the brain or spinal cord and innervate the opposite side. This is why after a stroke on one side of the brain, the opposite side of the body is typically affected. Fruit flies have this same body plan as us but in flies lacking the ROBO protein, instead of crossing over once, nerves keep crossing and recrossing, as if on a roundabout. They can no longer correctly sense where the midline of the body is and circle round like Winnie the Pooh and Piglet searching for Woozles. Genetic changes in the ROBO protein cause a similar defect in humans, leading to loss of specific movements because nerves do not form the right connections between different sides of the body.