One of the more interesting questions that science faces is: "Where does the brain end and the mind begin?"
This is complicated because our inner experience is so private. No one else can share your thoughts. We all live in a world of our own making inside our minds.
For most of human history, the origin of thought has remained a question securely in the realms of philosophy but in the past century or so, scientists have begun to explore the mind using both intrusive and non-intrusive techniques. The latter is the realm of chemistry and pharmacology.
The map that is emerging shows a brain and central nervous system that is primarily composed of two types of cells. Glial cells are the infrastructure, making up 85 per cent of the cells, and providing both the delivery of nutrients and the disposal of waste. The remaining 15 per cent are the neurons, which conduct and process electrical information. It is the neurons that are believed to be the centre for thought.
Or maybe a more accurate way of saying it is that all of the neurons are connected together in a network that generates thought and the mind. This is the image that we have of a neural network - a collective action in which all the neurons are firing together or not with a singular outcome that is a thought.
Each neuron in the network is a single cell composed of a body which contains the nucleus along with the organelles necessary for the proper function of the cell and a long axon or "wire" that dangles from the cell body. Axons can reach up to a metre in length in a human brain - from the head to the base of the spine. In giraffes, some of the axons are five metres in length.
A feature of the cell body of the neuron is that it is surrounded by dendrites or branches. The overall picture is a bit like a dandelion that has gone to seed, with the stalk being the axon, the core of the flower being the cell body, and the seeds representing the dendrites. The end of the axon is also split into a multitude of filaments, like the roots of a plant. These filaments interconnect with a multitude of dendrites from the cell bodies of a multitude of other neurons resulting in a multitude of connections.
Each neuron - through its dendrites and axon - might be connected to as many as 10,000 other neurons. Given that the brain consists of some 100 billion neurons, the number of possible connections is quite staggering - somewhere between 100 and 500 trillion connections in an adult brain!
Nerve impulses travel down the axon, from the cell body, using electrical impulses generated by the concentrations of sodium and potassium ions. The membranes of the axon for a neuron build up a high concentration of sodium ions on the outside and potassium ions on the inside.
When a nerve fires, a signal travels down the axon by the release of sodium into the neuron, followed by a flood of potassium out of the neuron. It's a bit like a series of dominoes that click over, one after another, except that in the neuron - once the flood of ions occurs - everything is set up again for the next impulse.
The net result is that an electrical potential or voltage propagates along the axon. But what happens at the connections between neurons?
These connections are called synapses and there are two types: electrical and chemical. In mammals, the most important and abundant are the chemical synapses. They are also the most interesting as there are about 40 different compounds that have been identified as signal carriers, called neurotransmitters.
The arrival of a signal causes the firing neuron to release neurotransmitters into the synapse which are picked up by the receiving neuron. Each neurotransmitter has a separate receptor site which it activates and this can result in different responses. The study of neurotransmitters is revealing a lot of information about how the brain is organized and why it works.
Neurotransmitters, such as serotonin, are the compounds that a number of the drugs for depression or mental disorders target. However, it is not just drugs but many compounds such as caffeine and nicotine that we ingest daily that depend on changes in the levels of neurotransmitters for their effect.
By controlling the release or re-uptake of neurotransmitters, the function of individual synapses can be modified. In turn, this changes the firing rate of the neurons which changes the electrical impulses and alters all of the connections. The whole neural net can be modified in this fashion.
But none of the science, so far, has been able to answer the question: "Where does the brain end and the mind begin?"
