One of the more interesting questions 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 emerging shows a brain and central nervous system primarily composed of two types of cells. Glial cells are the infrastructure making up 85 per cent of brain 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 neuroscientists believe to be the centre for thought.
Or maybe a more accurate way of saying it is all of the neurons are connected together in a network which generates thought and the mind. This is the image we have of a neural network - a collective action in which all the neurons are firing together or not with a singular outcome which is 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 the nervous system - from the head to the base of the spine.
In giraffes, some axons are five metres in length.
The cell body of a neuron is surrounded by dendrites or branches. The overall picture is a bit like a dandelion 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 dendrites like the roots of the 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 - might be connected to as many as 10,000 other neurons.
As the adult human brain consists of some 100 billion neurons, the number of possible connections is quite staggering - somewhere between 100 and
500 trillion connections.
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 a neuron actively 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 clicking over, one after another, except in the neuron - once the flood of ions occurs - everything is set up again for the next impulse.
The net result is the propagation of an electrical potential or voltage along the cell's membranes. 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 about 40 different compounds 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 a number of the drugs for depression or mental disorders target. But many of the compounds we consume in our daily lives, such as caffeine and nicotine, act by changing the levels or effectiveness of neurotransmitters.
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.
While the development of psycho-active compounds has resulted in some missteps, the ability to use chemical compounds to alter our mind has revealed a great deal of information about how the brain works.
But none of the science, so far, has been able to answer the question: "Where does the brain end and the mind begin?"
