"What lies behind us and what lies before us are tiny matters compared to what lies within us."
—Oliver Wendell Holmes
Embedded inside our head lies a seemingly ordinary piece of greyish matter called the human brain weighing on average between 1.5 and 1.6 kilograms for an adult.
The chemical building blocks of the human brain
Our brain is composed of a number of chemical building blocks. If our eyes could look at, say, the very atoms and molecules composing our brains, we would see the most abundant chemical substance in the human brain is water. The second most abundant chemical substance is protein, then comes the inorganic salts (a chemical mixture of metals and non-metals), lipids (mainly fats), carbohydrates (molecules having only carbon, hydrogen and oxygen in their chemical structure and which act as a source of energy for the brain), and nucleic acids.
These chemical substances come together to form the fundamental biological building blocks in the human brain called neurons or nerve cells.
The biological building blocks of the human brain
Neurons are categorised into three main types:
- Sensory neurons convey to other neurons information about the external environment ie. they produce the sensation of sight, sound, smell, taste and touch;
- Motor neurons utilise information from other neurons to activate muscle cells and biochemical reactions; and
- Integrative neurons transmit information from sensory neurons to motor neurons or other integrative neurons.
The two most important functions every neuron plays in the body are (i) to monitor and relay information or messages from one neuron to another using a combination of what are called nervous impulses and neurotransmitters; and (ii) to learn, as it were, by forming sophisticated networks with other nerve cells.
Nervous impulses are merely the flow of charged ions moving through the nerve cell. A charged ion is a chemical molecule that has lost an atom or two from its structure (usually temporarily), causing the molecule to develop a strong electrical charge. Neurotransmitters, on the other hand, are uncharged chemical molecules sent out by one nerve cell to the next to help with the transfer of information.
The number of neurons relaying information and creating networks in a typical brain has not been properly determined, but we are assured it is in the billions, perhaps hundreds of billions.
The structure of a typical neuron
Despite the numbers and types of neurons in the human brain, they all have a common cell structure.
Each neuron has a cell body containing a nucleus. Here, the nucleus determines whether or not an impulse is to be sent along an axon (the main conducting fibre of a nerve cell), depending on the impulses received by numerous branch-like extensions surrounding the surface of the cell body, called dendrites. At the end of the axon lies a conglomeration of root-like extensions, each having a tiny bulb at the terminal position called a synaptic knob. To transmit a signal from one neuron to the next, this synaptic knob must release special chemicals called neurotransmitters across a small gap known in the technical jargon as a synapse, which can be as narrow as 20 millionths of a millimetre. Once the chemicals arrive at the next neuron, they stimulate the neuron to generate an electrical impulse, which may or may not travel through the axon depending on the decisions made by the nucleus of this other neuron.
A nervous impulse travelling through an axon is not electrical, but a movement of ions. Ions are nothing more than tiny, free-moving electrically-charged molecules and atoms. The ions that are important in the transmission of an impulse are mainly calcium, potassium, sodium, chloride and protein ions.(1)
The totality of such neurons in the human body, at least over 10 billion or so as we are told, gives rise to the development of a large-scale neuron-like structure called the nervous system.
The entire nervous system consists of the brain (which acts like the nucleus of a neuron) and spinal cord (which acts like the axon of a neuron) called the central nervous system, and all the biological 'wires' or integrative neurons connecting the central nervous system with the sensory and motor neurons in the body.