Neuron Anatomy and physiology
The image above is a rough representation of a neuron (nerve cell). Each neuron has a cell body and tens of thousands of tiny branches called dendrites which receive information from other neurons. Each neuron also has a primary axon (a projection that can travel long distances to make contact with other neurons or with somatic cells such as muscle, taste buds, skin pain receptors, etc.) The neurons discussed on this page are located in the brain. The axon’s function is to send data out of the neuron. While each neuron has only a single primary axon, that axon can branch hundreds of times in order to make contact with many other neurons.
The neurons make contacts with other neurons at points called synapses. Each neuron makes between a thousand and ten thousand synapses with other neurons. These synapses can be either excitatory or inhibitory, that is they can either excite the neuron with which it makes contact, increasing its potential to “fire”, or it can inhibit the contacted neuron from firing.
Synapses do not actually make “electrical” connections between neurons and/or cells. Instead, the axon’s tip looks more like a cul-de-sac which contains tiny vacuoles containing neuron specific neurotransmitters. A neurotransmitter is a hormone-like protein which can stimulate either excitatory or inhibitory potential in the cell membrane of the receiving neuron. When a neuron “fires”, these vacuoles migrate to the cell membrane at the synapse and then migrate across the gap to react with the cell membrane of the receiving neuron.
By itself, the neurotransmitters released from a single synapse will not stimulate or inhibit the receiving neuron. Each synapse simply contributes to the potential that that neuron will fire. It is the collective potential–either inhibitory or excitatory– from all the impinging synapses that builds the impulse necessary to cause the neuron to send its own impulse to its collective network of connected “neighbors”.
In the image above, the oligodendrocyte serves the same function in the brain that Schwann cells serve in the peripheral nervous system.
A piece of brain the size of a grain of sand contains one hundred thousand neurons, two million axons, and one billion synapses. All of these neurons are talking to each other. It has been calculated that the number of “brain states”, the number of permutations and combinations of activity that are theoretically possible, exceeds the number of elementary particles in the universe.