Neurons

No one has hitherto laid down the limits to the powers of the body, that is, no one has as yet been taught by experience what the body can accomplish solely by the laws of nature, in so far as she is regarded as extension. No one hitherto has gained such an accurate knowledge of the bodily mechanism, that he can explain all its functions; nor need I call attention to the fact that many actions are observed in the lower animals, which far transcend human sagacity, and that somnambulists do many things in their sleep, which they would not venture to do when awake: these instances are enough to show, that the body can by the sole laws of its nature do many things which the mind wonders at. (Ethics, Book III, Proposition 2, Note)

The basic unit of brain functioning is the nerve cell or neuron, of which there are around 100 billion in the human brain. Each neuron connects to multiple others in complex pathways. The enormous number of neurons and connections between them is what makes the extraordinary human capacity to think and emote possible. The human brain can adopt more neural configurations than there are particles in the entire universe.

Real Neurons

Neurotransmitter being transported in vesicles (purple spheres) to
the synaptic cleft.

Basic Functional Anatomy of Neurons

Neurons convey information to other neurons by means of the generation of "action potentials." When electro-chemical stimulation of a neuron exceeds a certain critical threshold, a sodium (Na⁺) - potassium (K⁺) chemical pump changes the polarity, the distribution of electrical charges, across the cell membrane of the neuron. The result is a pulse-like wave of electrical energy that travels through the cell body and down a long projection called an axon.

The action potential causes vesicles filled with a chemical neurotransmitter to migrate to the end of the axon, and then dumped into the synaptic cleft (the gap between neurons). By binding to receptor sites on the far side of the cleft, the neurotransmitter, depending upon its chemical composition, contributes to either exciting or inhibiting an action potential in the next neuron down the line. That neuron will in turn "fire," in other words, generate its own action potential, if the sum of excitatory and inhibitory impulses from all of the neurons that synapse onto it reaches the critical threshold.

Drugs alter the mental condition of the user by modifying the various processes involved in synaptic transmission.

Simulated Neurons