The Nature of Mind

Neurons are the brain cells that a manifest all the properties of mind. The study of neurons could be considered ne plus ultra, the quantum mechanics of biology. Neurons come in different shapes and sizes but have the common property of receiving and sending information.

Neurons conduct discrete signals as electro-chemical pulses, known as action potentials or “spikes.” The signal is passes from one neuron to another by the secretion of chemical neurotransmitters in synapses. There are trillions of synaptic junctions in the human brain. Learning occurs at least in part by changes in the number, strength and kind of synaptic connections. Early studies of neurons focused on the on-off characteristic of action potentials and a misleading comparison has been made with the transistor binary switch in digital circuits.

Neurons are something like bushes or trees and have branches emerging from an axon trunk that transmit signals. Neurons have dendrites or roots that receive signals. Signals are transmitted along axons and dendrites by the movement of sodium and potassium ions across cell membranes. The movement of ions creates a wave of electrical charge something like the wavy motions of electrons in copper wire. To increase the speed of long transmission of signals, axons are insulted with myelin in interrupted sequences, something like strings of sausage. Excitation jumps across insulated sections from one uninsulated node to the next.

Where axons contact other neurons, the signal is transmitted across synapses by neurotransmitters such as acetyl choline, norephinephrine, serotonin and dopamine. The sending side of the synapse is called the presynaptic membrane and the receiving side is postsynaptic. Neurotransmitters are chemicals stored in packets or vesicles on the presynaptic side and are released in clusters to cross the synapse and dock with postsynaptic receptors. The postsynaptic receptor is activated and conveys its signal to chemical devices inside the cell that can propagate the activity started at the receptor surface.

When enough neurotransmitters activate enough receptors, the receiving neuron sends an action potential along its dendrites to other neurons downstream. You could argue that much of the computation in the brain is done by adding and subtracting voltage fluctuations on the surface of neurons and the action potentials or pulses carry the results over longer distances to other neurons. Neuronal computation cannot be understood by looking at single neurons but may be understood by examining neuronal networks that receive and send pulse-encoded information.

The growth of the brain is a remarkable process that reveals a prodigious ability of neurons to self-organize. We now know that the neurons in the growing brain form a much larger number of trial connections than will be preserved even a few years after birth. The strategy of neuronal growth is to populate the brain with a surplus of neurons and synaptic connection and then allow the activity of the brain select the neurons that are useful and reject others that are not. Yuanyuan and Smith suggest: “Neural activity modulates development through biasing this process of formation and elimination, promoting the formation and stabilization of appropriate synaptic connections on the basis of functional activity patterns.” [i] The removal of synapses and the death of inactive neurons is referred to as “pruning.”

Neurons live in the “grey matter” of the brain, surrounded by white matter, the point to point wiring bundles in the brain. The surface of the brain is a thin layer of grey matter with neurons arranged in vertical columns six layers deep. The input and output pathways to these columns lies in the white matter below. The long axons of neurons travel through the white matter, carrying signals to and from the brain and within the brain. Neurons are surrounded by glial cells that attend to their needs and provide protection. The white matter is formed by specialized supporting cells, oligodendrocytes; they have flat, myelin-containing extensions that wrap around axons, creating a fatty insulation. The myelinated axons are compared to copper wires coated with plastic insulation and are designed for longer distance signal transmission. In peripheral nerves, the insulation is provided by Schwann cells that make a single wrap around each axon.

[i] Jackie Yuanyuan Hua, Stephen J Smith. Neural activity and the dynamics of central nervous system development. Nature Neuroscience. April 2004 Volume 7 Number 4 pp 327 - 332