Neurons are the Cells of the Mind
Neurons and glial cells 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 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 have root-like inputs, dendrites, and tree- like outputs, axons that transmit signals. Spines on the dendrites make contact with axons from other neurons. 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. Where axons contact other neurons, the signal is transmitted across synapses by neurotransmitters such as glutamate, acetylcholine, 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 axon 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.
Glial cells intermingle with and closely embrace neurons. Glial cells are caretakers and custodians for neurons. Oligodendrocytes form myelinated conduction pathways that facilitate signal conduction. Some have immune cell activity. Neurons require a steady supply of energy in the form of glucose and/or lactate. Astrocytes extract nutrients from the blood and feed neurons. Blood vessels in the brain regulate flow to match oxygen and glucose delivery with metabolic demands determined by neural activity. Astrocytes connect neuronal synapses and blood vessels regulating blood flow in terms of synaptic activity. Glial cells form a slow conduction and biasing network that regulates brain function overall, but little is known about their role in detailed signal processing tasks.
Glial cells have structural functions and provide nurturing and defense services. Glial cells help to create the blood-brain barrier that limits access to neurons from the blood. Glial cells are active after injury in brain repair, but may contribute to neurodegenerative diseases. Microglial cells are resident macrophages in the brain. Microglia are trigged by foreign antigens and activate a variety of immune responses. Like macrophages in other tissues, their sensing and reacting ability is both defensive and destructive. Inherited forms of neurodegenerative diseases, such as amyotrophic lateral sclerosis, have been explained as death of neurons because of mutant proteins produced internally; however, more complexity is always revealed by more research. The disease process involves interactions of glial cells with neurons. Microglia may initiate or at least contribute to disease progression. Proliferation of microglia is often observed in many brain diseases. Bahareh et al suggest that resident progenitor cells give rise to new microglia. Unchecked proliferation of astrocytes produces a common form of brain cancer. The most malignant is Glioblastoma multiforme.