The Believing Brain - Michael Shermer [60]
This is what I mean when I say that the mind is what the brain does. The neuron and its actions are to psychology what the atom and gravity are to physics. To understand belief we have to understand how neurons work.
Synaptic States and Believing Neurons
The brain consists of about a hundred billion neurons of several hundred types, each of which contains a cell body, a descending axon cable, and numerous dendrites and axon terminals branching out to other neurons in approximately a thousand trillion synaptic connections between those hundred billion neurons. These are staggering numbers. A hundred billion neurons is 1011, or a 1 followed by 11 zeros: 100,000,000,000. A thousand trillion connections is a quadrillion, or 1015, or a 1 followed by 15 zeros: 1,000,000,000,000,000. The number of neurons in a human brain is about the same number of stars in the Milky Way galaxy—literally an astronomical number! The number of synaptic connections in the brain is equivalent to the number of seconds in 30 million years. Think about that for a moment. Start counting seconds as “one one thousand, two one thousand, three one thousand.…” When you get to 86,400 that is the number of seconds in a day; when you reach 31,536,000 that is the number of seconds in a year; and when you finally reach one trillion seconds you will have been counting for about 30,000 years; now, do that 30,000-year counting block one thousand more times and you will have counted the number of synaptic connections in your brain.
Large neuronal counts do generate greater computational power to be sure (like adding more processor chips or memory cards to your computer), but the action is in the individual neurons themselves. Neurons are elegantly simple and yet beautifully complex electrochemical information-processing machines. Inside a resting neuronal cell there is more potassium than there is sodium, and a predominance of anions—negatively charged ions—gives the inside of the cell a negative charge. Depending on which type of neuron it is, if you put a tiny electrode inside the neuronal cell body in a resting state it would read −70 mv (a millivolt is equal to one-thousandth of a volt). In this resting state the cell wall of the neuron is impermeable to sodium but permeable to potassium. When the neuron is stimulated by the actions of other neurons (or the electrical machinations of curious neuroscientists with electrodes), the permeability of the cell wall changes, allowing sodium to enter and thereby shift the electrical balance from −70 mv toward 0. This is called the excitatory postsynaptic potential, or EPSP. The synapse is the tiny gap between neurons, so postsynaptic means the neuron on the receiving end of the signal that travels across the synaptic cleft is the one being excited to reach its potential to fire. By contrast, if the stimulation comes from inhibitory neurons it causes the voltage to shift downward from −70 mv to −100 mv, making the neuron less likely to fire, and this is called the inhibitory postsynaptic potential, or IPSP. Although there are hundreds of different types of neurons, we can classify most of them as either excitatory or inhibitory in their actions.
If there are enough EPSPs built up (from numerous neuronal firings in sequence or from multiple connections from many other neurons) for the permeability of the neuron cell wall to reach a critical point, sodium rushes in, causing an instant spike in voltage to +50 mv, which spreads throughout the cell body and cascades down the axon into the terminals. Just as quickly, the neuron’s voltage collapses back down to −80 mv, then returns to the −70 mv resting state. This process of the cell wall becoming permeable to sodium with a corresponding shift in voltage from negative to positive that travels down the axon to the dendrites and their synaptic connections to other neurons is called an action potential. More colloquially, we say that the cell “fired.” The buildup of EPSPs