Origin of Brain Waves
The discharge of a single neuron or single nerve fiber in the brain can never be recorded from the surface of the head. Instead, many thousands or even millions of neurons or fibers must fire synchronously; only then will the potentials from the individual neurons or fibers summate enough to be recorded all the way through the skull. Thus, the intensity of the brain waves from the scalp is determined mainly by the numbers of neurons and fibers that fire in synchrony with one another, not by the total level of electrical activity in the brain. In fact, strong nonsynchronous nerve signals often nullify one another in the recorded brain waves because of opposing polarities. This is demonstrated in Figure 59–2, which shows, when the eyes were closed, synchronous discharge of many neurons in the cerebral cortex at a frequency of about 12 per second, thus causing alphawaves. Then, when the eyes were opened, the activity ofthe brain increased greatly, but synchronization of the signals became so little that the brain waves mainly nul-lified one another, and the resultant effect was very low voltage waves of generally high but irregular frequency, the beta waves.
Origin of Alpha Waves. Alpha waves willnotoccur in thecerebral cortex without cortical connections with the thalamus. Conversely, stimulation in the nonspecific layer of reticular nuclei that surround the thalamus or in “diffuse” nuclei deep inside the thalamus often sets up electrical waves in the thalamocortical system at a frequency between 8 and 13 per second, which is the natural frequency of the alpha waves. Therefore, it is believed that the alpha waves result from spontaneous feedback oscillation in this diffuse thalamocortical system, possibly including the reticular activating system in the brain stem as well. This oscillation pre-sumably causes both the periodicity of the alpha waves and the synchronous activation of literally millions of cortical neurons during each wave.
Origin of Delta Waves. Transection of the fiber tracts fromthe thalamus to the cerebral cortex, which blocks thal-amic activation of the cortex and thereby eliminates the alpha waves, nevertheless does not block delta waves in the cortex.This indicates that some synchronizing mech-anism can occur in the cortical neuronal system by itself—mainly independent of lower structures in the brain—to cause the delta waves.
Delta waves also occur during deep slow-wave sleep; this suggests that the cortex then is mainly released from the activating influences of the thalamus and other lower centers.