 Hello and welcome to Physiology Open. Electroencephalography is the procedure of recording electrical activity of the brain using surface electrodes which are placed on the scalp. So suppose this is a scalp and this is the cortex, for EEG we place electrodes here over the scalp. So actually what kind of electrical activity is recorded by EEG? See in the cerebral cortex there are six layers. Superficially it's layer 1 and as we go deeper it's layers 2, 3, 4, 5 and 6. Now the cell bodies of neurons are present in deep layers and their axons go further deep. Now this first layer that is molecular layer has very few cell bodies. Instead it consists of dendrites of other neurons which actually synapse with axons of neurons from other area especially from non-specific thalamic nuclei. So remember that we are recording EEG from the surface. So obviously the EEG electrodes will pick up electrical activity from the area which is near the electrodes. That is this superficial area of cortex. Now see action potentials in neurons arise from here, from the junction between the cell body and the axon. While at the dendrites there are only graded potentials known as postsynaptic potential due to signals from these synapses. Now these graded potentials can be either excitatory causing change in potential towards positive that is excitatory postsynaptic potential or it can be inhibitory known as inhibitory postsynaptic potential. So basically EEG electrodes pick up these excitatory and inhibitory postsynaptic potentials that too which are near the electrodes and the magnitude of voltage recorded is a sum of these EPSPs and IPSPs. So two things to remember here. One these electrodes don't pick up action potential and the electrical activity recorded by these electrodes is a sum of all excitatory and inhibitory postsynaptic potentials which these electrodes pick up. And secondly if the activity is near the electrode the effect of that activity on the sum will be more than if that activity is say little bit away from the electrode. See obviously if some activity is happening here far away from the electrode the voltage recorded by these electrodes will be lesser than if the same activity is happening somewhere here near the electrode. Okay before we proceed let's see some hypothetical examples of summing up of electrical activity. This will help you in understanding EEG waves. See in this figure there is equal positive and negative excitation near the electrode. That's just hypothetical we are talking. So it will sum up and the recorded activity will be like this. Or say this one in this there is simultaneous EPSPs in two postsynaptic neurons or we can say there is synchronized activation. So there is adding up of activity causing increased amplitude but in this other example excitation is similar but it is not happening at the same time or we can say it is desynchronized. So there is adding up of potential but the amplitude has not increased. Instead frequency of recorded waves has increased. Basically if there is synchronized activation of cortex amplitude of EEG waves is more while frequency is lesser and in case of desynchronized activation of cortex amplitude is not much but frequency is more. So that's why we refer EEG in terms of synchronized or desynchronized. Now we have told before that neurons from thalamus may contact with the adenbrides in the superficial layer of the cortex. So the way these projections from non-specific nuclei from thalamus activate the cortex affects the EEG. Alert there is desynchronized activation of cortex and this activity changes to more and more synchronized as we become relaxed to drowsy and is more synchronized in deep sleep. So with this you can very well guess that EEG activity will be desynchronized with lower amplitude and higher frequency when we are awake and alert while it will become too synchronized that is of higher amplitude and lower frequency when we are in deep sleep. So with these concepts let's discuss the EEG waves seen in different brain arousal states. Well remember it like this bat D, B for beta, A for alpha, T for beta and D for delta. From beta to delta the sequence of waves is arranged with decreasing frequency and increasing amplitude. So here beta waves have highest frequency and lowest amplitude while delta waves have opposite that is lowest frequency and highest amplitude. So these are their frequencies. Beta wave is from 13 to 30 hertz, alpha waves 8 to 13 hertz, theta waves 4 to 7 hertz and delta waves is frequency less than 4 hertz. So as already told desynchronized EEG waves with high frequency signify active and alert sleep while synchronized means decreased drowsy. So in order if we see beta waves are seen while we are awake, alert and concentrating on something and are most often recorded from frontal regions of brain. While alpha waves are seen in awake but relaxed stage and are seen most prominently in parietal and occipital regions. Then theta waves are seen in light sleep while delta waves are seen in deep sleep. Let's see a bit in detail about EEG waves during sleep. Sleep consists of cycles of NRM and REM sleep. NRM sleep consists of 4 stages itself beginning from lighter stages of sleep to deep sleep and as already discussed that as when progresses into deeper stages of sleep there is increased synchronization and hence increased amplitude and decreased frequency of EEG waves. So in stage one of sleeper that is the lighter sleep we see theta waves. In stage two we see similar waves but there is also appearance of sinusoidal waves known as sleep spindles. Note here these waves are sinusoidal so these are sleep spindles and there is also appearance of occasional high voltage biphasic waves known as k complexes. So these are biphasic waves here and k complexes. In stage three and stage four we see delta waves of decreased frequency and high amplitude. Due to these low frequency waves seen in these stages stage three and four are also known as slow wave sleep. Then in REM sleep we see beta wave like activity just like the one which is seen in awake and alert state. It appears paradoxical that even though we are sleeping EEG brain activity resembles a awake state. So REM sleep is also known as paradoxical sleep. So that's all on EEG fundamentals and waves. Thanks for watching the video if you liked it do like and share the video and don't forget to subscribe to the channel Physiology Open. Thank you.