APT Practice Parameter for Standard Polysomnography APT ...

Publication of the Association of Polysomnographic Technologists • 2006, Volume 15, Number 4 • www.aptweb.orgThey Come From the CortexBY WILL ECKHARDT, BS RPSGT CRT, ASSOCIATE EDITORWhere do scalp potentials come from and what produces thesevoltages? How do volume conduction, tissue dipoles and geometricorientation affect the electroencephalogram (EEG)? What informationcan we derive from these waves forms once conducted throughtissue and recorded through our amplifiers? We will explore these issues.We record the EEG from scalp electrodes commonly placed by whatis know as the International 10-20 System of Electrode Placement(albeit modified generally in sleep studies). Hans Berger recorded thefirst human EEG in the1920’s. We have come along way in the equipmentused in recording the EEG but the source remains the same. EEGis a means of looking at voltages derived from our cortex which vary asa function of time and their spatial distribution in relation to the recordingelectrode.EEG can be recorded via scalp electrodes or from intracranial electrodes.Scalp sites sample from a larger area than intracranial placement.Intracranial sites provide more local sampling giving generallydifferent data from that of the global scalp recordings. Scalp EEG isnow believed to be derived from postsynaptic potentials (postsynapticpotentials are changes in the electrical potential of the neuron thatreceives information at aneuronal junction orsynapse) from the cortexthat summate and reachthe scalp giving us ourEEG waveforms. Intrinsiccell currents (produced byionic channel activation)may contribute to the EEGbut is still under investigation.Action potentialswere once thought tocontribute to the EEG butrecently have been dismissedas their temporallimits are too short.Fig. 1 Pyramidal CellFig. 2 Dipole28The cortex is composedof a dense collectionof neuron cell bodieswith myelinated andunmyelinated fibers runningthrough it. It is lessthen 5 mm thick. The cortexcovers both cerebralhemispheres of the brain.There are millions of neuronswithin the cortex,each having contact withthousands of other neurons.The cortex hasareas with distinct functionsand EEG output. Theneurons receive inputfrom subcortical areas viathe thalamus. The cerebral cortex andthe thalamus often work together in generatingbrain rhythms 1 . These waveforms are derived from the summation ofdifferent rhythms rather than being arhythm generated by a single cell orgroup of cells. The cortex also sendsinput signals to other areas within thecortex via association fibers. EfferentWill Eckhardt(directed away) signals are sent to manyother brain structures e.g. the brainstem,thalamus, cerebellum, the basal nuclei and the spinal cord.Most of the cortex has six layers of neurons and is called the neocortex.Cytoarchitecture is the distribution of these neurons. Pyramidalcells (see Fig 2) the most common neurons within the cortex, arenamed such due to their cell body shape. Although they are found in alllayers other than layer 1, they are they are most predominant in layers2, 3, and 5. 2Pyramidal neurons have a cell body, an axon, a single apical dendriteand a number of basal dendrites. Their axon originating on the base ofthe cell body leaves the cortex being the output pathway of the cortex.Axons can branch many times contacting hundreds of other neurons.These neurons are layered and project into other areas via their axonsand axon collaterals. Pyramidal neurons are associated with excitatoryneurotransmitters. Other neurons in the cortex are local and stay withinthe area of their cell body. These are known as interneurons. Theseneurons are often inhibitory.Presently we believe EEG potentials are due to excitatory postsynapticpotentials (EPSP) and inhibitory postsynaptic potentials (IPSP) propagatedby the cell body and dendrites of thousands of synchronizedpyramidal neurons 3 . The summation of these potentials is facilitated buythe architecture of pyramidal neurons. These neurons are oriented in acolumnar structure with apical dendrites pointing toward the corticalsurface. These very small dipoles (see Fig 2 — a separation of unlikecharges) therefore have similar orientations. The Solid Angle (see Fig 3— a measure of the apparent cross-sectional area of an object asviewed from a distance) of the dipole and the actual voltage of the dipolegenerated by a single cell is too small to produce recordable EEG at thesurface. It is the summation of solid angles and synchronization of potentialsin groups of neuronal synapses that enables the EEG to be recordableat the surface of the head.There can be a great deal of difference in the recording from twoelectrodes spaced only millimeters apart which was previously thoughtto imply the activity was from the immediate proximity of the surfaceelectrode 4 . Those electrodes far apart and producing the same waveforms were considered linked to a common source. The solid angle theorem(discussed below) and summation of the potentials is now consideredto be the means by which we record postsynaptic potentials at thesurface electrode.The small area within the cortex created by summated activity inneighboring active cells has been referred to as a dipole layer (see Fig.➟