The impulses are responsible for the perception and localization of individual sensations. However, they must be processed in the awake brain to be perceived. At least in mammals, there is a spectrum of behavioral states ranging from deep sleep through light sleep, REM sleep, and the two awake states: relaxed awareness and awareness with concentrated attention. Discrete patterns of brain electrical activity correlate with each of these states. Feedback oscillations within the cerebral cortex and between the thalamus and the cortex serve as producers of this activity and possible determinants of the behavioral state. Arousal can be produced by sensory stimulation and by impulses ascending in the reticular core of the midbrain. Many of these activities have rhythmic fluctuations that are approximately 24 h in length; that is, they are circadian.
Thalamus, cerebral cortex, & reticular formation
The thalamus is a large collection of neuronal groups within the diencephalons; it participates in sensory, motor, and limbic functions. Virtually all information that reaches the cortex is processed by the thalamus, leading to its being called the “gateway” to the cerebral cortex. The thalamus can be divided into nuclei that project diffusely to wide regions of the neocortex and nuclei that project to specific discrete portions of the neocortex and limbic system. The nuclei that project to widen regions of the neocortex are the midline and intraluminal nuclei.
Reticular formation & reticular activating system
The reticular formation, the phylogenetically old reticular core of the brain, occupies the midventral portion of the medulla and midbrain. It is primarily an anatomic area made up of various neural clusters and fibers with discrete functions.
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Evoked cortical potentials
The electrical events that occur in the cortex after stimulation of a sense organ can be monitored with an exploring electrode connected to another electrode at an indifferent point some distance away. A characteristic response is seen in animals under barbiturate anesthesia, which eliminates much of the background electrical activity. If the exploring electrode is over the primary receiving area for a particular sense, a surface-positive wave appears with a latency of 5 to 12 ms.
Physiologic basis of the electroencephalogram
The background electrical activity of the brain in anesthetized animals was first described in the 19th century. Subsequently, it was analyzed in a systematic fashion by the German psychiatrist Hans Berger, who introduced the term electroencephalogram (EEG) to denote the record of the variations in brain potential. The EEG can be recorded with scalp electrodes through the unopened skull or with electrodes on or in the brain. The term electrocardiogram (ECoG) is used for the record obtained with electrodes on the pail surface of the cortex.
Clinical uses of the egg
The EEG is sometimes of value in localizing pathologic processes. When a collection of fluid overlies a portion of the cortex, activity over this area may be damped. This fact may aid in diagnosing and localizing conditions such as subdural hematomas. Lesions in the cerebral cortex cause the local formation of irregular or slow waves that can be picked up in the EEG leads. Epileptogenic foci sometimes generate high-voltage waves that can be localized.
Throughout NREM sleep, there is some activity of skeletal muscle. A theta rhythm can be seen during stage 1 of sleep. Stage 2 is marked by the appearance of sleep spindles and occasional K complexes. In stage 3, a delta rhythm is dominant. Maximum slowing with slow waves is seen in stage 4.