Sleep is a complex phenomenon that is not fully understood. While it was previously believed that reduced activity in the reticular activating system (located in the brainstem) caused sleep, recent evidence suggests that sleep is an active process. Stimulation in certain areas of the brainstem can induce sleep, while anesthesia of the lower brainstem results in arousal. Neurotransmitters such as serotonin, norepinephrine, and acetylcholine play a role in the sleep-wake cycle, but a detailed explanation of the neural mechanisms of sleep is still lacking.

Despite the unclear purpose of sleep, its importance is evident as a significant portion of our lives is spent sleeping, and lack of sleep can have debilitating effects. Various sleep disorders, including insomnia, bed-wetting, sleepwalking, sleep apnea, and narcolepsy, can disrupt the sleep-wake cycle and have medical implications.

Epilepsy, a neurological disorder, often presents with specific electroencephalographic (EEG) abnormalities. EEG patterns can help diagnose epilepsy, and different forms of the condition are associated with distinct EEG abnormalities. For example, tonic-clonic seizures (also known as grand mal seizures) display specific EEG patterns during the tonic and clonic phases. Absence seizures (also called petit mal seizures) exhibit spike and wave components on the EEG. Temporal lobe epilepsy can be identified through specific EEG tracings, while focal seizures are characterized by abnormal EEG patterns in a localized area.

In coma cases, the EEG shows dominant delta activity. Brain death is defined by a maintained flat EEG wave. Epilepsy can cause and be diagnosed by various EEG abnormalities. Different types of epilepsy exist, and EEG patterns can vary accordingly. Partial seizures, originating in the motor cortex, result in localized contractions of muscles that can spread to other muscles in a stereotypical progression known as a Jacksonian march. Complex partial seizures, associated with psychomotor epilepsy, arise from the limbic structures of the temporal lobe and lead to illusions and semi-purposeful motor activity. Scalp recordings during and between focal seizures may reveal EEG spikes.

Generalized seizures involve wide areas of the brain and lead to a loss of consciousness. The two major types of generalized seizures are petit mal seizures and grand mal seizures. Petit mal epilepsy is characterized by transient loss of consciousness, typically lasting less than 15 seconds, and the EEG displays spike and wave activity.

Cerebral dominance, which determines the hemisphere responsible for language communication, is assigned to the hemisphere opposite to the hand dominance. In the majority of both right-handed and left-handed individuals, the left hemisphere is dominant.
The mechanism of sleep is incompletely understood. Stimulation in the brainstem in a large region known as the reticular activating system causes arousal and low-voltage, fast EEG activity. Sleep was once thought to be caused by a reduced level of activity in the reticular activating system. However, substantial data, including the observations that anesthesia of the lower brainstem results in arousal and that stimulation in the medulla near the nucleus of the solitary tract can induce sleep, suggest that sleep is an active process. Investigators have tried to find a relationship between sleep mechanisms and brainstem networks in which particular neurotransmitters, including serotonin, norepinephrine, and acetylcholine, are used; manipulations of the levels of these transmitters in the brain can affect the sleep-wake cycle. However, a detailed neurochemical explanation of the neural mechanisms of sleep is not yet available.
Similarly, the purpose of sleep is still unclear. However, it must have a high value because so much of life is spent in sleep and because lack of sleep can be debilitating. Medically important disorders of the sleep-wake cycle include insomnia, bed-wetting, sleepwalking, sleep apnea, and narcolepsy.
•Fig. 10.6 Electroencephalographic (EEG) Abnormalities in Several Forms of Epilepsy. A, EEGtracingsduringthetonic(left) andclonic(right) phasesofatonic-clonic(grandmal)seizure.B, Spikeandwavecomponentsofanabsence(petitmal)seizure.C, EEGtracinginapersonwithtemporallobeepilepsy.D, EEGtracingofafocalseizure.(RedrawnfromEyzaguirreC,FidoneSJ.Physiology of the Nervous System. 2nded.St.Louis:Mosby;1975.)
TheEEGbecomesabnormalinavarietyofpathologicalcircumstances.Forexample,duringcoma,theEEGisdominatedbydeltaactivity.Brain death isdefinedbyamaintainedflatEEGwave.
Epilepsy commonlycauses,andcanbediagnosedby,specificEEGabnormalities.Therearemanyformsofepilepsy,andexamplesofEEGpatternsfromsomeofthesetypesofepilepsyareshownin
Figure10.6 .Epilepticseizurescanbeeitherpartialorgeneralized.
Oneformofpartialseizuresoriginatesinthemotorcortexandresultsinlocalizedcontractionsofcontralateralmuscles.Thecontractionsmaythenspreadtoothermuscles;suchspreadfollowsthesomatotopicsequenceofthemotorcortex(see ).ThisstereotypicalprogressioniscalledaJacksonian march. Complexpartialseizures(whichmayoccurinpsychomotor epilepsy)originateinthelimbicstructuresofthetemporallobeandresultinillusionsandsemipurposefulmotoractivity.Duringandbetweenfocalseizures,scalprecordingsmayrevealEEGspikes(see
Fig.
10.6C andD
Generalizedseizuresinvolvewideareasofthebrainandlossofconsciousness.Twomajortypesarepetit mal and
Although right-handedness represents a sensorimotor dominance of the left hemisphere and left-handedness represents a sensorimotor dominance of the right hemisphere, cerebral dominance is assigned to the hemisphere in which language is to communicate; in humans, the left hemisphere is the dominant hemisphere in more than 90% of both rightand left-handed people. This dominance grand mal seizures. Inpetitmalepilepsy,consciousnessislosttransiently(typicallyforlessthan15seconds),andtheEEGdisplaysspike and wave activity (see