Clinical Note

Seizures

Seizures represent the extreme in synchronicity of brain activity and are often associated with some underlying pathological condition. General seizures involve the entire cortex; partial seizures involve only limited areas of the cortex. During a seizure, sensory information is blocked and the individual retains no memory of the event. Motor units normally controlled by the areas involved in a seizure will become rigid during clonic activity and will contract rhythmically during clonic activity of the affected motor areas. ''Absence'' seizures occur during childhood and consist of short periods lasting several seconds, during which generalized 3-Hz EEC waves are generated. There is a loss of consciousness, with only subtle motor signs, such as fluttering of the eyelids or facial twitching. Ten percent of the population have one or two isolated seizures during their lifetime without apparent consequence. Epilepsy is described as the repeated occurrence of seizure activity and can result from a wide variety of causes, including tumors, trauma, metabolic dysfunction, infection, vascular disease, developmental abnormalities in brain structure, and drug interactions.

neurotransmitters at widely spread innervation sites across the brain. Noradrenergic fibers from the locus coeruleus represent a second ascending modulatory system, with perhaps the most diffuse connections known to occur in brain (see Fig. 5). A single locus coeruleus neuron makes as many as 250,000 synapses that spread from the cerebral cortex to the cerebellum. Serotonergic neurons located in the raphe nuclei located near the midline of the brain stem provide a third ascending system that innervates most of the brain in a manner similar to the locus coeruleus. Projections from the locus coeruleus and raphe nuclei together form the reticular-activating system, which serves to alert the brain to incoming sensory stimuli that are novel. The behavioral response to an abrupt presentation of a new stimulus, such as a loud noise, is an immediate reorientation of the head and perhaps the body toward the novel stimulus and a general increase in awareness and attentiveness. Most of the currently used psychoactive drugs—many of which are analogs of norepinephrine, dopamine, and serotonin—act directly on the neuro-transmitter systems that are components of the diffuse modulatory systems subserving the positive reinforcement and the ascending reticular-activating systems.

The reticular-activating system represents but one mechanism that controls the overall functional state of the brain. Its activity helps maintain the awake, interactive state. Different functional states are associated with specific patterns of electrical activity in the cortex that can be recorded using electrodes placed on the scalp. These patterns generate an electroencephalogram (EEG) that is the result of large populations of cortical pyramidal cells firing at rates and degrees of synchrony that change in response to different states of arousal. In general, high-frequency firing rates and low-amplitude

A. Self-stimulation sites in the rat brain

A. Self-stimulation sites in the rat brain

B. Areas in the human brain comparable to rat self- stimulation sites

FIGURE 4 Structures involved in the reward system. (A) Areas of brain identified as pleasure centers are based on the observation that rats self-stimulate electrodes implanted in these regions. (B) Comparable regions in humans have been shown to elicit pleasurable sensations when electrically stimulated during various clinical procedures.

FIGURE 4 Structures involved in the reward system. (A) Areas of brain identified as pleasure centers are based on the observation that rats self-stimulate electrodes implanted in these regions. (B) Comparable regions in humans have been shown to elicit pleasurable sensations when electrically stimulated during various clinical procedures.

A. Noradrenergic modulatory system B. Serotonergic modulatory system

A. Noradrenergic modulatory system B. Serotonergic modulatory system

C. Dopaminergic modulatory systems

FIGURE 5 Three diffuse modulatory systems of the brain. Brain stem nuclei that give rise to each system are labeled in bold italics on the lower left. (A) A small group of neurons in the locus coeruleus distribute norepinephrine throughout most of the brain, including the neocortex, cerebellum, thalamus, and spinal cord. (B) Neurons producing serotonin arise in the raphe nuclei and have an equally broad distribution. (C) Dopamine neurons arise from two nuclei: (1) the substantia nigra, which projects to the striatum of the basal ganglia, and (2) the ventral tegmental area, which projects to the frontal lobe.

FIGURE 5 Three diffuse modulatory systems of the brain. Brain stem nuclei that give rise to each system are labeled in bold italics on the lower left. (A) A small group of neurons in the locus coeruleus distribute norepinephrine throughout most of the brain, including the neocortex, cerebellum, thalamus, and spinal cord. (B) Neurons producing serotonin arise in the raphe nuclei and have an equally broad distribution. (C) Dopamine neurons arise from two nuclei: (1) the substantia nigra, which projects to the striatum of the basal ganglia, and (2) the ventral tegmental area, which projects to the frontal lobe.

responses are generated by pyramidal cells that are relatively desynchronized in their activity, indicative of responses to continuous sensory input in an awake, alert individual. During slow-wave sleep, pyramidal cells are in a state of reduced responsiveness as sensory input is diminished and they fire more synchronously. EEG patterns show beta rhythms (approximately 14 Hz) when the cortex is activated and theta rhythms (4 to 7 Hz) or delta rhythms (<4 Hz) during most stages of sleep. Alpha rhythms (8 to 13 Hz) are generated during quiet, waking states. Several times during a normal sleep cycle, EEGs change dramatically to patterns more characteristic of the aroused state. This stage of sleep is called rapid eye movement (REM) sleep because of the spontaneous eye movements that occur at this time. REM sleep is associated with episodes of dreaming.

Suggested Readings

Erickson RP. On the neural basis of behavior. Am Scientist 1984; 72:233-241.

Friedman MI, Stricker EM. The physiological psychology of hunger:

a physiological perspective. Psychol Rev 1976; 83:409-431. Stellar JR, Stellar E. The neurobiology of motivation and reward. New York: Springer, 1985.

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