The document discusses the reticular formation and reticular activating system (RAS), detailing their structure, function, and role in regulating brain activity and consciousness. It outlines how the RAS influences arousal, sensory signal processing, and the transition between sleep states, including slow-wave sleep and REM sleep. Additionally, it explains the classification of brain waves recorded via EEG, highlighting various types such as alpha, beta, theta, and delta waves associated with different states of consciousness.

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Reticular formation & reticular activating system
 reticular formation (RF) refers to the complex network of neurons
and nerve fibres, which occupy midventral portion of brain stem
around the central cavity
 the brain stem RF can be considered to comprise medullary RF,
pontine RF and mid brain RF
 structurally, brain stem reticular formation consists of:
 neuronal aggregates
afferent connections
 efferent connections

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Reticular formation & reticular activating system cont’d
Reticular activating system (RAS)
 reticular activating system, also known as ascending reticular
activating system, is a complex polysynaptic pathway that projects
diffusely from the brain stem reticular formation to the cerebral cortex
 this comprises excitatory area located in the reticular substance of the
pons and mesencephalon
 this area sends signals in the upward direction to the thalamus, where
they excite a different set of neurons that transmit nerve signals to all
regions of the cerebral cortex as well as to multiple subcortical areas
 the signals passing through the thalamus are of two types:
 rapidly transmitted action potentials that excite the cerebrum for only
a few milliseconds,
 these signals originate from large neuronal cell bodies that lie
throughout the brain stem reticular area, their nerve endings release
the excitatory neurotransmitter substance acetylcholine

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Reticular formation & reticular activating system cont’d
 second type of excitatory signal originates from large numbers of
small neurons spread throughout the brain stem reticular excitatory
area
 these pass to the thalamus, but this time through small, slowly
conducting fibers that synapse mainly in the intralaminar nuclei of
the thalamus and in the reticular nuclei over the surface of the
thalamus
 from thalamus, additional small fibers are distributed everywhere in
the cerebral cortex
 the excitatory effect caused by this system of fibers can build up
progressively for many seconds to a minute or more, which suggests
that its signals are especially important for controlling longer-term
background excitability level of the brain

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Reticular formation & reticular activating system cont’d

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Reticular formation & reticular activating system cont’d
 the level of activity of the excitatory area in the brain stem, and
therefore the level of activity of the entire brain, is determined to a
great extent by the number and type of sensory signals that enter the
brain from the periphery
 pain signals in particular increase activity in this excitatory area and
therefore strongly excite the brain to attention
 not only do excitatory signals pass to the cerebral cortex from the
bulboreticular excitatory area of the brain stem, but feedback signals
also return from the cerebral cortex back to this same area
 any time the cerebral cortex becomes activated by either brain
thought processes or motor processes, signals are sent from the
cortex to the brain stem excitatory area, which in turn sends still
more excitatory signals to the cortex

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Reticular formation & reticular activating system cont’d
 the reticular inhibitory area, located medially and ventrally in the
medulla, is another area that is important in controlling brain activity
 this area can inhibit the reticular facilitory area of the upper brain
stem and thereby decrease activity in the superior portions of the
brain as well
 one of the mechanisms for this is to excite serotonergic neurons; these
in turn secrete the inhibitory neurohormone serotonin at crucial points
in the brain

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Sleep
 sleep is defined as unconsciousness from which the person can be
aroused by sensory or other stimuli
 coma is unconsciousness from which the person cannot be aroused
Two types of sleep/stages of sleep
 slow-wave sleep
 rapid eye movement sleep (REM sleep)
Slow-wave sleep
 most sleep during each night is of the slow-wave variety; this is the
deep, restful sleep that the person experiences during the first hour of
sleep after having been awake for many hours
 it is associated with decrease in both peripheral vascular tone,
decreases in blood pressure, respiratory rate, and basal metabolic rate
 dreams and sometimes even nightmares do occur during slow-wave
sleep, the dreams of slow-wave sleep usually are not remembered

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Sleep cont’d
REM sleep (paradoxical sleep, desynchronized Sleep)
 it occupies about 25 per cent of the sleep time
 in a normal night of sleep, bouts of REM sleep lasting 5 to 30 minutes
usually appear on the average every 90 minutes
 important characteristics of REM sleep:
 it is usually associated with active dreaming and active bodily
muscle
movements
 the person is even more difficult to arouse by sensory stimuli than
during deep slow-wave sleep, and yet people usually awaken
spontaneously in the morning during an episode of REM sleep
 muscle tone throughout the body is exceedingly depressed,
indicating
strong inhibition of the spinal muscle control areas
 heart rate and respiratory rate usually become irregular

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Sleep cont’d
 despite the extreme inhibition of the peripheral muscles, irregular
muscle movements do occur
 the brain is highly active in REM sleep, and overall brain metabolism
may be increased as much as 20 per cent
 the electroencephalogram (EEG) shows a pattern of brain waves
similar to those that occur during wakefulness; it is also called
paradoxical sleep because it is a paradox that a person can still be
asleep despite marked activity in the brain
 it is not so restful, and it is usually associated with vivid dreaming

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Sleep cont’d
Sleep is believed to be caused by an active inhibitory process
 transecting the brain stem at the level of the midpons creates a brain
whose cortex never goes to sleep; indicates some center located below
the midpontile level of the brain stem that is required to cause sleep by
inhibiting other parts of the brain
 stimulation of several specific areas of the brain can produce sleep
 the raphe nuclei in the lower half of the pons and in the medulla
 nerve fibers from these nuclei spread locally in the brain stem
reticular formation and also upward into the thalamus,
hypothalamus, most areas of the limbic system
 many nerve endings of fibers from these raphe neurons secrete
serotonin
 it has been assumed that serotonin is a transmitter substance
associated with production of sleep

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Sleep cont’d
 stimulation of some areas in the nucleus of the tractus solitarius can
also cause sleep
 stimulation of several regions in the diencephalon can also promote
sleep, including the rostral part of the hypothalamus, mainly in the
suprachiasmal area, and an occasional area in the diffuse nuclei of
the thalamus

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Brain waves
 electrical recordings from the surface of the brain or even from the
outer surface of the head demonstrate that there is continuous
electrical activity in the brain
 the undulations in the recorded electrical potentials are called brain
waves, and the entire record is called an EEG (electroencephalogram)
 the intensities of brain waves recorded from the surface of the scalp
range from 0 to 200 microvolts, and their frequencies range from
once
every few seconds to 50 or more per second
 the character of the waves is dependent on the degree of activity in
respective parts of the cerebral cortex, and the waves change markedly
between the states of wakefulness and sleep and coma

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Brain waves cont’d
 in normal healthy people, most waves in the EEG can be classified as
alpha, beta, theta, and delta waves
 alpha waves, waves that occur at frequencies between 8 and 13 cycles
per second and are found in the EEGs of almost all normal adult
people when they are awake and in a quiet, resting state of cerebration
 their voltage usually is about 50 microvolts, during deep sleep, the
alpha waves disappear
 beta waves occur at frequencies greater than 14 cycles per second and
as high as 80 cycles per second
 they are recorded mainly from the parietal and frontal regions during
specific activation of these parts of the brain
 when the awake person’s attention is directed to some specific type of
mental activity, the alpha waves are replaced higher-frequency but
lower-voltage beta waves

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Brain waves cont’d
 theta waves have frequencies between 4 and 7 cycles per second
 they occur normally in the parietal and temporal regions in
children,
but they also occur during emotional stress in some adults,
particularly during disappointment and frustration
 theta waves also occur in many brain disorders, often in
degenerative
brain states
 delta waves include all the waves of the EEG with frequencies less
than 3.5 cycles per second, and they often have voltages two to four
times greater than most other types of brain waves
 they occur in very deep sleep, in infancy, and in serious organic brain
disease

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Brain waves cont’d

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Brain waves cont’d
Progressive change in the characteristics of the brain waves during different stages of
wakefulness and sleep

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Reading assignment
local, conditioned and unconditioned reflexes; spinal reflexes
superficial, deep, and visceral
 clonus
 vertigo