Transcript Chapter 12 C

Chapter 12
The Central Nervous
System
Part C
Shilla Chakrabarty, Ph.D.
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Electroencephalogram (EEG)
• Records electrical activity that accompanies brain function
• Measures electrical potential differences between various cortical areas
(a) Scalp electrodes are used to record brain wave
activity (EEG).
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Brain Waves
• Patterns of neuronal electrical activity
• Generated by synaptic activity in the cortex
• Each person’s brain waves are unique
• Can be grouped into four classes based on frequency measured as
Hertz (Hz)
 Alpha waves (8–13 Hz)—regular and rhythmic, low-amplitude,
synchronous waves indicating an “idling” brain
 Beta waves (14–30 Hz)—rhythmic, less regular waves occurring when
mentally alert
 Theta waves (4–7 Hz)—more irregular; common in children and
uncommon in adults
 Delta waves (4 Hz or less)—high-amplitude waves seen in deep sleep
and when reticular activating system is damped, or during anesthesia;
may indicate brain damage
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Brain Waves Indicate State Of The Brain
1-second interval
Alpha waves—awake but relaxed
Beta waves—awake, alert
Theta waves—common in children
Delta waves—deep sleep
(b) Brain waves shown in EEGs fall into
four general classes.
• Brain waves change with age, sensory stimuli, brain disease, and chemical state of the body
• EEGs are used to diagnose and localize brain lesions, tumors, infarcts, infections, abscesses,
and epileptic lesions
• A flat EEG (no electrical activity) is clinical evidence of death
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Epilepsy
• Epilepsy occurs in 1% of the population
• A victim of epilepsy may lose consciousness, fall stiffly, and have
uncontrollable jerking
• Epilepsy is not associated with intellectual impairments
• Epileptic seizures are categorized as:
 Absence seizures, or petit mal
 Mild seizures seen in young children where the expression goes blank
 Tonic-clonic (grand mal) seizures
 Victim loses consciousness, bones are often broken due to intense
contractions, may experience loss of bowel and bladder control, and
severe biting of the tongue
Control of Epilepsy
• Anticonvulsive drugs
• Vagus nerve stimulators implanted under the skin of the chest can keep
electrical activity of the brain from becoming chaotic
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Consciousness
• Conscious perception of sensation
• Voluntary initiation and control of movement
• Capabilities associated with higher mental processing (memory, logic,
judgment, etc.)
• Clinically defined on a continuum that grades behavior in response to
stimuli
 Alertness
 Drowsiness (lethargy)
 Stupor
 Coma
• Loss of consciousness (e.g., fainting or syncopy) is a signal that brain
function is impaired
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Sleep
• State of partial unconsciousness from which a person can
be aroused by stimulation
• Two major types of sleep (defined by EEG patterns)
 Nonrapid eye movement (NREM)
 Rapid eye movement (REM)
• First two stages of NREM occur during the first 30–45
minutes of sleep
• Fourth stage is achieved in about 90 minutes, and then
REM sleep begins abruptly
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Awake
REM: Skeletal
muscles (except
ocular muscles
and diaphragm)
are actively
inhibited; most
dreaming occurs.
NREM stage 1:
Relaxation begins;
EEG shows alpha
waves, arousal is easy.
NREM stage 2: Irregular
EEG with sleep spindles
(short high- amplitude
bursts); arousal is more
difficult.
NREM stage 3: Sleep
deepens; theta and
delta waves appear;
vital signs decline.
(a) Typical EEG patterns
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NREM stage 4: EEG is
dominated by delta
waves; arousal is difficult;
bed-wetting, night terrors,
and sleepwalking may
occur.
Figure 12.21a
Sleep Patterns
• Alternating cycles of sleep and wakefulness reflect a natural circadian
(24-hour) rhythm
• RAS activity is inhibited during, but RAS also mediates, dreaming
sleep
• The suprachiasmatic and preoptic nuclei of the hypothalamus time the
sleep cycle
• A typical sleep pattern alternates between REM and NREM sleep
Awake
REM
Stage 1
Stage 2
Non
REM Stage 3
Stage 4
Time (hrs)
(b) Typical progression of an adult through one
night’s sleep stages
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Importance of Sleep
• Slow-wave sleep (NREM stages 3 and 4) is presumed to be the
restorative stage
• People deprived of REM sleep become moody and depressed
• REM sleep may be a reverse learning process where superfluous
information is purged from the brain
• Daily sleep requirements decline with age
• Stage 4 sleep declines steadily and may disappear after age 60
Sleep Disorders
• Narcolepsy : Lapsing abruptly into sleep from the awake state
• Insomnia: Chronic inability to obtain the amount or quality of sleep
needed
• Sleep apnea: Temporary cessation of breathing during sleep
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Language
• Language implementation system
o Basal nuclei
o Broca’s area and Wernicke’s area (in the association
cortex on the left side)
o Analyzes incoming word sounds
o Produces outgoing word sounds and grammatical
structures
• Corresponding areas on the right side are involved with
nonverbal language components
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Memory
• Storage and retrieval of information
• Two stages of storage
• Short-term memory (STM, or working memory)—temporary
holding of information; limited to seven or eight pieces of
information
• Long-term memory (LTM) has limitless capacity
• Factors that affect transfer from STM to LTM
 Emotional state—best if alert, motivated, surprised, and aroused
 Rehearsal—repetition and practice
 Association—tying new information with old memories
 Automatic memory—subconscious information stored in LTM
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Memory: STM and LTM
Outside stimuli
General and special sensory receptors
Afferent inputs
Temporary storage
(buffer) in
cerebral cortex
Automatic
memory
Data permanently
lost
Data selected
for transfer
Short-term
memory (STM)
Forget
Forget
Data transfer
influenced by:
Excitement
Retrieval
Rehearsal
Association of
old and new data
Long-term
memory
Data unretrievable
(LTM)
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Categories of Memory
1.
Declarative memory (factual knowledge)
•
Explicit information
•
Related to our conscious thoughts and our language ability
•
Stored in LTM with context in which it was learned
2. Nondeclarative memory
•
Less conscious or unconscious
•
Acquired through experience and repetition
•
Best remembered by doing; hard to unlearn
•
Includes procedural (skills) memory, motor memory, and
emotional memory
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Brain Structures Involved in Declarative Memory
• Hippocampus and surrounding temporal lobes function in consolidation and
access to memory
• ACh from basal forebrain is necessary for memory formation and retrieval
Thalamus
Basal forebrain
Touch
Prefrontal cortex
Hearing
Vision
Taste
Smell
Hippocampus
(a) Declarative
memory circuits
Sensory
input
Association
cortex
Thalamus
Medial temporal lobe
(hippocampus, etc.)
Prefrontal
cortex
ACh
ACh
Basal
forebrain
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Brain Structures Involved in Nondeclarative
Memory
• Procedural memory
• Basal nuclei relay sensory and motor inputs to the thalamus
and premotor cortex
• Dopamine from substantia nigra is necessary
• Motor memory—cerebellum
• Emotional memory—amygdala
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Sensory and
motor inputs
Association
cortex
Basal
nuclei
Thalamus
Dopamine
Premotor
cortex
Premotor
cortex
Substantia
nigra
Thalamus
Basal nuclei
Substantia nigra
(b) Procedural (skills) memory circuits
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Figure 12.23b
Protection of the Brain
• Bone (skull)
• Membranes (meninges)
• Watery cushion (cerebrospinal fluid)
• Blood-brain barrier
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Superior
sagittal sinus
Subdural
space
Subarachnoid
space
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Skin of scalp
Periosteum
Bone of skull
Periosteal Dura
Meningeal mater
Arachnoid mater
Pia mater
Arachnoid villus
Blood vessel
Falx cerebri
(in longitudinal
fissure only)
Figure 12.24
Dura Mater
• Strongest meninx
• Two layers of fibrous connective tissue
(around the brain) separate to form dural
sinuses
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Dura Mater
• Dural septa limit excessive movement of the
brain
• Falx cerebri—in the longitudinal fissure;
attached to crista galli
• Falx cerebelli—along the vermis of the
cerebellum
• Tentorium cerebelli—horizontal dural fold over
cerebellum and in the transverse fissure
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Superior
sagittal sinus
Straight
sinus
Crista galli
of the
ethmoid
bone
Pituitary
gland
Falx cerebri
Tentorium
cerebelli
Falx
cerebelli
(a) Dural septa
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Figure 12.25a
Arachnoid Mater
• Middle layer with weblike extensions
• Separated from the dura mater by the subdural space
• Subarachnoid space contains CSF and blood vessels
• Arachnoid villi protrude into the superior sagittal sinus
and permit CSF reabsorption
Pia Mater
• Layer of delicate vascularized connective tissue that
clings tightly to the brain
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Superior
sagittal sinus
Subdural
space
Subarachnoid
space
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Skin of scalp
Periosteum
Bone of skull
Periosteal Dura
Meningeal mater
Arachnoid mater
Pia mater
Arachnoid villus
Blood vessel
Falx cerebri
(in longitudinal
fissure only)
Figure 12.24
Cerebrospinal Fluid (CSF)
• Composition
 Watery solution
 Less protein and different ion concentrations than plasma
 Constant volume
• Functions
 Gives buoyancy to the CNS organs
 Protects the CNS from blows and other trauma
 Nourishes the brain and carries chemical signals
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Superior
sagittal sinus
4
Choroid
plexus
Arachnoid villus
Interventricular
foramen
Subarachnoid space
Arachnoid mater
Meningeal dura mater
Periosteal dura mater
1
Right lateral ventricle
(deep to cut)
Choroid plexus
of fourth ventricle
3
Third ventricle
1 CSF is produced by the
Cerebral aqueduct
Lateral aperture
Fourth ventricle
Median aperture
Central canal
of spinal cord
(a) CSF circulation
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2
choroid plexus of each
ventricle.
2 CSF flows through the
ventricles and into the
subarachnoid space via the
median and lateral apertures.
Some CSF flows through the
central canal of the spinal cord.
3 CSF flows through the
subarachnoid space.
4 CSF is absorbed into the dural venous
sinuses via the arachnoid villi.
Figure 12.26a
Choroid Plexuses
• Produce CSF at a constant rate
• Hang from the roof of each
ventricle
• Clusters of capillaries enclosed
by pia mater and a layer of
ependymal cells
• Ependymal cells use ion pumps
to control the composition of the
CSF and help cleanse CSF by
removing wastes
Ependymal
cells
Capillary
Section
of choroid
plexus
Connective
tissue of
pia mater
Wastes and
unnecessary
solutes absorbed
CSF forms as a filtrate
containing glucose, oxygen,
vitamins, and ions
(Na+, Cl–, Mg2+, etc.)
(b) CSF formation by choroid plexuses
Cavity of
ventricle
Figure 12.26b
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Blood-Brain Barrier
• Helps maintain a stable environment for the brain
• Separates neurons from some blood-borne substances
• Composition
 Continuous endothelium of capillary walls
 Basal lamina
 Feet of astrocytes
Provide signal to endothelium for the formation of tight junctions
• Selective barrier
 Allows nutrients to move by facilitated diffusion
 Allows any fat-soluble substances to pass, including alcohol,
nicotine, and anesthetics
• Absent in some areas, e.g., vomiting center and the hypothalamus,
where it is necessary to monitor the chemical composition of the blood
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Capillary
Neuron
Astrocyte
(a) Astrocytes are the most abundant
CNS neuroglia.
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Figure 11.3a
Homeostatic Imbalances of the Brain
Traumatic brain injuries
• Concussion—temporary alteration in function
• Contusion—permanent damage
• Subdural or subarachnoid hemorrhage—may force
brain stem through the foramen magnum, resulting in
death
• Cerebral edema—swelling of the brain associated with
traumatic head injury
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Homeostatic Imbalances of the Brain
• Cerebrovascular accidents (CVAs)(strokes)
 Blood circulation is blocked and brain tissue dies, e.g.,
blockage of a cerebral artery by a blood clot
 Typically leads to hemiplegia, or sensory and speed
deficits
 Transient ischemic attacks (TIAs)—temporary
episodes of reversible cerebral ischemia
 Tissue plasminogen activator (TPA) is the only
approved treatment for stroke
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Homeostatic Imbalances of the Brain
• Degenerative brain disorders
• Alzheimer’s disease (AD): a progressive degenerative
disease of the brain that results in dementia
• Parkinson’s disease: degeneration of the dopaminereleasing neurons of the substantia nigra
• Huntington’s disease: a fatal hereditary disorder
caused by accumulation of the protein huntingtin that
leads to degeneration of the basal nuclei and cerebral
cortex
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