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The Central Nervous System:
Part C
Functional Brain Systems
• Networks of neurons that work together and span wide areas of
the brain
• Limbic system
• Reticular formation
Limbic System
• Structures on the medial aspects of cerebral hemispheres and
diencephalon
• Includes parts of the diencephalon and some cerebral
structures that encircle the brain stem
Limbic System
• Emotional or affective brain
• Amygdala—recognizes angry or fearful facial expressions,
assesses danger, and elicits the fear response
• Cingulate gyrus—plays a role in expressing emotions via
gestures, and resolves mental conflict
• Puts emotional responses to odors
• Example: skunks smell bad
Limbic System: Emotion and Cognition
• The limbic system interacts with the prefrontal lobes, therefore:
• We can react emotionally to things we consciously understand to
be happening
• We are consciously aware of emotional richness in our lives
• Hippocampus and amygdala—play a role in memory
Reticular Formation
• Three broad columns along the length of the brain stem
• Raphe nuclei
• Medial (large cell) group of nuclei
• Lateral (small cell) group of nuclei
• Has far-flung axonal connections with hypothalamus, thalamus,
cerebral cortex, cerebellum, and spinal cord
Reticular Formation: RAS and Motor Function
• RAS (reticular activating system)
• Sends impulses to the cerebral cortex to keep it conscious and
alert
• Filters out repetitive and weak stimuli (~99% of all stimuli!)
• Severe injury results in permanent unconsciousness (coma)
Reticular Formation: RAS and Motor Function
• Motor function
• Helps control coarse limb movements
• Reticular autonomic centers regulate visceral motor functions
• Vasomotor
• Cardiac
• Respiratory centers
Electroencephalogram (EEG)
• Records electrical activity that accompanies brain function
• Measures electrical potential differences between various
cortical areas
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)
Types of Brain Waves
• 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
Brain Waves: State of the Brain
• Change with age, sensory stimuli, brain disease, and the
chemical state of the body
• EEGs 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
Epilepsy
• A victim of epilepsy may lose consciousness, fall stiffly, and
have uncontrollable jerking
• Epilepsy is not associated with intellectual impairments
• Epilepsy occurs in 1% of the population
Epileptic Seizures
• 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
Consciousness
• Conscious perception of sensation
• Voluntary initiation and control of movement
• Capabilities associated with higher mental processing
(memory, logic, judgment, etc.)
• Loss of consciousness (e.g., fainting or syncopy) is a signal
that brain function is impaired
Consciousness
• Clinically defined on a continuum that grades behavior in
response to stimuli
• Alertness
• Drowsiness (lethargy)
• Stupor
• Coma
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)
Sleep
• 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
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
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
Language
• Language implementation system
• Basal nuclei
• Broca’s area and Wernicke’s area (in the association cortex on the left
side)
• Analyzes incoming word sounds
• Produces outgoing word sounds and grammatical structures
• Corresponding areas on the right side are involved with nonverbal
language components
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
Transfer from STM to LTM
• Factors that affect transfer from STM to LTM
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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
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
Categories of Memory
2.Nondeclarative memory
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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
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
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
Molecular Basis of Memory
• During learning:
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Altered mRNA is synthesized and moved to axons and dendrites
Dendritic spines change shape
Extracellular proteins are deposited at synapses involved in LTM
Number and size of presynaptic terminals may increase
More neurotransmitter is released by presynaptic neurons
Molecular Basis of Memory
• Increase in synaptic strength (long-term potentiation, or LTP) is
crucial
• Neurotransmitter (glutamate) binds to NMDA receptors,
opening calcium channels in postsynaptic terminal
Molecular Basis of Memory
• Calcium influx triggers enzymes that modify proteins of the
postsynaptic terminal and presynaptic terminal (via release of
retrograde messengers)
• Enzymes trigger postsynaptic gene activation for synthesis of
synaptic proteins, in presence of CREB (cAMP responseelement binding protein) and BDNF (brain-derived neurotrophic
factor)
Protection of the Brain
• Bone (skull)
• Membranes (meninges)
• Watery cushion (cerebrospinal fluid)
• Blood-brain barrier
Meninges
• Cover and protect the CNS
• Protect blood vessels and enclose venous sinuses
• Contain cerebrospinal fluid (CSF)
• Form partitions in the skull
Meninges
• Three layers
• Dura mater
• Arachnoid mater
• Pia mater
Dura Mater
• Strongest meninx
• Two layers of fibrous connective tissue (around the brain)
separate to form dural sinuses
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
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
Cerebrospinal Fluid (CSF)
• Composition
• Watery solution
• Less protein and different ion concentrations than plasma
• Constant volume
Cerebrospinal Fluid (CSF)
• Functions
• Gives buoyancy to the CNS organs
• Protects the CNS from blows and other trauma
• Nourishes the brain and carries chemical signals
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
Blood-Brain Barrier
• Helps maintain a stable environment for the brain
• Separates neurons from some bloodborne substances
Blood-Brain Barrier
• Composition
• Continuous endothelium of capillary walls
• Basal lamina
• Feet of astrocytes
• Provide signal to endothelium for the formation of tight
junctions
Blood-Brain Barrier: Functions
• 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
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
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
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 dopamine-releasing 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