Download Chapter 12_Part II

Functional Brain System
Networks of neurons working together and
spanning wide areas of the brain
The two systems are:
Limbic system
Reticular formation
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Limbic System
Structures located on the medial aspects of cerebral
hemispheres and diencephalon
Includes the rhinencephalon, amygdala,
hypothalamus, and anterior nucleus of the
thalamus
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Limbic System
Parts especially important in emotions:
Amygdala – deals with anger, danger, and fear
responses
Cingulate gyrus – plays a role in expressing
emotions via gestures, and resolves mental conflict
Puts emotional responses to odors – e.g., the aroma
of a particular food reminds you of home
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Limbic System: Emotion and Cognition
The limbic system interacts with the prefrontal lobes, therefore:
One can react emotionally to conscious understandings
One is consciously aware of emotion in one’s life
Hippocampal structures – convert new information into longterm memories
Psychosomatic illnesses interfere with these structures
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Reticular Formation
Composed of three broad columns along the length of the brain stem
Raphe nuclei
Medial (large cell) group
Lateral (small cell) group
Has far-flung axonal connections with hypothalamus, thalamus, cerebellum,
and spinal cord
Arouses the brain as a whole
Filters out weak sensory inputs (familiar inputs)
e.g. rings and watches
99% of all sensory input is unimportant
(drugs like LSD interfere with these sensory
dampers)
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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
Motor function
Helps control coarse motor movements
Autonomic centers regulate visceral motor
functions – e.g., vasomotor, cardiac, and
respiratory centers
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Types of Sleep
There are two major types of sleep:
Non-rapid eye movement (NREM)
Rapid eye movement (REM)
One passes through four stages of NREM during
the first 30-45 minutes of sleep
REM sleep occurs after the fourth NREM stage
has been achieved
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Sleep….zzzzzzzzzz
Sleep wake cycle: state of partial unconsciousness
from which a person can be aroused by stimulation
Coma: can not be aroused by stimulation
Cortical level is depressed
Brain stem functions (breathing, heartbeat,
blood pressure) continue
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Types and Stages of Sleep: NREM
Awake
?
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Figure 12.21a.2
Types and Stages of Sleep: REM
Characteristics of REM sleep
EEG pattern reverts through the NREM stages to
the stage 1 pattern
Vital signs increase
Increase in O2 use by the brain (greater than when
awake!!)
Skeletal muscles (except ocular muscles) are
inhibited
Most dreaming takes place
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Sleep Patterns
Alternating cycles of sleep and wakefulness reflect
a natural circadian rhythm
Although RAS activity declines in sleep, sleep is
more than turning off RAS
The brain is actively guided into sleep
The suprachiasmatic and preoptic nuclei of the
hypothalamus regulate the sleep cycle
A typical sleep pattern alternates between REM
and NREM Stage 4 sleep
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Importance of Sleep
Those 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
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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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Memory
Memory is the storage and retrieval of information
The three principles of memory are:
Storage – occurs in stages and is continually
changing
Processing – accomplished by the hippocampus
and surrounding structures
Memory traces – chemical or structural changes
that encode memory
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Memory Processing
7-8 pieces of data at a time
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Figure 12.22
Categories of Memory
The two categories of memory are fact memory
and skill memory
Fact (declarative) memory:
Entails learning explicit information, e.g. names,
faces, words
Is related to our conscious thoughts and our
language ability
Is stored with the context in which it was learned
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Nondeclarative (Skill) Memory
Skill memory is less conscious than fact memory
and involves motor activity
It is acquired through practice
Skill memories do not retain the context in which
they were learned
…but do you remember who taught you to
ride a bike and that breakthrough moment?!
It’s not all or none
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Structures Involved in Fact Memory
Fact memory involves the following brain areas:
Hippocampus and the amygdala, both limbic
system structures
Specific areas of the thalamus and hypothalamus of
the diencephalon
Ventromedial prefrontal cortex and the basal
forebrain
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Structures Involved in Skill Memory
Skill memory involves:
Corpus striatum – mediates the automatic
connections between a stimulus and a motor
response
Portion of the brain receiving the stimulus
Premotor and motor cortex
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Mechanisms of Memory
Neuronal RNA content is altered
Dendritic spines change shape
Extracellular proteins are deposited at synapses
involved in long-term memory
Number and size of presynaptic terminals may
increase
More neurotransmitter is released by presynaptic
neurons
New hippocampal neurons appear
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Brain Protection
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Protection of the Brain
The brain is protected by bone, meninges, and
cerebrospinal fluid
Harmful substances are shielded from the brain by
the blood-brain barrier
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Meninges
Three connective tissue membranes lie external to
the CNS – dura mater, arachnoid mater, and pia
mater
Functions of the meninges
Cover and protect the CNS
Protect blood vessels and enclose venous sinuses
Contain cerebrospinal fluid (CSF)
Form partitions within the skull
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Meninges
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Figure 12.24a
Dura Mater (one tough mother!)
Strongest menige
2 layered sheet of fibrous connective tissue
Periosteal layer (superficial/external) attached to the periosteum
(but is not associated with the spinal cord)
Menigeal layer (deep/internal) continues to the spinal cord as the
dural sheath
Fused except at the dural sinuses that collect venous blood from
the brain and direct it into the internal jugular veins in the neck.
Three dural septa extend inward and limit excessive movement
of the brain
Falx cerebri – fold that dips into the longitudinal fissure
Falx cerebelli – runs along the vermis of the cerebellum
Tentorium cerebelli – horizontal dural fold extends into the
transverse fissure
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Dura Mater
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Figure 12.25
Arachnoid Mater
The middle meninge, which forms a loose brain covering
It is separated from the dura mater by the subdural space
Beneath the arachnoid is a wide subarachnoid space filled with CSF and
large blood vessels
Beneath the subarachnoid space the arachnoid is attached to the pia mater
Arachnoid villi protrude superiorly and permit CSF to be absorbed into
venous blood
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Cerebrospinal Fluid (CSF)
Watery solution similar in composition to blood plasma
Contains less protein and different ion concentrations than
plasma
Forms a liquid cushion that gives buoyancy to the CNS
organs
Prevents the brain from crushing under its own weight
Protects the CNS from blows and other trauma
Nourishes the brain and carries chemical signals
throughout it
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Circulation of CSF
CSF moves freely through the ventricles and central canal of the
spinal cord, but most enters the subarachnoid space via the lateral
and median apertures in the walls of the 4th ventricle
In the subarchnoid space, CSF baths the outer surface of the brain
and spinal cord and then returns to the blood via the dural sinuses
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Figure 12.26b
Choroid Plexuses
Clusters of capillaries that form tissue fluid filters, which hang from the roof
of each ventricle
Enclosed by pia mater and then ependymal cells
Volume in adults = 150 ml and is continuously replaced (around 800 ml/day)
Cilia of the ependymal cells lining the ventricles circulate the CSF
Have ion pumps that allow them to alter ion concentrations of the CSF
Help cleanse CSF by removing wastes
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Blood-Brain Barrier
Protective mechanism that helps maintain a stable
environment for the brain
Bloodborne substances are separated from neurons
by:
Continuous endothelium of capillary walls (main
barrier; least permeable capillaries in the body)
Relatively thick basal lamina
Bulbous feet of astrocytes (stimulate the capillary
endothelial cells to form tight junctions)
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Blood-Brain Barrier: Functions
Selective barrier that allows nutrients to pass freely
Glucose, amino acids, electrolytes (some)
Metabolic waste, proteins, toxins, drugs (some)
are denied
Is ineffective against substances that can diffuse
through plasma membranes
Absent in some areas (vomiting center and the
hypothalamus), allowing these areas to monitor the
chemical composition of the blood
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Spinal Cord
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Figure 12.29a
Spinal Cord
CNS tissue is enclosed within the vertebral column
from the foramen magnum to L1
About 17” long
Major reflex center
Provides two-way communication to and from the
brain
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Spinal Cord
Protected by bone, meninges, and CSF
Epidural space – space between the vertebrae and the
dural sheath (dura mater) filled with fat and a
network of veins
Cerebral spinal fluid fills the subarachnoid space
between the arachnoid and pia mater
Dural and Arachnoid membranes extend to S2
Lumbar punctures: done below L3 (see first bullet)
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Lumbar Puncture
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Figure 12.30
Spinal Tap
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Spinal Cord
Spinal cord terminates at the conus medullaris (@L1)
Filum terminale – fibrous extension of the pia mater; anchors the spinal cord
to the posterior surface of the coccyx
Denticulate ligaments – delicate shelves of pia mater; attach the spinal cord
to the vertebrae
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Spinal Cord
Spinal nerves – 31 pairs attach to the cord by
paired roots
Each nerve exits thru intervertebral foramina
superiorly (cervical vertebrae) or inferiorly (the
rest) at its corresponding numbered vertebra
Cervical and lumbar enlargements – sites where
nerves serving the upper and lower limbs emerge
Cauda equina – collection of nerve roots at the
inferior end of the vertebral canal
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Cross-Sectional Anatomy of the Spinal Cord
Anterior median fissure – separates anterior funiculi
Posterior median sulcus – divides posterior funiculi
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Figure 12.31a
Gray Matter and Spinal Roots
Gray matter consists of soma, unmyelinated processes, and
neuroglia
Gray commissure – connects masses of gray matter;
encloses central canal
Posterior (dorsal) horns – interneurons
Anterior (ventral) horns – interneurons and somatic motor
neurons
Lateral horns – present in the thoracic and superior lumbar
segments of the spinal cord and contain sympathetic nerve
fibers of the autonomic nervous system
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Gray Matter and Spinal Roots
All neurons whose cell bodies are in the gray matter are
multipolar
The amount of gray matter present at any given level of the
spinal cord reflects the amount of skeletal muscle innervated at
that level
Lateral horn neurons are autonomic (sympathetic division)
motor neurons that serve the visceral organs
E.g. ventral horns are the largest in the limb-innervating
cervical and lumbar regions
-their axons leave the spinal cord via the ventral root
The dorsal roots of the spinal cord are formed from afferent
fibers carrying impulses from peripheral sensory receptors
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Gray Matter: Organization
Dorsal half – sensory roots
Ventral half – motor roots
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Gray Matter: Organization
Dorsal and ventral roots fuse laterally to form spinal
nerves
Four zones are evident within the gray matter –
somatic sensory (SS), visceral sensory (VS), visceral
motor (VM), and somatic motor (SM)
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White Matter in the Spinal Cord
Composed of myelinated and unmyelinated nerve fibers
allowing communication between different parts of the spinal
cord and between the spinal cord and brain
Fibers run in three directions –
Ascending: abundant
Descending: abundant
Transversely (commissural fibers)
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White Matter in the Spinal Cord
Divided into three funiculi (columns): each contain
several fiber tracts
Posterior
Lateral
Anterior
Fiber tract names reveal their origin and
destination
Fiber tracts are composed of axons with similar
functions
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White Matter: Pathway Generalizations
Pathways decussate at some point
Most consist of two or three neurons
Most exhibit somatotopy (precise spatial relationships, mapping of the
body)
Pathways are paired (one on each side of the spinal cord or brain)
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Main Ascending Pathways
Conduct sensory impulses upward usually thru
chains of three successive neurons (1st, 2nd, and 3rd
order neurons; the latter two are interneurons) to
various areas of the brain
1st order neurons: cell bodies reside in a ganglion
(dorsal root or cranial) and conduct impulses from
cutaneous receptors of the skin & proprioceptors to
the spinal cord or brain stem where they synapse
w/ 2nd order neurons
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Main Ascending Pathways
2nd order neurons: cell bodies reside in the dorsal
horn of the spinal cord and transmit impulses
(from 1st order neurons) to the thalamus or to the
cerebellum
3rd order neurons: cell bodies reside in the
thalamus and conduct impulses (from 2nd order
neurons) to the somatosensory cortex of the
cerebrum
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Three Ascending Pathways
Somatosensory info. is conducted along 3 main
pathways on each side of the spinal cord
The nonspecific and specific ascending pathways
send impulses to the sensory cortex
These pathways are responsible for discriminative
touch and conscious proprioception
3rd pathway consists of spinocerebellar tracts that
send impulses to the cerebellum and do not
contribute to sensory perception
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Nonspecific Ascending Pathway
Evolutionarily older
Nonspecific pathway for pain,
temperature, and crude touch
within the lateral spinothalamic
tract
Cross over occurs in the spinal
cord
We are aware of the senses but
have difficulty localizing it on the
body surface
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Figure 12.34b
Specific and Posterior Spinocerebellar Tracts
Specific ascending pathways:
Mediate precise, straight-thru transmission
of inputs from a single type of sensory
neuron that can be localized precisely on
the body surface
Formed by the paired tracts of the dorsal
white column (fasciculus cuneatus &
fasciculus gracilis ) and the medial
lemniscal tracts
The medial lemniscal tracts arise in the
medulla and terminate in the ventral
posterior nuclei of the thalamus and then to
the somatosensory cortex
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The Anterior/Posterior Spinocerebellar Tract
Convey info. about muscle or tendon
stretch to the cerebellum which uses this
info. to coordinate skeletal muscle
activity
Do not contribute to conscious sensation
Either do not cross or double decussate
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Descending (Motor) Pathways & Tracts
Descending tracts deliver efferent impulses from the brain to the
spinal cord, and are divided into two groups:
1. Direct pathways equivalent to the pyramidal tracts
2. Indirect pathways, essentially all others
Motor pathways involve two neurons (upper and lower motor
neurons)
Upper motor neuron: pyramidal cells of the motor
cortex, neurons of the subcortical motor nuclei
Lower motor neuron: ventral horn motor neurons that
innervate the skeletal muscle
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The Direct (Pyramidal) System
Direct pathways originate with the
pyramidal neurons in the precentral gyri
Impulses are sent through the corticospinal
tracts and synapse in the ventral horn
Stimulation of ventral horn neurons activates
skeletal muscles
The direct pathway regulates fast and fine
(skilled) movements
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Indirect (Extrapyramidal) System
Includes the brain stem, motor nuclei, and all
motor pathways not part of the pyramidal
system
These motor pathways are complex and
multisynaptic
Involved in regulating:
Axial muscles that maintain
balance/posterior
Coarse limb movements
Head, neck, eye movements
This system includes the rubrospinal,
vestibulospinal, reticulospinal, and
tectospinal tracts
Heavily dependent on reflex activity
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Cranial Nerves
Twelve pairs of cranial nerves arise from the brain
I & II attach to the forebrain
III-XII originate from the brain stem
They serve the head and neck (except for X, the
vagus nerve, serving the abdomen)
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Cranial Nerves
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Figure 13.5a
Cranial Nerve I: Olfactory
Arises from the olfactory epithelium
Fibers run through the olfactory bulb and terminate in the primary olfactory
cortex
Functions solely by carrying afferent impulses for the sense of smell
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Cranial Nerve II: Optic
Arises from the retina of the eye
Really a brain tract (outgrowth of brain) and NOT a nerve
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Cranial Nerve III: Oculomotor
Supplies 4 of the 6 extrinsic muscles that move the eyeball in the orbit
Functions in raising the eyelid, directing the eyeball, constricting the iris, and
controlling lens shape
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Cranial Nerve IV: Trochlear
Innervate the superior oblique muscle
Primarily a motor nerve that directs the eyeball
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Cranial Nerve V: Trigeminal
Three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3)
Largest cranial nerve
Conveys sensory impulses from various areas of the face (V1) and (V2), and
supplies motor fibers (V3) for mastication
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Cranial Nerve VI: Abdcuens
Primarily a motor nerve innervating the lateral
rectus muscle (abducts the eyeball)
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Figure VI from Table 13.2
Cranial Nerve VII: Facial
Fibers leave the pons, travel through the internal
acoustic meatus, and emerge through the
stylomastoid foramen to the lateral aspect of the
face
Mixed nerve with five major branches
Motor functions include facial expression, and the
transmittal of autonomic impulses to lacrimal and
salivary glands
Sensory function is taste from the anterior twothirds of the tongue
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Cranial Nerve VIII: Vestibulocochlear
Fibers arise from the hearing and equilibrium apparatus of the inner ear, pass
through the internal acoustic meatus, and enter the brainstem at the ponsmedulla border
Two divisions – cochlear (hearing) and vestibular (balance)
Functions are solely sensory – equilibrium and hearing
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Cranial Nerve IX: Glossopharyngeal
Fibers emerge from the medulla, leave the skull via the jugular foramen, and
run to the throat
Nerve IX is a mixed nerve with motor and sensory functions
Motor – innervates part of the tongue and pharynx, and provides motor fibers
to the parotid salivary gland
Sensory – fibers conduct taste and general sensory impulses from the tongue
and pharynx
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Cranial Nerve X: Vagus
The only cranial nerve that extends beyond the head and neck
Fibers emerge from the medulla via the jugular foramen
The vagus is a mixed nerve
Most motor fibers are parasympathetic fibers to the heart, lungs, and visceral
organs
Its sensory function is in taste
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Cranial Nerve XI: Accessory
Formed from a cranial root emerging from the
medulla and a spinal root arising from the superior
region of the spinal cord
The spinal root passes upward into the cranium via
the foramen magnum
The accessory nerve leaves the cranium via the
jugular foramen
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Cranial Nerve XI: Accessory
Primarily a motor nerve
Supplies fibers to the larynx, pharynx, and soft palate
Innervates the trapezius and sternocleidomastoid, which
move the head and neck
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Cranial Nerve XII: Hypoglossal
Fibers arise from the medulla and exit the skull via the
hypoglossal canal
Innervates both extrinsic and intrinsic muscles of the tongue,
which contribute to swallowing and speech
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