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CHAPTER 12
CENTRAL NERVOUS SYSTEM
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CENTRAL NERVOUS SYSTEM
Embryonic Development
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Figure 12.1 Development of the neural tube from embryonic ectoderm (1 of 4).
Surface
ectoderm
Head
Neural
plate
Tail
1 The neural plate forms from surface ectoderm.
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Figure 12.1 Development of the neural tube from embryonic ectoderm (2 of 4).
Neural folds
Neural
groove
2 The neural plate invaginates, forming the neural
groove, flanked by neural folds.
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Figure 12.1 Development of the neural tube from embryonic ectoderm (3 of 4).
Neural crest
3
Neural fold cells migrate to form the neural crest,
which will form much of the PNS and many other
structures.
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Figure 12.1 Development of the neural tube from embryonic ectoderm (4 of 4).
Head
Surface
ectoderm
Tail
Neural
tube
4 The neural groove becomes the neural tube, which
will form CNS structures.
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Neural tube formation involves all of these
stages except…
1)
2)
3)
4)
5)
Neural plate
Neural ring
Neural groove
Surface ectoderm
Neural tube
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Figure 12.2 Embryonic development of the human brain.
(a)
Neural
tube
Anterior
(rostral)
(b) Primary brain
vesicles
(c) Secondary brain
vesicles
(d) Adult brain
structures
(e) Adult
neural
canal
regions
Telencephalon
Cerebrum: cerebral
hemispheres (cortex,
white matter, basal nuclei)
Lateral
ventricles
Prosencephalon
(forebrain)
Diencephalon
Diencephalon
(thalamus, hypothalamus,
epithalamus), retina
Third ventricle
Mesencephalon
(midbrain)
Mesencephalon
Brain stem: midbrain
Cerebral
aqueduct
Metencephalon
Brain stem: pons
Rhombencephalon
(hindbrain)
Cerebellum
Myelencephalon
Posterior
(caudal)
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Brain stem: medulla
oblongata
Spinal cord
Fourth
ventricle
Central canal
Which of these adult brain structures is
derived from the prosencephalon?
1)
2)
3)
4)
Brain stem
Cerebellum
Cerebrum
All of the above
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CENTRAL NERVOUS SYSTEM
Regions and Organization
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Figure 12.3d Effect of space restriction on brain development.
Cerebral
hemisphere
Diencephalon
(d) Birth
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Cerebellum
Brain stem
• Midbrain
• Pons
• Medulla
oblongata
Figure 12.4 Pattern of gray and white matter in the CNS (highly simplified).
Cortex of
gray matter
Inner gray
matter
Central cavity
Migratory
pattern of
neurons
Cerebrum
Cerebellum
Region of cerebellum
Outer white
matter
Gray matter
Central cavity
Inner gray matter
Outer white matter
Brain stem
Gray matter
Central cavity
Outer white matter
Spinal cord
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Inner gray matter
CENTRAL NERVOUS SYSTEM
Ventricles
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Figure 12.5 Ventricles of the brain.
Lateral ventricle
Septum pellucidum
Anterior horn
Posterior
horn
Inferior
horn
Lateral
aperture
Interventricular
foramen
Third ventricle
Inferior horn
Cerebral aqueduct
Fourth ventricle
Central canal
(a) Anterior view
(b) Left lateral
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Median
aperture
Lateral
aperture
view
White matter represents…
1)
2)
3)
4)
5)
High density areas of neuron cell bodies
Muscle
Blood vessels
High density areas of unmyelinated axons
High density areas of myelinated axons
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True or false: The central canal of the spinal
Cord is continuous with the ventricles of the
Brain.
1) True
2) False
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CENTRAL NERVOUS SYSTEM
Cerebral Hemispheres:
Cerebral Cortex
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Figure 12.6a Lobes and fissures of the cerebral hemispheres.
Precentral
gyrus
Frontal
lobe
Central
sulcus
Postcentral
gyrus
Parietal lobe
Parieto-occipital sulcus
(on medial surface
of hemisphere)
Lateral sulcus
Occipital lobe
Temporal lobe
Transverse cerebral fissure
Cerebellum
Pons
Medulla oblongata
Spinal cord
Fissure
(a deep
sulcus)
Gyrus
Cortex (gray matter)
Sulcus
White matter
(a)
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Figure 12.6b Lobes and fissures of the cerebral hemispheres.
Frontal lobe
Gyri of insula
Temporal lobe
(pulled down)
(b)
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Central
sulcus
Figure 12.6c Lobes and fissures of the cerebral hemispheres.
Anterior
Longitudinal
fissure
Frontal lobe
Cerebral veins
and arteries
covered by
arachnoid
mater
Parietal
lobe
Right cerebral
hemisphere
Occipital
lobe
Left cerebral
hemisphere
(c)
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Posterior
Figure 12.7 Functional neuroimaging (fMRI) of the cerebral cortex.
Central sulcus
Longitudinal
fissure
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Left frontal
lobe
Left temporal
lobe
Areas active
in speech and
hearing (fMRI)
Figure 12.8a Functional and structural areas of the cerebral cortex.
Motor areas
Central sulcus
Primary motor cortex
Premotor cortex
Frontal eye field
Broca’s area
(outlined by dashes)
Prefrontal cortex
Working memory
for spatial tasks
Executive area for
task management
Working memory for
object-recall tasks
Solving complex,
multitask problems
(a) Lateral view, left cerebral hemisphere
Motor association cortex
Primary sensory cortex
Primary motor cortex
Sensory association cortex
Multimodal association cortex
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Sensory areas and related
association areas
Primary somatosensory
cortex
Somatic
Somatosensory
sensation
association cortex
Gustatory cortex
(in insula)
Taste
Wernicke’s area
(outlined by dashes)
Primary visual
cortex
Visual
association
area
Auditory
association area
Primary
auditory cortex
Vision
Hearing
Figure 12.8b Functional and structural areas of the cerebral cortex.
Premotor cortex
Corpus
callosum
Cingulate
gyrus
Primary
motor cortex
Frontal eye field
Prefrontal
cortex
Occipital
lobe
Processes emotions
related to personal
and social interactions
Orbitofrontal
cortex
Olfactory bulb
Olfactory tract
Fornix
Visual
association
area
Temporal lobe
(b) Parasagittal view, right hemisphere
Uncus
Primary
olfactory cortex
Motor association cortex
Primary sensory cortex
Primary motor cortex
Sensory association cortex
Multimodal association cortex
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Central sulcus
Primary somatosensory
cortex
Parietal lobe
Somatosensory
association cortex
Parieto-occipital
sulcus
Primary
visual cortex
Calcarine sulcus
Parahippocampal
gyrus
Figure 12.9 Body maps in the primary motor cortex and somatosensory cortex of the cerebrum.
Posterior
Motor
Motor map in
precentral gyrus
Sensory
Anterior
Sensory map in
postcentral gyrus
Toes
Genitals
Jaw
Tongue
Swallowing
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Primary motor
cortex
(precentral gyrus)
Primary somatosensory cortex
(postcentral gyrus)
Intraabdominal
Quiz Q1: All of these are true of the primary
motor cortex except it…
1)
2)
3)
4)
Is located anterior to the central sulcus
Is located on the precentral gyrus
Controls voluntary muscle movement
Coordinates repeated/learned complex
muscle movements
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Quiz Q2: The sensory homunculus has the
fingers mapped to a large area of the
brain because…
1) The fingers have a very large surface area
2) The fingers have lots of muscles required
to move them
3) The fingers contain many sensory
receptors
4) All of the above
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Quiz Q3: True or false: The neural crest
develops into the central nervous system.
1) True
2) False
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Quiz Q4: Information from the premotor cortex
mostly gets sent to…
1)
2)
3)
4)
muscles directly.
the primary somatosensory cortex
the somatosensory association cortex
the primary motor cortex
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Quiz Q5: Gray matter includes…
1)
2)
3)
4)
myelinated axons
neuron cell bodies
Schwann cells
ganglia
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Figure 12.10 Types of fiber tracts in white matter.
Longitudinal fissure
Lateral
ventricle
Basal nuclei
• Caudate
• Putamen
• Globus
pallidus
Thalamus
Superior
Association fibers
Commissural fibers
(corpus callosum)
Corona
radiata
Fornix
Internal
capsule
Gray matter
Third
ventricle
White matter
Pons
Projection
fibers
Medulla oblongata
(a)
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Decussation
of pyramids
(b)
Figure 12.11a Basal nuclei.
Fibers of
corona radiata
Caudate
nucleus
Lentiform
nucleus
Corpus
striatum • Putamen
• Globus pallidus
(deep to putamen)
(a)
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Projection fibers
run deep to
lentiform nucleus
Thalamus
Tail of
caudate
nucleus
Figure 12.11b Basal nuclei (1 of 2).
Anterior
Cerebral cortex
Cerebral white matter
Corpus callosum
Anterior horn
of lateral ventricle
Caudate nucleus
Putamen
Lentiform
Globus
nucleus
pallidus
(b)
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Posterior
Thalamus
Tail of caudate nucleus
Third ventricle
Inferior horn
of lateral ventricle
Figure 12.11b Basal nuclei (2 of 2).
Cerebral cortex
Cerebral white matter
Corpus callosum
Anterior horn
of lateral ventricle
Caudate nucleus
Lentiform nucleus
Thalamus
Third ventricle
Inferior horn
of lateral ventricle
(b)
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Basal nuclei are…
1)
2)
3)
4)
5)
Part of the cerebral cortex
Part of the cerebral hemispheres
Gray matter
2 and 3 only
All of the above
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CENTRAL NERVOUS SYSTEM
Cerebral Hemispheres:
Lateralization and
Contralateralization
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CENTRAL NERVOUS SYSTEM
Diencephalon & Brainstem
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Figure 12.12 Midsagittal section of the brain illustrating the diencephalon (purple) and brain stem (green).
Cerebral hemisphere
Septum pellucidum
Interthalamic
adhesion
(intermediate
mass of
thalamus)
Interventricular
foramen
Anterior
commissure
Hypothalamus
Optic chiasma
Pituitary gland
Mammillary body
Pons
Medulla oblongata
Spinal cord
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Corpus callosum
Fornix
Choroid plexus
Thalamus
(encloses third
ventricle)
Posterior commissure
Pineal gland
(part of epithalamus)
Corpora
quadrigemina Midbrain
Cerebral
aqueduct
Arbor vitae (of
cerebellum)
Fourth ventricle
Choroid plexus
Cerebellum
Figure 12.13 Selected structures of the diencephalon.
Paraventricular
nucleus
Dorsal nuclei
Medial Lateral Lateral
dorsal posterior
Pulvinar
Anterior
nuclear
group
Medial
geniculate
body
Reticular
nucleus
Lateral
geniculate
body
Ventral
Ventral Ventral posteroanterior lateral lateral
Ventral nuclei
(a) The main thalamic nuclei. (The reticular
nuclei that “cap” the thalamus laterally are
depicted as curving translucent structures.)
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Anterior
commissure
Preoptic
nucleus
Anterior
hypothalamic
nucleus
Supraoptic
nucleus
Suprachiasmatic
nucleus
Optic
chiasma
Infundibulum
(stalk of the
pituitary gland)
Fornix
Arcuate
nucleus
Pituitary
gland
(b) The main hypothalamic nuclei.
Dorsomedial
nucleus
Posterior
hypothalamic
nucleus
Lateral
hypothalamic
area
Ventromedial
nucleus
Mammillary
body
The diencephalon is comprised of all of the
Following except…
1)
2)
3)
4)
Hypothalamus
Thalamus
Epithalamus
Midbrain
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Figure 12.15a Three views of the brain stem (green) and the diencephalon (purple).
View (a)
Optic chiasma
Optic nerve (II)
Crus cerebri of
cerebral peduncles
(midbrain)
Diencephalon
• Thalamus
• Hypothalamus
Mammillary body
Hypothalamus
Diencephalon
Midbrain
Oculomotor nerve (III)
Trochlear nerve (IV)
Pons
Medulla
oblongata
Trigeminal nerve (V)
Pons
Facial nerve (VII)
Middle cerebellar
peduncle
Abducens nerve (VI)
Vestibulocochlear
nerve (VIII)
Pyramid
Glossopharyngeal nerve (IX)
Hypoglossal nerve (XII)
Vagus nerve (X)
Ventral root of first
cervical nerve
Decussation of pyramids
Accessory nerve (XI)
Spinal cord
(a) Ventral view
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Thalamus
Brainstem
Figure 12.15b Three views of the brain stem (green) and the diencephalon (purple).
Crus cerebri of
cerebral peduncles
(midbrain)
Thalamus
View (b)
Infundibulum
Pituitary gland
Superior colliculus
Inferior colliculus
Trochlear nerve (IV)
Trigeminal nerve (V)
Pons
Superior cerebellar peduncle
Middle cerebellar peduncle
Facial nerve (VII)
Abducens nerve (VI)
Glossopharyngeal nerve (IX)
Hypoglossal nerve (XII)
Inferior cerebellar peduncle
Vestibulocochlear nerve (VIII)
Olive
Thalamus
Vagus nerve (X)
Hypothalamus
Midbrain
Accessory nerve (XI)
Pons
Medulla
oblongata
(b) Left lateral view
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Diencephalon
Brainstem
Figure 12.15c Three views of the brain stem (green) and the diencephalon (purple).
Thalamus
View (c)
Diencephalon
Pineal gland
Anterior wall of
fourth ventricle
Choroid plexus
(fourth ventricle)
Dorsal median sulcus
Dorsal root of
first cervical nerve
Midbrain
• Superior
Corpora
colliculus quadrigemina
• Inferior
of tectum
colliculus
• Trochlear nerve (IV)
• Superior cerebellar peduncle
Pons
• Middle cerebellar peduncle
Medulla oblongata
• Inferior cerebellar peduncle
• Facial nerve (VII)
• Vestibulocochlear nerve (VIII)
• Glossopharyngeal nerve (IX)
• Vagus nerve (X)
• Accessory nerve (XI)
Thalamus
Hypothalamus
Midbrain
Pons
(c) Dorsal view
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Diencephalon
Medulla
oblongata
Brainstem
Figure 12.16a Cross sections through different regions of the brain stem.
Tectum
Periaqueductal gray
matter
Oculomotor
nucleus (III)
Medial
lemniscus
Red
nucleus
Substantia
nigra
Fibers of
pyramidal tract
(a) Midbrain
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Dorsal
Superior
colliculus
Cerebral aqueduct
Reticular formation
Ventral
Crus cerebri of
cerebral peduncle
Figure 12.16b Cross sections through different regions of the brain stem.
Fourth
ventricle
Superior cerebellar
peduncle
Trigeminal main
sensory nucleus
Reticular
formation
Trigeminal
motor nucleus
Middle
cerebellar
peduncle
Trigeminal
nerve (V)
Medial lemniscus
(b) Pons
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Pontine
nuclei
Fibers of
pyramidal
tract
Figure 12.16c Cross sections through different regions of the brain stem.
Reticular formation
Fourth ventricle
Choroid
Hypoglossal nucleus (XII)
plexus
Dorsal motor nucleus
of vagus (X)
Inferior cerebellar
peduncle
Lateral
nuclear
group
Medial
nuclear
group
Raphe
nucleus
Medial lemniscus
(c) Medulla oblongata
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Solitary
nucleus
Vestibular nuclear
complex (VIII)
Cochlear
nuclei (VIII)
Nucleus
ambiguus
Inferior olivary
nucleus
Pyramid
Mike the Headless Chicken
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46
The brainstem is comprised of all of the
Following except…
1)
2)
3)
4)
Midbrain
Cerebellum
Pons
Medulla oblongata
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CENTRAL NERVOUS SYSTEM
Cerebellum
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Figure 12.17b Cerebellum.
Anterior lobe
Cerebellar cortex
Arbor
vitae
Cerebellar
peduncles
• Superior
• Middle
• Inferior
Medulla
oblongata
(b)
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Flocculonodular
lobe
Posterior
lobe
Choroid
plexus of
fourth
ventricle
Figure 12.17a Cerebellum.
Anterior lobe
Arbor vitae
Cerebellar
cortex
Pons
Fourth
ventricle
Medulla
oblongata
(a)
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Posterior
lobe
Flocculonodular lobe
Choroid plexus
Figure 12.17c–d Cerebellum.
Anterior
lobe
Primary
fissure
Posterior
lobe
Horizontal
fissure
Vermis
(c)
Anterior
lobe
Posterior
lobe
(d)
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Vermis
Which of the following is most directly
required for basic survival of the body?
1)
2)
3)
4)
Cerebrum
Cerebellum
Brainstem
Diencephalon
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Which of the following is used in refining
movements of the body?
1)
2)
3)
4)
Cerebrum
Cerebellum
Brainstem
Diencephalon
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CENTRAL NERVOUS SYSTEM
Functional Brain Systems
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Figure 12.18 The limbic system.
Septum pellucidum
Diencephalic structures
of the limbic system
•Anterior thalamic
nuclei (flanking
3rd ventricle)
•Hypothalamus
•Mammillary
body
Olfactory bulb
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Corpus callosum
Fiber tracts
connecting limbic
system structures
•Fornix
•Anterior commissure
Cerebral structures of the
limbic system
•Cingulate gyrus
•Septal nuclei
•Amygdala
•Hippocampus
•Dentate gyrus
•Parahippocampal
gyrus
Figure 12.19 The reticular formation.
Radiations
to cerebral
cortex
Visual
impulses
Auditory
impulses
Reticular formation
Ascending general
sensory tracts
(touch, pain, temperature)
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Descending
motor projections
to spinal cord
CENTRAL NERVOUS SYSTEM
Protection of the Brain: Meninges
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Figure 12.24 Meninges: dura mater, arachnoid mater, and pia mater.
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 12.25 Dural septa and dural venous sinuses.
Straight
sinus
Crista galli
of the
ethmoid
bone
Pituitary
gland
Superior
sagittal sinus
Skull
Falx cerebri
Scalp
Occipital lobe
Tentorium
cerebelli
Falx
cerebelli
Cerebellum
Arachnoid
mater over
medulla
oblongata
(a) Dural septa
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(b) Dural venous sinuses
Dura mater
Transverse
sinus
Temporal
bone
Which brain covering is the most closely
associated (in contact) with the brain?
1) Arachnoid mater
2) Dura mater
3) Pia mater
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CENTRAL NERVOUS SYSTEM
Protection of the Brain:
Cerebrospinal Fluid (CSF)
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Figure 12.26a Formation, location, and circulation of CSF.
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
Cerebral aqueduct
Lateral aperture
Fourth ventricle
Median aperture
Central canal
of spinal cord
(a) CSF circulation
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2
1 CSF is produced by the
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 12.26b Formation, location, and circulation of CSF.
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
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Cavity of
ventricle
Figure 12.27 Hydrocephalus in a newborn.
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CENTRAL NERVOUS SYSTEM
Blood-brain barrier (BBB)
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True or false: the blood-brain barrier keeps
everything bad out of the brain.
1) True
2) False
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CENTRAL NERVOUS SYSTEM
Spinal Cord
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Figure 12.29a Gross structure of the spinal cord, dorsal view.
Cervical
enlargement
Dura and
arachnoid
mater
Lumbar
enlargement
Conus
medullaris
Cauda
equina
Filum
terminale
(a) The spinal cord and its nerve
roots, with the bony vertebral
arches removed. The dura mater
and arachnoid mater are cut
open and reflected laterally.
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Cervical
spinal nerves
Thoracic
spinal nerves
Lumbar
spinal nerves
Sacral
spinal nerves
Figure 12.29c Gross structure of the spinal cord, dorsal view.
Spinal cord
Vertebral
arch
Denticulate
ligament
Denticulate
ligament
Arachnoid
mater
Dorsal
median
sulcus
Dorsal root
Spinal dura
mater
(c) Thoracic spinal cord, showing
denticulate ligaments.
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Figure 12.30 Diagrammatic view of a lumbar tap.
T12
Ligamentum
flavum
Lumbar puncture
needle entering
subarachnoid
space
L5
L4
Supraspinous
ligament
L5
Filum
terminale
S1
Intervertebral
disc
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Arachnoid
matter
Dura
mater
Cauda equina
in subarachnoid
space
CENTRAL NERVOUS SYSTEM
Spinal Cord:
Cross-sectional anatomy
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Figure 12.31a Anatomy of the spinal cord.
Epidural space
(contains fat)
Subdural space
Subarachnoid
space
(contains CSF)
Pia mater
Arachnoid
mater
Dura mater
Spinal
meninges
Bone of
vertebra
Dorsal root
ganglion
Body
of vertebra
(a) Cross section of spinal cord and vertebra
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Figure 12.31b Anatomy of the spinal cord.
Dorsal median sulcus
Dorsal funiculus
White
Ventral funiculus
columns Lateral funiculus
Dorsal root
ganglion
Gray
commissure
Dorsal horn Gray
Ventral horn matter
Lateral horn
Spinal nerve
Dorsal root
(fans out into
dorsal rootlets)
Ventral root
(derived from several
ventral rootlets)
Central canal
Ventral median
fissure
Pia mater
Arachnoid mater
Spinal dura mater
(b) The spinal cord and its meningeal coverings
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Which nerve root has a ganglion?
1)
2)
3)
4)
Dorsal
Ventral
Anterior
Posterior
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Figure 12.32 Organization of the gray matter of the spinal cord.
Dorsal root (sensory)
Dorsal root ganglion
Dorsal horn (interneurons)
Somatic
sensory
neuron
Visceral
sensory
neuron
Visceral
motor
neuron
Spinal nerve
Somatic
motor neuron
Ventral root
(motor)
Ventral horn
(motor neurons)
Interneurons receiving input from somatic sensory neurons
Interneurons receiving input from visceral sensory neurons
Visceral motor (autonomic) neurons
Somatic motor neurons
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Figure 12.33 Major ascending (sensory) and descending (motor) tracts of the spinal cord, cross-sectional view.
Ascending tracts
Fasciculus gracilis
Dorsal
white Fasciculus cuneatus
column
Dorsal
spinocerebellar
tract
Ventral
spinocerebellar
tract
Lateral
spinothalamic tract
Ventral spinothalamic
tract
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Descending tracts
Ventral white
commissure
Lateral
reticulospinal tract
Lateral
corticospinal tract
Rubrospinal
tract
Medial
reticulospinal
tract
Ventral corticospinal
tract
Vestibulospinal tract
Tectospinal tract
Figure 12.34a Pathways of selected ascending spinal cord tracts (2 of 2).
Dorsal
spinocerebellar
tract (axons of
second-order
neurons)
Medial lemniscus (tract)
(axons of second-order neurons)
Nucleus gracilis
Nucleus cuneatus
Medulla oblongata
Fasciculus cuneatus
(axon of first-order sensory neuron)
Axon of
first-order
neuron
Muscle spindle
(proprioceptor)
(a) Spinocerebellar
pathway
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Joint stretch
receptor
(proprioceptor)
Cervical spinal cord
Fasciculus gracilis
(axon of first-order sensory neuron)
Lumbar spinal cord
Dorsal column–medial
lemniscal pathway
Touch
receptor
Figure 12.34a Pathways of selected ascending spinal cord tracts (1 of 2).
Primary
somatosensory
cortex
Axons of third-order
neurons
Thalamus
Cerebrum
Midbrain
Cerebellum
Pons
(a) Spinocerebellar
pathway
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Dorsal column–medial
lemniscal pathway
Figure 12.35a Three descending pathways by which the brain influences movement (1 of 2).
Pyramidal cells
(upper motor
neurons)
Primary motor cortex
Internal capsule
Cerebrum
Midbrain
Cerebral
peduncle
Cerebellum
Pons
(a) Pyramidal (lateral and ventral corticospinal) pathways
Copyright © 2010 Pearson Education, Inc.
Figure 12.35a Three descending pathways by which the brain influences movement (2 of 2).
Ventral
corticospinal
tract
Pyramids
Decussation
of pyramid
Lateral
corticospinal
tract
Medulla oblongata
Cervical spinal cord
Skeletal
muscle
Lumbar spinal cord
Somatic motor neurons
(lower motor neurons)
(a) Pyramidal (lateral and ventral corticospinal) pathways
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