Transcript No. 22

No. 22
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Spinal cord (2)
Brain stem (1)
Ⅱ. Internal Structure
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In transverse section, the spinal cord
is seen to consist of the gray matter
and the white matter. The gray
matter of the spinal cord is centrally
located and is surrounded by the
white matter.
Ⅰ) Gray Matter
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The gray matter forms a butterfly-shaped core
of the spinal cord, roughly in the form of a letter
H.
The transverse bar of gray matter that connects
the two lateral gray areas is the gray
commissure.
Within the gray commissure is the narrow, fluidfilled central canal, which is continuous with the
fourth ventricle. The anterior and posterior to the
gray matter are respectively anterior gray
commissure and posterior gray commissure.
The anterior and posterior gray commissures are
also called central gray.
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The vertical bars of the gray H, on either side of
the gray commissure, are separated into:
a pair of posterior horns (columns),
a pair of anterior horns (columns).
In the thoracic and upper lumbar (L1-3) regions,
the spinal cord also has a pair of lateral horns
(columns). The area located between the
anterior and posterior horns is the intermediate
zone.
The gray matter of the spinal cord contains an
enormous number of neurons of varying size and
shape.
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Rexed lamina structure of gray matter of
spinal cord.
1. Lamina Ⅰ
It is also called marginal layer or Walderyer
layer. The nucleus posteromarginalis is in this
lamina, forming a thin layer of cells covering the
tip of the posterior horn.
2. Lamina Ⅱ
It is also named the substantia gelatinosa
anterior to marginal area and is the correlation
between the spinal segments.
3. Lamina Ⅲ
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4. Lamina Ⅳ
In lamina Ⅲ and Ⅳ, there are larger cell groups—
nucleus proprius. They receive the numerous
afferent fibers from posterior root.
Fibers from the nucleus proprius, crosses in the
anterior white commisure and contribute to the
spinothalamic tract.
5. Lamina Ⅴ
It is equivalent to the neck of posterior horn. It
has reticular nucleus.
6. Lamina Ⅵ
It lies the base part of posterior horn.
Lamina Ⅴ and Ⅵ receive the afferent fibers of
proper sensation from the posterior roots.
7. Lamina Ⅶ
 It occupies the great part of the
intermediate zone.
1) The nucleus thoracicus (nucleus
dorsalis of Clarke) is situated in the
medial portion of the base of posterior
horn. It is well defined in the segments of
C8—L3. The axons contribute fibers to the
posterior spinocerebellar tracts of
ipsilateral side.
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2) The intermediomedial nucleus lies in
the most medial portion of lamina Ⅶ,
lateral to the lamina Ⅹ, anterior to the
nucleus thoracicus., receiving the
It receives the visceral afferent fibers from
the posterior roots and gives rise to fibers
which constitute the anterior
spinocerebellar tract of both sides of the
spinal cord.
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3) The intermediolateral nucleus
It is located between the anterior and posterior
horns, lateral to the intermediomedial nucleus.
The intermediolateral nucleus of the lateral horn
from C8—L3) segments, is the lower center of
sympathetic nerve. It is the site where the bodies
of sympathetic neurons lie.
In segments S2—S4, the site equivalent to the
intermediolateral nucleus, the sacral
parasympathetic nucleus is the lower center of
parasymthetic nerve.
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8. Lamina Ⅷ
It is in the base part of anterior horn,
consisting of intermediate neurons. In
cervical and lumbosacral enlargements it
is only located in the medial part of
anterior horn.
It receives the fibers from joining laminae
and some descending funiculi and sends
out fibers to the lamina Ⅸ.
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9. Lamina Ⅸ
It is composed of anterior horn motor neurons
and intermediate neurons.
In the cervical and lumbosacral enlargements,
the anterior horn motor neurons are divided into
two great groups. The medial group is also
called medial nucleus of anterior horn
responsible for innervating the intrinsic muscles
of the and trunk.
While the lateral group is also called the lateral
nucleus of anterior horn responsible for
supplying the muscles of the limbs.
The composition of anterior horn motor
neurons:
1) Theα-motor neuron
 It is large multipolar neuron. Its axon
passes through the white matter to
contribute to the respective anterior
rootlet of the spinal nerve, which
innervates the extrafusal fibers of the
skeletal muscle and produces the motion
of joints.
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2) Theγ-motor neuron
The smaller γ-motor neuron gives rise to
γ-efferent fiber which also emerges via
the anterior rootlet and supplies the
intrafusal muscle fibers of neuromuscular
spindles, which play important role in the
reflex regulation of muscle tonus.
The Renshaw cell
This recurrent inhibitory pathway from
motor axon collaterals via Renshaw cell
back to α-motor neuron is regarded as a
negative feed-back mechanism.
10. Lamina Ⅹ
 It is located around the central canal,
including anterior and posterior
commissures.
Ⅱ) White Matter
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The white matter completely surrounds
the gray matter. It is composed of
longitudinal fibrous tracts.
Anterior to the gray commissure is a
bundle of transverse fibers, called the
anterior white commissure.
In each half of the spinal cord the white
matter is divided by the gray matter into
three areas: the anterior, posterior, and
lateral funiculi.
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The anterior funiculus lies between the
anterior median fissure and the
posterolateral sulcus.
The posterior funiculus lies between the
posterolateral sulcus and posterior median
sulcus.
The lateral funiculus lies between the
anterolateral and posterolateral sulci.
Each funiculus is composed of many
tracts or fasciculi.
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The main tracts contained within the white
matter are as follows:
1. Ascending (sensory) spinal tracts
The ascending tracts carry sensory
impulses from spinal cord to the
supeasegmental structures.
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1) Fasciculus gracilis and fasciculus cuneatus
Location:
They are in the posterior funiculus. The fasciculus
gracilis is lateral to the fasciculus cuneatus.
Arision:
They are both composed of the central processes
(branches) of pseudounipolar neurons in spinal
ganglion.
The fibers arising from the lower half body (under
T5, sacral, lumbar and lower six thoracic
segments) make up the fasciculus gracilis, while
the fibers arising from the upper half body
(above T4, upper six thoracic and cervical
segments) make up the fasciculus cuneatus.
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Termination:
After reaching the medulla oblongata, the
fasciculi gracilis and cuneatus, terminate upon
the nuclei gracilis and cuneatus, respectively.
Functions:
These two tracts conduct the deep senses
(position, vibration and movement) and the
discriminating tactile (ability to recognize the size,
shape and texture of an object).
Symptom after lesion:
Lesion of posterior funiculus naturally abolishes
or diminishes discriminating tactile and
kinesthetic senses (sensory ataxia) and the
symptoms appear and vibratory sense are lost or
diminished.
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2) Spinocerebellar tract
It includes posterior and anterior spinocerebellar
tract.
①Posterior spinocerebellar tract
Location:
It is situated along the posterolateral periphery of
the spinal cord.
Journey:
It arises from the ipsilateral dorsal nucleus of
lamina Ⅶ and ascends through the spinal cord to
the medulla oblongata in which it becomes
incorporated in the inferior cerebellar peduncled.
Function: It conveys subconscious proprioceptive
impulses to the cerebellum.
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②Anterior spinocerebellar tract
Location: It is situated along the lateral periphery
of the spinal cord, anterior to the posterior
spinocerebellar tract.
Journey: Its fibers arise from the scattered cells
in the intermediate zone of lamina Ⅴ--Ⅶ, cross
the spinal cord and ascend through the spinal
cord, medulla oblongata and pons, then enter the
cerebellum by coursing along the dorsal surface
of the superior cerebellar peduncle.
Function: Its function is the same as that of the
posterior spinocerebellar tract.
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3) Spinothalamic tract
It includes lateral and anterior spinothalamic
tracts.
Location:
They are situated in the lateral funiculus and
anterior funiculus.
Arision and route and function:
The fibers of tracts arise from the nucleus
proprius (laminaeⅠand Ⅳ--Ⅶ).
Most fibers of the tract cross obliquely in the
anterior white commissure to opposite side,
ascend within the anterior and lateral funiculus,
terminate to the ventral posterolateral nucleus of
thalumus.
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Fibers ascending in the opposite
(ascending one to two segments before
crossing) lateral funiculus are called as
lateral spinothalamic tract.
The lateral spinothalamic tract transmits
the impulses of pain and thermal sense.
Fibers of the lateral spinothalamic tract
are somatotopically arranged, those
originating from the most caudal
segments of the spinal cord are situated
laterally with respect to those from more
rostral spinal segments.
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Fibers ascending in the opposite anterior
funiculus (including undecussating fibers) are
called as anterior spinothalamic tract. It
transmits the impulses associated with what is
called “light touch”.
Clinical points:
In the patient with hemisection of the spinal cord
(Brown-Sequard’s Syndrome), there is loss of
pain and thermal senses on the opposite half of
the body below 1-2 segments of the level of the
lesion, while the senses of position and
movement, two-point discrimination and
vibration are lost on the same side as the lesion.
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2. Descending (motor) spinal tracts
1) Corticospinal tract
This tract arise from the cerebral cortex
(precentral gyrus, anterior part of paracentral
lobule), descends through the internal capsule
and brain stem, and divides into two tracts:
The lateral corticospinal tract:
The most fibers (75-90%) of corticospinal tract
decussate in the medulla oblongata and descend
medial to the posterior spinocerebellar tract in
the spinal cord, forming the lateral corticospinal
tract. The tract extends to the most caudal part
of the spinal cord and progressively diminishes in
size as more and more fibers leave to terminate
in the anterior horn of the gray matter.
This tract has somatotopical arrangement from
medial to lateral in cervical, thoracic, lumbar and
sacral order.
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The anterior corticospinal tract:
It occupies a strip adjacent to the anterior
median fissure. It normally extends only to the
upper thoracic spinal segments. Most of these
fibers decussate in the anterior white commissure
before they terminate in the anterior horn.
The corticospinal tract control the voluntary
movements of skeletal muscles.
2) Tectospinal tract
3) Rubrospinal tract
4) Vestibulospinal tract
5) Reticulospinal tract
3 Short ascending and descending tracts
 The short ascending and descending tracts
are restricted in the spinal cord; they
connect the various segments of the spinal
cord and make them act as a whole.
1) Fasciculus proprius
 It surrounds the grey matter of the spinal
cord and takes part in the intrinsic reflex
mechanism of the spinal cord.
2) Dorsolateral fasciculus (of Lissauer)
 Locaciton:
 It is situated between the tip of the
posterior grey column and the surface of
the spinal cord, close to the posterior
rootlets.
 It belongs to one of the segmental
apparatus and relates to transmit
impulses of pain and thermal senses.
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Reticular formation: It appears mainly
in the upper cervical cord, lying laterally
between posterior and anterior horns. The
fibers contributing to the reticular
formation interlace each other to form
widespread network.
Ⅲ. Functions of Spinal Cord
①One of the principal functions of the
spinal cord is to convey afferent impulses
which initiates from the somatic and
visceral receptors to the brain and to
conduct efferent impulses from the brain
to the effectors.
②A second principal function is related to
the reflexes. It plays the part of the center
of the segmental reflex arc, e. g. the
stretch reflex, the reflex of bladder and
rectal emptying.
Section 2 The Brain
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The brain (encephalon) is one of the
organs of the body, weighing about
1,400g.
It is divided into four principal parts:
brain stem, consisting of the: medulla
oblongata, pons and midbrain.
cerebellum,
diencephalon,
telencephalon.
Ⅰ. The Brain Stem
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Brain stem is the stalk-like part of the brain,
which connects the cerebrum, cerebellum with
the spinal cord.
Ⅰ) External Features
1. The medulla oblongata
The shape of inferior part of the medulla
oblongata is familiar with the spinal cord. The
central canal of the spinal is prolonged upwards
into the lower half of the medulla oblongata, and
then it expands to form the lower half of the floor
of fourth ventricle, the medulla oblongata may
therefore be divided into a lower closed part
and an upper open part.
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1) Ventral surface of medulla obongata
①Anterior median fissure and anterolateral sulci
②Pyramid and decussation of pyramid:
On each side of the anterior median fissure is an
oblongated elevation, the pyramid. Near the
lower extremity of the medulla oblongata a great
number of fibers leave the pyramids in successive
bundles, and decussate with those of the
opposite side in anterior median fissure. These
interdigitating bundles of fiber are known as the
decussation of pyramid.
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③Olive
Posterolateral to the pyramid is an oval elevation,
called olive. Embed in it is inferior olivary
nucleus.
④Emerging of the cranial nerves:
Four pairs of cranial nerves emerge from the
surface of the medulla oblongata. Hypoglossal
nerve emerges in the anterolateral sulcus.
Glossopharyngeal, vagus, and accessory
nerve emerge in the posterolateral sulcus.
⑤bulbopontine sulcus: the bounary between
the medulla oblongata and pons.
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2) Dorsal surface of medulla oblongata
①Gracile tubercle and cuneate tubercle
At the lower angle of the fourth ventricle,
the fasciculus gracilis ends in an elongated
swelling, the gracile tubercle. Lateral
and adjacent to it, there is another
swelling, the cuneate tubercle, in which
the fasciculus cuneatus ends.
②Inferior cerebellar peduncle
Rostral to the gracile and cuneate
tubercles a thick rounded ridge is the
inferior cerebellar peduncle.
③Posterior median sulcus and
posterolateral sulcus
④The lower half of the rhomboid fossa
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2. The pons
1) The ventral surface of the pons
The ventral surface (basilar part of pons) of the
pons is markedly convex from side to side and
shows many broad transverse bands of nerve
fibers across the median plane.
①Basilar sulcus:
A marked shallow median groove on the ventral
surface of the pons is called the basilar sulcus
in which the basilar artery is lodged.
②Middle cerebellar peduncle:
The transverse fibers converge on each side into
a compact mass which forms the middle
cerebellar peduncle and finally enter the
corresponding hemisphere of the cerebellum.
③Cranial nerves emerging in the pons
 Four pairs of the cranial nerves make their
exits in the ventral surface of the pons.
 In the bulbopontine sulcus there emerge
the abducent, facial and
vestibulocochlear nerves ranged medial
laterally.
 The trigeminal nerves emerge from the
junction of the basilar part of pons and
middle cerebellar peduncles.
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2) The dorsal surface of the pons
Superior cerebellar peduncles
The dorsal part of the pons is opened to
form the upper half of the floor of fourth
ventricle, along the lateral borders of
which there are two prominent and rather
large strands of nerve fiber called the
superior cerebellar peduncles.
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3.The rhomboid fossa
The floor of the fourth ventricle is
rhomboidal in shape, named rhomboid
fossa.
It is formed by the posterior surface of
both the pons and the open part of
medulla oblongata.
1) The boundaries:
 Superolateral boundary: the superior
cerebellar peduncles.
 Inferolateral boundary: the inferior
cerebellar peduncle, the cuneate and
gracile tubercles from above downwards.
 The caudal angle of rhomboid fossa is
continuous with the central canal in the
closed part of medulla oblongata, the
rostral angle with the mesencephalic
aqueduct of midbrain.
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2) Morphology:
Median sulcus:
Sulcus limitans:
Mmedial eminence:
Striae medullares:
Facial colliculus:
At the middle of the medial eminence
above the striae medullares a rounded
swelling is referred to as facial colliculus.
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Hypoglossal and vagal triangle:
Below the striae medullares on each side
of the median sulcus there are two
triangular there are two triangular areas.
Of these the most medial one is called the
hypoglossal triangle; the lateral one is the
vagal triangle.
Vestibular area:
There is a triangular field to the sulcus
limitans, called the vestibular area.
Acoustic tubercle:
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4. The fourth ventricle
1) Location:
The fourth ventricle lies behind the pons and
upper half of the medulla oblongata and in front
of the cerebellum.
2) Formation:
It possesses a roof and a floor.
The floor: the rhomboid fossa
The roof :
Anterior part—superior cerebellar peduncle and
superior (or anterior) medullary velum.
Posterior part—inferior medullary velum and tela
choroidea of fourth ventricle.
In the inner surface of the inferior medullary
velum is lined with pia matter and ependyma.
Choroids plexus of fourth ventricle:
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3) Communications:
It gets into the subarachnoid space
through one median aperture of fourth
ventricle and two lateral apertures of
fourth ventricle (Luschka aperture).
Its tapering upper and lower ends become
continuous, respectively, with the
mesencephalic aqueduct and the central
canal in the lower half of the medulla
oblongata.
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5. The midbrain (mesencephalon)
1) Ventral surface of the midbrain
① Cerebral peduncles and interpeduncular fossa
The ventral surface of the midbrain is a pair of
longitudinal columns of nerve fibers, the
cerebral peduncles.
A deep depression bounded by the cerebral
peduncles is known as interpeduncular fossa.
② Posterior perforated substance
③ Cranial nerve emerging in the midbrain:
The oculomotor nerve emerges from a groove on
the medial side of each peduncle.
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2) The dorsal surface of the midbrain
Superior and inferior colliculi,
Brachium of superior and inferior
colliculi: getting to the lateral geniculate
and medial bodies.