THE BRAIN ANATOMY in animals veterinary.pdf

THE BRAIN ANATOMY
MAGEJA.T@VETHACKING.LECTURE NOTES.CO.BVSc.UZ

The brain
oThe brain is composed of the embryologically segmented brain stem
and two suprasegmental portions, the cerebrum (telencephalon) and
the cerebellum (dorsal metencephalon) and the
diencephalon(interbrain epithalamus,thalamus and hypothalamus)
in between the brain stem and the cerebrum.
oThe diencephalon and telencephalon are collectively known as
forebrain.
oThe brain stem includes the myelencephalon (medulla), the ventral
metencephalon (pons) and the mesencephalon (midbrain).

The brain stem
o The brain stem is the brain anatomical structurethat links different parts of the nervous system –the
forebrain(diencephalonand telencephalon),thecerebellumand spinal cord.
o The brainstem plays a vital role in many essential functions that its nuclei regulate such as
breathingand heart rate as well as consciousness.
o It is also a passage way to sensory and motor pathways and the pain controlcentre.
o It is provides the emergency to most cranial nerves(CN)
o There are three parts of the brain stem-the myelencephalon(medulla), the ventral metencephalon
(pons) and the mesencephalon(midbrain).

o The brainstem
plays a vital
role in many
essential
functions that
its nuclei
regulatesuch
as breathing
and heart rate
as well as
consciousness.
BRAINSTEM

The brain stem
o The mid brain or mesencephalonis the area of the brain that is directly connected to the forebrain
through a pair of cerebral peduncles.
o Behind these peduncleswe find the tegmentum with a hole on its back side –the cerebral
aqueduct also known as the aqueductof sylvius.
o It connectsthe third to fourth ventricle .
o On the back of the midbrain is the tectum with four colliculi-where lies the reflex centers involved
in hearing and vision.
o The pons or pons varolli is the middle part of brain stem-plays an important role in motor functions
with its relay position between forebrain and cerebellum.
o It also contributesto autonomousfunctions and facial sensitivity-it is the emergency of the
trigeminal nerve(CN V)
o The pons is connectedto the cerebellum(dorsal metencephalon)by the middle cerebellarpeduncle.
o It houses the front face of the forth ventricle.

o The mid brain or
mesencephalonis
the area of the
brain that is
directlyconnected
to the forebrain
through a pair of
cerebral
peduncles(shown
in orange)
Aqueduct of sylvius.

Behind these peduncles we find the
tegmentum with a hole on its back
side –the cerebral aqueduct also
known as the aqueduct of sylvius.
AQUEDUCT OF SILVIUS.
CEREBRAL PEDUNCLES
TEGMENTUM

tectum
THE FOUR COLLICULI IN BLUE

It also contributes to
autonomous functions
and facial sensitivity-it is
the emergency of the
trigeminal nerve(CN V)

The pons is
connected to the
cerebellum(dorsal
metencephalon)
by the middle
cerebellar
peduncle.
the middle cerebellar peduncle.

It houses the
front face of the
forth ventricle.

Part that houses the fourth
ventricle

The brain stem
o The medulla is the part of brainstem which links the pons and the spinal cord
o Medulla contains the olivary and a pair of pyramids that contain corticospinal
fibres of the pyramidal tracts (ref to physiology)
o Contains vital autonomic control centers for functions such as breathing,heartrate
and many reflex functions –e.g vomiting,coughing,sneezing and swallowing.
o Its ends at the bottom contain pyramidal decussation (the crossing region of
corticospinal fibres).

pyramids

Brain stem dissection in a dog
o Gently separate the two cerebral hemispheres at the longitudinalfissure.
o Expose the band of fibers that course transversely from one hemisphere to the otherin the depth of
the fissure.
o This structureis the corpus callosum.
o Completelydivide the corpus callosumlongitudinallyalong the median plane in the depth of the
longitudinalfissure.
o Cut deep enough to includethe hippocampalcommissure and body of the fornix, but do not cut
into the thalamus.
o Continuethecut rostrallyand ventrally through the rostral commissure just dorsal to the optic
chiasm and rostral to the thalamus.
o On the ventral surface, follow the optic tract in a dorsocaudal direction from the optic chiasm and
cut the fibers of the internal capsule rostral and medial to this tract.

Brain stem dissection in a dog
o This is an obliquecut. The fibers of the internal capsule attach the cerebral hemisphere to the brain
stem.
o Gently lift the medial side of the cerebrumoff the thalamus and continuethis separation over the
dorsal aspect of the diencephalon.
o Cut any remaining attachments, and remove the cerebral hemisphere from the diencephalon.
o Examine and recognize the structuresyou have cut to remove the cerebral hemisphere.
o The internal capsuleis the only structureconnectingthe cerebral hemisphere with the brain stem.
o It consists of neuronal processes projectingfrom the cerebral hemisphere to the brain stem and
neuronalprocesses projecting from the thalamus to the cerebral hemisphere.
o This will be dissected further later.
o Examine the surface of the brain stem and locatethe following structures.

The cerebrum
o The cerebrum is divided into two cerebral hemispheres by the longitudinal fissure.
o Each cerebral hemisphere has outward folds (convolutions) called gyri and inward folds called sulci.
o rhinal sulcus(rostral and caudal parts)
o pseudosylvian fissure
o rostral and caudal sylvian gyri,
o ectosylvian sulcus and gyrus,
o suprasylvian sulcus and gyrus,
o cruciate sulcus,
o postcruciate and precruciate gyri,
o coronal sulcus,
o ectomarginal sulcus and gyrus,
o marginal sulcus and gyrus.

The cerebrum
o Each cerebral hemisphere may be divided into lobes named for that portion of the calvaria that
covers them.
o The relationship is not precise and varies among species.
o The frontal lobeis that portion of each cerebral hemisphere rostral to the cruciate sulcus.
o The precruciategyrus is part of this lobeand functionsas part of the motor cortex.
o The parietal lobe is caudal to the cruciate sulcus and dorsal to the sylvian gyri.
o It extends caudallyto approximatelythe caudal third of the cerebral hemisphere.
o The postcruciateand rostral suprasylvian gyri are found in this lobeand function as part of the
motor and somesthetic sensory cerebral cortex.

The cerebrum
o The occipital lobe includesthe caudal third of the cerebral hemisphere.
o Caudal portionsof this lobeon both medial and lateral sides function as the visual cortex.
o The temporal lobe is composed of the gyri and sulci on the ventrolateralaspect of the cerebral
hemisphere.
o Parts of the sylvian gyri are located here and function as the auditory cortex.
o The rhinal sulcus separates the phylogenetically new cerebrum or neopallium, above, from the
olderolfactory cerebrum, the paleopallium, below.
o Portionsof the paleopalliumthat are visible are the olfactory bulb, which rests on the cribriform
plate, and the olfactory peduncle, which joins the bulb to the cerebral hemisphere.
o The olfactory pedunclecourses caudallywith a band of fibers on its ventral surface.
o Caudally, this band divides into lateral and medial olfactory tracts the lateral olfactory tract passing
caudallyto the piriform lobe, which forms a ventral bulge just lateral to the pituitary gland and
medial to the temporal lobeof the neopallium.

The cerebrum
o The medial olfactory tract cannot be observed.
o Each gyrus containsgray matter superficially and white matter in its center.
o The gray matter, or cerebral cortex of the neopallium, is composed of six layers of neuronalcell
bodies.
o The white matter, coronaradiata, containsthe processes of neuronscoursing to and from the
overlying cortex.

The cerebellum
o The cerebellum is derived from the dorsal portion of the metencephalon and lies caudal to the cerebrum
and dorsal to the fourth ventricle.
o The transverse cerebral fissure separates it from the cerebrum.
o The dural and osseous tentorium cerebelli is located in this fissure.
o The cerebellum is connected to the brain stem by three cerebellar peduncles on each side of the fourth
ventricle and by portions of the roof of the fourth ventricle.
o The choroid plexus is a compact mass of pia, blood vessels, and ependyma.
o A choroid plexus develops where neural tube neuroepithelium did not proliferate to form parenchyma
but remained as a single layer of the neuroepithelial cells, a roof plate.
o These areas are found in the medulla (roof plate of the fourth ventricle), the diencephalon (roof plate of
the third ventricle), and telencephalon (roof plate of the lateral ventricle).
o At these sites the vessels in the pia covering the single layer of neuroepithelial cells proliferate to form a
dense plexus of capillaries intimately related to the neuroepithelial cells.

The cerebellum
o These cells and bloodvessels are involved in passive and active secretion of cerebrospinal fluid
into the ventricular system.
o The choroid plexus of the fourth ventricle protrudesinto the lumen of the fourth ventricle and is
visible caudolateralto the cerebellumon the dorsal surface of the medulla.
o Identify the transverse fibers of the pons on the ventral surface of the brain stem.
o Followthese fibers laterallyas they course dorsocaudallyinto the cerebellumon each side as the
middle cerebellarpeduncle.
o At the point where they merge into the cerebellum, cut this pedunclewith a scalpel.
o Continuethecut slightly rostral to cut the rostral cerebellarpeduncle.
o Gently lift the caudal part of the cerebellumfrom the medulla.
o Remove the choroid plexus so you can see the caudal cerebellarpeduncleconnectingthe medulla
and cerebellum.

The cerebellum
o Cut this peduncleand detach the cerebellumfrom the pons on that side.
o Cut these peduncleson the oppositeside and remove the cerebellum.
o The rostral cerebellarpedunclecontainsmainly efferent axons from the cerebellumto the brain
stem.
o Afferent axons to the cerebellumfrom the brain stem and spinal cord pass primarily through the
middle and caudal cerebellarpeduncles.
o The cerebellumis composed of lateral cerebellarhemispheres and a middle portion, thevermis.
o The convolutionsof the cerebellumare known as folia.
o These are grouped into three lobes and numerous cerebellarlobulesthat have specific names.
o The vermis comprises the entire middle portion of the cerebellumdirectly above the fourth
ventricle.

The cerebellum
o Some of its lobules are found on the ventral surface of the cerebellum facing the roof plate of the fourth
ventricle.
o Each hemisphere projects over the cerebellar peduncles and the adjacent brain stem.
o A lateral component lies in the cerebellar fossa of the petrosal part of the temporal bone.
o Make a median incision through the vermis, hemisectioning the cerebellum.
o Examine the cut surface.
o Note the pattern of white matter as it branches and arborizes from the medulla of the cerebellum into
the folia .
o The medulla of the cerebellum is the white matter in its central portion that contains the cerebellar nuclei
and connects with all the folia and the cerebellar peduncles.
o Observe the laminae of foliate white matter and the cerebellar cortex.
o Make a transverse section of one half of the cerebellum through its medulla to observe the lateral extent
of the medullary white matter and its nuclei.

Diencephalon
o The diencephalonconsists of a large, centrallylocated thalamus; a smaller hypothalamusbelow;
and a very small epithalamuson the dorsal midline.
o It is the most rostral part of the brain.
o The optic or second cranial nerves form the optic chiasm of the diencephalonrostral to the
hypophysis or pituitarygland .
o The optic tracts course laterallyand dorsocaudallyfrom the chiasm, pass over the lateral surface of
o the diencephalon, and enterthe lateral geniculate nucleusof the thalamus.
o In this pathway, each tract curves around the caudal edge of the internal capsule
o Caudal to the opticchiasm on the median planeis the hypophysis (pituitarygland), which is
attached by the infundibulumto the tubercinereum of the hypothalamus.
o If the gland is missing, the lumen of the infundibulumwill be evident.
o This lumen communicates with the overlying third ventricle of the diencephalon.

Diencephalon
o The mamillary bodies of the hypothalamusbulge ventrally caudal to the tubercinereum.
o They demarcate the most caudal extent of the hypothalamuson the ventral surface of the
diencephalon.
o The internal capsuleboundsthe diencephalonlaterallyand was cut when the left cerebral
hemisphere was removed.
o The thalamus and epithalamuscan be seen on the dorsal aspect of the diencephalon.
o Three structurescompose the epithalamus.
o They all are located adjacent to the median plane.
o The stria habenularis lies on either side of the midline, coursing dorsallyand caudallyfrom the
rostroventral aspect of the hypothalamusover the thalamusto the dorsocaudal aspect of the
diencephalon.
o Here the stria enters the habenularnucleus.
o The habenula nuclei on each side are connectedby a small commissure.

Diencephalon
o Caudal to the habenularnucleusis the small, unpaired pineal body.
o This caudal projection from the diencephalonis small in the dog but very prominent in larger
domestic animals.
o A space is located between the stria habenularis of each side.
o This is the dorsal part of the third ventricle.
o It is covered by a thin remnant of the roof plateof the neural tube, a layer of ependyma that extends
from one stria habenularisto the other.
o Branches of the caudal cerebral artery course over the diencephalonand form the choroid plexus of
the third ventricle.
o This is usually pulledout when the calvaria is removed to expose the brain.
o Rostrally, the choroid plexus of the third ventricle is continuouswith the choroid plexus of the
lateral ventricle at the interventricularforamen.

Diencephalon
o This foramen is caudal to the column of the fornix at the level of the rostral commissure.
o These structureswill be seen in the dissection of the telencephalon.
o The thalamus lies between the internal capsuleon each side and dorsal to the hypothalamus.
o It is covered by pia, arachnoid trabeculations, and the subarachnoidspace.
o It consists of a multitudeof nuclei, two of which are readily recognized on the caudal surface.
o A lateral eminence on the caudodorsal surface of the thalamusis the lateral geniculate nucleus,
which receives fibers of the optic tract and functionsin the visual system.
o The lateral geniculate nucleusis connectedwith the rostral colliculusof the midbrain.
o Caudoventralto the lateral geniculate nucleusis the medial geniculate nucleusof the thalamus.
o This nucleusfunctions in the auditorysystem and is connectedto the caudal colliculusof the
midbrain by the brachiumof the caudal colliculus.

Diencephalon
o In the third ventricle, between the stria habenularisof each side, observe the interthalamic adhesion
between the right and left sides of the thalamus.
o This area appears round on median section because the third ventricle encircles it
o In transverse section the narrow, vertically oriented third ventricle appears as a perpendicularslit
below the interthalamic adhesion
o Its lateral and ventral walls are formed by the hypothalamus.
o The dorsal portion of the third ventricle is small and tubular.
o It passes over the interthalamicadhesion, but its thin roof plate,whichis attached on each side to
the stria habenularis, cannot be observed grossly.

Diencephalon

Mesencephalon
o The mesencephalon is relatively short.
o It is nearly round on transverse section with a canal, the mesencephalic aqueduct , passing through it.
The mesencephalon consists of a tectum or roof dorsal to the aqueduct, which is composed of four
groups of neuronal cell bodies:the colliculi.
o That portion of the mesencephalon ventral to the aqueduct is the cerebral peduncle,which consists of a
tegmentum (reticular formation), substantia nigra, and crus cerebri, from dorsal to ventral.
o Between the mamillary bodies of the hypothalamus and the transverse fibers of the pons is the ventral
surface of the mesencephalon (midbrain).
o The transverse fibers of the pons cover part of the caudal mesencephalon ventrally.
o Descending tracts of projection processes that connect portions of the cerebral cortex with brain stem
centers and the spinal cord course on the ventral surface of the midbrain.
o These are grouped together on each side as the crus cerebri.
o The oculomotor,or third cranial, nerve leaves the midbrain medial to the crus.

Mesencephalon
o The mesencephalicstructuresdorsal to the mesencephalicaqueductcompose the tectumof the
midbrain.
o The mesencephalicaqueduct is a short, narrow tube, derived from the neural canal in the midbrain,
that connectsthe third ventricle rostrallywith the fourth ventricle caudally.
o Four dorsal bulges, the corporaquadrigemina, are evident on the dorsal side.
o The rostral pair are the rostral colliculi, which function with the visual system.
o The smaller caudal pair are the caudal colliculi, which function in the auditorysystem.
o The trochlear,or fourth cranial, nerve courses laterallyout of the roof of the fourth ventricle, the
rostral medullary velum, adjacent to the caudal colliculus.
o It continuesrostroventrallyon the lateral surface of the midbrain.
o The lateral lemniscus is a band of auditory system axons on the lateral side of the midbrain.

Mesencephalon
o It courses rostrodorsallyfrom the level of the cochlearnucleusto the caudal colliculusand emerges
medial to the middle cerebellarpeduncle.
o Many of these fibers arise from the cochlearnucleus.
o The brachiumof the caudal colliculusruns rostroventrallyfrom the caudal colliculusto the medial
geniculate nucleusof the thalamus.
o On the dorsal surface the commissure of the caudal colliculican be seen crossing between these
two structures.
o The rostral colliculusis connectedto the lateral geniculate nucleusof the thalamus by a short
brachiumof the rostral colliculus

Ventral Metencephalon
o The metencephalic portion of the rhombencephalon includes a segment of the brain stem, the pons, and
the dorsal development, the cerebellum.
o The ventral surface of the pons includes the transverse fibers of the pons, which course laterally into the
middle cerebellar peduncles.
o This large band of transverse fibers borders the trapezoid body of the medulla caudally.
o Its rostral border covers part of the ventral surface of the midbrain.
o The trigeminal nerve is associated with the pons and can be found entering the pons along the
caudolateral aspect of the transverse fibers.
o The descending fibers of the crus cerebri enter the pons dorsal to the transverse fibers, where they are
called the longitudinal fibers of the pons.
o These longitudinal fibers are covered ventrally by the transverse fibers.
o The longitudinal fibers that do not terminate in pontine nuclei continue caudally on the ventral surface
of the trapezoid body of the medulla as the pyramids.

o Many of the axons in the crus cerebri and the longitudinalfibers of the pons, and most of those in
the transverse fibers of the pons, make up a large cerebropontocerebellar pathway.
o Synapse occurs in the pontinenucleithat are covered by the transverse fibers, and crossing occurs
through the transverse fibers to enterthe cerebellumthrough the middle cerebellarpeduncle.
o Therefore impulses that arise in the left cerebrumare projected to the right cerebellarhemisphere.
o The rostral medullaryvelum forms the roof of the fourth ventricle between the caudal colliculiof
the mesencephalonrostrallyand the midventral surface of the cerebellumcaudally.
o The crossing fibers of the trochlearnerves course through this velum.
o The velum in the preserved specimen lies on the floor of the fourth ventricle and covers the caudal
openingof the mesencephalic aqueduct.
o Insert a probeinto the slitlike fourth ventricle under the caudal cut edge of this velum and raise the
velum to demonstrateits attachmentsand the continuityof the fourth ventricle with the aqueduct.

o This velum lies between the fourth ventricle, which it covers, and the rostral cerebellarvermis.
o Between the velum and the vermis is subarachnoid space.

Myelencephalon (Medulla)
o The myelencephalon, or medulla, extends from the transverse fibers of the pons to the level of the
ventral rootlets of the first cervical spinal nerve.
o The trapezoid body is the transverse band of fibers rostrally that course parallel but caudal to the
transverse pontine fibers.
o It is continuous with the vestibulocochlear nerve and cochlear nuclei laterally on the side of the medulla
and functions in the auditory system.
o The pyramids are a pair of longitudinally coursing fiber bundles on either side of the ventral median
plane.
o They emerge from the transverse fibers as the caudal continuations of axons from the longitudinal fibers
of the pons that did not terminate in pontine nuclei.
o They course caudally across the trapezoid body to continue on the ventral surface of the medulla.
o They are separated by the ventral median fissure.
o This fissure can be followed caudally until it is obliterated over a short distance by the decussation of
the pyramids located at the level of the emerging hypoglossal nerve fibers.

Myelencephalon (Medulla)
o The decussation itself is difficult to see because it occurs as the pyramidal fibers are passing
dorsallyinto the parenchyma of the medulla.
o Pyramidal axons continuein the spinal cord as the corticospinaltracts.
o These are cerebral projection processes that project to the spinal cord.

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