📚 Question Bank

The oculomotor nerve is not related to the floor of the third ventricle.

The structures related to the floor of the third ventricle (from anterior to posterior) include:

• Optic chiasma
• Infundibulum
• Mamillary bodies
• Posterior perforating substance
• Tegmentum of the midbrain.

The VII cranial nerve nucleus is not present on the floor of the fourth ventricle.

The floor of the fourth ventricle is diamond-shaped and lies on the dorsal surface of the pons and superior half of the medulla.

It is divided into two symmetrical halves by the longitudinal median sulcus. Each half is further divided by sulcus limitans into a medial region, medial eminence, and a lateral region, vestibular area. The vestibular nuclei lie beneath the vestibular area.

At the superior end of the sulcus limitans, a bluish-gray area is produced by a cluster of nerve cells containing melanin pigment called substantia ferruginea.

At the inferior end of the medial eminence, the fibers from the motor nucleus of the facial nerve looping over the abducens nucleus are represented by the facial colliculus.

Inferiorly, the medial eminence is represented by the hypoglossal triangle overlying the hypoglossal nucleus and the vagal triangle overlying the dorsal nucleus of the vagus.

The facial nerve fibers, which cover the abducent nucleus, lie deep to the facial colliculus.

Facial colliculus is a small bilateral bulge in the floor of the 4th ventricle. Beneath this, the fibers from the motor nucleus of the facial nerve loop around the abducens nucleus. This is due to a phenomenon called neurobiotaxis - a shift of nerve cells during the development over the course of evolution.

Substantia ferruginea is present in the gray matter.

It is a cluster of nerve cells containing melanin pigment located at the superior end of sulcus limitans in the floor of the 4th ventricle.

The image is of the floor of the 4th ventricle, and the area marked 'a' is the hypoglossal triangle.

The floor of the 4th ventricle is diamond-shaped and lies on the dorsal surface of the pons and superior half of the medulla. It is divided into two symmetrical halves by the longitudinal median sulcus. Each half is further divided by sulcus limitans into a medial region, medial eminence, and a lateral region, vestibular area.

The vestibular nuclei lie beneath the vestibular area.

At the superior end of the sulcus limitans, a bluish-gray area is produced by a cluster of nerve cells containing melanin pigment called substantia ferruginea/locus coeruleus.

At the inferior end of the medial eminence, the fibers from the motor nucleus of the facial nerve looping over the abducens nucleus are represented by the facial colliculus.

Inferiorly, the medial eminence is represented by the hypoglossal triangle overlying the hypoglossal nucleus and the vagal triangle overlying the dorsal nucleus of the vagus.

The area marked "A" is the quadrigeminal cistern, and it contains the great cerebral vein of Galen.

Aneurysmal malformation of the great cerebral vein of Galen is a congenital anomaly occurring due to the persistence of the embryonic prosencephalic vein of Markowski.

The image below is a CT cerebral angiogram of a vein of Galen malformation and an abnormal falcine sinus.

The largest subarachnoid cistern is the cerebellomedullary cistern.

The clinical scenario described is of Blake's pouch cyst, caused due to an imperforate foramen of Magendie.

The roof of the fourth ventricle communicates posteriorly with the cisterna magna through a median aperture called the foramen of Magendie.

During fetal development, the roof of the fourth ventricle is formed by the anterior and posterior membranous areas (AMA, PMA).

Blake's pouch is a temporary finger-like growth of the PMA which communicates with the fourth ventricle. During the 9th week of the development, Blake's pouch perforates to form the foramen of Magendie.

Communications of the ventricular system:

• Foramen of Monro - Lateral ventricle to the third ventricle
• Cerebral aqueduct of Sylvius - III ventricle to IV ventricle
• IV ventricle to subarachnoid space:
• Foramen of Magendie (midline) - Cisterna magna
• Foramina of Luschka (laterally) - Subarachnoid space

The outflow of CSF from the IV ventricle is via the medial outlet. This is covered by a membrane in the fetus which perforates later. The persistence of this membrane causes cystic obstruction.

In Dandy-Walker malformation, there is the presence of many posterior fossa anomalies such as:

• Cystic dilation of the fourth ventricle
• Hydrocephalus
• Hypoplasia of the cerebellar vermis
• Enlarged posterior fossa.

Arachnoid villi act like valves. If venous pressure exceeds CSF pressure, arachnoid villi close the tubules to prevent the entry of blood into the subarachnoid space.

Arachnoid villi are diverticulums of the subarachnoid space that pierces the dura mater. Grouped arachnoid villi produce elevations called arachnoid granulations.

When the CSF pressure is higher than the venous pressure of the sinus, CSF flows from the subarachnoid space into the venous sinuses via the arachnoid villi.

If the venous pressure exceeds CSF pressure, the arachnoid villi close and prevent the reflux of blood into the arachnoid space. Some CSF is also directly absorbed into the veins in the subarachnoid space.

The CSF production from choroid plexuses is usually constant. Therefore, the rate of absorption through the arachnoid villi controls the CSF pressure.

The image below shows the pulsatile CSF movement.

Pacchionian bodies (arachnoid granulations) are focal pouches of arachnoid mater and subarachnoid space protruding through the walls of dural venous sinuses.

Option B: Tuft of capillaries enclosed by pia mater lined by ependymal cells forms tela choroidea. It secretes CSF.

Option C: Calcareous deposits in the astrocytes of the pineal gland are known as brain sand or corpora arenacea.

Option D: Shrunken, yellow-brown, cystic lesions in the gyri and subarachnoid space associated with head injury are called plaque jaune.

The collateral sulcus is a complete sulcus. A complete sulcus is one that is deep enough to indent the wall of a ventricle.

The posterior part of the calcarine sulcus is an axial sulcus and the anterior part is a complete sulcus.

Therefore, among the options provided, the collateral sulcus is the best answer.

The anterior part of the calcarine sulcus is considered a complete sulcus because its depth creates an elevation in the medial wall of the occipital horn of the lateral ventricle. This is called the calcar avis.

However, the posterior part of the calcarine sulcus is considered an axial sulcus as its axis is along the visual cortex.

Heschl's gyrus is located in the primary auditory cortex (area 41, 42).

Area marked D is the frontal eye field (8) and is located in the posterior part of the middle frontal gyrus. It controls the conjugate movements of the eyes. It is connected to the visual part of

the occipital cortex.

Lesions here cause loss of horizontal conjugate movements.

Brodman Area Wernicke'sarea (22) (A)

Broca'sarea (44,45)

Visual associationarea (1

8,19) (B)

Primary visual cortex(17)

Sensory associationarea (

5,7) (C)

Location

Present with auditory ass ociation areas in the supe rior temporal gyrus

Pars triangularis and par s opercularis in the inferi or frontal gyrus.

Present surrounding the striate area

Around the post-calcarin e sulcus

Between the sensory and visual areas in the superi or parietal lobule

Function Sensorylanguage area

Motorspeech area

Correlation of visual imp ulses with memory, recog nition of objects seen, an d depth perception

Reception and perceptio n of isolated visual impre ssions of color, size form, motion, illumination, an d transparency

Stereognosis and sensory speech

Lesion produces Sensory aphasia

Motor aphasia.

Visual agnosia.

Homonymous hemianopi a with macular sparing

Astereognosis and sensor y aphasia

The structure marked by the arrow is the insular lobe. The insular cortex is not entirely agranular. The posterior part of the insular cortex is granular and the anterior part is agranular.

The insula forms the base of the Sylvian cistern and lies between the frontal and temporal lobes. It receives somatosensory, viscerosensory, homeostatic, and nociceptive information from the entire body. It receives inputs from the thalamus.

The marked structure is the internal capsule, which is a form of projection fiber.

The area marked 'A' in the image is rostrum of the corpus callosum. It connects the orbital surfaces of the frontal lobes.

The cingulum of the brain is classified under long association fibers.

The cingulum lies within the cingulate gyrus, connecting the frontal and parietal lobes with the parahippocampal and temporal cortical regions

Corona radiata is not classified under association fibers. It an example of projection fiber.

Option A: Cingulum is a long association fiber connecting the frontal and parietal lobes with the parahippocampal and temporal cortical region.

Option C: Fornix is an example of association fiber but only Hippocampal commissure of fornix is a commisural fibre as it has transverse fibers connecting the two crura of the fornix.

Option D: Arcuate fasciculus is a short association fiber.

The given clinical scenario is suggestive of tumour-associated psychosis. The thalamus lies medial to the posterior limb of the internal capsule and is most likely to be affected in this patient.

The internal capsule separates the thalamus and the caudate nucleus from the lentiform nucleus (globus pallidus + putamen). It is V-shaped in horizontal section and is divided into the anterior limb, genu, posterior limb, retrolenticular part, and sublenticular part. The anterior limb is located between the head of the caudate nucleus medially and the lentiform nucleus laterally. The posterior limb is located between the thalamus medially and the lentiform nucleus laterally.

The corticonuclear fibres pass through the genu of the internal capsule and hence, is most likely to be affected in this patient.

The majority of corticonuclear fibers cross in the median plane before supplying the cranial nerve nuclei. All cranial motor nuclei receive bilateral corticonuclear connections except, facial nucleus supplying the lower part of the face and the hypoglossal nucleus supplying the genioglossus

muscle. Hence, a lesion of these fibers causes contralateral clinical manifestations.

Option A: Anterior limb of the internal capsule carries fronto-pontine fibers. A lesion here produces confusion, impaired attention, agitation, and dysarthria.

Option C: Posterior limb of the internal capsule carries cortico-spinal motor fibers in the anterior part and a lesion here produces contralateral hemiparesis. The posterior part of the posterior limb of the internal capsule carries sensory fibers and lesions here may produce contralateral

hemi-sensory deficits.

The correct answer is A, B, and C only.

The structure marked as 'D' is the body of the fornix and it mainly contains association fibres. The commissure of the fornix (hippocampal commissure) contains the commissural fibers.

A- Corpus callosum

B- Anterior commissure

C- Posterior commissure

D- Body of the Fornix

The locations of the commissural fibers are:

• Corpus callosum- Medial surface of the brain
• Anterior commissure- Beneath the rostrum of the corpus callosum
• Posterior commissure- In the inferior lamina of the pineal gland
• Habenular commissure- In the superior lamina of the pineal gland
• Hippocampal commissure- Between the right fornix and the left fornix

(Note: Varied information regarding the fibers present in the Fornix. 41st edition Gray's Clinical anatomy mentions Fornix under association fibers, the commissure of the fornix under commissural fibers. Whereas, Snell's Neuroanatomy textbook mentions the Fornix under commissural fibers.)

The habenular commissure is located in the superior lamina of the pineal gland. The locations of the commissural fibers are:

• Corpus callosum- Medial surface of the brain
• Anterior commissure- Beneath the rostrum of the corpus callosum
• Posterior commissure- In the inferior lamina of the pineal gland
• Habenular commissure- In the superior lamina of the pineal gland
• Hippocampal commissure- Between the right fornix and the left fornix

The arrow indicates the fornix. It arises from the hippocampus and ends in the mamillary body of the hypothalamus.

Fornix is an example of association fiber. It is supplied by the anterior cerebral artery.

A is anterior commissure, which is a type of commissural fiber, not association fiber.

B is lamina terminalis

C is the mamillary body and is a content of interpeduncular fossa.

D is the pineal gland, which does not have blood-brain barrier.