Answers to clinically based questions

Neuroanatomy I

1. Answer B: Numbness on the face, resulting from a lesion in the cerebral cortex,

indicates a lesion in the lower one-third of the postcentral gyrus (face area of the

somatosensory cortex). The anterior paracentral gyrus and the precentral gyrus are

somatomotor areas of the cerebral cortex. The upper extremity is represented in the

middle one-third of the postcentral gyrus and the lower extremity is represented in

the posterior paracentral gyrus.

2. Answer D: Tic douloureux (trigeminal neuralgia) is agonizing pain that originates

from the branches of the trigeminal nerve, primarily its V2 or V3 territories. The

trigger zone is frequently around the corner of the mouth. There is a geniculate

neuralgia (related to the ear) and a glossopharyngeal neuralgia (related to the throat),

but neither of these originates from the surface of the face near the oral cavity. The

hypoglossal nerve is the motor for the tongue and the vagus is the motor for most of

the pharynx and larynx, visceromotor for much of the gut, and contains

viscerosensory fibers from the gut.

3. Answer A: In most cases (85–100%), the labyrinthine artery, also called the

internal auditory artery, originates from the anterior inferior cerebellar artery (a

branch from the basilar artery). It enters the internal acoustic meatus, serves bone

and dura of the canal, the nerves of the canal, and vestibular and cochlear structures.

In a few cases (15% or less), this artery originates from the basilar artery. None of

the other choices gives rise to vessels that serve the inner ear.

4. Answer E: The quadrigeminal artery is the primary blood supply to the superior

and inferior colliculi: this vessel originates from posterior cerebral artery. The

geniculate bodies receive their blood supply from the thalamogeniculate arteries, and

the pineal and habenula from the posterior medial choroidal artery. The superior

cerebellar peduncle receives its blood supply via the medial branch of the superior

cerebellar artery, and branches of the cerebral circle (of Willis) supply the

mammillary bodies.

5. Answer C: Vestibular schwannomas larger than 2.0 cm in diameter may impinge

on the root of the trigeminal nerve and cause numbness on the same side of the face.

Although the other deficits listed are not seen in these patients, diplopia

(involvement of oculomotor, abducens or trochlear nerves, singularly or in

combination) may be present, but in fewer than 10% of these individuals.

6. Answer B: The internal acoustic meatus contains the vestibulocochlear nerve, the

facial nerve, and the labyrinthine artery, a branch of the anterior inferior cerebellar

artery. A vestibular schwannoma located in the meatus would likely affect the facial

nerve and result in facial weakness. The vagus and glossopharyngeal nerves exit the

skull via the jugular foramen (along with the accessory nerve). The cerebellar

arteries originate within the skull and distribute to structures within the skull.

(The so-called acoustic neuroma, a tumor associated with the eighth nerve, is

actually a vestibular schwannoma since it arises from the neurilemma sheath

of the vestibular root. Most patients with this tumor have hearing loss, tinnitus

and equilibrium problems, or vertigo. As the tumor enlarges (to more than

about 2 cm) it may cause facial weakness (seventh nerve root), numbness

(fifth nerve root), or abnormal corneal reflex (fifth or seventh nerve roots).

Treatment is usually by surgery, radiation therapy, or a combination thereof.)

7. Answer C: The lingual gyrus is the lower bank of the calcarine sulcus; the upper

(cuneus) and lower banks (lingual) of this sulcus are the location of the primary

visual cortex. The precuneus is the medial aspect of the parietal lobe, and the

angular gyrus is a portion of the inferior parietal lobule on the lateral aspect of the

hemisphere. The cingulated and parahippocampal gyri are located on the medial

aspect of the hemisphere and are parts of the limbic lobe.

8. Answer A: The cerebral aqueduct is about 1.5–2.0 mm in diameter, and connects

the third ventricle with the fourth ventricle. When this part of the ventricular system

appears in a sagittal MRI, the plane of the scan is at the midline. Neither the

interventricular foramen nor the superior colliculus are on the midline. Both the

interpeduncular fossa and the corpus callosum are on the midline, but extend off the

midline well beyond the width of the cerebral aqueduct.

9. Answer A: The only portion of the ventricular system that does not contain

choroid plexus is the cerebral aqueduct. The choroids plexus in the lateral ventricle

is continuous from the inferior horn into the atrium and into the body of the

ventricle, and through the interventricular foramen with the choroid plexus located

along the roof of the third ventricle. There is a tuft of choroid plexus in the fourth

ventricle, a small part of which extends into the lateral recess and through the lateral

foramen (of Luschka) into the subarachnoid space at the cerebellopontine angle.

10. Answer E: Branches of the superior cerebellar artery are most frequently

involved in cases of trigeminal neuralgia that are presumably of vascular origin. The

posterior cerebral artery and its larger branches serve the midbrain-diencephalic

junction or join the medial surface of the hemisphere. The basilar artery serves the

basilar pons and the anterior inferior cerebellar artery serves the caudal midbrain,

inner ear, and the inferior surface of the cerebellar surface. The basal vein drains the

medial portions of the hemisphere and passes through the ambient cistern to join the

great cerebral vein (of Galen).

11. Answer C: The afferent limb of the corneal reflex is via the ophthalmic division

of the trigeminal nerve (V); the cell body of origin is in the trigeminal ganglion and

the central terminations in the pars caudalis of the spinal trigeminal nucleus. The

efferent limb originates in the motor nucleus of the facial nerve (VII) and distributes

to the facial muscles around the eye. None of the other choices contains fibers

related to the corneal reflex.

12. Answer B: The callosomarginal artery, a branch of the anterior cerebral artery,

supply the medial aspect of the superior frontal gyrus and that portion of this gyrus

on the superior and lateral aspects of the hemisphere. The middle cerebral artery

serve the lateral aspects of the hemisphere; the posterior cerebral artery serve the

medial aspects of the hemisphere caudal to the parietoccipital sulcus, and the angular

artery (branch of middle cerebral artery) serves the angular gyrus of the inferior

parietal lobule. The lenticulostriate arteries are branches of middle cerebral that

supply the internal structures of the hemisphere.

13. Answer B: The limbic lobe, consisting primarily of the cingulated gyrus and the

parahippocampal gyrus, is located on the most medial aspect of the hemisphere; the

cingulate gyrus is located immediately adjacent to the corpus callosum. None of the

other lobes of the cerebral cortex borders directly on the corpus callosum.

14. Answer B: The inferior frontal gyrus consists of the pars orbitalis (Brodmann

area 47), pars triangularis (area 45), and pars opercularis (area 44). A lesion located

primarily in areas 44 and 45 (in the inferior frontal gyrus) in the dominant

hemisphere will result in a nonfluent (Broca) aphasia. The supramarginal (area 40)

and angular (area 39) gyri represent what is called the Wernicke area. The middle

frontal gyrus contains areas 6 and 8. The lower lateral one-third of the precentral

gyrus is the face area of the somatomotor cortex.

15. Answer A: Areas 3, 1, 2 collectively represent the somatosensory cortex. Area 4

is the primary somatomotor cortex, area 17 the primary visual cortex, and area 22

the primary auditory cortex. Area 40 is in the supramarginal gyrus, a large part of

which is called the Wernicke area.

16. Answer B: The body is represented in the somatomotor cortex (precentral gyrus,

anterior paracentral gyrus) in the following pattern: the face in about the lower one-

third of the precentral gyrus above the lateral sulcus; the hand and upper extremity

in about its middle third; and the trunk and hip in about its upper third. The lower

extremity and foot are represented in the anterior paracentral gyrus. Posterior

portions of the middle frontal gyrus are the location of the frontal eye field.

17. Answer C: The L4-L5 interspace is commonly used for a lumbar puncture. The

L3-L4 space may also be used. Levels T12 to L2-L3 are too high. Because the

caudal end of the spinal cord (the conus medullaris) may be as low as L2 in some

individuals, levels. T12-L1 to L2-L3 are not used, as this would most likely result in

damage to the spinal cord. The S1-S2 vertebrae are fused so there is no

intervertebral space through which a needle can pass. Furthermore, the dural sac

ends at about S2.

18. Answer B: The oculomotor nerve (III) exits from the medial aspect of the

midbrain into the interpeduncular fossa/cistern. It traverses this space, courses

through the lateral wall of the cavernous sinus to eventually enter (along with the

trochlear [IV] and abducens [VI] nerves) the superior orbital fissure. Cranial nerves

IV, VI, and V1 (the ophthalmic portion of the trigeminal nerve), along with III, pass

through the cavernous sinus. Cranial nerve II (optic) is quite rostral to the

interpeduncular fossa.

19. Answer D: A lesion in area 44 that spreads will affect the lower portions of the

precentral gyrus in which the face is represented. This will result in weakness of

facial muscles, accompanied by other cranial nerve deficits. The cortical areas for

hearing and vision are far separated from area 44. Also, a lesion in the primary

auditory cortex will not result in a hearing loss in one ear. The hand area of the

sensory cortex and the upper extremity area of the motor cortex are not adjacent to

Brodmann area 44.

20. Answer A: As they descend in the dural sac from their origin from the spinal

cord to their exit at their respective intervertebral foramen, the anterior and posterior

roots form the cauda equina. The conus medullaris is the most caudal end of the

spinal cord, and the filum terminale internum is the strand of pia that extends from

the conus caudally to attach to the inner aspect of the dural sac at about S2. The

denticulate ligament anchors the spinal cord laterally to the inner surface of the dural

sac, and the filum terminale externum anchors the dural sac caudally to the inner

aspect of the coccyx.