Forty Four

Question 1

Explain the brains dependence on blood flow and oxygen. Explain how blood is distributed with the CNS.

Answer 1

The neurons of the central nervous system, unlike the primary cells of most organsystems, are very dependent on aerobic metabolism. When deprived of blood flow for
only 20 seconds, the brain is reduced to a state of unconsciousness; if circulation is not reestablished in 4 to 5 minutes, this state is usually irreversible. The brain itself makes up approximately 2% of the body weight (1500 gm) but uses 15% of the total cardiac output (5L/min) and consumes 20% (50 mL/min) of the total available O2. This enormous blood

flow and O2 consumption demands an extensive yet smoothly functioning delivery

system, the cerebrovascular system.

Different areas of the cerebrum and spinal cord receive different amounts of blood

depending on metabolic activity. Under most circumstances, the more metabolically

active gray matter has a greater flow than the white matter (75 mL versus 25 mL/100

g/min). In addition, certain neurons in the CNS (i.e., selected layers of the hippocampus

and the cerebellar and cerebral cortices) display a selective vulnerability to O2 loss such

that they are affected first in states of acute hypoxia.

Question 2

Explain the autoregulation of cerebral blood flow. What effects do O2 and CO2 have on it? How can these effects be used clinically?

Answer 2

The cerebrovasculature autoregulates to maintain a constant amount of blood flow

to the neuraxis despite fluctuations in systemic blood pressure. The larger extracerebral

vessels possess a readily identifiable adventitial plexus of nerves, but autoregulation

persists even after their complete removal, because (unlike the peripheral vascular

system) the sympathetic and parasympathetic influences on cerebrovascular tone are

quite limited.

Cerebral autoregulation is closely related to local metabolic processes and many

metabolites affect cerebral blood flow. The most important metabolites that affect

cerebral blood flow are the local concentrations of O2 and CO2. Hypoxia or hypercarbia

or both result in cerebral vasodilation and increased cerebral blood flow, whereas

hypocarbia results in vasoconstriction and diminished blood flow.

Question 3

How do intracranial arteries differ from extracranial arteries? Intracranial veins? Explain what happens to the different arterial layers at large intracranial artery branches. What is the clinical result?

Answer 3

Intracranial arteries differ considerably in histologic composition from those

found elsewhere in the body. The intima of intracranial vessels possesses a well-
developed internal elastic membrane (IEM), which is actually thicker than that found in

extracranial vessels. The media (composed of muscle and elastica), however, is much

less prominent than that of extracranial arteries. The adventitia is thin and contains no

paravascular supporting tissue, no external elastic lamina, and no vasovasorum.

Histologically, intracranial veins are thin-walled structures consisting mostly of collagen

with minimal elastic tissue, little muscle, and no valves.

In primates, small discontinuities of the media occur at the points where

larger intracranial arteries branch. In these areas the adventitia actually

abuts the IEM. Clinically, these so-called “media gaps” relate to the

location of saccular aneurysms formed as the IEM is damaged by

progressive atherosclerosis. With the congenital absence of the media

and with developmental damage to the IEM, the vessel wall is supported

by only the endothelium and adventitia. This weak support progressively

balloons to form an aneurysm.

Question 4

Where are intracranial extracerebral vessels located? What is the Virchow-Robin space? What is its clinical significance?

Answer 4

The intracranial extracerebral vessels are contained within the subarachnoid

space. As these vessels and their branches penetrate the brain, they become intracerebral.

A small perivascular extension of the subarachnoid space is formed alongside these

penetrating vessels. This Virchow-Robin space extends from the general subarachnoid

space and gradually thins as the vessel penetrates deep into the brain substance.

Disease processes in the subarachnoid space such as subarachnoid

hemorrhage and meningitis may gain entrance into the brain tissue itself as

they fill the perivascular spaces surrounding the penetrating vessels.

Question 5

What is the blood brain barrier? What is its anatomical composition? What does the passage of substances across it depend on? How is intracerebral volume controlled and by what? What happens if this structure is damaged?

Answer 5

The concept of a selective barrier between the intravascular space and the brain is

suggested by the results of dyes (such as trypan blue) being introduced into the blood

stream—most of the body tissues including the meninges are stained, but not the brain.

The blood-brain barrier selectively prevents the penetration of certain substances into the

cerebral space. The anatomical composition of the blood-brain barrier consists of the

capillary endothelium, the astrocytic foot processes, and a shared basement membrane.

The selective permeability of the blood-brain barrier probably rests along the capillary

endothelium. The tight junctions and nonfenestrated composition of the capillary

endothelium impede the passage of many substances.

Physiologically, the passage of substances across the blood-brain barrier depends

on the molecular size, lipid miscibility, and degree of ionic dissociation. Many drugs that

are useful in the treatment of systemic disorders are ineffective in identical CNS

disorders due to their inability to cross the blood-brain barrier. The astrocytic foot

processes control the intracerebral volume by regulating the quantity of substances such

as sodium, water, glucose, etc. that enter this space. Disruptions of the astrocytic foot

processes generally result in leakage of fluid into the brain with the development of

cerebral edema. This condition occurs commonly with trauma and tumors.

Question 6

Describe the pathway of the left and right common carotid arteries from their beginning to their entrance in the cranium.

Answer 6

The common carotid artery begins on the right as the brachiocephalic trunk

bifurcates into the common carotid and the subclavian arteries. The left common carotid

artery branches from the arch of the aorta at its highest point. Each common carotid

artery lies within the carotid sheath, with the internal jugular vein lateral and the vagus nerve dorsal (lying between the artery and vein). Near the upper border of the thyroid

cartilage, the common carotid artery bifurcates into the internal and external carotid

arteries. The carotid sinus and carotid body, which influence blood pressure and

respiratory regulation, respectively, are located at the bifurcation and extend along the

proximal few millimeters of the internal carotid artery (ICA).

From the bifurcation, the external carotid artery proceeds medially to divide into

its many extracranial branches, whereas the ICA proceeds posterolaterally (without

branching) to enter the carotid canal in the petrous portion of the temporal bone.

Question 7

What are the 4 segments of the ICA? Where do they end and begin? What branches exist off each one?

Answer 7

Radiographically, the course of the ICA can be subdivided into four segments:

cervical, petrous, cavernous, and cerebral. The cervical segment extends from the

common carotid bifurcation to the point where the artery pierces the carotid canal. The

petrous segment is contained within the carotid canal of the petrous portion of the

temporal bone. This portion of the artery has several small branches to the inner ear.

The cavernous segment is contained within the cavernous sinus and extends from the

point at which it enters the dura near the anterior clinoid process.

The cerebral segment is the terminal portion of the ICA and ends as the internal

carotid bifurcates into the anterior and middle cerebral arteries. Other major branches of

the cerebral segment include the ophthalmic artery, the superior hypophysial arteries, the

posterior communicating artery, and the anterior choroidal artery.

Question 8

Describe the pathway and branches of the opthalmic artery.

Answer 8

The ophthalmic artery leaves the ICA beneath the optic nerve and enters the orbit

through the optic foramen with the optic nerve. It gives rise to the central artery of the

retina and eventually communicates freely with the external carotid artery via its

lacrimal, ethmoidal, supraorbital, supratrochlear, and nasal branches.

Question 9

Describe the pathway of the superior hypophysial arteries? What do they supply?

Answer 9

The superior hypophysial arteries exit from the internal carotid arteries to form a

plexus around the pituitary stalk.

The capillaries of these vessels aid in the formation of the

hypophysial-portal system that supplies the anterior lobe of the pituitary gland.

Question 10

What is the pathway of the posterior communicating artery? What does it join with? What is its clinical significance?

Answer 10

The posterior communicating artery leaves the dorsolateral surface of the ICA just

before its terminal branching and joins the proximal portion of the posterior cerebral

artery, thus connecting the anterior and posterior circulations.

Clinically, one of the most frequent sites for aneurysm formation is

where the posterior communicating artery arises from the ICA.

Question 11

Where does the anterior choroidal artery arise? Generally, where does it go? What does it supply?

Answer 11

The anterior choroidal artery usually arises from the internal carotid just proximal

to its bifurcation. Sometimes, however, it arises from the MCA, the posterior

communicating artery, or the bifurcation of the middle and anterior cerebral arteries. The

anterior choroidal artery crosses the optic tract and passes toward the medial surface of

the temporal lobe. The penetrating branches of the anterior choroidal artery supply the

hippocampal formation, the amygdaloid nucleus, and the ventral and entire retrolenticular

part of the posterior limb of the internal capsule. In addition, the anterior choroidal artery

supplies the choroid plexus of the inferior horn of the lateral ventricle.

Question 12

What are the two portions of the Anterior cerebral artery? Where does the division take place? What does the first segment supply?

Answer 12

The anterior cerebral artery (ACA) is divided by the anterior communicating artery into proximal or precommunicating (A-1) and distal or postcommunicating (A-2) segments.

A-1 Segment

The A-1 segment begins at the carotid bifurcation and passes over the optic tract and chiasm to reach the anterior communicating artery. Along its course, branches supply portions of the anterior hypothalamus.

Question 13

Where does the recurrent artery of Heubner arise? What does it supply?

Answer 13

The recurrent artery of Heubner is conspicuous by its large size. It arises either

from the distal part of the A-1 segment or the proximal part of the A-2 segment and

courses laterally along the A-1 segment to join the lateral striate arteries as they enter the

anterior perforated substance. The recurrent artery supplies the ventral parts of the head

of the caudate nucleus, the anterior pole of the putamen, the anterior part of the globus

pallidus, and the anterior limb of the internal capsule as far dorsal as the top of the globus

pallidus.

Question 14

What is the pathway of the anterior communicating artery? What is it like? What does it supply? What its clinical significance?

Answer 14

The anterior communicating artery joins the two anterior cerebral arteries with the

A-1 segments of these vessels located proximally and the A-2 segments distally.

Anatomically, the anterior communicating artery is seldom a distinct vessel but more

often constitutes a complex network or web of vessels. Small perforators from the

anterior communicating artery supply the genu of the corpus callosum, septum

pellucidum, and septal nuclei.

The anterior communicating artery forms an important potential

source of blood flow between the two hemispheres, particularly

when one ICA occludes. In addition the anterior

communicating artery is another one of the frequent sites of

saccular aneurysm formation.

Question 15

Where does the A-2 Segment begin? What do its proximal branches supply? What are its two terminal bifurcations?

Answer 15

The A-2 segment of the ACA begins at the anterior communicating artery.

Proximal branches of the A-2 segment include the orbital artery, which supplies the gyrus

rectus and olfactory bulb and tract, and the frontopolar artery, which supplies the anterior

part of the superior frontal gyrus. The A-2 segment ends by bifurcating into the

callosomarginal artery and the pericallosal trunk near the genu of the corpus callosum.

Question 16

What is the pathway of the collosomarginal artery? What does it supply?

Answer 16

Callosomarginal Artery

The callosomarginal artery follows the course of the callosomarginal sulcus,

supplying anterior, middle, and posterior frontal branches to the superior frontal gyrus. It

ends as the paracentral artery to the paracentral lobule. All of these branches anastomose

with pre-rolandic and post-rolandic branches of the MCA as they turn onto the convexity

of the hemisphere.

Question 17

What does the pericallosal trunk supply? What are the symptoms of ACA stroke?

Answer 17

The pericallosal trunk artery is regarded as a continuation of the ACA. It passes

posteriorly in close relation to the corpus callosum, supplying penetrating vessels to the

corpus callosum, septum pellucidum, and fornix. Terminal branches include the

precuneal artery, which supplies the precuneus, and the posterior callosal artery, which

supplies the splenium of the corpus callosum.

A stroke in the cortical distribution of one ACA results in

sensorimotor deficit in the opposite foot and leg. Urinary

incontinence and contralateral frontal lobe signs may also be

observed.

Question 18

Describe the pathway of the M-1 segment? What does it supply? What is the clinical importance of some of its branches?

Answer 18

The proximal portion of the MCA is related to the lowest portion of the insula as

the artery travels to reach the lateral or Sylvian fissure. From this segment, 10 to 15

penetrating vessels, the lateral striate arteries or the lenticulostriate arteries, arise and

supply the dorsal part of the head and the entire body of the caudate nucleus, most of the

lentiform nucleus, and the internal capsule above the level of the globus pallidus. Like

the recurrent artery of Heubner, these penetrating arteries run a recurrent course back

along the M-1 segment to penetrate the lateral two-thirds of the anterior perforated

substance.

The other M-1 segment branches include the anterior temporal artery, which

supplies the most anterior portion of the temporal lobe, and the orbitofrontal artery,

which supplies the lateral portions of the orbital surface of the frontal lobe.

Clinically, the lenticulostriate vessels are the most common site of spontaneous hypertensive hemorrhage in individuals with long-standing hypertension.

Question 19

Describe the pathway of the M-2 segment including its bifurcations. What does it supply? What are the results of an MCA stroke?

Answer 19

The bifurcation of the MCA is located at the base of the insula and it forms the

M-2 segment, which consists of the superior and inferior trunks. These trunks travel deep

in the lateral (Sylvian) fissure along the insula. At the insula, branches travel along the

frontal and temporal opercula to exit the lateral fissure and proceed along the convexity of the hemisphere. Generally, the superior trunk supplies branches to the frontal and

parietal lobes, and the inferior trunk supplies the temporal and occipital lobes. The

angiographic shape of the superior and inferior trunks and their branches is called the

middle cerebral candelabra. The branches of both the superior and inferior trunks are

named according to the region they supply. These include the precentral or pre-rolandic,

the central or rolandic, the postcentral or post-rolandic, the anterior and posterior parietal,

the angular, the posterior temporal, and the posterior occipital arteries. The precentral,

central, postcentral, anterior and posterior parietal, and angular arteries leave the lateral

fissure and supply most of the cerebral convexity, anastomosing with the branches of the

ACA near the anterior and dorsal margins of the convexity. The posterior temporal and

posterior occipital branches supply most of the temporal and occipital convexity,

anastomosing with branches of the posterior cerebral artery at the posterior and ventral

margins of the hemisphere.

A stroke in the cortical distribution of the MCA results in a severe

sensorimotor deficit in the contralateral face and upper limb. With

dominant hemisphere involvement, global aphasia also results; with

nondominant hemisphere involvement, the neglect syndrome or

amorphosynthesis results.

Question 20

Describe the pathway of the vertebral arteries up to the cranium? Describe their pathway once they reach the cranium including any branches.

Answer 20

The vertebral arteries are the first branches of the subclavian arteries. They

generally enter the transverse foramina of CV6 and travel upward through the transverse

foramina of the other cervical vertebrae to reach the superior margin of CV1, where they

pierce the atlantooccipital membrane. They then enter the cranial cavity through the

foramen magnum ventral to the hypoglossal nerves, travel along the anterior or lateral

surfaces of the medulla, and join to form the basilar artery near the pontomedullary

junction.

After entering the cranial cavity, each vertebral artery gives rise to a posterior

spinal artery that descends along the posterolateral aspect of the spinal cord. The

vertebral arteries, 1 to 2 cm before joining to form the basilar artery, give rise to their

largest branches, the posterior inferior cerebellar arteries (PICA).

Question 21

Describe the pathway of the PICA. What does it supply? Waht does a stroke in the vertebral artery or PICA result in?

Answer 21

The PICA curve around the medulla ventral to the roots of IX CN, X CN, and

XI CN. The PICA reach the region of the cerebellar tonsil and proceed along the

posterior inferior cerebellar surface. Multiple penetrating vessels supplying the

posterolateral medulla arise from the PICA as they curve around this region. Other

branches supply the choroid plexus of the fourth ventricle before the PICA terminate as

inferior vermian and tonsillar-hemispheric branches, which supply all of the posterior and

inferior parts of the cerebellum.

A stroke in the distribution of the vertebral artery (or the PICA)

results in an ipsilateral loss of pain and temperature sensations in the

face, contralateral loss of pain and temperature sensation in the limbs,

trunk, and neck, an ipsilateral Horner syndrome, hoarseness,

dysphagia, nystagmus, vertigo, diplopia, ipsilateral ataxia, and

ipsilateral loss of taste. This combination of signs is the lateral

medullary or Wallenberg syndrome.

Question 22

What is the course of the basilar artery? What are its branches? What des it supply?

Answer 22

The basilar artery begins near the pontomedullary junction and travels in the

shallow median groove on the ventral surface of the pons to end at the midbrain. At the

midbrain it divides into the posterior cerebral arteries (PCA). As the basilar artery travels

along the pons, it supplies multiple penetrating vessels to the pons itself. These vessels

penetrate the pons as paramedian, short circumferential, and long circumferential arteries.

Symmetrical large branches arising at about the middle of the basilar artery are called the

anterior inferior cerebellar arteries (AICA). Similar large paired vessels arising just

proximal to the termination of the basilar artery are called the superior cerebellar arteries

(SCA).

Question 23

What is the course of the AICA? Waht do they supply?

Answer 23

The AICA emerge from the basilar artery and travel along the course of VII CN

and VIII CN. At times, these vessels may actually enter the internal auditory meatus for

a short distance, but ultimately they reach the anterior and inferior portions of the

cerebellum, their principal area of supply. The labyrinthine or internal auditory arteries

may arise from the AICA or directly from the basilar artery.

Question 24

What is the course of the SCA? What do they supply?

Answer 24

Just proximal to the bifurcation of the basilar artery into the PCA, the basilar

artery gives off the SCA. These vessels encircle the midbrain and end by dividing into

hemispheric and superior vermian branches that supply the superior aspects of the

cerebellum and most of the cerebellar nuclei and superior cerebellar peduncles.

Question 25

Describe the pathway of the PCA? What do they supply? What does a stroke in the PCA result in?

Answer 25

A stroke in the cortical distribution of the posterior cerebral artery results

in a contralateral homonymous hemianopsia. With dominant (usually left)

hemisphere involvement, reading and writing abnormalities also result.

The PCA begin at the basilar bifurcation near the tip of the dorsum sellae. A

short distance after arising, the PCA anastomose with the posterior communicating

arteries, thus connecting the anterior and posterior cerebral circulations. Each of the PCA

swings around the anterior aspect of the oculomotor nerve, passes laterally along the

surface of the cerebral crus to reach the dorsal surface of the free margin of the tentorium,

and then proceeds posteriorly along the inferomedial surface of the temporal lobe.

The PCA give rise to brainstem and cortical branches. The chief brainstem

branches are named according to their areas of supply as follows: thalamoperforate,

medial posterior choroidal, and quadrigeminal, which arise medial to the anastomosis

with the posterior communicating artery and supply the midbrain; and the

thalamogeniculate, lateral posterior choroidal, and peduncular, which arise lateral to the

posterior communicating anastomosis and supply the lateral parts of the posterior

NER-44-9

Revised 06-‘11

diencephalon. Cortical branches arise as the PCA courses along the inferomedial surface

of the temporal lobe to reach the occipital lobe, and they supply the hippocampus and the

medial and inferior surfaces of the temporal and occipital lobes. The PCA ends by

forming the parieto-occipital and calcarine arteries found in the respective sulci. The

calcarine artery supplies the primary visual area. The cortical branches of the PCA

extend slightly onto the lateral surfaces of the temporal and occipital lobes where they

anastomose with branches of the MCA

Question 26

What are the parts of the cerebral arterial circle of willis? Why is it important? Waht developmental changes are there in it?

Answer 26

The cerebral arterial circle, described by Sir Thomas Willis in 1664, consists of

the larger cerebral vessels and their interconnections located on the ventral surface of the

brain. The arteries of the circle of Willis include anterior communicating, left anterior

cerebral, left internal carotid, left posterior communicating, left posterior cerebral,

basilar, right posterior cerebral, right posterior communicating, right internal carotid, and

tight anterior cerebral. A perfectly symmetrical circle of Willis in which each component

vessel is of the same caliber occurs only in a minority of instances. More commonly, one

or more of the arteries (most frequently the anterior cerebral, posterior cerebral, anterior

communicating, or posterior communicating) are, to some degree, atrophic.

The function of the cerebral arterial circle of Willis is debated, but it probably

serves as a potential vascular shunt, assisting in the development of collateral circulation

to the brain should one of the proximal vessels (such as the carotid or basilar) become

temporarily or permanently occluded.

Question 27

What are the 4 perforating central branches of the circle of willis?

Answer 27

The branches of the cerebral arterial circle of Willis that penetrate the ventral

surface of the brain are called the perforating, penetrating, central, or ganglionic branches

and are divided into four groups: medial striate, lateral striate, thalamoperforate, and

thalamogeniculate.

Question 28

Describe the medial striate arteries including their origin, pathway, destination, and what they supply.

Answer 28

The medial striate arise chiefly from the A-1 segment of the ACA, although some

may arise from the most proximal part of the A-2 segment, the anterior communicating

artery, or the most terminal part of the ICA. Collectively referred to as the medial striate

arteries, they enter the brain in the medial third of the anterior perforated substance. The

largest and most lateral of these arteries to enter the brain is the recurrent artery of

Heubner. The medial striate arteries are the principal sources of the blood supply to the

supraoptic and preoptic regions of the hypothalamus and to the ventral part of the head of

the caudate nucleus and the adjacent parts of the anterior limb of the internal capsule and

putamen.

Question 29

Describe the lateral striate arteries including their origin, pathway, destination, and what they supply. Clinical importance?

Answer 29

The lateral striate arteries usually arise entirely from the M-1 segment of the

MCA, although sometimes a few may come from the initial part of the ACA. They are

frequently called the lenticulostriate arteries and they enter the brain in the lateral

two-thirds of the anterior perforated substance. The lateral striate arteries supply the

dorsal part of the head of the caudate nucleus, most of the putamen and adjacent part of

the globus pallidus, and the dorsal part posterior limb of the internal capsule.

As mentioned previously, these vessels are the most common sites of spontaneous

hemorrhage in individuals with long-standing hypertension. For this reason, collectively

they are called the “artery of cerebral hemorrhage.”

Question 30

Describe the thalamoperforate arteries including their origin, pathway, destination, and what they supply.

Answer 30

Thalamoperforate arteries arise along the posterior communicating artery and the

posterior cerebral artery proximal to the point at which these two vessels join. These

penetrating arteries enter the brain in the posterior perforated substances. The more

anterior vessels supply the tuberal region of the hypothalamus and the anteromedial part

of the thalamus, including the anterior and medial dorsal nuclei. The more posterior

vessels supply the mamillary region of the hypothalamus, the subthalamus, the adjacent

parts of the thalamus, and the medial parts of the rostral midbrain tegmentum and

cerebral crus.

Question 31

Describe the thalamogeniculate arteries including their origin, pathway, destination, and what they supply.

Answer 31

The thalamogeniculate arteries arise from the posterior cerebral artery distal to its

anastomosis with the posterior communicating, and penetrate the brain at the geniculate

bodies. They supply the most posterior parts of the thalamus, including the ventral lateral

and ventral posterior nuclei and the medial three-fourths of the metathalamic nuclei.