Forty Four Flashcards
Explain the brains dependence on blood flow and oxygen. Explain how blood is distributed with the CNS.
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.
Explain the autoregulation of cerebral blood flow. What effects do O2 and CO2 have on it? How can these effects be used clinically?
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.
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?
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.
Where are intracranial extracerebral vessels located? What is the Virchow-Robin space? What is its clinical significance?
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.
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?
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.
Describe the pathway of the left and right common carotid arteries from their beginning to their entrance in the cranium.
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.
What are the 4 segments of the ICA? Where do they end and begin? What branches exist off each one?
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.
Describe the pathway and branches of the opthalmic artery.
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.
Describe the pathway of the superior hypophysial arteries? What do they supply?
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.
What is the pathway of the posterior communicating artery? What does it join with? What is its clinical significance?
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.
Where does the anterior choroidal artery arise? Generally, where does it go? What does it supply?
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.
What are the two portions of the Anterior cerebral artery? Where does the division take place? What does the first segment supply?
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.
Where does the recurrent artery of Heubner arise? What does it supply?
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.
What is the pathway of the anterior communicating artery? What is it like? What does it supply? What its clinical significance?
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.
Where does the A-2 Segment begin? What do its proximal branches supply? What are its two terminal bifurcations?
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.