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In order to understand how stroke occurs and the damage
it can cause, it is useful to understand the basic anatomy
of the brain. The signs and symptoms of a stroke depend
on which region of the brain is affected and how severely.
The brain has three primary components. Each is responsible
for different functions:
The Cerebrum:
The cerebrum is the largest and most developmentally
advanced portion of the brain. It controls a number
of higher functions, including speech, emotion, the
integration of sensory stimuli, initiation of the final
common pathways for movement, and fine control of movement.
It is divided into a left and a right hemisphere. The
left hemisphere controls the majority of functions on
the right side of the body, while the right hemisphere
controls most of functions on the left side of the body.
Thus, injury to the left cerebral hemisphere produces
sensory and motor deficits on the right side, and vice
versa.
The cerebrum is composed of the frontal, parietal,
temporal, and occipital lobes:
- The frontal lobe is involved in planning, organizing,
problem solving and selective attention. The portion
known as the prefrontal cortex controls personality
and various higher cognitive functions such as behavior
and emotions. The back of the frontal lobe consists
of the pre-motor and motor areas, which produce and
modify movement.
- The left and right parietal lobes contain the primary
sensory cortex, which controls sensation (touch and
pressure), and a large association area that controls
fine sensation (judgment of texture, weight, size,
and shape). Damage to the right parietal lobe can
cause visuo-spacial deficits, making it hard for the
patient to find his/her way around new or even familiar
places. Damage to the left parietal lobe may disrupt
a patient’s ability to understand spoken and/or
written language.
- The left and right temporal lobes, located around
ear level, allow a person to differentiate smells
and sounds. They also help in sorting new information
and are believed to be responsible for short-term
memory. The right lobe is primarily involved in visual
memory (i.e., memory for faces and pictures). The
left lobe is primarily involved in verbal memory (i.e.,
memory for words and names).
- The occipital lobe processes visual information.
It is mainly responsible for visual reception and
contains association areas that help in the visual
recognition of shapes and colors. Damage to this lobe
can cause visual deficits.
The Cerebellum:
The cerebellum is the second largest area of the brain.
It controls reflexes, balance and certain aspects of
movement and coordination.
The Brain Stem:
The brain stem is responsible for a variety of automatic
functions that are critical to life, such as breathing,
digestion and heart beat – as well as alertness
and arousal (the state of being awake).
Symptoms Point to Stroke
Location
A stroke can occur anywhere in the brain or just outside
it. The symptoms that a stroke victim experiences depend
on which area(s) of the brain are involved.
When a stroke occurs in the right hemisphere of the
cerebrum, the result may be paralysis on the left side
of the body, difficulty reasoning or thinking out solutions
to even the simplest problem. A stroke in the left hemisphere
can result in paralysis of the right side of the body
and may disrupt the ability to speak.
A stroke involving the cerebellum may result in a lack
of coordination (ataxia), clumsiness and balance problems,
shaking, or other muscular difficulties. This can interfere
with a person’s ability to walk, talk, eat and
perform other self-care tasks.
Brain stem strokes are the most devastating and life
threatening because they can disrupt the involuntary
functions essential to life. People who survive may
remain in a vegetative state or be left with severe
impairments.
Blood Flow
to the Brain
The heart pumps oxygen- and nutrient-laden blood to
the brain, face, and scalp via two major sets of vessels:
the carotid arteries and the vertebral arteries. The
jugular and other veins bring blood out of the brain.
The carotid arteries run along the front of the neck
– one on the left and one on the right. They are
what you feel when you take your pulse just under your
jaw. The carotid arteries split into external and internal
arteries near the top of the neck.
The external carotid arteries supply blood to the face
and scalp. The internal carotid arteries supply blood
to the front (anterior) three-fifths of cerebrum, except
for parts of the temporal and occipital lobes.
The vertebral arteries travel along the spinal column
and cannot be felt from the outside. They join to form
a single basilar artery (hence the name vertebrobasilar
arteries) near the brain stem at the base of the skull.
The arteries supply blood to the posterior two-fifths
of the cerebrum, part of the cerebellum, and the brain
stem.
Because the brain relies on only two sets of major
arteries for its blood supply, it is very important
that these arteries are healthy. Often when an ischemic
stroke occurs, the carotid or vertebral artery system
is blocked with a fatty buildup called plaque, allowing
little or no blood to flow to the brain. During a hemorrhagic
stroke, an artery in or on the surface of the brain
has ruptured or is leaking, causing bleeding and damage
in or around the brain.
These arteries that conduct blood to the brain —
the internal-carotid and vertebral arteries —
connect through the Circle of Willis, which loops around
the brainstem at the base of the brain. From this circle,
other arteries — the anterior cerebral artery
(ACA), the middle cerebral artery (MCA), and the posterior
cerebral artery (PCA) — arise and travel to all
parts of the brain.
Because the carotid and vertebrobasilar arteries form
a circle, if one of the main arteries is blocked, the
smaller arteries that the circle supplies can receive
blood from the other arteries. This phenomenon is called
collateral circulation.
Collateral circulation is a process in which small
(normally closed) arteries open up and connect two larger
arteries or different parts of the same artery. They
can serve as alternate routes of blood supply. Sometimes
when an artery in the brain is blocked due to ischemic
stroke or transient ischemic attack (TIA), open “collateral”
vessels can allow blood to "detour" around
the blockage, restoring blood flow to the affected part
of the brain. Everyone has collateral vessels, at least
in microscopic form. These vessels normally aren't open.
However, they grow and enlarge in some people with coronary
heart disease or other blood vessel disease. While everyone
has collateral vessels, they don't open in all people.
The Circle of Willis has a downside, however. Cerebral
aneurysms tend to occur at the junctions between the
arteries that make up the Circle.
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Stroke & The Brain
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