BRAIN

The Brain  - Chapter 12 of Marieb Textbook, 7th Ed.  p. 431-432.

The brain and spinal cord first develops as a neural tube. The embryonic brain is laid
out in a linear fashion, much like a fish or frog brain. See the illustration on the following.

At five weeks of development the most cephalad of portion of the neural tube becomes the
olfactory lobes and the telencephalon, which divides into the cerebral hemispheres
containing the cavities which become the first and second ventricles. The two lateral ventricles
become separated somewhat by a membrane called the septum pellucidum.

The next cavity caudad is the diencephalon. The roof of the diencephalon outpockets to become the
pineal gland and the floor outpockets to become the hypothalamus and, further below, the infundibulum
or pituitary stalk. In the center of the diencepahlon, the thalamus will form. Laterally the diencephalon
outpockets to produce the optic stalks which become the optic nerves. At the ends of the optic stalks, the
optic cups form. They become the retina of the eye (the lens forms from epithelial tissues just distal to the
optic cups). The third ventricle is the lumen of the diencephalon.

To which part of the diencephalon do the optic stalks connect?

The next cavity caudad is the mesencephalon, which becomes the midbrain which is the top of the
brain stem. The lumen of the midbrain is the cerebral aqueduct.

Following caudally is the metencephalon which becomes the cerebellum and pons. Its lumen becomes
the fourth ventricle.

Last is the myelencephalon which becomes the medulla oblongata. Further caudally the spinal cord forms.
As the brain develops the cerebral hemispheres enlarge and fold over the thalamus and brain stem.
A question mark-like shape ( ? ) is formed.
 

Please label the illustration.
 

Meninges - the same three layers of connective tissues cover the brain and spinal cord, serving protection
and aiding the circulation of cerebrospinal fluid around and within both structures.  The layers are in order,
the dura mater, arachnoid and pia mater. The cerebrospinal fluid is formed by pressure filtration from
the choroid plexi (blood vessels) of the ventricles. The CSF flows back into the venous circulation around
the brain through the arachnoid villi.  See p. 463 and http://www.epub.org.br/cm/n02/fundamentos/circulation_i.htm .

What is an epidural hematoma? Compare a brain concussion and a contusion. See below.

If the flow of the cerebrospinal fluid in the ventricles and subarachnoid space is impeded by a "birth defect"
or inflammation in or around the brain, hydrocephalus results. Treatment may attempt to reduce the
inflammation or by installation of a shunt into a vein which drains the fluid, thus relieving the expansive
pressure.

The most common forms of meningitis may are bacterial (meningococcal, such as that spreading through
schools), or viral (transmitted by mosquitoes)-.

BRAIN STEM

Medulla Oblongata - See p. 451. It connects the spinal cord to the higher brain.
Decussations of the pyramids (cross-overs of many descending motor fibers occur
here giving rise to the saying "that the right half of the higher brain controls the left
side of the body", and "left handed people are in their right brains!"

The nucleus cuneatus and the nucleus gracilis relay sensory impulses from the arms
and legs, respectively, on one side to the opposite cerebral hemisphere.
The ascending reticular activating system, described below, ascends from this level.
The medulla also contains the breathing rhythmicity center which controls
relaxed exhalation and inhalation, the sympathetic cardiovascular (heartbeat
and vasoconstriction) centers. Other centers include vomiting, swallowing,
coughing, sneezing and hiccupping. The medulla contains nuclei for cranial nerves
VIII - XII.

The Pons is a "bridge” to both cerebellar hemispheres and to the midbrain
that connects the medulla and pons to other brain centers such as the cerebellum
and thalamus. The pons contains the apneustic center that causes gasping by
prolonging inhalation and the pneumotaxic center which stimulates exhalation
when stretch receptors in the lung are activated. The pons contains nuclei for
cranial nerves V-VII.  See p. 449.

Dorsally, the midbrain contains relay centers for reflexes in response to auditory
and visual reflexes, respectively,  the inferior and superior colliculi, as well as
other motor centers. Parkinson's disease, that is marked loss of muscle
coordination and function, is caused by the death of dopamine neurotransmitter
producing cells in the substantia nigra of the midbrain and the higher basal nuclei
of the cerebral hemispheres. See p. 448.

The red nucleus also coordinates postural muscle movements controlled by the
descending rubrospinal tracts as well as aiding the higher basal nuclei of the
brain to produce discrete movements of muscles. Ventrally, the cerebral peduncles
conduct impulses up and down the brain stem. The midbrain contains nuclei for
cranial nerves III and IV.
-

-

Label the thalamus, the hypothalamus, the pituitary,
the midbrain, the pons and the the medulla.
-

Cerebellum - the cerebellum is responsible for integrating impulses for muscle
coordination, posture, and balance keeping.  The inner ear relays information
to the cerebellum on turning or head-up or head-down motions. The cerebellum
constantly compares the intended movement with the actual movement. Error
messages are sent back to the cerebral hemisphere motor areas
where modification of muscle movements is planned and directed. Damage
to the cerebellum can result in an inability to close your eyes and touch your
nose. When the movement is attempted again, there is an over correction to
the opposite side of the "miss." Also, a sing-song speech pattern
may result.

DIENCEPHALON - See p. 453.

The thalamus is called the "gatekeeper of the higher brain."  Sitting at the top
of lower structures called the ascending reticular activating system. It alerts
the higher brain only to those stimuli designated appropriate.  Together with
the hypothalamus, the thalamus shuts down the higher cerebral cortex for
various stages of sleep or stimulates the cortex during the awake state.  The
thalamus also contains relay centers for auditory and visual sensory impulses
(from the optic nerves or cranial nerve II), the medial and lateral
geniculate nuclei respectively, that will be perceived by the cerebral hemispheres.
The roof of the diencephalon is marked by an outpocketing called the pineal
gland or epiphysis. Secretions of melatonin from it lighten the skin and may
induce S.A.D. - seasonal affective disorder (depression) (by depleting
serotonin?) in those afflicted. Light transmitted to the brain by the eyes seems
to decrease melatonin secretion.

So the treatment for S.A.D. is what?

Hypothalamus - this is the center for mind over body phenomena and
homeostasis.  The centers for the regulation of feeding (hunger), thirst,
satiety, body temperature (hypothalamic thermostat), and blood pressure
are regulated here.  The mammillary bodies are reflex relays for smell.
The hypothalamus indirectly controls the anterior pituitary gland (hat is
an outpocketing of the embryonic pharynx)  with releasing hormones sent
through the pituitary portal veins. The ant. pit. then controls growth (hGH),
sexual reproduction organ functions (FSH, LH) the adrenal cortex secretions
(cortisol, aldosterone, and androgens), and basal metabolism (Thyroid
Stimulating Hormone). The posterior pituitary or hypophysis releases
hormones that are made in the hypothalamus and transported down the
pituitary stalk; ADH increases water reclamation by the kidney and thus
increases blood volume and oxytocin causes the uterus to contract during
labor. The hypothalamus is the source of rage and aggression and
it assists the thalamus in maintaining awake or sleep states.

CEREBRUM

Cerebral Hemispheres - See pg. 432//454. Selected functions follow.

White matter occurs in the center of the cerebral hemispheres as association
fibers that transmit impulses within a hemisphere, commissural fibers that
transmit impulses from one cerebral hemisphere to the opposite side, and
projection fibers which communicate with lower parts of the brain and the
spinal cord.

The basal ganglia ( p. 441-442//443-444) include the caudate nucleus and the lenticular
nucleus of which the putamen and globus pallidus are parts. They control large
muscular movements such as arm swinging during walking, and producing
muscle tone.

The limbic system (see pg. 454) consists of structures derived from the floor
of the diencephalon and the inner gyri of the cerebral hemispheres. It is associated
with strong emotions like aggression, rage, fear and anger and their opposites.
docility and affection. It is called the "emotional brain." The parahippocampal
gyrus, the hippocampus and the amygdaloid body are also associated with
memory formation. Notice how the mammillary body connects to the limbic
system. Damage to the amygdala or hippocampal gyrus interferes with the
ability to make memories which last for more than a few minutes. The affected
patients cannot make new memories but keep their old ones.

How is it that our strongest memories are associated with strong emotions or smells?

Olfactory Lobes - receive the lock and key chemical perceptions of nasal sensory
cells to create a "sense of smell." The olfactory nerves are cranial nerves # I.

Frontal Lobes - contain areas responsible for the elaboration and ethical control of
emotions and so-called animal drives which originate in a more primitive core area
called the limbic system. In 1849, Phineas T. Gage was a railroad supervisor when
a spike was blasted through his skull, from maxilla upward through the frontal bones,
severing some connections between the ethical areas and limbic system.   Specifically,
his venteromedial prefrontal lobe areas, especially the left, were damaged. These areas
control the elaboration of emotions or actions once they have been initiated. A recent
study suggests that such damage would prevent a person from stopping a behavior like
drawing a half circle and then stopping and drawing a half square. A person would be
compelled to finish the circle before drawing the square. His personality changed from
sober/responsible to drunken/childish/impulsive (more animal like?) This was a landmark
event for Science as Phineas was studiedextensively at Harvard U., where his skull,
death mask and spike (1.25 inches in diameter) reside at this day. His personality
changed from sober/responsible to drunken/childish/impulsive.  This was an landmark
event for Science as Phineas was studied extensively at Harvard U., where his skull,
death mask and spike reside at this day.  This began serious attempts of scientists to
relate brain structure to behavior.

The frontal lobes also plan voluntary muscle activities in the frontal lobe premotor area
which are directed by the primary motor areas; both are anterior to the central sulcus
dividing the frontal and parietal lobes. The frontal lobes also develop the initial sensations
of taste, hearing and smell, and are the sites of moral and ethical decision-making. The
prefrontal areas elaborate (strengthen) emotions like affection and rage. Frontal
lobotomies, made infamous by the West Virginia Experiment of 1950, destroyed
these areas and made the patients permanently uncouth and docile. Why?

Functional Areas - See p. 437-440.

Broca's area controls muscle movements in speech. It is found usually on the left
cerebral hemisphere. Damage to this area produces nonfluent aphasia, the patient
cannot form and speak words.

Parietal Lobes - these develop perceptions of skin touch, pressure
and pain, and muscle/joint positions in the somatosensory association and primary
somatosensory areas.  Memories of past sensory experiences are held there.
Also in the parietal cortex is the General or Gnostic Interpretative area which
relates various sensory information (like associating a smell with a visual image).

Temporal Lobes - the primary auditory area processes sounds; the auditory
(Wernike's) association area translates thought into words. damage to the
gnostic or auditory association areas results in fluent aphasia: a person can say
a string of words but they are meaningless.

Occipital Lobes - develop and associate visual sensations.  Damage to the
more anterior visual association/interpretative areas can lead to a syndrome
called prosopagnosia where people cannot recognize faces.

-

Learning areas include the hippocampal and amygdala areas of the limbic system.  In
one syndrome of amygdala and hippocampal damage, long term memory is unimpaired
while a person cannot remember what happened five minutes ago even when presented
pictures or written evidence of events.

The right and left cerebral hemispheres are connected by a bridge of white matter the
corpus callosum.  In some cases of epilepsy the corpus callosum is cut and many functions
of the right and left brain are separated.  When these 'split brains' are studied it has been
discovered that one side or the other dominates in motor control for left handedness and
right handedness. See the following table.

-

.
Label as appropriate to the discussion above.

In one syndrome of amygdala and hippocampal damage, long term memory is
unimpaired while a person cannot remember what happened five minutes ago even
when presented pictures or written evidence of events.
 

                           There is "plasticity in the hippocampus. One is the ‘shrinking’ effect
                           seen when people are placed under a severe degree of
                           stress, such as that suffered by holocaust survivors or Gulf
                           war veterans or when people suffer from diseases such as
                           Cushing’s syndrome, depression, Post Traumatic Stress Syndrome,
                           dementia and schizophrenia which are all caused by a long-term
                           over-production of adrenaline (epinephrine) from the adrenal gland.
                           The cause is due to adrenaline binding to receptors in the
                           hippocampus. Chronic overproduction causes the cells in this
                           area to die and neuroscientists have now found that the
                           shrinking effect that stress induces on the rat hippocampus
                           can be accelerated by other factors such as diseases like
                           diabetes that also interfere with hormonal balance. The
                           other main feature McEwan described was the reverse effect
                           conferred by estrogen. Oddly enough, this hormone seems to
                           protect against the loss of hippocampal neurons."
                           A. Gillings, BioMedNet News, Apr. 21, 1999.

In females, the anterior portions of the hemispherical bridge or corpus callosum are
enlarged, perhaps meaning that there is less hemispherical specialization in function.
Also, there is an area of the hypothalamus that is enlarged in females.

One treatment for severe epilepsy, an "electrical storm" of depolarizations in one
hemisphere that spreads into the opposite one, involves cutting the corpus callosum.
"Split brain" studies then discovered function specialization in each hemisphere. For
instance, we have known for some time that the opposite hemisphere controls the
muscle movements of one side, leading to a saying that, left handed people are in their
"right minds." See medulla decussations above. Also, the left brain specializes in
scientific thinking; that is, reasoning, calculations and language. The right brain is
our artistic hemisphere.

Hemispherical Specialization -
 
 

Left cerebral hemisphere	Right C.H.
right side motor (85% of population have a left hemisphere dominance = right handed)	left side motor
reasoning	artistic abilities
language	spacial relationships
scientific skills	insight and imagination

-
Brain Waves - See p. 457-459.

1. alpha - seen when awake and eyes closed
2. beta - seen when eyes are open and brain awake
3. theta - seen when sleep begins, abnormal when awake
4. delta - deep sleep waves, normal in awake infant, abnormal in awake adult

What is meant by the term "flat lines"?
 

Diseases

Brain contusion - watch for gradual and inappropriate loss of consciousness after
a head injury. Blood may pool in epidural or subdural spaces. Brain may be forced
through the foramen magnum resulting in compression of the medulla and death.
Why would death result?

Brain concussion - temporary loss of consciousness because the brain bounces
against the skull case.

Brain laceration - brain surface is cut.

Hydrocephalus - blockage of the cerebrospinal fluid circulation through the
ventricles of the brain. See pg. 467.

Cerebrovascular accident (CVA or stroke) - a clot blocks a blood vessel
or a blood vessel has a weak place in the wall (aneurysm) that ruptures -
a PET scan will show the damage. Anticoagulants are used to break up the clots. See p. 468.

Transient ischemic attacks (TIA) - a slowing or reduction of blood flow to
the brain can cause temporary confusion or loss of consciousness - a signal
of impending stroke (CVA).

Alzheimer's disease - diagnosis depends on microscopic examination showing
neurofibrillary tangles in affected nerurons and A68 beta amyloid protein in
brain tissue. Loss of memory, confusion and dementia result. Ach secretions
decline in the brain.  See p. 469.

Senile dementia - deterioration of brain tissue due to aging and blood poor circulation.

Cerebral palsy - brain damage to motor areas due to rubella virus or deprivation
of oxygen during birth.

Parkinson's disease - see above, characterized by slow movement and
decreases in the range of movement. Tremor or shaking is seen - sometimes
with a "pill rolling" movement of the fingers.

Multiple sclerosis -  a demyelinating, autoimmune disease; it is set off
by virus infection. Symptoms include vision disturbance, tingling or lack of feeling
in appendages, and difficul;y walking. Symptoms may progess consistently deteriorating
nervous functiopn or may be the remitting/relapsing type when there are periods of time
between attacks that generally worsen. Suppression  of herpes eruptions with acylovir
prevents subsequent attacks in some patients.

A.L.S. or Lou Gehrig's disease, a disease that destroys
motor neurons, also follows virus infections. Weakness, blurred vision and
tingling sensations occur as axons are demyelinated. Total paralysis results. See p. 481. 

Anencephaly - failure of the neural tube to develop the upper half of the cranial
vault and the cerebral hemispheres. Newborns cannot survive. See p. 483.

Study Questions

1. How do drugs, like dilantin that enhance the effects of neuroinhibitors, help
epileptics?
2. Explain the "split brain" surgery for epilepsy.
3. Explain the accident and symptoms of Phineas Gage.
4. Explain how the brain sees and interprets objects - concentrate on the path
and areas.
5. Compare the two causes of strokes and how each is treated. See pg. 460.
6. Compare the pathology and symptoms of concussion and contusion of the brain.
7. If the vagus nerve were cut in the lower neck, you would expect what symptoms?
8. Pretend you are a neurologist. Decribe the cranial nerve functions that can be
observed simply.
 

 jaliff @ gpc.edu