Download Lab 12 - The Brain and Cranial Nerves

Week 13 Lab
The Brain and Cranial Nerves
LEARNING OUTCOMES:
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To identify the following brain structures on a dissected specimen (or slices), human brain model, or
appropriate diagram, and to state their functions:
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Cerebral hemisphere structures: lobes, important fissures, lateral ventricles, corpus callosum, fornix
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Brain stem structures: corpora quadrigemina, cerebral peduncles of the midbrain, pons, medulla
Diencephalon structures: thalamus, intermediate mass, hypothalamus, optic chiasma, pituitary
gland, mammillary bodies, pineal body, choroid plexus of the third ventricle
Cerebellum structures: cerebellar hemispheres, arbor vitae
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To define gyri, fissures, and sulci.
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To identify the cranial nerves by number and name on a model or diagram, stating the origin and
function of each.
To identify at least four pertinent anatomical differences between the human brain and that of the sheep
(or other mammal).
BEFORE WE BEGIN:
When it comes to dissections, there are three types of people: cutters (people who don’t mind cutting things),
touchers (people who don’t like to cut, but will touch afterwards), and lookers (who won’t touch anything).
❍ Every group needs to have at least one cutter. Otherwise you’ll just sit there and stare at the brain.
❍ Only cutters and touchers needs gloves.
❍ Lookers are good for turning pages and taking notes.
ACTIVITY 1: Sheep Brain Dissection — External Features
In this activity, you will study the gross superficial anatomy of the sheep brain and its meninges.
In Lab:
1. The tough outer covering of the sheep brain is the dura mater, one of three meninges that cover the
brain. While you will need to remove the dura mater to see most of the brain, you can see some
structures on the brain before you remove it. Take special note of the pituitary gland and the optic
chiasma, optic nerves and optic tracts. The pituitary gland and attached infundibulum are likely to be
pulled off when you remove the dura mater. (This will demonstrated for you.)
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2. The most prominent feature of the brain is the cerebrum. It is divided into nearly symmetrical left and
right cerebral hemispheres by the longitudinal fissure.
sheep brain, superior view
(dura mater partially intact)
sheep brain, superior view
(dura mater removed)
3. The surface of the cerebrum is covered with large folds of tissue called gyri. The grooves between the
gyri are sulci. The deeper indentations are called fissures and are used as landmarks to divide the surface
of the cerebrum (the cerebral cortex) into regions: frontal lobe, parietal lobe, occipital lobe, and
temporal lobes.
sheep brain, left lateral view
4. The smaller, rounded structure at the back of the brain is the cerebellum. The cerebellum has smaller
gyri that are roughly parallel to one another. Compare the gyri of the cerebellum to that of the cerebrum.
5.
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6. Turn the brain over so that the cerebrum is down. The most prominent structure visible on the ventral
side of the brain is the optic chiasma, where the two optic nerves cross over each other and form an “X”
shape. The optic tracts run from the optic chiasma to the brain. Just posterior to the optic chiasma is the
mammillary body. You may also be able to see where the infundibulum attached the pituitary gland to
the brain.
7. Toward the front of the brain are two prominent round structures, the olfactory bulbs, that connect to the
olfactory tracts. The olfactory nerves (CN I) enter the olfactory bulbs from the nasal cavity through the
cribriform plate of the ethmoid bone.
8. Identify the brain stem structures: the midbrain, the pons, and the medulla oblongata.
9. Carefully bend the cerebellum to see the dorsal surface of the midbrain. The four domes are the two
superior colliculi and the two inferior colliculi. If you gently push those structures down, you can see the
rounded pea-sized pineal body (also called the pineal gland).
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ACTIVITY 2: Sheep Brain Dissection — Internal Features
In this activity, you will make a midsagittal cut through the sheep brain to observe its internal anatomy.
In Lab:
1. Use a knife or long-bladed scalpel to cut the specimen along the longitudinal fissure. This will allow you
to separate the brain into left and the right parts. Lay one side of the brain on your tray to locate the
structures visible on the inside:
❍ The corpus callosum is a commissure that connects the right and left cerebral hemispheres.
❍ You may see a membrane—the septum pellucidum—covering the opening to the lateral ventricle.
❍ Underneath it, you can find the third ventricle, which surrounds the intermediate mass of the
thalamus. (This is just labeled as the thalamus in the figure above.)
❍ The white area between those two ventricles is the fornix.
❍ The fourth ventricle is the space under the cerebellum.
❍ Just behind the thalamus is the pineal body. The hypothalamus is the area below the thalamus that
points to the area of the optic chiasma.
❍ The pons, medulla, cerebellum and spinal cord are also visible in the side view of the brain.
❍ The transverse fissure separates the cerebellum from the cerebrum.
❍ Within the cerebellum, you can see the arbor vitae, named such because the pattern of white matter
resembles a tree.
2. Compare the sizes of the cerebral hemispheres, olfactory lobes and brain stems in the sheep and the
preserved human brain (or one of the brain models in the lab).
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3. When you’re finished:
❍ Please wash and dry all tools, and return them to the tray of tools.
❍ You are welcome to put your dissected brain in a sealable bag and keep in the back room for future
study for Lab Exam 2.
❍ Any biological waste that you don’t want to keep must go in the large hazardous waste container in
the prep room.
❍ Gloves and paper towels can be discarded in the regular lab trash.
❍ When you’ve put all dissection materials away, please spray and wipe down your lab bench.
ACTIVITY 3: The Cranial Nerves
The cranial nerves, like the spinal nerves, are technically part of the peripheral nervous system, but their
association with the brain means that studying them along with brain anatomy is most appropriate.
Clinically, many of the tests that physicians perform during routine checkups are designed to test cranial
nerve function.
1. Below is a table that summarizes the twelve pairs of cranial nerves. You should be able to do the
following, using either a brain model or an image of the cranial nerves:
❍ identify the cranial nerves by name and by number (Roman numeral);
❍ identify the functions of the cranial nerves as primarily sensory, motor, or mixed; and
❍ state the brain association and peripheral association for each cranial nerve. For example:
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The optic nerve connects the retina of the eye to the thalamus.
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The vagus nerve connects the medulla oblongata to organs of the thorax and abdomen.
2. The last page of this handout contains an empty table that you can use to summarize this information for
your study.
Number and name
Origin and course
Function*
Testing
I. Olfactory
Fibers arise from olfactory
epithelium and run through the
cribriform plate of ethmoid bone
to synapse in olfactory bulbs.
Purely sensory—carries afferent
impulses associated with sense of
smell.
Person is asked to sniff aromatic
substances, such as oil of cloves
and vanilla, and to identify each.
II. Optic
Fibers arise from retina of eye to
form the optic nerve and pass
through optic canal of orbit.
Fibers partially cross over at the
optic chiasma and continue on to
the thalamus as the optic tracts.
Final fibers of this pathway travel
from the thalamus to the visual
cortex as the optic radiation.
Purely sensory—carries afferent
impulses associated with vision.
Vision and visual fields are
determined with eye chart and by
testing the point at which the
person first sees an object (finger)
moving into the visual field.
Fundus of eye viewed with
ophthalmoscope to detect
papilledema (swelling of optic
disc, or point at which optic
nerve leaves the eye) and to
observe blood vessels.
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Number and name
Origin and course
Function*
Testing
III. Oculomotor
Fibers emerge from dorsal
midbrain and course ventrally to
enter the orbit. They exit from
skull via superior orbital fissure.
Primarily motor—somatic motor
fibers to inferior oblique and
superior, inferior, and medial
rectus muscles, which direct
eyeball, and to levator palpebrae
muscles of superior eyelid;
parasympathetic fibers to iris and
smooth muscle controlling lens
shape (reflex responses to varying
light intensity and focusing of eye
for near vision).
Pupils are examined for size,
shape, and equality. Pupillary
reflex is tested with penlight
(pupils should constrict when
illuminated). Convergence for
near vision is tested, as is
subject’s ability to follow objects
with the eyes.
IV. Trochlear
Fibers emerge from midbrain and
exit from skull via superior orbital
fissure.
Primarily motor—provides
somatic motor fibers to superior
oblique muscle that moves the
eyeball.
Tested in common with cranial
nerve III.
V. Trigeminal
Fibers emerge from pons and
form three divisions, which exit
separately from skull: mandibular
division through foramen ovale in
sphenoid bone, maxillary
division via foramen rotundum in
sphenoid bone, and ophthalmic
division through superior orbital
fissure of eye socket.
Mixed—major sensory nerve of
face; conducts sensory impulses
from skin of face and anterior
scalp, from mucosae of mouth
and nose, and from surface of
eyes; mandibular division also
contains motor fibers that
innervate muscles of mastication
and muscles of floor of mouth.
Sensations of pain, touch, and
temperature are tested with safety
pin and hot and cold objects.
Corneal reflex tested with wisp of
cotton. Motor branch assessed by
asking person to clench his teeth,
open mouth against resistance,
and move jaw side to side.
VI. Abducens
Fibers leave inferior pons and exit
from skull via superior orbital
fissure to run to eye.
Carries somatic motor fibers to
lateral rectus muscle that moves
the eyeball.
Tested in common with cranial
nerve III.
VII. Facial
Fibers leave pons and travel
through temporal bone via
internal acoustic meatus, exiting
via stylomastoid foramen to
reach the face.
Mixed—supplies somatic motor
fibers to muscles of facial
expression and parasympathetic
motor fibers to lacrimal and
salivary glands; carries sensory
fibers from taste receptors of
anterior portion of tongue.
Anterior two-thirds of tongue is
tested for ability to taste sweet
(sugar), salty, sour (vinegar), and
bitter (quinine) substances.
Symmetry of face is checked.
Subject is asked to close eyes,
smile, whistle, and so on. Tearing
is assessed with ammonia fumes.
VIII. Vestibulocochlear
Fibers run from inner-ear
equilibrium and hearing
apparatus, housed in temporal
bone, through internal acoustic
meatus to enter pons.
Purely sensory—vestibular
branch transmits impulses
associated with sense of
equilibrium from vestibular
apparatus and semicircular
canals; cochlear branch transmits
impulses associated with hearing
from cochlea.
Hearing is checked by air and
bone conduction using tuning
fork.
IX. Glossopharyngeal
Fibers emerge from medulla and
leave skull via jugular foramen to
run to throat.
Mixed—somatic motor fibers
serve pharyngeal muscles, and
parasympathetic motor fibers
serve salivary glands; sensory
fibers carry impulses from
pharynx, tonsils, posterior tongue
(taste buds), and from
chemoreceptors and pressure
receptors of carotid artery.
A tongue depressor is used to
check the position of the uvula.
Gag and swallowing reflexes are
checked. Subject is asked to
speak and couch. Posterior third
of tongue may be tested for taste.
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Number and name
Origin and course
Function*
Testing
X. Vagus
Fibers emerge from medulla and
pass through jugular foramen and
descend through neck region into
thorax and abdomen.
Mixed—fibers carry somatic
motor impulses to pharynx and
larynx and sensory fibers from
same structures; very large
portion is composed of
parasympathetic motor fibers,
which supply heart and smooth
muscles of abdominal visceral
organs; transmits sensory
impulses from viscera.
As for cranial nerve IX (IX and X
are tested in common, since they
both innervate muscles of throat
and mouth).
XI. Accessory
Fibers arise from the superior
aspect of spinal cord, enter the
skull, and then travel through
jugular foramen to reach muscles
of neck and back.
Mixed (but primarily motor in
function)—provides somatic
motor fibers to
sternocleidomastoid and
trapezius muscles and to muscles
of soft palate, pharynx, and
larynx (spinal and medullary
fibers respectively).
Sternocleidomastoid and
trapezius muscles are checked for
strength by asking person to
rotate head and shrug shoulders
against resistance.
XII. Hypoglossal
Fibers arise from medulla and
exit from skull via hypoglossal
canal to travel to tongue.
Mixed (but primarily motor in
function)—carries somatic motor
fibers to muscles of tongue.
Person is asked to protrude and
retract tongue. Any deviations in
position are noted.
*Does not include sensory impulses from proprioceptors.
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