The 12 Cranial Nerves
The 12 Cranial Nerves
The 12 Cranial Nerves
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Visual learning: The figure below is especially useful to visual learners:
There are 12 pairs of cranial nerves in humans.
I Olfactory
II Optic
III Oculomotor
IV Trochlear
V Trigeminal
VI Abducens
VII Facial
VIII
Vestibulocochlear
(auditory)
IX Glossopharyngeal
X Vagus
XI Accessory (spinal accessory)
XII Hypoglossal
The cranial nerves mostly innervate structures in the head and neck.
The cartoon codes for at least one major functional innervation of each cranial nerve. This drawing is
based on one done by Beatrice Humphris, R.N. published in Nursing 1984.
http://www.sci.uidaho.edu/med532/cranialnervebackgroundinfo.html
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12 pairs of cranial nerves are associated with the ventral aspect of the brain.
—The first 2 pairs attach to the forebrain, the rest originate from the brainstem.
**Note: The Intermediate nerve included in the diagram above is actually the smaller root of the facial nerve (VII).
Most images do not illustrate it separately. This one just happens to do so.
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Test Your Cranial Nerves
Now that you know the names and functions of the cranial nerves, let's test them. These tests will help you
understand how the cranial nerves work. These tests are not meant to be a "clinical examination" of the cranial
nerves.
You will need to get a partner to help...both of you can serve as the experimenter (tester) and the subject.
Record your observations of what your partner does and says.
Cranial nerves III, IV and VI are tested together. They each control muscles involved with eye movement.
In addition, the oculomotor also functions in pupil dilation.
Oculomotor Nerve (III), Trochlear Nerve (IV) and Abducens Nerve (VI): Motor
These three nerves control eye movement and pupil diameter. Hold up a finger in front of your partner.
Tell your partner to hold his or her head still and to follow your finger, then move your finger up and
down, right and left. Do your partner's eyes follow your fingers?
Check the pupillary response (oculomotor nerve): look at the diameter of your partner's eyes in dim light
and also in bright light. Check for differences in the sizes of the right and left pupils.
To test the sensory part of the trigeminal nerve, lightly touch various parts of your partner's face with
piece of cotton or a blunt object. Be careful not to touch your partner's eyes. Although much of the
mouth and teeth are innervated by the trigeminal nerve, don't put anything into your subject's mouth.
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Facial Nerve (VII) Sensory for taste on anterior 2/3 of tongue and salivary glands.
Sensory from external auditory meatus and ear drum but NOT sensory from inner ear for hearing.
Motor for controlling muscles used in facial expression
The motor part of the facial nerve can be tested by asking your partner to smile or frown or make
funny faces. Lack of symmetry when smiling or frowning may indicate damage to this nerve or a
portion of the motor cortex – might indicate a stroke.
The sensory part of the facial nerve is responsible for taste on the front part of the tongue. You could
try a few drops of sweet or salty water on this part of the tongue and see if your partner can taste it.
Cranial nerves IX and X can be tested together since they both serve to innervate the pharynx.
Glossopharyngeal Nerve (IX): Sensory for taste on the posterior 1/3 of the tongue and conveying
information from the tongue, tonsils and pharynx
Motor for swallowing
Have your partner drink some water and observe the swallowing reflex. Also the glossopharyngeal
nerve is responsible for taste on the back part of the tongue. You could try a few drops of salty (or sugar)
water on this part of the tongue and see if your partner can taste it.
Vagus Nerve (X): Sensory from throat, larynx, thoracic and abdominal organs
Motor for swallowing, esophagus and to stimulate stomach and intestines (digestion).
Motor to slow HR.
Have the subject speak or cough. Her voice should be clear and strong. Cough should be strong. Subject
should have a strong gag reflex.
Spinal Accessory Nerve (XI): Motor to control the sternocleidomastoid and trapezius muscles involved in head
movement
Put your hands on your partner’s shoulders from the back. As you apply slight resistance, have him
shrug his shoulders upward. Contraction should be symmetrical.
Now place one hand on one side of your partner's jaw. Tell your partner to move his or her head toward
your hand while you apply slight resistance. Repeat on the opposite side. Strength in the
sternocleidomastoid should be symmetrical.
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DO YOU KNOW……?
Which cranial nerve is the largest? _______________________________
Seasickness? _____________________________________________
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PAIN
HOW THE BODY BLOCKS PAIN
The Medical community has observed that a person can block or reduce the amount of perceived pain. Over
time there have been many theories of the physiology of this phenomenon. The current theory states: the brain,
in response to pain, will activate analgesic systems in the brainstem. These in turn will relay pain-suppressing
signals to the spinal cord to release endogenous opioids such as ENDORPHINS and ENKEPHALINS. These create
IPSPs to cancel out EPSPs (Substance P) produced by the pain receptors.
ACETAMINOPHEN (Tylenol) blocks pain but not inflammation by inhibiting activation of the COX
enzyme.
NARCOTICS such as morphine, Demerol, and heroin reduce pain by mimicking the effects of
endorphins and enkephalins.
Applying continuous or repetitive stimulation through the skin of an appropriate area using TENS
‘annoys’ the brain with a flood of sensory input. In response, the brain increases the frequency of
impulses down the spinal cord to create IPSPs to “block” both the incoming messages from the
skin as well as the pain messages.
Electrical stimulation of nerves using TENS also stimulates the release of endorphins so that
fewer pain generating impulses reach the brain, so pain is not felt or at least is felt less.
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REFERRED PAIN
A painful sensation felt in a region of the body that is
not the origin of the stimuli.
Examples:
1. Heart attack – the origin of the impulses is the heart
muscle but the pain is often felt in the upper chest, left
side of the mandible, and on the medial surface of L arm.
Happens because the visceral sensory neurons and somatic sensory neurons converge on the same
ascending tracts of the spinal cord. When messages arrive on these tracts the brain can’t always
distinguish between the two points of origin.
Hyperalgesia is pain amplification and is common in amputations. In order to reduce this hyperalgesia
during amputations, epidural anesthetics (in addition to general anesthesia) are used to block
neurotransmission in the spinal cord and thus prevent the spinal cord from “learning” hyperalgesia.
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Dissection of Cow Brain with Dura Mater
Use images from the dissection manuals in lab and your knowledge of the brain to identify the following.
Recall the functions of those with an *.
Your group will be evaluated on the quality of your dissection. FOLLOW INSTRUCTIONS CAREFULLY.
OPTIC NERVES and OPTIC CHIASMA. Cut the optic nerves (cranial nerve #2) and other motor nerves as
far out as you can clearly see them to get rid of those dangling lumps of fatty tissue that were
located behind the eyes.
Pituitary gland - usually appears as a nodule or “lump” approximately ¼ inch in diameter on the ventral
side of the brain.)
TRIGEMINAL NERVES - The lateral edges of the dura mater containing the pituitary gland also contain the
large trigeminal nerves (cranial nerve #5). Cut the nerves near the dura mater to leave stumps on the
lateral walls of the pons. Remove the dura mater containing any branches of this nerve but do not
remove the pituitary gland.
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OCULOMOTOR NERVES – Gently lift the pituitary gland from its anterior edge while looking for the
oculomotor nerves (cranial nerve #3) to be raised with the dura mater and the pituitary gland. Go
ahead and remove the dura mater and pituitary gland. The stumps of the oculomotor nerves will
either be lying on the midbrain, projecting forward or they will have torn loose and be attached to the
dura mater above the pituitary gland.
BRAIN STEM
Spinal cord – function?
Medulla Oblongata – functions?
Pons – functions?
**Confirm identifications of everything up to this point with your instructor.
2. Bisect the brain on the mid-sagittal plane along the longitudinal fissure all the way through the brain stem.
Make one clean, straight cut all the way through. Try to cut as precisely in the middle as possible. Identify the
following.
Corpus Callosum and Fornix– recall function?
Lateral ventricles - Use your metal probe to separate the corpus callosum and the fornix in order to see
better into the lateral ventricles. The corpus callosum will be the roof and the fornix
will be the floor of each ventricle.
Third ventricle – Look above the thalamus but below the fornix.
Choroid plexus - specialized capillaries for production of CSF. They are delicate and dark brown in
color similar to the vessels on the surface of the cerebrum. Use your forceps to gently reach deep into
the posterior portion of a lateral ventricle and pull some of this out. You might also find some in the
3rd and 4th ventricles.
Cerebral aqueduct – passageway for CSF to flow from 3rd to 4th ventricle.
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