4.1 Cranial nerves Flashcards
How many pair of spinal nerves and cranial nerves are in the PNS?
Cranial Nerves: 12 pairs
Spinal Nerves: 31 pairs
What is a collection of cell bodies in the CNS vs PNS called?
What is the exception?
CNS = Nuclei PNS = Ganglia
Cranial nerves are peripheral nerves but are termed nuclei
Which cranial nerves are ‘atypical’ and why?
Typical peripheral nerves are normal spinal nerves 3-12
1-2 are atypical nerves because they are extensions of the brain. Compared to typical peripheral nerves, so it doesn’t have a cranial nerve nucleus in the brainstem like nerves 3-12
Which cranial nerves carry autonomic fibres?
3, 7, 9 and 10
(III, VII, IX, X)
What are the names of the 12 cranial nerves?
State which cranial nerves are sensory, motor or mixed
Label the select Foramina in the Skull
Cranial nerve emerge from holes 1-12 in that order from front to back
State which foramina in the skull each CN emerges from
State which region of the brainstem each CN nuclei resides
What are the 2 exceptions
3 and 4 come from the midbrain
5,6,7,8 come from the pons
9, 10, 11, 12 come from the medulla
Excepton: 1 and 2 do NOT come from the brainstem, they come directly from the brain
What is name and function of CN I
Describe its pathway
Olfactory nerve: sense of smell (sensory)
Starts at the olfactory mucosa where the olfactory receptor neurons are located (sense of smell is detected here)
The axons assemble into small bundles of olfactory nerves and assend through the cribiform plate of the ethmoid bone
Once in the cranial cavity, the fibres form the olfactory bulb which then pass into the olfactory tract.
Why are CN I and II not considered ‘true nerves’
because they do NOT come from the brainstem, they come directly from the brain
considered a ‘a brain tract’
What structure is indicated on the image?
Which bone is this structure found in?
Cribriform Plate in the Ethmoid bone
What is the clinical term for a loss of sense of smell?
Anosmia
Why might a fractured cribriform plate lead to a loss of sense of smell and a runny nose?
Loss of smell: compression of the olfactory receptor neurons ➞ can’t feedback to olfactory bulb
Runny nose: CSF leakage from nose!
Give 4 causes for a lack of smell
1) Upper resp. tract infection
2) Nasal polyps/rhinosinusitis
3) Trauma
4) Meningitis
How do you test the Olfactory Nerve?
First, the patient should be asked if they have noticed any changes in their taste or sense of smell.
Then each nostril should be tested, asking the patient to identify a certain smell (coffee, orange, vanilla).
The eyes should be closed for this part of the examination.
What is name and function of CN II
Describe its pathway
Optic nerve: innervates the eye, sight (sensory)
Complex pathway but simplified:
Nerve comes in through the back of the eye
Fibres cross over from the eye at the optic chiasm
Name 5 ways to examine the integrity of the optic nerve
1) Visual acuity: snellen chart
2) Colour vision: Ishihara test
3) Visual Fields
4) Pupillary reflexes
5) Fundoscopy
Where will visual field loss be produced if there is a lesion on the optic chiasm?
lesion on the optic chiasm will produce a visual field defect laterally on both sides
What is the pupillary reflex? (In response to light)
It is a neural pathway which has an afferent limb (from the optic nerve CN II) and two efferent limbs (from the oculomotor nerve CN III)
The afferent limb carries sensory input and when light is shone, cells of the retina project fibers through the optic nerve
The efferent limb is the pupillary motor output and controls the ciliary sphincter muscle of the iris. CN III is under PNS control and causes constriction of the pupil in response to light
What is name and function of CN III
Describe its pathway (Incl what structures are innervated)
Occulomotor nerve: moves the eyeball/eyelid and pupil constriction (motor + PNS)
Starts in the occulomotor nucleus of the midbrain
It exits the brain through the superior orbital fissue and splits into 2 branches: superior and inferior
The superior branch provides motor innervation to the superior rectus and levator palpabrae superioris.
The Inferior branch provides motor innervation to middle rectus, inferior rectus, inferior oblique, sphincter of the iris (PNS to constrict) and ciliary muscle of the eye
What are the 4/6 extraocular eye muscles innervated by CN III
State the movement each allows
Superior Rectus Muscle (upwards movement)
Medial Rectus Muscle (inward movement)
Inferior Rectus (downwards movement)
Inferior Oblique (upwards and outwards movement)
What nerve innvertates the other 2/6 extraocular eyes that are not innervated by CN III
LR6: Lateral rectus innervated by CN VI
- Outward movement
SO4: Superior oblique innervated by CN IV
- depresses, internally rotates and abducts the eye
- ability to move eye downward and out
What is the function of Levator Palpebrae Superioris?
Lifts the upper eyelid (CN III)
Label the image of the eye
What other nerves are tested along with the Oculomotor Nerve?
How can these be tested
CN IV and CN VI are examined together by drawing an H to test if all eye movements are working
Also test Pupillary Light Reflex and Accommodation reflex (absent in paralysis of ciliary muscle)
What are the typical features of a complete third cranial nerve palsy?
Explain why each occurs
‘Down and Out eye’ (while other eye is looking straight ahead): No innervation by CN III muscles, hence only CN IV and VI are working which pull the eye down and out
Closed eyelid: CN III supplies levator palpabrae superioris which lifts the eyelid
Enlarged pupil: normally PNS fibers go to sphincter of the eye which constrict pupil
List 3 reasons why autonomic damage to CN III may occur
1) Aneurysm
2) Increased intracranial pressure
3) Mydriasis or ‘Blown’ pupil
What is name and function of CN IV
Describe its pathway
Trochlear nerve: eye movements ‘down and ‘in’ (motor)
Starts at the Trochlear nucleus and passes dorsal to Midbrain through the superior orbital fissure to Superior Oblique Eye Muscle
Where do V1, V2, and V3 all exit the skull?
Ophthalmic (V1) through Superior Orbital Fissure
Maxillary (V2) through Foramen Rotundum
Mandibular (V3) through Foramen Ovale
How can we test the sensory and motor branch of the Trigeminal nerve?
Sensory:
Ask the patient to close their eyes and introduce a cotton wisp to areas of the face supplied by the 3 divisions to detect tactile sensory competence.
Motor:
Ask the patient to clench their jaw as you palpate superior to the zygomatic arch to feel for contraction of the temporalis and then repeat palpating inferiorly for the masseter.
Ask the patient to open their mouth and deviate their mandible to the right and left to check for competence of the medial and lateral pterygoid muscles
What is the name and function of CN IX
Describe its pathway
Glossopharyngeal Nerve (both + PNS)
From the medulla oblongata, through the jugular foramen to the tongue- parotid gland, carotid body and sinus and stylopharyngeus
What nerves does the ‘Gag’ or pharyngeal reflex assess?
What is considered a normal vs abnormal reflex and what may this indicate?
The affernt (sensory) limb is mediated by CN IX (glossopharyngeal nerve)
The efferent (motor) limb by CN X (vagus nerve).
Normally palate should rise when patient opens mouth and says ahhhh. With a CN IX and X Palsy the palate away from the lesion.
What is the name and function of CN XI
Describe its course and Incl state 2 parts of this nerve
Accessory nerve: SCM and Trapezius (motor)
1) The spinal portion (main part)
2) The cranial portion (short)
The spinal portion is the main part and its cell bodies are located in the spinal cord roots C1-C5
They enter the skull through the Foramen magnum
Then exit the skull through the Jugular Foramen with CN IX and X, along the internal carotid artery to reach the Trapezius and SCM
What is the name and function of CN XII
Describe its pathway
Hypoglossal nerve (motor)
From the hypoglossal nucleus in the medulla, through the hypoglossal canal to the tongue
