Cranial Nerves I-VI

Question 1

Central nervous system = brain and spinal cord

Answer 1

Collections of cell bodies in the CNS are called nuclei (singular = nucleus)

Question 2

Peripheral nervous system (PNS) = nervous system outside the CNS

Answer 2

12 pairs of cranial nerves: head and neck*

31 pairs of spinal nerves

Collections of cell bodies in the PNS are called ganglia (singular = ganglion)

Question 3

What are cranial nerves?

Answer 3

12 pairs; named and numbered I - XII

Peripheral nerves
Most are attached to the brainstem

Carry sensory / motor /autonomic info between the brain and the head and neck*

Contain different combinations of fibre types (motor, general sensory, special sensory, autonomic); some contain just 1 type, others contain several.

Question 4

Autonomic fibres

Answer 4

Parasympathetic nevres: III, VII, IX, X

Sympathetic fibres also innervate the head and neck, but they’re not carried in cranial nerves

Question 5

Cranial nerve nuclei

Answer 5

Nerves III – XII
In the brainstem (MB, pons, MO)

Either receive sensory / afferent input from the periphery (blue in this image) or
contain cells whose axons convey motor / efferent signals to the periphery (red in this image)

Some nerves with motor and sensory components have two separate nuclei – a motor nucleus and a sensory nucleus

Parasympathetic fibres arise from specific nuclei in the brainstem

Question 6

CN I: Olfactory

Answer 6

Attached to the brain, not the brainstem.
Function: smell
Receptors - nasal cavity

Axons travel through the cribriform plate
> olfactory bulb
> tracts
> temporal lobe

Connections with the limbic system

Question 7

CN II: Optic

Answer 7

Attached to the brain, not the brainstem.

Function: vision
Fibres travel from the retina to the primary visual cortex (calcarine sulcus, medial aspect of occipital lobe)

Question 8

CN II: Visual pathway

Answer 8

Temporal retina and nasal retina of each eye form images from different parts of the visual field.

= Each optic nerve conveys info about the right and left visual fields

Question 9

CN II: Visual Pathway 2

Answer 9

Optic chiasm:
-Info from nasal retinae decussates
-Info from temporal retinae remains ipsilateral

Each optic tract contains fibres carrying info about the contralateral visual field

= Right and left visual cortices receive information about the contralateral visual field

Question 10

CN II: Testing

Answer 10

1) Visual acuity
2) Visual fields
3) Pupillary light reflex
4) Fundoscopy

Pupillary light reflex

-Normal reflex – both pupils constrict when light is shone into either eye
-Involves CN II, and parasympathetic fibres in CN III

Question 11

CNs III, IV, VI = Eyes

Answer 11

All motor

Control the extra ocular muscles

Pass through the superior orbital fissure to enter the orbit

Question 12

CN III: Oculomotor

Answer 12

Innervates MR, SR, IR, IO, LPS
Carries parasympathetic fibre into the orbit —> constrict pupil

Nuclei: Midbrain
Nerves exit at the junction between the midbrain and pons
Close to posterior cerebral arteries

Question 13

Pupillary light reflex

Answer 13

Afferent limb (input)= CN II (lessens activity)
Efferent limb (output)= Parasympathetic fibres CN III

Shine light into one eye:
-Ipsilateral pupil constricts= direct response
-Contralateral pupil constricts= consensual response

E.g. shine light into right eye:
-Right pupil constricts (direct)
-Left pupil constricts (consensual)

Question 14

CN IV: Trochlear

Answer 14

Innervates Superior Oblique

Nuclei in the midbrain
Isolated lesions rare

Paralysis of SO= diplopia on looking down

Question 15

CN VI: Abducens

Answer 15

Innervates lateral rectus

Nuclei in the pons
Paralysis of LR= Medial deviation of the eye (unopposed action of medial rectus), unable the abduct the eye on examination

Question 16

CN V: Trigeminal

Answer 16

Attached to the pons

3 branches
-Opthalmic- V1- superior orbital fissure
-Maxillary- V2- foramen rotundum
-Mandibullar- V3- foramen ovale

All 3 carry sensory fibres- extensive distribution in the head

Mandibular division carries motor fibres to the muscles of mastication

Question 17

CN V: Sensory fibres

Answer 17

Branches of CN V carry general sensation (touch, pressure, pain, temp) from the
-Dura, face and scalp, cornea, nose and mouth
-Tongue- general sensation anterior 2/3

Also carries proprioception from TMJ (Temporomandibular joint) and muscles of mastication

Question 18

CN V: Testing sensation

Answer 18

Test general sensation over the face- compare left and right

Test the corneal reflex= (between CN V (corneal sensation) and CN VII (muscles of facial expression))

Gently touching the cornea should result in blinking (contraction of orbicularis oculi)