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Introduction

  • Peripheral nervous system is made up of spinal and cranial nerves
    • Spinal nerves supply limbs and trunk
    • Cranial nerves supply the head and neck
  • Cranial nerves are numbered in order as they arise from the brain from superior to inferior
  • There are 12 pairs of cranial nerves (numbered based on anterior to posterior)
    • CN1: Olfactory
    • CN2: Optic
    • CN3: Oculomotor
    • CN4: Trochlear
    • CN5: Trigeminal
    • CN6: Abducens
    • CN7: Facial
    • CN8: Vestibulocochlear
    • CN9:Glossopharyngeal
    • CN10: Vagus
    • CN11: Accessory
    • CN12: Hypoglossal
  • Brainstem recall:
    • See diagram
    • Called the pyramid because of 1) the pyramidal tract running through it and 2) pyramidal cells lie within it

Functional divisions of cranial nerves

  • supply structures mostly in head and neck
  • motor (III, IV, VI, XI, XII)
  • sensory (I, II, VIII)
  • motor and sensory (V, VII, IX, X)
  • parasympathetic (III, VII, IX, X)
  • For each cranial nerve, know origin, course and whether it's motor, snsory, etc
  • For CN3, we can divide it into special sensory and somatosensory

CN1 – Olfactory nerve

  • Sensory nerve responsible for smell
  • Originates from the forebrain
    • Formed by axons of specialised olfactory hair cells, that originate from olfactory mucosa
      • Olfactory hairs that lie in mucus. Impulses pass through bipolar nerve, through cribriform plate, and synapse with cells in the olfactory bulb. Olfactory tracts from the olfactory bulb run back to the temporal lobe.
  • Enter the nasal cavity through holes in the cribriform plate of the ethmoid bone
  • Down-hanging dendrites extend in the olfactory epithelium of the nasal cavity
    • These axons terminate in the olfactory bulb on the inferior surface of the frontal lobe
  • Primitive, thus doesn’t go through the thalamus (weird cranial nerve)
    • Feeds directly into the limbic system (temporal lobe) via olfactory stria to the olfactory area of the temporal lobe
    • Temporal lobe also holds hearing and emotion

CN2 – Optic nerve

  • Transmits visual impulses from the retina to the thalamus
  • Enters the cranial cavity through the optic canal (foramen; under the anterior clinoid process) where it travels with the ophthalmic artery
    • The pair of nerves meet in the midline in the cranial cavity where they form the optic chiasm
    • Optic nerve is very thick, receiving millions of neurons from the corresponding eye
    • Some of the fibres (not all of the fibres) cross over at the optic chiasm
    • Note that the olfactory nerve is just superior to the midbrain
  • After the optic chiasm, the part that projects backwards is the optic tract
  • So we have optic nerve --> optic chiasm --> optic tract

Muscles of the orbit

  • Superiorly: frontal bone, medially: nasal bone, posterior: superior orbital fissure (where CN 3,4,6 enter)
  • Extraocular muscles are attached to the globe of the eye
    • NB: there are also muscles within the eye: sphincter pupillae
  • Back from the eyeball is the optic nerve
  • Supplied by CN3, 4, 6
    • Both extraocular (skeletal) and intrinsic (smooth)
  • 6 extraocular muscles:
  • 4 rectus (L. straight) muscles
    • Superior inferior, medial and lateral rectus
    • Insert into the superior, inferior, medial and lateral sides of the eye sclera
    • Arise from the common tendinous ring (around the superior orbital fissure)
      • Surrounds the orbital opening of the optic canal and the medial end of the superior orbital fissure
    • Lateral rectus abducts the eyeball; medial rectus adducts the eye
    • Superior rectus moves the eyeball upwards and medially. Similarly for inferior rectus (see diagram)
  • 2 oblique muscles
    • Superior oblique
      • Arises from the medial wall of the orbit (sphenoid bone)
      • Follows the medial wall before becoming a tendon that passes through a pulley (trochlear)
      • Inserts into the sclera on the superolateral aspect
    • Inferior oblique
      • Arises from the floor of the front of the orbit
      • Inserts into the sclera on the inferolateral aspect of the eye
  • The superior rectus and superior oblique both attach to the posterior surface of the globe of the eye, so they actually cause the opposite movement to what you think. I.e. superior oblique causes the pupil to move laterally and inferiorly. Superior rectus helps you carry out reading etc.
  • Levator palpabrae superioris muscle
    • Arises from the sphenoid bone just above the optic foramen
    • Inserts into the upper eyelid
  • 3 intrinsic (smooth muscle under autonomic control)
    • Ciliary muscle – thickness of the lens of the eye, supplied by the parasympathetic (CN3)
    • Sphincter pupillae muscles – close the pupil, supplied by the parasympathetic (CN3)
    • Dilator pupillae muscles – open the pupil, supplied by the sympathetic
  • Axis of orbit is different to axis of eyeball; this restricts movements of the eye. Important in diagnosing cranial nerve issues using eye movements
  • In summary, memorise the movements of the eyeball and the muscles that cause it, and the nerves that supply those muscles.
  • Rotundum = for maxillary nerve (part of CN5).
  • Ovale =
  • See diagram showing where CN 3, 4 and 6 arise from the brainstem. CN3 arises from within the interpeduncular fossa and the trochlear nerve arises from the posterior surface of the midbrain, just below the inferior colliculus. The abducent nerve arises from the junction between the pons and the medulla
  • All these nerves head towards the orbit via the superior orbital fissure

CN3 – Oculomotor Nerve

  • Made up of 2 types of fibres (functionally split)
    • Somatic motor fibres
      • Supply: all extraocular muscles (except superior oblique and lateral rectus), as well as levator palpebrae superioris m (elevates the superior eyelid: note ptosis)
    • Preganglionic pararsympathetic fibres (NB: in other cranial nerves, they increase glandular secretions; there are ganglia inside the orbit)
      • Supply: intrinsic eye muscles
      • Ciliary muscle in the eye (lens fatter)
        • Accommodation
      • Constrictor (sphincter) pupillae in eye: 1) bright light, 2) accommodation
  • Both arise from different nuclei in the midbrain
    • Emerge at the interpeduncular fossa and pierce the dura
    • Run along the wall of the cavernous sinus and enter the orbit through the superior orbital fissure at the common tendinous ring
  • After entering the orbit, the nerve splits into branches that innervate the deep surface of each muscle (including levator palpebrae superioris)
    • The preganglionic fibres split off the inferior rectus branch and synapse with postganglionic cells in the ciliary ganglion
      • Located lateral to the optic nerve and the size of a pinhead
    • Supply the sphincter pupillae muscles (close pupils against light) and the ciliary muscle (lens thickness)

CN4 – Trochlear nerve

  • Smallest cranial nerve
  • Arises from the trochlear nucleus in the midbrain
    • Exits the brainstem on the dorsal surface below the inferior colliculus
    • Pierces the dura and enters the cavernous sinus
      • Travels inferior to CN3
    • Enters the orbit through the superior orbital fissure (along with CN3) inside the common tendinous ring and supplies superior oblique (the muscle associated with the trochlea)

CN6 – Abducens nerve

  • Nerve that abducts the eye – supplies the lateral rectus muscle
    • 3x the size of CN4
  • Originates from cells in the caudal pons (abducens nucleus)
  • Emerges at the pontomedullary junction lateral to the midline
  • Pierces the dura, passes through the cavernous sinus
  • Enters the orbit via the superior orbital fissure inside the common tendinous ring, then supplies lateral rectus
  • Raised intracranial pressure can compress this nerve and thus we get clinical signs of the eyes
    • Ie: the person can look cross-eyed