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Anatomy of the Nervous System and Principles of Localization

Peripheral Nervous System

Cranial Nerves

• Definition: Cranial nerves are peripheral nerves connected to the brain.
• Number: 12 pairs.

Cranial Nerve	Exit from Skull	Brain Connection	Function
Olfactory (I)	Olfactory Foramen	Cerebrum (olfactory sulcus)	Olfaction
Optic (II)	Optic Canal	Diencephalon (optic chiasm)	Vision
Oculomotor (III)	Superior Orbital Fissure	Midbrain (interpeduncular fossa)	Eye movement, pupil constriction
Trochlear (IV)	Superior Orbital Fissure	Midbrain (superior medullary velum)	Control of upward eye deviation
Trigeminal (V)	Superior Orbital Fissure, Foramen Rotundum, Foramen Ovale	Pons (pons peduncle)	Facial sensation, mastication
Abducens (VI)	Superior Orbital Fissure	Pons (middle of pontomedullary sulcus)	Lateral eye movement
Facial (VII)	Internal Acoustic Meatus	Pons (lateral pontomedullary sulcus)	Facial expression, taste
Vestibulocochlear (VIII)	Internal Acoustic Meatus	Pons (lateral pontomedullary sulcus)	Hearing and balance
Glossopharyngeal (IX)	Jugular Foramen	Medulla oblongata (posterior groove of olive)	Taste, pharyngeal sensation
Vagus (X)	Jugular Foramen	Medulla oblongata (posterior groove of olive)	Visceral control, pharyngeal muscles
Accessory (XI)	Jugular Foramen	Medulla oblongata (posterior groove of olive)	Control of sternocleidomastoid and trapezius muscles
Hypoglossal (XII)	Hypoglossal Canal	Medulla oblongata (anterolateral sulcus)	Tongue movement

• One for smell, two for sight, three for eye movement, four for tilt, five for touch and chew, six for lateral gaze, seven for face and taste, eight for hearing and balance, nine for tongue and throat, ten for vagus, eleven for accessory, twelve for hypoglossal.
• Sensory nerves: I, II, VIII (Dad really dares).
• Mixed nerves: V, VII, IX, X (My wife is mixed).
• Without parasympathetic component: II, V, VI, XI (Second grade math problem 5+6=11).

Olfactory Nerve (Olfactory Nerve, 1)

• Origin: Olfactory epithelium in the upper nasal cavity.
• Bipolar neurons pass through the cribriform plate into the anterior cranial fossa, terminating at the olfactory bulb, whose neurons send fibers through the olfactory tract to the olfactory center.
• Special visceral sensory fibers; sensory nerve conducting olfaction.
• Localization Diagnosis:
  • Nasal mucosa: Local lesions affecting both sides can cause decreased or absent sense of smell.
  • Olfactory nerve, bulb, and tract: Abnormalities (decreased or absent sense of smell); tumors in the frontal lobe or olfactory sulcus can cause unilateral or bilateral loss of smell.
  • Olfactory center: Lesions generally do not cause loss of smell but may cause olfactory disturbances (parosmia).

Optic Nerve (Optic Nerve, 2)

• Origin: Axons of retinal ganglion cells.
• The retina has three layers of neurons:
  • Outermost: Rods and cones.
  • Middle: Bipolar cells.
  • Innermost: Retinal ganglion cells.
• Pathway: Ganglion cells → optic canal → middle cranial fossa → above the sella turcica forming the optic chiasm → nasal half fibers cross to the contralateral side, temporal half remains ipsilateral → form the optic tract → lateral geniculate nucleus → decussate → posterior limb of internal capsule → optic radiation → occipital visual cortex.
• Localization Diagnosis:

Affected Area	Clinical Presentation	Common Causes
Optic nerve	Unilateral blindness, direct pupillary reflex (-), consensual reflex (+)	Optic neuritis, optic atrophy, tumor compression
Optic chiasm	Bitemporal hemianopia	Pituitary adenoma, craniopharyngioma, aneurysm
Optic tract	Contralateral homonymous hemianopia in both eyes, direct pupillary reflex (-) on the affected side	Stroke, brain tumor, head trauma
Optic radiation	Upper quadrantanopia if damaged in the parietal lobe, lower quadrantanopia if damaged in the temporal lobe	Stroke, brain tumor, head trauma
Occipital visual center	Contralateral homonymous hemianopia, macular sparing, visual agnosia	Occipital infarction, occipital tumor, head trauma

Oculomotor, Trochlear, and Abducens Nerves (Oculomotor Nerve, Trochlear Nerve, Abducens Nerve, 3, 4, 6)

• Origins:
  • Oculomotor: Oculomotor nucleus located below the aqueduct in the periaqueductal gray matter.
  • Trochlear: Originates from the trochlear nucleus in the inferior colliculus.
  • Abducens: Originates from the abducens nucleus in the midline of the pons.
• The oculomotor, trochlear, and abducens nerves, through their innervated muscles, collectively control eye movements and various reflexes.
• Localization Diagnosis:
  • Ocular Muscle Paralysis and Diplopia:
    • Causes: When the oculomotor, trochlear, abducens nerves and their nuclei, or connecting fibers are diseased.

Type	Ocular Muscle Paralysis	Description	Clinical Presentation	Possible Causes
Peripheral nerve paralysis
⤷ Oculomotor nerve paralysis	Motor fiber damage	Ptosis, paralysis of levator palpebrae superioris, superior rectus, medial rectus, inferior oblique, and inferior rectus; paralysis of the sphincter pupillae	Ptosis, restricted eye movement inward, upward, and downward, dilated pupil, loss of light reflex	Intracranial aneurysm, tuberculous meningitis
⤷ Trochlear nerve paralysis	Paralysis of the superior oblique muscle	Restricted downward and outward eye movement	Restricted downward and outward eye movement on the affected side, diplopia when looking down or descending stairs	-
⤷ Abducens nerve paralysis	Paralysis of the lateral rectus muscle	Esotropia, limited or no lateral eye movement	Esotropia, limited or no lateral eye movement on the affected side	Nasopharyngeal cancer intracranial infiltration, diabetes
Nuclear paralysis	Brainstem lesion	Ocular motility disorder	Similar to peripheral ocular muscle paralysis, selective damage to individual nuclei, often accompanied by other brainstem lesions, can be bilateral	Brainstem lesion
Internuclear paralysis
⤷ One-and-a-half syndrome	Pontine tegmental lesion	Horizontal gaze limitation	The affected eye cannot move medially or laterally; the contralateral eye cannot move medially but can move laterally with nystagmus	Pontine lesion
Pupillary changes
Pupillary phenomena	Pupillary size regulation	The oculomotor nerve parasympathetic fibers innervate the sphincter pupillae; postganglionic fibers from the cervical sympathetic ganglia innervate the dilator pupillae	Normal pupil diameter 3-4 mm in ordinary light; less than 2 mm is miosis, greater than 5 mm is mydriasis	Intracranial aneurysm compressing the oculomotor nerve; Horner's syndrome

Trigeminal Nerve (Trigeminal Nerve, 5)

• Origin: Pons trigeminal motor nucleus, exiting the brainstem ventrally, passing through the foramen ovale.
• Mixed nerve:
  • Motor fibers innervate: Masseter, temporalis, medial and lateral pterygoid, and tensor tympani muscles.
  • Sensory nerve participation:
    • Originating structure: Trigeminal ganglion located at the trigeminal impression on the petrous apex, containing pseudounipolar neurons, divided into three peripheral branches.
    • Distribution