Cranial Nerves I-VI

Cranial Nerves I-VI

• Cranial nerves I-VI, including optical pathways and lesions, and visual reflexes are covered in Year 2, Semester 1 of the Medicine course.
• The lecture was given by Dr. Vijayalakshmi S B on 19/11/2024.
• Learning outcomes for the lecture included describing the visual pathway and its potential lesions, comparing and contrasting pupillary light and accommodation reflexes, the corneal reflex, common pathologies causing visual disturbances, cranial nerves I-VI, their nuclei, components, and courses.
• Foramina, relations, and functions of cranial nerves I-VI, effects of lesions on cranial nerves I-VI, and testing cranial nerves I-VI are also key topics to be covered.

Cranial Nerves (Detailed)

• The first two cranial nerves (olfactory and optic) are not part of the peripheral nervous system (PNS) but instead are extensions of the forebrain.
• Olfactory (I): Sensory for smell.
• Optic (II): Sensory for vision.
• Oculomotor (III): Motor: ciliary muscles, pupils, extrinsic eye muscles (except those for IV and VI).
• Trochlear (IV): Motor: superior oblique muscle of the eye.
• Trigeminal (V): Sensory: face, oral/nasal mucosa, teeth, anterior two-thirds of tongue; Motor: muscles of mastication and 4 other muscles.
• Abducens (VI): Motor: lateral rectus muscle of the eye.
• Facial (VII): Motor: muscles of facial expression and 3 other muscles. Sensory: taste from anterior two-thirds of tongue, soft palate. Sensory and intermediate nerves connected.
• Vestibulocochlear (VIII): Vestibular nerve: equilibrium, motion; Cochlear nerve: hearing.
• Glossopharyngeal (IX): Motor: stylopharyngeus, parotid gland. Sensory: taste from posterior third of tongue, pharynx, tonsillar sinus, pharyngotympanic tube, middle ear cavity.
• Vagus (X): Motor: palate, pharynx, larynx, trachea, bronchi, heart, gastrointestinal tract to the left colic flexure. Sensory: pharynx, larynx, sensory from tracheo-bronchial tree, lungs, heart, gastrointestinal tract to the left colic flexure.
• Spinal accessory (XI): Motor: sternocleidomastoid and trapezius.
• Hypoglossal (XII): Motor: all intrinsic and extrinsic tongue muscles (excluding palatoglossus).

Cranial Nerve Origins

• Cranial nerves originate from different parts of the developing brain.
• Developmentally, CN I and II develop from the prosencephalon, while CN III to VII develop from the midbrain and the brain stem. The remaining cranial nerves (VIII-XII) develop further from the brain stem.

Cranial Nerves – Emergence from CNS

• The image shows the point of emergence from the central nervous system and the various cranial nerves.
• Olfactory (I)
• Optic (II)
• Oculomotor (III)
• Trochlear (IV)
• Trigeminal (V)
• Abducent (VI)
• Facial (VII)
• Vestibulocochlear (VIII)
• Glossopharyngeal (IX)
• Vagus (X)
• Spinal accessory (XI)
• Hypoglossal (XII)

Functional Components

• Somatic Motor (GSE): Eyeball and tongue muscles (CNs 3, 4, 6, and 12), and innervation to skeletal muscles (CNs V3/Vc, 7, 9, 10, and 11).
• Branchial Motor (SVE/BE): Muscles of mastication, Facial muscles, muscles of pharynx, larynx, and palate (CNs 5, 7, 9, 10 and 11).
• Visceral Motor (GVE): Lacrimal, Salivary glands, ciliary and sphincter pupillae muscles, heart, lungs, and gastrointestinal (GI) organs (CNs 3, 7, 9, and 10).
• Sensory (GSA) and sensory input from viscera (GVA): Pain, temperature, touch, pressure, and proprioception (CN 5,7,9 and 10), sensory input from viscera (CN 9 and 10).
• Special Sensory (SA): Sensory input for smell, vision, taste, sound, and balance (CN 1, 2, 7, 8, 9 and 10).

Motor Cranial Nerves

• Somatic Motor (GSE): CN III, IV, VI, and XII, controlling eyeball, eyelid, and tongue.
• Branchiomotor (SVE): CN V (branchial arches), VII, IX, X, XI: involved in mastication, facial expression, etc.
• Parasympathetic (GVE): CN III, VII, IX, and X: smooth muscle, heart muscle, ciliary muscles, and secretomotor functions (glands).

Sensory Cranial Nerves

• Somatic Sensory (GSA): Perception of touch, pain, temperature, and proprioception (CN 5, 7, 9, and 10).
• Special Visceral Sensory (SA): Smell, vision, sound, balance, and taste (CN I, II, VII, VIII, IX, and X).
• Visceral Sensory (GVA): Sensory input from viscera (CN 9 and 10).

Olfactory Nerve – (I)

• The olfactory nerve is a sensory nerve for smell.
• Olfactory receptors in the nasal cavity send signals to the olfactory bulb.

Optic Nerve (II)

• Optic nerve (II): Sensory for vision.
• Visual pathway tracing light to visual cortex, including optic nerves, optic tract, optic chiasm, lateral geniculate nuclei, optic radiation.

Photoreceptors and Bipolar Cells

• Photoreceptors and bipolar cells in the retina process light signals. Primary bipolar cells in the retina receive stimulus from photoreceptors. Secondary ganglion cells send signals in the optic nerve (decussation is partial in the optic chiasm). Finally, tertiary cells in the lateral geniculate nucleus (thalamus) project to the visual cortex.

Ganglion Cells and Optic Nerve

• Ganglion cells receive signals from bipolar cells and transmit signals along the optic nerve through the optic chiasm. Light stimuli are transmitted through the retina to the optic nerve. The nerve fibers continue to the optic chiasm for partial decussation (crossing). Tertiary cells are in the lateral geniculate nucleus for projections to the visual cortex.

Optic Chiasm and Optic Tract

• Optic chiasm: Partial decussation (crossing) of optic nerve fibers.
• Optic tract: continuation of optic nerve after the chiasm.

Lateral Geniculate and Optic Radiation

• Lateral geniculate nucleus (LGN): Part of the thalamus that processes visual information.
• Optic radiation: Fibers carrying visual information from the LGN to the visual cortex.

Projection to Visual Cortex

• Projection of visual information to the visual cortex in the brain. The visual cortex is located in the occipital lobe. Tertiary cells are in the lateral geniculate nucleus for projections to the visual cortex.

Cranial Nerve III - Oculomotor

• Eye movements
• Voluntary ocular muscles
• Parasympathetic supply.
• Includes accommodation (changes in lens curvature for focusing on nearby objects), pupil constriction, and convergence (eyes move medially).
• Damage to the oculomotor nerve can cause double vision or dilated pupils.

Clinical Aspects

• Various clinical examples of damage to cranial nerves (e.g., visual agnosia, achromatopsia, prosopagnosia).
• Includes disorders like Argyll Robertson pupil to illustrate nerve damage manifestations.

Trochlear (IV) and Abducens (VI)

• Control eye movements specifically for superior oblique (CN IV) and lateral rectus (CN VI) muscles or eye movements.
• Testing and dysfunction examples explained.

Trigeminal Nerve (V)

• The largest cranial nerve (sensory and motor).
• Sensory: face, oral/nasal mucosa, teeth, anterior two-thirds of tongue.
• Motor: Muscles of mastication, and additional muscles (V3).
• Nuclei: main sensory, spinal nucleus, mesencephalic nucleus, and motor nucleus.
• Ganglion, divisions and branches covered.

Trigeminal Ganglion

• In the middle cranial fossa, containing cell bodies for sensory neurons of the trigeminal nerve.
• Semilunar shape, located within the trigeminal or Meckel's cave.
• Three terminal divisions emerge from this ganglion (ophthalmic, maxillary, and mandibular).

Corneal Reflex

• A reflex involving the trigeminal (sensory) and facial (motor) nerves.
• Gently touching the cornea should cause a blink response initiated by the facial nerve's motor nucleus.

Practice Questions

• There are practice questions available.