Summary

• Cranial nerves are a set of 12 peripheral nerves emerging from the brain, innervating structures in the head, neck, thorax, and abdomen
• Modalities of cranial nerves:

  • Efferent: information goes from brain to the periphery (motor)
  • Afferent: information from periphery to brain (sensory)
  • Mixed nerves carry out both functions
  • Information is classified as special if it travels from special senses (vision, smell, taste)
  • Somatic: related to skin and skeletal muscles
  • Visceral: related to internal organs
• Olfactory nerve (CN I):

  • Carries special sensory information only
  • Originates in the Telencephalon
  • Transmits sense of smell from nasal cavity
  • Penetrates the small foramina in the cribriform plate of the ethmoid bone
  • Olfactory receptors synapse with mitral cells forming synaptic glomeruli
  • Olfactory tract divides into lateral and medial stria
• Optic nerve (CN II):

  • Responsible for transmitting special sensory information for vision
  • Arises from bipolar ganglion cells in the retina
  • Divided into extra cranial and intracranial pathways
  • Fibers from nasal half of each retina cross over at the optic chiasm
  • Optic radiation carries visual information to the visual cortex
• Oculomotor nerve (CN III):

  • Provides motor and parasympathetic innervation to structures within the bony orbit
  • Originates from oculomotor nucleus within the midbrain and Edinger-Westphal nucleus
  • Functions in eye movement and pupil constriction
• Trochlear nerve (CN IV):

  • Arises from the trochlear nucleus in the dorsal aspect of the midbrain
  • Innervates the superior oblique muscle causing depression and intrusion of the eye
  • Exits via the superior orbital fissure
  • Decussates and emerges from the dorsal aspect
• Trigeminal nerve (CN V) innervates the superior oblique muscle, causing depression and intrusion of the eye
• Trigeminal nerve exits via the superior orbital fissure
• Trigeminal nerve is the only nerve that decussates and emerges from the dorsal aspect of the midbrain
• Trigeminal nerve is mainly General Somatic Afferent (GSA) for the head and has a small Special Visceral Efferent (SVE) function
• Trigeminal nerve has four nuclei:

  • One motor nucleus
  • Three sensory nuclei (mesencephalic, principal sensory, and spinal nuclei of trigeminal nerve)
• Trigeminal nerve emerges from the ventrolateral aspect of the pons, with a large sensory root and a small motor root
• In the middle cranial fossa, the sensory nerves form the trigeminal ganglion housed in Meckel’s cave
• Nuclei of Trigeminal nerve (CN V) arise from 4 nuclei:

  • Spinal nucleus of trigeminal nerve responsible for exoreceptive sensation: pain, temperature, from face and upper part of neck
  • Principal sensory nucleus in the upper pons responsible for crude sensation
  • Mesencephalic nucleus in the midbrain responsible for proprioceptive sensations
  • Motor nucleus for muscles of mastication
• Course of exoreceptive sensations from the face:

  1. From ophthalmic, maxillary, mandibular divisions to trigeminal ganglion

  2. Then to spinal nucleus of trigeminal nerve

  3. Crosses to the opposite side and ascends with trigeminal lemniscus

  4. Passes by VPM nucleus of thalamus

  5. Thalamic radiation to primary somatosensory area
• Lingual branch of the mandibular division provides general sensations to the anterior 2/3 of the tongue and floor of the mouth
• Abducens nerve (CN VI) provides General Somatic Efferent (GSE) innervation to the lateral rectus muscle of the eye
• Vestibulocochlear nerve (CN VIII) is composed of vestibular (balance) and cochlear (hearing) fibers and exits via the internal acoustic meatus
• Abducens nerve arises from the Abducens nucleus in the pons and exits the brainstem via the superior orbital fissure
• Abducens nerve function is to provide innervation to the lateral rectus muscle, one of the extraocular muscles
• Glossopharyngeal nerve (CN IX) leaves the skull via the jugular foramen and leaves the medulla through the postolateral fissure
• Vagus nerve (CN X) exits via the jugular foramen and has various nuclei including Nucleus Ambiguus, Dorsal motor nucleus, Vagal ganglion, inferior and superior, Rostral nucleus solitaries (special), and caudal nucleus solitaries (general)
• Accessory nerve (CN XI) arises mainly from the first 5 to 6 cervical parts of the spinal cord and caudal medulla, innervating sternocleidomastoid and trapezius muscles (SVE)
• Hypoglossal nerve (CN XII) provides General Somatic Motor (GSE) function, innervating extrinsic muscles of the tongue and exiting the medulla through the anterolateral fissure
• Facial nerve (CN VII) is both a motor and sensory nerve with three nuclei: main motor nucleus, parasympathetic nucleus, and sensory nucleus
• Facial nerve leaves the skull through the internal acoustic meatus, enters the inner ear cavity, and forms the geniculate ganglion in the middle ear cavity
• Facial nerve branches in the middle ear cavity include:

  1. Greater superficial petrosal nerve (parasympathetic)

  2. Nerve to stapedius

  3. Chorda tympani (taste to anterior 2/3 of the tongue)
• Facial nerve motor branch leaves the skull via the stylomastoid foramen
• UMN lesion of the facial nerve would cause weakness in the contralateral lower part of the face
• Parasympathetic nuclei of the facial nerve lie posterolateral to the main motor nucleus and include the superior salivatory nucleus concerning taste and lacrimal nucleus for emotional response and reflex lacrimation
• Facial nerve emerges from the anterior surface between the medulla and pons, exits via the internal auditory meatus, and leaves the skull through the stylomastoid foramen
• Oculomotor nerve (CN III) is responsible for most eye movements: up, down, and pupil constriction
• Oculomotor nerve consists of the main motor nucleus supplying all extrinsic eye muscles except the superior oblique and lateral rectus
• Accessory parasympathetic nucleus (Edinger-Westphal nucleus) of the oculomotor nerve supplies intrinsic eye muscles and is involved in accommodation and light reflex
• Midbrain houses CN III, CN IV, and CN V, pons houses CN V, CN VI, CN VII, and CN VIII, and medulla oblongata houses CN IX and CN XII
• Branches of the Trigeminal nerve include:

  • Ophthalmic division (V1) for GSA above the lower eyelid
  • Maxillary division (V2) for GSA from lower eyelid to upper lip
  • Mandibular division (V3) for GSA and SVE below the upper lip for muscles of mastication