Cranial Nerves: Anatomy and Function

Cranial Nerves Overview

• Twelve paired nerves originate directly from the brain, unlike spinal nerves from the spinal cord.
• Cranial nerves traverse skull foramina to reach their target destinations.
• These nerves facilitate motor and sensory innervation of head and neck structures, and also distribute autonomic fibers.

Naming and Numbering

• Cranial nerves are designated by Roman numerals (I-XII) based on their anterior-to-posterior order in the brain.
• Their names are indicative of their primary roles or anatomical paths.

Functional Components

• Cranial nerves may possess one or more of these functional components:
  • General Sensory (GSA): Conveys sensory data from skin, muscles, and joints.
  • Special Sensory (SSA): Deals with sensory input for vision, hearing, balance, taste, and smell.
  • General Visceral Sensory (GVA): Relays sensory information from internal organs.
  • General Somatic Motor (GSE): Manages motor signals to skeletal muscles.
  • Branchial Motor (SVE): Supplies motor innervation to muscles derived from pharyngeal arches; also known as Special Visceral Efferent.
  • General Visceral Motor (GVE): Delivers autonomic motor input to smooth muscle, cardiac muscle, and glands; also known as Parasympathetic.

Olfactory Nerve (I)

• Function: Special Sensory (SSA) for smell.
• Pathway: Olfactory receptors in nasal mucosa project to olfactory bulb, then olfactory tract, and finally olfactory cortex.
• Clinical Significance: Anosmia, or loss of smell, may arise from head trauma, nasal congestion, or neurodegenerative conditions.

Optic Nerve (II)

• Function: Special Sensory (SSA) for vision.
• Pathway: Originates in the retina, extends via the optic nerve to the optic chiasm, then the optic tract, the lateral geniculate nucleus (LGN) of the thalamus, and ultimately the visual cortex.
• Clinical Significance: Lesions along the optic pathway can lead to visual field defects, like scotomas or hemianopsia; optic neuritis can induce pain and vision impairment.

Oculomotor Nerve (III)

• Function:
  • General Somatic Motor (GSE): Provides motor innervation to most extraocular muscles, including the superior rectus, inferior rectus, medial rectus, inferior oblique, and levator palpebrae superioris.
  • General Visceral Motor (GVE): Supplies parasympathetic innervation to the pupillary constrictor and ciliary muscle.
• Pathway: From the midbrain through the superior orbital fissure to the extraocular muscles; the Edinger-Westphal nucleus projects to the ciliary ganglion, then to the pupillary constrictor and ciliary muscle.
• Clinical Significance:
  • Oculomotor nerve palsy can manifest as ptosis (drooping eyelid), mydriasis (pupil dilation), and diplopia (double vision).
  • Affected individuals may exhibit a "down and out" gaze.

Trochlear Nerve (IV)

• Function: General Somatic Motor (GSE) for the superior oblique muscle.
• Pathway: Emerges from the midbrain, passes through the superior orbital fissure, and innervates the superior oblique muscle.
• Clinical Significance: Trochlear nerve palsy can result in vertical diplopia, especially during downward gaze.

Trigeminal Nerve (V)

• Function:
  • General Sensory (GSA): Affords sensory innervation to the face, oral cavity, nasal cavity, and anterior two-thirds of the tongue.
  • Branchial Motor (SVE): Governs motor innervation for the muscles of mastication.
• Branches:
  • Ophthalmic (V1): Sensory input from the forehead, upper eyelid, cornea, and nose.
  • Maxillary (V2): Sensory input from the lower eyelid, cheek, nasal mucosa, upper lip, and upper teeth.
  • Mandibular (V3): Sensory input from the lower lip, chin, and anterior two-thirds of the tongue (excluding taste), as well as motor control of the muscles of mastication.
• Pathway: Originates in the trigeminal ganglion, leading to sensory nuclei in the brainstem and the motor nucleus in the pons, which connects to the muscles of mastication.
• Clinical Significance: Trigeminal neuralgia (tic douloureux) results in intense facial pain; trigeminal nerve lesions can lead to loss of sensation or weakness in mastication.

Abducens Nerve (VI)

• Function: General Somatic Motor (GSE) to the lateral rectus muscle.
• Pathway: Originates in the pons, traverses the superior orbital fissure, and terminates at the lateral rectus muscle.
• Clinical Significance: Abducens nerve palsy causes horizontal diplopia and impairs the ability to abduct the eye.

Facial Nerve (VII)

• Function:
  • Branchial Motor (SVE): Controls motor innervation of facial expression muscles, stylohyoid, and stapedius.
  • Special Sensory (SSA): Facilitates taste sensation from the anterior two-thirds of the tongue.
  • General Visceral Motor (GVE): Provides parasympathetic innervation to the lacrimal, submandibular, and sublingual glands.
• Pathway: From the pons through the internal acoustic meatus to the facial canal, exiting via the stylomastoid foramen to reach the muscles of facial expression; the chorda tympani extends to the anterior two-thirds of the tongue; the greater petrosal nerve targets the lacrimal gland; the submandibular ganglion links to the submandibular and sublingual glands.
• Clinical Significance:
  • Bell's palsy induces unilateral facial paralysis.
  • Other potential effects include loss of taste on the anterior two-thirds of the tongue and dry eyes.

Vestibulocochlear Nerve (VIII)

• Function: Special Sensory (SSA) for hearing and balance.
• Branches:
  • Vestibular: Responsible for balance and spatial orientation.
  • Cochlear: Facilitates hearing.
• Pathway: From the inner ear through the internal acoustic meatus to the vestibular and cochlear nuclei in the brainstem.
• Clinical Significance: Vestibular lesions may lead to vertigo, nystagmus, and imbalance; cochlear lesions can cause hearing loss and tinnitus.

Glossopharyngeal Nerve (IX)

• Function:
  • Branchial Motor (SVE): Provides motor innervation to the stylopharyngeus muscle.
  • Special Sensory (SSA): Conveys taste sensations from the posterior one-third of the tongue.
  • General Sensory (GSA): Relays sensory information from the posterior one-third of the tongue and pharynx.
  • General Visceral Sensory (GVA): Gathers sensory input from the carotid body and sinus.
  • General Visceral Motor (GVE): Provides parasympathetic innervation to the parotid gland.
• Pathway: From the medulla through the jugular foramen to the stylopharyngeus muscle, posterior one-third of the tongue, carotid body and sinus, and the otic ganglion en route to the parotid gland.
• Clinical Significance: Lesions of the glossopharyngeal nerve can result in loss of taste on the posterior one-third of the tongue, difficulty swallowing, and loss of the gag reflex.

Vagus Nerve (X)

• Function:
  • Branchial Motor (SVE): Manages motor innervation for the muscles of the pharynx and larynx.
  • General Sensory (GSA): Collects sensory data from the dura of the posterior cranial fossa and the external auditory canal.
  • General Visceral Sensory (GVA): Conveys sensory information from the thoracic and abdominal viscera.
  • General Visceral Motor (GVE): Handles parasympathetic innervation for the thoracic and abdominal viscera.
• Pathway: From the medulla through the jugular foramen to the pharynx, larynx, and the thoracic and abdominal viscera.
• Clinical Significance: Vagus nerve lesions may cause hoarseness, dysphagia, and an impaired gag reflex; cardiac and gastrointestinal dysfunction can also manifest.

Accessory Nerve (XI)

• Function: General Somatic Motor (GSE) for the sternocleidomastoid and trapezius muscles.
• Pathway: Originates in the spinal cord and medulla, passes through the jugular foramen, and then innervates the sternocleidomastoid and trapezius muscles.
• Clinical Significance: Accessory nerve lesions can lead to weakness or paralysis of the sternocleidomastoid and trapezius muscles, resulting in difficulty turning the head and shrugging the shoulders.

Hypoglossal Nerve (XII)

• Function: General Somatic Motor (GSE) for the muscles of the tongue.
• Pathway: From the medulla through the hypoglossal canal to the muscles of the tongue.
• Clinical Significance: Hypoglossal nerve lesions can cause tongue weakness and deviation towards the affected side during protrusion.