Cranial Nerves Flashcards
This deck to help us understand in detail the 12 cranial nerves.
List the 12 cranial nerves, stating whether the nerve has sensory functions, motor functions or both.
CN I - Olfactory nerves: Sensory
CN II - Optic nerves: Sensory
CN III - Occulomotor nerves: Motor
CN IV - Trochlear nerves: Motor
CN V - Trigeminal nerves: (Both)
V1 - Ophthalmic: Sensory
V2 - Maxillary: Sensory
V3 - Mandibular: Both
CN VI - Abducens nerves: Motor
CN VII - Facial nerves: Both
CN VIII - Vestibulocochlear nerves: Sensory
CN IX - Glossopharygneal nerves: Both
CN X - Vagus nerves: Both
CN XI - Accessory nerves: Motor
CN XII - Hypoglossal nerves: Motor
Mnemonic:
Some Say Make Money But My Brother Says Big Brains Matter More. [S - sensory, M - motor, B - both]
State the distribution, function, and clinical relevance of the olfactory nerve.
Distribution: Roof of nasal cavity
Function: Smell sensation
Clinical relevance: Anosmia - may be as a result of fracture of anterior cranial fossa
NOTES:
- Anosmia: Loss of sense of smell
- If there is damage to olfactory epithelium (which is a neuroepithelium), it regenerates. This is the only example of neuronal cell body, when damaged, is capable of regeneration.
State the distribution, function, and clinical relevance of the optic nerve.
Distribution: Retina
Function: Vision
Clinical relevance: Anopia (blindnes) [but there are many other clinical relevances]
NOTE:
- Optic nerve is a second order sensory neuron
State the course, distribution, function, and clinical relevance of the oculomotor nerve.
Course: The nerve emerges from the ventral aspect of the midbrain. It passes forwards into the middle cranial fossa and lies in the lateral wall of cavernous sinus. It divides into superior and inferior divisions and enters the orbital cavity through the superior orbital fissure.
Distribution: (1) extraocular muscles of the eye i.e. levator palpebrae superioris, superior rectus, inferior rectus, medial rectus and inferior oblique, (2) intraocular muscles in the iris that control the size of the pupil
Function (1): Eye movements
Clinical relevance (1): Strabismus [vision disorder in which the eyes do not look at the same place at the same time
Function (2): Pupillary constriction
Clinical relevance (2): Mydriasis [dilated pupils that do not change in response to changes in light levels]
State the course, distribution, function, and clinical relevance of the trochlear nerve.
Course: emerges from the dorsal aspect of the midbrain at the level of inferior colliculus and immediately decussates with the nerve of the other side. In the middle cranial fossa it lies in the lateral wall of cavernous sinus. The nerve enters the orbit through superior orbital fissure.
Distribution: Superior oblique muscle
Function: Eye movements
Clinical relevance: Strabismus
State the course, distribution, function, and clinical relevance of the trigeminal nerve.
Course: The trigeminal nerve is attached to the ventrolateral surface of the pons by two roots, a very large lateral sensory root, and a small medial motor root. The sensory root contains a ganglion (trigeminal/Gasserian ganglion) which lies at the apex of petrous part of the temporal bone in the Meckel’s cave. The trigeminal ganglion divides into three branches: (1) ophthalmic, (2) maxillary, and (3) mandibular nerves. The ophthalmic and maxillary nerves are sensory nerves, while the mandibular nerve has both motor and sensory fibres. The foramen of exit of the ophthalmic nerve is superior orbital fissure. The foramen of exit of the maxillary nerve is foramen rotundum, then through the inferior orbital fissure. Mandibular nerve exits through foramen ovale.
[Diagram: Meckel’s cave]
Distribution (sensory): face and scalp [Territories of each trigeminal nerve: V1, forehead; V2, between the eyes and mouth; V3, mandible going down]
Function (sensory): general sensations
Distribution (motor): 1st pharyngeal arch muscles attached to the mandible
Function (motor): mastication
State the course, distribution, function, and clinical relevance of the abducens nerve.
Course: it emerges in the groove between lower border of pons and medulla oblongata. It traverses forward in the medial wall of the cavernous sinus, lying below, and lateral to internal carotid artery. The nerve enters the orbit through superior orbital fissure.
Distribution: lateral rectus muscle (adducts the eye)
Function: eye movements
Clinical relevance: strabismus (specifically medial strabismus)
State the course, distribution, function, and clinical relevance of the facial nerve.
Course:
* Intraneural course: Fibres of facial nerve which emerge from the motor nucleus first run dorsally within the pons and wind around the nucleus of abducens nerve (internal genu of facial nerve) [Diagram] and then run forward and laterally to emerge at the pontomedullary junction medial to vestibulocochlear nerve. The sensory root of facial nerve is between the motor root of facial nerve and vestibulocochlear nerve.
* Extraneural course: The facial nerve and the vestibulocochlear nerve pass through the cerebellomedullary cistern and enter the internal acoustic meatus. Facial nerve runs along the roof of the inner ear and enters the facial canal in the middle ear. In the facial canal, the facial nerve traverses forward and laterally, forms a bend, the (external) genu of facial nerve where geniculate ganglion is located. The greater petrosal nerve arises from the geniculate ganglion. The nerve is then directed backward and is related to medial wall of the middle ear, above lateral semicircular canal. The second turn is downward, in the posterior wall of middle ear. In this segment it gives rise to the nerve to stapedius and chorda tympani nerve. The facial nerve leaves the middle ear through the stylomastoid foramen.
Distribution, function and clinical relevance:
1. 2nd pharyngeal arch muscles - facial expression. Clinical relevance: Bell’s palsy [temporary weakness or paralysis that usually affects one side of the face (contralateral)]
2. Anterior 2/3 of tongue (via chorda tympani nerve) - taste sensation. Clinical relevance: Ageusia [loss of sense of taste]
3. Submandibular and sublingual salivary glands (via chorda tympani) - salivation [Diagram: salivary glands]
4. Lacrimal gland - lacrimation [Diagram: lacrimal gland]
Clinical relevance of number 3 and 4: crocodile tears syndrome/Bogorad syndrome [the shedding of tears while eating or drinking in patients recovering from Bell’s palsy]
State the course, distribution, function, and clinical relevance of the vestibulocochlear nerve.
Course: The two components of the vestibulocochlear nerve emerge from the ventral aspect of the brainstem between lower border of pons and medulla, in the cerebellopontine angle. They traverse the posterior cranial fossa and then enter the internal acoustic meatus.
Distribution, function and clinical relevance [in that order]:
a. Vestibular apparatus, equilibrium sensation, vertigo [sensation of spinning or being off balance]
b. Cochlear duct, hearing sensation, hearing loss
State the course, distribution, function, and clinical relevance of the glossopharyngeal nerve.
Course: emerges from medulla as a series of rootlets between the olive and inferior cerebellar peduncle. It traverses the posterior cranial fossa and exits through the jugular foramen.
Distribution, function and clinical relevance:
~ Posterior 1/3 of tongue, gustation/taste sensation, ageusia
~ 3rd arch muscle (stylopharyngeus muscle), swallowing
~ Parotid gland, salivation, xerostomia
Further notes:
xerostomia: sensation of oral dryness which can result from diminished saliva production
State the course, distribution, function, and clinical relevance of the vagus nerve.
Course: The vagus nerve emerges as a series of rootlets in a groove between the olive and inferior cerebellar peduncle. It traverses the posterior cranial fossa and exits the skull through jugular foramen. The superior sensory ganglion of the nerve is located in the jugular foramen.
Distribution, function, clinical relevance [in that order]:
1. 6th pharyngeal arch muscles, phonation, dsyarthria [a condition characterized by difficuly in speech due to weakness or lack of control over the speech muscles]
2. Epiglottis, taste sensation, ageusia
3. Visceral organs, parasympathetic functions, multiple clinical relevances [The vagus nerve travels widely throughout the body affecting several organ systems and regions of the body, such as the tongue, pharynx, heart and gastrointestinal system.]
State the course, distribution, function, and clinical relevance of the accessory nerve.
Course: The spinal root is from the axons of nerve cells in the spinal nucleus from the upper five cervical segments. The nerve ascends into the skull through foramen magnum, traverses a short distance with the cranial root as they pass through jugular foramen.
[The fibers arising from the intracranial component are known as the cranial accessory nerve, while those fibers arising from the spinal nucleus are called the spinal accessory nerve.]
Distribution, function, clinical relevance [in that order]:
(a) Cranial accessory: Muscles of the 4th pharyngeal arch, swallowing, dysphagia [medical term for difficulty in swallowing]
[NB: Some books say that what is supplied by the cranial accessory is supplied by the vagus nerve, because the cranial accessory joined the vagus nerve, forming vago-accessory nerve. Until the debate is solved, either cranial accessory, vagus or vago-accessory nerve is correct for the 4th arch muscles.]
(b) Spinal accessory nerve: Trapezius and sternocledomastoid, shoulder and neck movements, torticollis [Torticollis, also known as wry neck, is a condition characterized by an abnormal, assymetrical head or neck position, usually due to tight muscles on one side of the neck.]
State the course, distribution, function, and clinical relevance of the hypoglossal nerve.
Course: The hypoglossal nerve emerges as rootlets from the ventral aspect of the medulla between pyramid and olive. It traverses the posterior cranial fossa and exits through hypoglossal canal.
Distribution: Genioglossus, hyoglossus, styloglossus and intrinsic muscles of tongue [Palatoglossus is a muscle of the 4th pharyngeal arch hence not supplied by CN XII. It is supplied by the vagus nerve.]
Function: tongue movements
Clinical relevance: dysarthria [a condition characterized by difficuly in speech due to weakness or lack of control over the speech muscles], tongue deviation towards paralysed side, tongue atrophy (ipsilateral)
What are cranial nerve nuclei?
These are the brainstem nuclei that give rise to the cranial nerves.
The cranial nerve nuclei contain the cell bodies of what neurons?
- lower motor neurons whose axons come out from the brainstem to form a cranial nerve
OR - preganglionic autonomic neurons within cranial nerves
OR - 2nd order sensory neurons that relay (synapse) with the central (axonal) processes of the 1st order sensory neurons within cranial nerves [sensory nuclei]
NOTE: There’s an exception! That is the mesencephalic nucleus of the trigeminal ganglion which contain cell bodies of 1st order sensory neurons because the sensory nerves which carry proprioception from the brain do not have their cell bodies within the trigeminal ganglion. They are unipolar neurons; their cell bodies are within the midbrain.
