General topography and anterior nerves
The roots of the cranial nerves are located sequentially on the inferior (basal) surface of the brain. They are conventionally designated by Roman numerals from I to XII.
- Pair I — olfactory nerves (nervi olfactorii). They penetrate the cranial cavity through the cribriform plate and proceed directly to the olfactory bulb.
- Pair II — optic nerves (nervi optici). These nerves emerge from the optic canals and form the characteristic optic chiasm within the cranial cavity.
- Pair III — oculomotor nerves (nervi oculomotorii). Their exit point is located on the medial surface of the cerebral peduncles.
- Pair IV — trochlear nerves (nervi trochleares). They curve around the lateral surface of the cerebral peduncles. Their main anatomical feature is that they are the only cranial nerves that emerge on the dorsal (posterior) surface of the brainstem.
Nerves emerging in the region of the pons
The pons (pons) serves as the exit site for several important pairs of cranial nerves that provide sensation and movement in the head region.
- Pair V — trigeminal nerves (nervi trigemini). They emerge from the lateral parts of the pons. They are mixed nerves, comprising sensory and motor components. They innervate the facial skin, mucous membranes, and masticatory muscles.
- Pair VI — abducens nerves (nervi abducentes). They leave the brain near the posterior border of the pons, located relatively close to the midline.
- Pair VII — facial nerves (nervi faciales). They also emerge from the posterior border of the pons, but lie more laterally than the sixth pair. Their task is to provide facial expression and, partially, taste sensation.
- Pair VIII — vestibulocochlear nerves (nervi vestibulocochleares). They emerge from the posterior border of the pons next to the facial nerve. They consist of two specialized parts (vestibular and cochlear) and provide the vital senses of hearing and balance.
Nerves of the medulla oblongata
The lower part of the brainstem, the medulla oblongata, is the origin of the last four pairs of cranial nerves.
- Pair IX — glossopharyngeal nerves (nervi glossopharyngei). They are located closer to the posterior surface of the medulla oblongata. They are responsible for taste sensation in the posterior third of the tongue.
- Pair X — vagus nerves (nervi vagi). They emerge posteriorly, play a key role in regulating internal organ activity, and provide autonomic innervation.
- Pair XI — accessory nerves (nervi accessorii). They emerge from the medulla oblongata, yet their roots originate even further down from the upper segments of the spinal cord. They innervate the muscles of the neck and shoulder girdle.
- Pair XII — hypoglossal nerves (nervi hypoglossi). The roots of this pair emerge closer to the anterior surface of the medulla oblongata. Their main function is motor innervation of the tongue muscles.
Role of the cerebral cortex
All neural pathways are ultimately connected to higher centers. The cerebral cortex (cortex cerebri or pallium) represents the most highly differentiated part of the human nervous system.
The complex structure of the cortex is due to long phylogenetic development. In the course of evolution and adaptation, various types of cortex have formed, which allow for the processing of information coming, among other sources, from the cranial nerves.
Briefly: answers to topical questions
Which muscles does the trigeminal nerve (Pair V) innervate?
The trigeminal nerve (its third branch — the mandibular nerve, n. mandibularis) innervates the masticatory muscles and the tensor veli palatini muscle. These include:
- Masseter muscle (m. masseter) — elevates the mandible.
- Temporalis muscle (m. temporalis) — elevates the mandible; posterior fibers perform retraction.
- Medial pterygoid muscle (m. pterygoideus medialis) — elevates the mandible.
- Lateral pterygoid muscle (m. pterygoideus lateralis) — protracts the mandible.
- Tensor veli palatini muscle (m. tensor veli palatini).
Through which skull openings do the branches of the trigeminal nerve exit?
The branches of the trigeminal nerve leave the cranial cavity through three main foramina.
- Ophthalmic nerve (branch I) — exits through the superior orbital fissure (fissura orbitalis superior).
- Maxillary nerve (branch II, n. maxillaris) — exits through the foramen rotundum (foramen rotundum).
- Mandibular nerve (branch III, n. mandibularis) — exits through the foramen ovale (foramen ovale).
Additionally, the terminal branches of the maxillary nerve pass through other openings: the zygomatic nerve goes through the orbital fissure, and the infraorbital nerve through the inferior orbital fissure.
What functions does the facial nerve (Pair VII) perform?
The facial nerve (n. facialis) is a mixed nerve performing motor, secretory, gustatory, and sensory functions.
- Motor function — innervation of the facial muscles of expression.
- Secretory (parasympathetic) function — innervation of the lacrimal gland, nasal cavity glands, and salivary glands (via the nervus intermedius).
- Gustatory function — provision of taste sensitivity for the anterior 2/3 of the tongue (via the chorda tympani).
- Sensory function — provision of general sensitivity.
Which cranial nerves provide taste sensitivity of the tongue?
Taste sensitivity of the tongue is provided by three pairs of cranial nerves.
- Facial nerve (n. facialis, Pair VII) — innervates the anterior 2/3 of the tongue via its branch, the chorda tympani (chorda tympani).
- Glossopharyngeal nerve (n. glossopharyngeus, Pair IX) — provides taste sensitivity for the posterior 1/3 of the tongue and vallate papillae (papillae vallatae).
- Vagus nerve (n. vagus, Pair X) — innervates taste buds predominantly in the posterior regions (root of the tongue).
What nuclei does the glossopharyngeal nerve (Pair IX) have?
The glossopharyngeal nerve (n. glossopharyngeus) has the following nuclei located in the medulla oblongata:
- Motor nucleus — nucleus ambiguus (nucleus ambiguus), which is shared by cranial nerve pairs IX, X, and XI.
- Sensory nucleus — nucleus of the solitary tract (nucleus tractus solitarii), shared with the vagus nerve.
- Parasympathetic (autonomic) nucleus — inferior salivatory nucleus.
Also, a portion of the central sensory fibers of the nerve passes to the upper part of the dorsal nucleus of the vagus nerve.
Class and exam questions
How many pairs of cranial nerves are there in humans?
Humans have exactly 12 pairs of cranial nerves, which emerge on the inferior surface of the brain.
Which cranial nerve emerges on the dorsal surface of the brainstem?
The only cranial nerve emerging on the dorsal (posterior) surface of the brainstem is Pair IV — the trochlear nerves (nervi trochleares).
Where do the rootlets of the accessory nerve (XI pair) originate?
The roots of the accessory nerves emerge from the medulla oblongata, but originate from the upper segments of the spinal cord.
What else to study on this topic
- Anatomy of the cribriform plate and olfactory bulb
- Formation of the optic chiasm
- Topography of the cerebral peduncles and emergence of the III and IV cranial nerve pairs
- Phylogenetic development and types of the cerebral cortex (cortex cerebri)
- Autonomic innervation of internal organs by the vagus nerve
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