Sensory cranial nerves

List

The cranial nerves (CN) are twelve pairs of nerves that, with the exception of the spinal accessory nerve (CN XI), originate in the brain and contribute to the peripheral nervous system (PNS), supplying the head and neck.

These 12 paired nerves, and their main branches, include:

• The olfactory nerve (CN I)
• The optic nerve (CN II)
• The oculomotor nerve (CN III)
• The trochlear nerve (CN IV)
• The trigeminal nerve (CN V)
  • Ophthalmic branch (V1)
  • Maxillary branch (V2)
  • Mandibular branch (V3)
• The abducens nerve (CN VI)
• The facial nerve (CN VII)
  • Temporal branch
  • Zygomatic branch
  • Buccal branch
  • Mandibular branch
  • Cervical branch
• The vestibulocochlear nerve (CN VIII)
• The glossopharyngeal nerve (CN IX)
• The vagus nerve (CN X)
• The spinal accessory nerve (CN XI)
• The hypoglossal nerve (CN XII)

Types

Of these nerves, some have special sensory functions, some have somatic sensory functions, some have autonomic functions, some have somatic motor functions, and some have a combination of the aforementioned functions. The olfactory nerve, the optic nerve, the facial nerve, the vestibulocochlear nerve, the glossopharyngeal nerve, and the vagus nerve each play roles in special sensory functions (i.e. olfaction, vision, gustation, audition, and balance).

The trigeminal nerve (all three branches: the ophthalmic, maxillary, and mandibular) and the glossopharyngeal nerve play roles in somatic sensory functions.

The oculomotor nerve, the facial nerve, the glossopharyngeal nerve, and the vagus nerve have important autonomic functions.

Finally, the oculomotor nerve, the trochlear nerve, the mandibular branch of the trigeminal nerve (V3), the abducens nerve, the facial nerve, the glossopharyngeal nerve, the vagus nerve, the spinal accessory nerve, and the hypoglossal nerve are responsible for motor functions.

One way to simplify efforts to understand each nerve and each of its separate functions is to separate the 12 cranial nerves into smaller groups. This can be done by first dividing the overarching group of 12 nerves into nerves which are considered to have primarily motor functions; nerves which are considered to have primarily sensory functions; and nerves which have a combination of both motor and sensory components.

To remember the names of the cranial nerves and whether they are primarily motor, sensory (or both) in nature, check out this cranial nerve mnemonics video:

This article will provide an introduction to the cranial nerves which are considered primarily sensory nerves, which includes the olfactory nerve, the optic nerve, and the vestibulocochlear nerve. The terminal nerve (CN 0, or CN N), although not conventionally included in the list of cranial nerves, will also be discussed among this group.

Pathway and parts

The optic nerve (CN II) is the cranial nerve that carries visual information, allowing us to see the world around us. It is made up of special sensory afferent fibers which transmit visual information from the retina, the innermost layer of the eye, to the brain. The signals transmitted by this nerve are first received by specialized receptors in the retina called photoreceptors, of which there are two types: rods, which respond to light, and cones, which detect color.

The retina represents a map of the visual field in which everything is reversed and upside down due to the angles at which light enters the eyes. As a result, the left visual field falls on the right side of the retina, the right visual field falls on the left side of the retina, the upper visual field falls on the lower side of the retina, and the lower visual field falls on the upper side of the retina. As such, the inferior, inner (nasal) quadrant of the retina corresponds to the upper, outer quadrant of the visual field, etc.

The axonal processes of rods and cones synapse in the retina with other cells known as horizontal cells and bipolar cells, which process signals from the photoreceptors. Ganglion cells are the only retinal cells able to fire action potentials, and it is their axons which travel toward the posterior pole of the eye and coalesce, preparing to exit the eye at the optic disc.

The optic disc is a pale circular region in the retina, colloquially known as the blind spot because it has no photoreceptors and thus is incapable of picking up visual stimuli. Despite this, we do not have an obvious hole in our visual fields because our brain essentially fills in the blank (although the blind spot can be demonstrated by looking straight forward with one eye closed and holding up one’s pointed index finger on the hand on the opposite side as the closed eye approximately a foot from one’s open eye. Then, moving the index finger from the midline slowly into the periphery; when the top of the finger disappears, you have found your blind spot).

At the optic disc, the ganglionic axons become myelinated and exit each eye as the optic nerves. The optic nerves are additionally covered by extensions of the meninges which cover the brain. The optic nerves enter the cranial cavity via the optic canals, and enter the brain at the pre-optic region of the diencephalon. Upon entering the brain, the optic nerves join to form an “x”-shaped structure called the optic chiasm. It is here that the fibers extending from the inner (nasal) half of each retina cross to the contralateral side, whereas fibers extending from the outer (temporal) half of each retina remain ipsilateral and uncrossed. This ultimately means that each cerebral hemisphere receives information on the opposite visual field, and does so by receiving some information from both eyes.

The fibers then depart the optic chiasm as optic tracts, with the left optic tract containing axons from the temporal hemiretina of the left eye and nasal hemiretina of the right eye, and the right optic tract containing axons from the temporal hemiretina of the right eye and nasal hemiretina of the left eye.

These tracts project to the lateral geniculate nuclei (LGN) of the thalamus on their corresponding sides, where they synapse on layers of cell types finely attuned to detect different types of visual stimuli. The ventral magnocellular layers contains large cells focused on movement and location, while the dorsal parvocellular layers contains smaller cells focused on detail, color, etc. Axons of cells in the magnocellular layers project to more dorsal (superficial) regions of the primary visual cortex (the “where” pathway), whereas axons of cells in the parvocellular layers project to more ventral (deep) regions of the primary visual cortex (the “what” pathway).

The primary visual cortex (Brodmann’s area 17) is the cortical region ultimately dedicated to the perception of the visual stimuli transmitted by the optic nerve.

Optic Nerve: Pupillary light reflex

The pupillary light reflex is the reflexive constriction of the pupils in response to light. There are two main components of this reflex: an afferent (sensory) component involving the optic nerve and an efferent component involving the oculomotor nerve. The reflex is triggered when cells in the retina are activated by light, sending signals down the optic nerve (the afferent component of the reflex). Optic nerve fibers project and synapse in the pretectal region of the midbrain, which contributes to the circuit for the pupillary light reflex.

Neurons from the pretectal region then project and synapse in the oculomotor nucleus. From here, preganglionic parasympathetic neurons from the Edinger-Westphal nucleus send axons via the oculomotor nerve to synapse with postganglionic neurons in the ciliary ganglion, which sends signals to the sphincter pupillae muscle to contract and leads to constriction of the pupil (the efferent component of the reflex).

Auditory Signals and the cochlear component

Our ability to perceive sound begins with specialized receptors called hair cells, located in a structure called the Organ of Corti in the inner ear. These cells are triggered when air pressure waves incite vibrations in the tympanic membrane, which subsequently oscillate the stapes bone against the vestibular window of a structure called the cochlea.

The cochlea is filled with fluid called perilymph that is non-compressible, so movement of the stapes against the vestibular window by air pressure waves  transmits those pressure waves into the cochlea, leading to oscillatory movement of the perilymph. This leads to vibration of a membrane within the cochlea called the basilar membrane, which contains the Organ of Corti.

There are two types of hair cells in the Organ of Corti: outer hair cells, and inner hair cells. The outer hair cells are called stereocilia and lie between two membranes: their tips are situated within a membrane called the tectorial membrane, and their bodies lie on the basilar membrane. When the basilar membrane is displaced upward by pressure waves, the stereocilia are displaced.

Lateral displacement depolarizes the cell, triggering the generation of a signal; whereas medial displacement hyperpolarizes the cell, preventing the generation of a signal. The frequency triggering signal generation by a hair cell depends on the hair cell’s location in the basilar membrane. This is referred to as a tonotopic distribution: hair cells located in the basal portion respond to high frequencies, whereas those located in the apical portion respond to low frequencies.

Once signals are generated by stereocilia in the Organ of Corti, they are transmitted to the peripheral processes of bipolar neurons that form a structure called the spiral ganglion. Projections from these bipolar neurons form the cochlear nerve.

Balance/movement and the vestibular component

The anatomical components of the vestibular system are the otolith organs (the saccule and utricle) which detect linear acceleration, and three semicircular canals, which detect angular acceleration.

Like the cochlea of the auditory system, these organs are filled with endolymph; and like the receptors of the auditory system, the receptors of the vestibular system are hair cells. These hair cells also come in two types: type I hair cells, which are goblet shaped; and type II hair cells, which are cylindrical; but in the vestibular system, stereocilia project from the apical part of all types of hair cells, with small kinocilium extending slightly above each stereocilium. The bodies of these hair cells are surrounded by supporting cells connected by tight junctions, separating the hair cell from the endolymph surrounding the cilia while the apical stereocilia project into a gelatinous matrix.

Displacement of the cilia occurs when the endolymph moves in a particular direction, i.e. when there is a change in position or motion of the head. Displacement of the stereocilia toward the kinocilia results in excitation of the hair cells; whereas displacement of the stereocilia away from the kinocilia results in inhibition of the hair cells.

The hair cells are innervated by peripheral processes of vestibular (Scarpa’s) ganglia within the vestibular labyrinth (the maculae of the utricle and saccule, and the ampullae of the semicircular canals). The central processes of these bipolar neurons in the vestibular ganglia make up the vestibular part of the vestibulocochlear nerve.

Pathway of the vestibulocochlear nerve

The vestibular and cochlear components of the vestibulocochlear nerve travel as separate nerves through the internal auditory meatus, along with the facial nerve and labyrinthine artery.

They then cross the posterior cranial fossa. It is in the posterior cranial fossa, within the petrous part of the temporal bone, that the cochlear nerve and the vestibular nerve join to form a unified vestibulocochlear nerve.

This unified nerve reaches the lateral surface of the brainstem at the medullopontine angle. At this point, the vestibulocochlear nerve re-divides into vestibular and cochlear branches where they  enter the brainstem at the rostral medulla.

Cochlear branches

The axons making up the cochlear nerve project to cochlear nuclei in the rostral medulla in a tonotopic manner similar to that observed in the basilar membrane. Axons carrying high-frequency sound signals from the basal part of the basilar membrane project deep into the nuclei, while axons carrying low-frequency sound signals from the apical part of the basilar membrane project into more superficial regions of the nuclei.

From here, signals are transmitted through various pathways to a number of brain areas, including:

• the superior olivary nucleus
• the inferior colliculi in the midbrain
• the medial geniculate nucleus of the thalamus
• the primary auditory cortex (in the transverse temporal gyri of Heschl in the medial superior temporal gyrus)

Vestibular branches

The axons extending from afferent neurons in the vestibular ganglia in the labyrinth project to various vestibular nuclei in the rostral medulla and caudal pons in the brainstem. The nuclei to which they project depends on the vestibular structures from whence they originate.

Axons of afferent neurons from the ampullae of the semicircular canals project to the superior vestibular nucleus (SVN) and rostral portion of the medial vestibular nucleus (MVN).

Many axons of afferent neurons from the maculae of the utricle and saccule project to the lateral vestibular nucleus (LVN), but some axons from the macula of the saccule project to the inferior vestibular nucleus (IVN)  as well. Axons originating in these nuclei provide input to various regions of the brain, including:

• the trochlear nuclei
• oculomotor nuclei
• abducens nuclei
• cerebellum
• parts of the cerebral cortex

Vestibulocochlear nerve and the vestibulo-oculomotor reflex

Signals received and transmitted by the vestibular branches of the vestibulocochlear nerve influence eye movements by way of the vestibular nuclei, which in turn project axons to the oculomotor, trochlear, and abducens nuclei which influence movement of the extraocular muscles. This works such that when the head moves in one direction, the eyes move in the opposite direction, staying fixed on an object until the object leaves the visual field. The eyes then move rapidly with the movement until fixing on another object until that new object is out of vision. This rapid eye movement between fixations is called a saccade.

This sequence of slower eye movement in the opposite direction to head movement with fixation followed by rapid eye movement in the same direction as head movement until fixation is again achieved is called nystagmus or the vestibulo-ocular reflex. The slower eye movement in the direction opposite to head movement is controlled by vestibular input from the vestibular nuclei, and involves the medial longitudinal fasciculus (MLF). The rapid saccadic eye movement is controlled by the paramedian pontine reticular formation (PPRF).