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Mixed cranial nerves: want to learn more about it?

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Mixed cranial nerves

Mixed cranial nerves are the cranial nerves that contain sensory and motor nerve fibers. There are four of such nerves in our peripheral nervous system;

We know that the task of learning neuroanatomy and the anatomy of the head and neck may feel insurmountable. This is why we are here, to help - let us be your guide! We are happy you decided to read this article which will introduce you to the innervation of the head and neck by explaining the anatomy of the mixed cranial nerves.

Key facts
Definition Mixed cranial nerves are the nerves that consist of motor and sensory nerve fibers.
Nerves

Trigeminal nerve (CN V)

Facial nerve (CN VII)

Glossopharyngeal nerve (CN IX)

Vagus nerve (CN X)

Clinical relations Trigeminal neuralgia, surgical removal of the parotid Gland, facial nerve palsy, inferior medial pontine syndrome, glossopharyngeal neuralgia, lateral pontine syndrome, lateral medullary syndrome, syringobulbia

Cranial nerves

Cranial nerves are the 12 nerves of the peripheral nervous system that innervate the structures of the head and neck. Vagus nerve (CN X) is the only cranial nerve that innervates the structures beyond the head and neck region. Except for the spinal accessory nerve (CN XI) which has origin in the spinal cord, all the other cranial nerves emerge from the brain

These 12 paired nerves are summarized in this table.

12 cranial nerves
Cranial nerve 1 Olfactory nerve (CN I) - sensory
Cranial nerve 2 Optic nerve (CN II) - sensory
Cranial nerve 3 Oculomotor nerve (CN III) - motor
Cranial nerve 4 Trochlear nerve (CN IV) - motor
Cranial nerve 5 Trigeminal nerve (CN V) - mixed 
Ophthalmic branch (V1)
- Maxillary branch (V2)
- Mandibular branch (V3)
Cranial nerve 6 Abducens nerve (CN VI) - motor
Cranial nerve 7 Facial nerve (CN VII) - mixed
Cranial nerve 8 Vestibulocochlear nerve (CN VIII) - sensory
Cranial nerve 9 Glossopharyngeal nerve (CN IX) - mixed
Cranial nerve 10 Vagus nerve (CN X) - mixed
Cranial nerve 11 Spinal accessory nerve (CN XI) - motor
Cranial nerve 12 Hypoglossal nerve (CN XII) - motor

Cranial nerves have various 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).
  • Trigeminal (all three branches) and glossopharyngeal nerves play roles in somatic sensory functions.
  • Oculomotor, facial, glossopharyngeal, and vagus nerves have important autonomic functions.
  • Oculomotor nerve, trochlear, mandibular branch of the trigeminal nerve (V3), abducens, facial, glossopharyngeal, vagus, spinal accessory and hypoglossal nerves are responsible for motor functions.

Understand better the anatomy of cranial nerves with our cranial nerves quizzes and labeling exercises.

According to their functions, cranial nerves are either motor, sensory or both (mixed). To remember the names of the cranial nerves and whether they are sensory, motor or both in numerical order, check out this cranial nerves mnemonics video:

Cranial nerves anatomy starter pack is waiting for you here.

Trigeminal nerve (CN V)

The trigeminal nerve (CN V) is a mixed nerve containing both general sensory (afferent) fibers and somatic motor (efferent) fibers. The fibers originate from the nuclei in the brainstem and spinal cord; principal sensory nucleus of trigeminal nerve, spinal nucleus of trigeminal nerve, mesencephalic nucleus of trigeminal nerve and the motor nucleus of trigeminal nerve. CN V forms the trigeminal ganglion near the apex of the petrous part of the temporal bone

From the trigeminal ganglion, the trigeminal nerve divides into three divisions; ophthalmic nerve (CN V1), maxillary nerve (CN V2) and mandibular nerve (CN V3). Ophthalmic divison exits the skull through the superior orbital fissure, maxillary through the foramen rotundum and the mandibular nerve exits via the foramen ovale.

The general sensory component sends information about pain, touch, pressure, and temperature sensation from the anterior two-thirds of the head, including the face. The smaller somatic efferent component innervates the skeletal muscles derived from the first branchial arch; the mylohyoid muscles, tensor tympani, tensor veli palatini, anterior belly of the digastric muscle and the muscles of mastication (the masseter, temporalis, medial pterygoid, and lateral pterygoid muscles). Because of its size, the trigeminal nerve can be easily seen where it emerges from the pons near the middle cerebral peduncle.

Ophthalmic division (CN V1)

The ophthalmic divison of the trigeminal nerve (CN V1) transmits sensory signals from receptors on the: forehead, cornea, upper eyelid, dorsal surface of the nose and the mucous membranes of the nasal and frontal sinuses.

The signals then travel along nerve fibers which enter the skull through the superior orbital fissure (along with the oculomotor, trochlear, and abducens nerves).

Find out more about the ophthalmic nerve here.

Maxillary division (CN V2)

The maxillary division of the trigeminal nerve (CN V2) transmits sensory signals from receptors on the: lateral surface of the nose, upper teeth, hard palate, upper cheek and mucous membranes of the upper teeth, nose, and roof of the mouth.

Signals generated by these receptors then travel along the nerve fibers into the skull via the foramen rotundum.

Learn more about the maxillary nerve here.

Mandibular division (CN V3)

The mandibular division of the trigeminal nerve (CN V3) transmits sensory signals from receptors on the: lower jaw, lower teeth, chin, parts of the posterior cheek, temple, external ear, anterior two-thirds of the tongue and the floor of the mouth.

It also supplies motor innervation to the muscles of mastication and a few other muscles in the lower face (listed previously). These fibers enter the skull via the foramen ovale.

Cover the anatomy of the mandibular branch here.

Facial nerve (CN VII)

Facial nerve carries general and special fibers. It originates from the brainstem as two separate divisions; a larger primary motor root, and a smaller intermediate nerve carrying sensory and parasympathetic fibers. The former originates from the motor nucleus of facial nerve, while the latter stemms from the nuclei of solitary tract, spinal nucleus of trigeminal nerve and the superior salivatory nucleus.

The two divisions leave the cranial cavity through the internal acoustic meatus and then travel through the facial canal. Here they join and leave the cranium together through the stylomastoid foramen. Facial nerve innervates the muscles of facial expression and salivary glands via its major branches; temporal, zygomatic, buccal, mandibular and cervical branches. It also provides the taste sensation from the anterior two thrids of the tongue.

Sensory branches innervate the middle ear, nasal cavity, soft palate (general visceral afferent); anterior two-thirds of the tongue (special visceral afferent); external auditory meatus (general somatic afferent). Motor fibers supply lacrimal, submandibular, sublingual, basal, palatine glands (general visceral efferent); muscles of facial expression (special visceral efferent).

Learn more about the facial nerve with our study materials.

Glossopharyngeal nerve (CN IX)

Glossopharyngeal nerve is another multimodal nerve. It originates from the brainstem and leaves the skull through the jugular foramen. Its fibers originate from four nuclei; nucleus ambiguus, inferior salivatory nucleus, nuclei of solitary tract and spinal nucleus of trigeminal nerve. This nerve enables swallowing, salivation, taste sensation and blood gas levels regulation.

Its motor fibers supply the stylopharyngeus and pharyngeal constrictors (special visceral efferent); parotid gland (general visceral efferent). Sensory fibers supply posterior one-third of the tongue (special visceral afferent); middle ear, pharynx, epiglottis, carotid body, carotid sinus (general visceral afferent); posterior one-third of the tongue and soft palate (general somatic afferent).

Learn more about the glossopharyngeal nerve here.

Vagus nerve (CN X)

Vagus nerve is also a multimodal nerve, containing somatic and visceral fibers. It originates from multiple nuclei in the brainstem, and exits the skull through the jugular foramen. Its nuclei are the posterior nucleus of vagus nerve (dorsal motor nucleus), nucleus ambiguus, nuclei of solitary tract and spinal nucleus of trigeminal nerve. 

Vagus nerve provides parasympathetic supply to the thoracic and abdominal viscera and it is the only cranial nerve that leaves the head and neck region. Its motor fibers supply the thoracic and abdominal viscera (general visceral efferent); laryngeal and pharyngeal muscles (special visceral efferent). Sensory fibers supply the epiglottis (special visceral afferent); thoracic and abdominal viscera and carotid body (general visceral afferent); external acoustic meatus, retroauricular skin and posterior part of meninges (general somatic afferent).

We have covered the vagus nerve anatomy in detail here.

Mixed cranial nerves and reflexes

Corneal reflex

The corneal reflex, also called the blink reflex, is the involuntary response of blinking the eyelids when the cornea is stimulated. The trigeminal nerve comprises the afferent (sensory) limb of the corneal reflex, while the facial nerve comprises the efferent (motor) limb.

Stimulation of sensory receptors in the cornea sends signals along the ophthalmic division of the trigeminal nerve and into the brainstem. The trigeminal nerve axons descend via the spinal trigeminal tract and synapse with neurons in the pars caudalis of the spinal trigeminal nucleus. Axons from these neurons subsequently project to the contralateral VPM thalamic nucleus.

Collateral axons from pars caudalis neurons are sent bilaterally to synapse with neurons in the facial nerve motor nuclei. As part of the facial nerve, the axons of motor neurons in these nuclei exit the skull via the stylomastoid foramen, and innervate the orbicularis oculi muscles in the eyelids as part of the zygomatic branch of the facial nerve. Innervation of the orbicularis oculi muscles leads the eyes to blink. Because both left and right facial nerve motor nuclei receive input from sensory stimulation of the trigeminal nerve on either side, the corneal reflex is both direct (in the stimulated eye) and indirect (in the opposite eye, also called a consensual reflex). The blink does, however, tend to be stronger on the stimulated side.

Stimulation of the cornea, of course, is also ultimately perceived as painful; this occurs due to transmission of the noxious information via ascending fibers in the anterior trigeminothalamic tract.

Mandibular reflex

The mandibular reflex, otherwise known as the jaw jerk reflex, is a version of the muscle stretch reflex mediated through the trigeminal nerve.

Tapping on the chin stretches muscle spindle fibers in the temporalis and masseter muscles, which triggers action potentials in A-alpha (primary) muscle spindle fibers and A-beta (secondary) muscle spindle fibers. These afferent fibers travel along the sensory root of the trigeminal nerve to both synapse on cell bodies in the mesencephalic nucleus, and send collaterals bilaterally to synapse on motor neurons in the trigeminal motor nuclei. As part of the motor root of the trigeminal nerve, axons of these motor neurons innervate the temporalis and masseter muscles, resulting in contraction of these muscles and closure of the jaw.

Gag reflex

The gag reflex has an afferent limb mediated by the glossopharyngeal nerve and an efferent limb mediated by the glossopharyngeal and vagus nerves. The gag reflex allows for constriction and elevation of the pharynx in response to irritation in the back of the throat, at the base of the tongue and/or in the soft palate in the back of the roof of the mouth, functioning to push out the object that is irritating the area. These regions between the mouth and pharynx are called the fauces; for this reason, the gag reflex may also be referred to as the faucial reflex.

When there is stimulation of A-delta fibers and C fibers in the fauces, signals are sent along the glossopharyngeal nerve to cell bodies in its superior ganglion. The signals are then transmitted via interneurons to the nucleus ambiguus, the origination of the efferent limb of the reflex. Efferent signals then travel along the glossopharyngeal nerve to innervate the stylopharyngeus muscle, and along the vagus nerve to innervate the pharyngeal constrictor muscles and other muscles which move the palate.

Carotid body chemoreceptor reflex

Increase in carbon dioxide levels, decrease in oxygen levels, or alterations in pH in the blood stimulate afferent fibers in the glossopharyngeal nerve, ultimately activating reticulospinal neurons in the reticular formation. Fibers then descend in the spinal cord to synapse on ventral horn cells in the cervical spinal cord, specifically in cervical levels 3, 4, and 5.

Axons from these ventral horn cells form the phrenic nerve, which innervates the muscles of the diaphragm and causes reflex contractions of the diaphragmatic muscles. This increases respiratory rate, which ultimately reduces the amount of carbon dioxide in the blood.

Baroreceptor reflex

The baroreceptor reflex functions to maintain a person’s blood pressure and cardiac output when mean arterial pressure changes. For example, when a person suddenly stands up from a sitting or lying position, blood pressure drops; this leads to decreased firing by receptors in the carotid body and aortic arch. Signals originating in the carotid body are transmitted by the glossopharyngeal nerve, whereas signals originating in the aortic arch are transmitted by the vagus nerve. The decreased signalling rate ultimately results in disinhibition of the sympathetic nervous system, which leads to an increase peripheral vascular tone, cardiac rate, and cardiac output.

Infant reflexes

A number of infantile reflexes are mediated by the trigeminal, facial, glossopharyngeal, and vagus nerves, as well as the hypoglossal nerve. The snout, sucking, and rooting reflexes, known as the primitive reflexes, typically disappear within the first few months of life; although they have been observed to reappear in some individuals with dementia, or degeneration or dysfunction of the frontal lobe. In infants, however, these reflexes are essential for survival by facilitating feeding.

The trigeminal nerve makes up the afferent limb of the primitive reflexes, and is activated by touching around or in the mouth. Signals travel along afferent trigeminal fibers to the spinal trigeminal ganglion in the brain stem, terminating in the spinal trigeminal nucleus and principal sensory nucleus. Fibers from these nuclei, as they travel to the VPM nucleus of the thalamus, give off collaterals which either travel directly or indirectly via interneurons to the facial nucleus, nucleus ambiguus, accessory nucleus, and hypoglossal nucleus. This leads to innervation of the infant’s facial muscles via the facial nucleus; orientation of the head toward or away from the stimulus via the accessory nucleus; and contraction of the laryngeal and pharyngeal muscles to allow for sucking via the hypoglossal nucleus.

Mixed cranial nerves: want to learn more about it?

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