Unlock the secrets of the sensory innervation of the head, face, and neck with this comprehensive guide. Dive deep into the intricate pathways that carry mechanoceptive, proprioceptive, thermal, and nociceptive information through the cranial nerves V, IX, and X. Learn about the special sensory inf...
Oropharynx and larynx
Sensory innervation of the Head, Face, Neck
The sensory innervation of the head, face and neck carries mechanoceptive, proprioceptive, thermal
and nociceptive information, through the V, IX and X cranial nerves, and special sensory information
through the I (olfactory), II (optic), VIII (auditory), VII and IX (taste).
Aside of special senses, all sensory information from the face and the mouth is carried by
the trigeminal nerve, with the contribution of the IX nerve for mechanoception from the posterior
part of the tongue. (vd. trigeminal system in neuroanatomy book II)
Mechanoception from the oro-facial district is conveyed through the three branches of the trigeminus
to the chief sensory nucleus of the trigeminus.
- CN V1 (ophthalmic division), carries information from the forehead, the upper lids and the
anterior face of the nose;
- CN V2 (maxillary division), from the temples, the zygomatic and maxillary areas, and the sides
of the nose;
- CN V3 (mandibular division), from the lateral, auricular, and mandibular regions. Only V3
contains a motor component, for masticatory muscles.
Thermal information from the orofacial district is also carried by the CN V and reaches the spinal
trigeminal nucleus; from the buccal mucosa it is perceived as hot/spicy taste (V2).
Nociception also reaches the spinal trigeminal nucleus; V2 and V3 carry nociception from the
teeth and the periodontal structures.
The primary neurons involved in all these sensations are located in the trigeminal (semilunar,
Gasser) ganglion.
Proprioceptive information is carried by the V cranial nerve and constitutes a particular case: crucial
proprioceptive information comes from the masticatory muscles (V3) and the periodontal receptors (V2,
V3); it is indispensable for the correct regulation of masticatory activity; it reaches the mesencephalic
trigeminal nucleus, where the cell bodies of the primary sensory neurons are located (bypassing
the trigeminal ganglion).
A particular case is constituted by proprioception from the facial muscles, which constitutes an important
input to the limbic system for emotional elaboration. Mimic muscles are innervated by CN VII (facial)
and do not possess muscle spindles. It has been suggested that acid-sensing ion channels type 2 (ASIC2),
transient receptor potential vanilloid 4 (TRPV4), and Piezo2 receptors initiate mechanotransduction as
proprioceptors in these muscles.
Facial expressions are mostly regulated in two different ways depending on whether we are dealing with
the upper or lower part of the face. The lower part of the face is slightly less implied in expressing emotion
when compared to the upper part. This is accounted for by the fact that facial nuclei have lateral and medial
portions which respectively control the upper and the lower face in an almost independent way. Each of the portions
of the nucleus, and therefore all the outputs to facial muscles, are controlled by different areas: not only
by the premotor and motor cortex (where the face is represented), but also by the mid cingulate gyrus,
implied in regulating emotions. This accounts for the fact that we have a large component of almost
involuntary control on the face. Whereas the information coming from the motor and premotor cortex
tends to be relatively unilateral, the emotional regulation is mostly bilateral.
It should be noted that quite extensive communication and intermixing exists between the terminal
branches of trigeminus and facial nerves, which suggests that the proprioceptive information from facial
muscles may also be conveyed to the trigeminal nucleus. Trigeminal nerve terminals reach the muscles
by running in the terminals of facial nerve (interchange of fibers). The proprioception of the state of the
mimic muscle is much more important than one may think because, in the regions of the brain that
control emotions, it tends to generate the emotion linked to the face expression à bidirectional control:
an emotion produces a mimic expression, and a mimic expression tends to produce the emotion.
The trigeminus also has a motor component that mostly controls biting, chewing and swallowing muscles
(masseter and temporal muscle, medial and lateral pterygoids); the mylohyoid, tensor veli palatini and
tensor tympani muscles, and the anterior belly of the digastric are also innervated by the trigeminus.
91 Body At Work II
, Enrico Tiepolo
A lesion of the trigeminus may impair the pterygoid muscles (deeply located muscles that pull up and
back the mandible to close the mouth – medial pterygoid – or protrude the mandible pushing it
contralaterally – lateral pterygoid); this produces deviation of the mandible toward the paralyzed side.
The different types of sensory information and the different paths
Recall the organization of different sensory modalities and the ascendent path they follow:
- Dorsal column - medial lemniscus system: proprioceptive sensations, fine touch and vibration: the
centripetal axon of primary neurons ascends ipsilaterally in the gracile (sacro-lumbar, medial) and
cuneate (thoraco-cervical, lateral) fasciculi; after the gracile and cuneate nuclei the path decussates
giving rise to the medial lemniscus
- Anterolateral system – thermal and nociceptive receptors (and crude touch) synapse on
secondary sensory neurons in the dorsal horns; ascending axons decussate (Lissauer tract, it takes a
few metamers) and ascend contralaterally (cervical to sacral, medial to lateral)
Consider that whereas the information ascending through the medial lemniscus mostly requires
analytical elaboration to precisely localize, detect changes and local differences, and is therefore mostly
directed to the somatosensory cortex, the information carried by the anterolateral system has lower
analytical content but generally more vital relevance, and will therefore need larger elaboration by the
limbic cortical and subcortical structures.
In the brainstem, the chief (main) sensory nucleus of the trigeminus receives ipsilateral
discriminative and proprioceptive information from the face; it plays a role similar to that of the gracile
and cuneate nuclei; the output crosses, and gives rise to the trigeminal lemniscus.
Conversely, the spinal trigeminal nucleus, that collects thermal and nociceptive information carried
in CN V, VII, IX and X, corresponds to second order sensory neurons in the posterior horn of the spine
and Lissauer tract (before decussation). It ascends (still ipsilateral) parallel to the spinothalamic tract (that
carries contralateral info instead).
In summary:
• Fine touch from the body ascends in the posterior columns, crosses after gracile/cuneate
nuclei, travels in the medial lemniscus to the VPL n. of the thalamus and S1 in the cortex. From
the face, this info reaches the main V nucleus
through CN V, VII, IX, X, crosses to the trigeminal
lemniscus and to the VPM n. of the thalamus
toward S1 in the cortex.
• Nociceptive/thermal info from the body
synapses in the dorsal horn, crosses a few metamers
above, ascends in the spinothalamic tract and
reaches the nuclei VPL (to S1), VM (to insula), MD
(to cingulate ctx) and IL (diffuse projections) of the
thalamus. From the face, it reaches the spinal V
nucleus through CN V, VII, IX, X, crosses to the
trigeminal lemniscus and to VPM (to S1) and VM,
MD, and IL nuclei as above.
Toothache
One source of tooth pain is the periodontal ligament, very rich of sensory
nerve terminals. Its sensitivity stems from the fact that the tooth can move
very little, but it is not immobile. The periodontal ligament sensors
transduce with high sensitivity any vertical, lateral, or diagonal force the
tooth may be exposed to. This information is treated as proprioceptive and
reaches, together with information from the spindles of masticatory muscles,
the mesencephalic nucleus of the trigeminus, which is positioned in close
relation with the site of origin of the motor efferences from the same nucleus
92 Body At Work II
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