SOMATOSENSORY PATHWAYS FROM THE FACE
In the last section, we covered the three modalities
of sensation from the body: discriminative touch, proprioception,
and pain and temperature. These same modalities need to be addressed
in the face as well, but sensory input from the face does not
enter the spinal cord. In fact, it all enters the brainstem via
the trigeminal nerve. Just as in the spinal cord, these
three modalities have different receptors, travel along different
tracts, and have different targets in the brainstem. Once the
pathways synapse in the brainstem, they join the pathways from
the body on their way up to the thalamus.
A. The trigeminal nucleus complex:
The collection of cells in the brainstem that can
be called the trigeminal nucleus is huge - it stretches from midbrain
to medulla. If we could see it in a transparent brainstem, it
would look something like this:
Most of the sensory fibers enter the trigeminal
ganglion, regardless of which trigeminal division they are
coming from. Their cell bodies, like those of all somatosensory
neurons, lie outside the CNS in the ganglion, and their proximal
processes enter the brainstem in the mid-pons. From there they
fan out to their different targets. Each modality will be described
B. Discriminative touch:
The large diameter (Ab)
fibers enter directly into the main sensory nucleus of the
trigeminal (V), also called the principal nucleus.
Just like the somatosensory neurons of the body, they SYNAPSE,
then CROSS. The secondary afferents can then join the medial
lemniscus on its way to the thalamus.
C. Pain and temperature:
The small diameter fibers carrying pain and temperature enter at mid pons, and then do something unusual - they turn down the brainstem. They travel down the pons and medulla until they reach the caudal medulla, which is where they finally synapse and cross.
The tract that the descending axons travel in is
called the spinal tract of V, and the long tail of a nucleus
that they finally synapse in is called the spinal nucleus of
V. These names come from the fact that they actually reach
as far down as the upper cervical spinal cord. The spinal nucleus
of V can be divided into three regions along its length; the region
closest to the mouth is called subnucleus oralis, the middle
region is called subnucleus interpolaris, and the region
closest to the tail is called subnucleus caudalis. The
pain fibers actually synapse in subnucleus caudalis, so you may
hear that term used instead of the spinal nucleus of V.
The secondary afferents from subnucleus caudalis
cross to the opposite side, and join the spinothalamic tract
on its way to the thalamus.
The proprioceptive axons in the trigeminal nerve
are the stretch and tendon receptors from the muscles of mastication.
(Recall that all of the muscles of facial expression are controlled
by the facial nerve. ) These axons coming from the face have
a strange characteristic unique among primary somatosensory neurons:
their cell bodies are inside the CNS. They are the only
exception to the rule. Although their cells look similar to cells
in the dorsal root ganglion (the cell body does not come between
the distal and proximal axon processes), they are located inside
the brainstem in a nucleus called the mesencephalic nucleus.
The mesencephalic nucleus is essentially a dorsal root ganglion
that has been pushed into the CNS, so there are no synapses within
it. The fibers enter the brainstem via a small branch of the
trigeminal that bypasses the trigeminal ganglion, turn up towards
the mesencephalic nucleus, pass by the cell body, and leave the
nucleus immediately. Most then synapse in the nearby motor
nucleus where they can initiate the stretch reflexes for the
muscles of mastication. The stretch reflex in the face behaves
exactly like that in the body, and tapping on the tendon of the
masseter (for example) will produce a twitch.
E. Motor innervation:
Motor or efferent control is not considered a sensory
modality, but it is the fourth component of the extensive trigeminal
complex. The motor nucleus of V lies just medial to the
main sensory nucleus, and in it reside the a-motor
neurons that control the muscles of mastication. The two principal
muscles involved are the masseter (in your cheek) and the
temporalis (over your temple), both of which tighten when
you clench your teeth. The motor axons leave the mid-pons and
bypass the trigeminal ganglion, and reach their targets via the
mandibular division of the trigeminal nerve.
F. On to the thalamus:
The somatosensory information from the face joins
that from the body and enters the thalamus with it. However,
face information actually enters a different nucleus in the thalamus.
Recall that information from the body enters the ventroposterior
lateral nucleus (VPL). Information from the face actually enters
the ventroposterior medial nucleus (VPM). The thalamocortical
afferents take all of the signals, whether from VPL or VPM, to
primary somatosensory cortex. Once there, it is distributed in
a somatotopic (body-mapped) fashion, with the legs represented
medially, at the top of the head, and the face represented laterally.
G. A review:
To test your knowledge of the sensory modalities,
complete this chart.
|Tract with primary afferents, for the legs:|
|for the arms:|
|for the face:|
|Location of first synapse, legs:|
|Location of crossing, legs:|
|Name of secondary afferent tract:|
|Target of secondary afferents, body:|
If you can complete this table on your own, you know
the somatosensory system inside and out. When you are done, click
here to check your answers.