NBIO 401
Fall 2013
Cranial Nerve Nuclei A: Overview and Nuclei III – V
Class 5 – Friday, October 4, 2013
Robinson
Objective for the first two cranial nerve lectures, those about both cranial nerve nuclei:
-Be able to summarize the function and location of cranial nerve nuclei associated with nerves III-XII.
Objective for the 3rd cranial nerve lecture, about cranial nerves:
-Be able to summarize the sensory and motor information that travels in every cranial nerve i-XII.
I. Cranial Nerve Nuclei vs. Cranial Nerves (Please understand the distinction between cranial
nerve nuclei and cranial nerves.)
A. Basic facts
1. Cranial nerves (bundles of axons attached to the brainstem) have sensory, motor or both
sensory and motor functions. When they have both, we call them mixed nerves.
2. Cranial nerve nuclei are either sensory or motor but never both.
3. There is not a one-to-one correlation between cranial nerves and cranial nerve nuclei. This
means that a particular cranial nerve can carry several distinct types of signals associated with
more than one cranial nerve nucleus.
For our purposes in NBIO 401, we will first learn about the cranial nerve nuclei in two lectures and
then, in a third lecture, learn about the cranial nerves.
B. We group cranial nerve motor nuclei according to their targets
1. Nuclei whose axons innervate striated muscle derived from somites (embryonic structures that
develop into muscle) are called General Somatic Efferents (GSE). GSE targets are the
oculomotor muscles and the tongue. These nuclei are medial and dorsal in the brainstem.
2. Nuclei whose axons innervate striated muscle derived from branchial arch (an embryonic
structure in the head, separate from somites, that develops into muscle) are called Special
Visceral Efferents (SVE – I don’t know why these are called “visceral” at all because they
innervate striated muscle, not smooth muscle or autonomic ganglia. Nonetheless, there it is.
We have to remember it.) SVE targets are the jaw muscles, face muscles, and muscles of the
larynx and pharynx. SVE nuclei are of intermediate laterality in the brainstem, i.e., not very
medial and not very lateral and ventral to GSE nuclei.
3. Nuclei whose axons innervate autonomic ganglia that drive glands (e.g., salivary glands) or
smooth muscle (i.e., the ciliary muscle that controls pupil diameter) are called General
Visceral Efferents (GVE). These nuclei are slightly more lateral in the brainstem than GSE
and can be at the same dorsal-ventral location or more ventral.
C. We group cranial nerve sensory nuclei according to the information they receive from the
periphery.
1. Nuclei that receive somatosensory (SS) information, i.e., touch, pain, or proprioception
(signals related to the length of muscle or the angle of joints) are called General Somatic
Afferents (GSA). GSA nuclei receive SS information from the skin and muscles of the head.
2. Nuclei that receive vestibular and auditory information are called Special Somatic Afferents
(SSA).
3. Nuclei that receive pressure and chemical signals for autonomic control are called General
Visceral Afferents (GVA).
4. Nuclei that receive information about taste are called Special Visceral Afferents (SVA).
In remembering the cranial nerves it is helpful to organize them into the groups described above. The
nuclei within each group share their position in the brainstem and their general function. Below we
describe each group.
(NOTE: Two cranial nerves, the olfactory nerve (I), and the optic nerve (II), do not have cranial
nerve nuclei. Nonetheless, these nerves are classified in the 3-letter system. The olfactory
nerve is considered SVA and the optic nerve is considered SSA. We will not cover cranial nerves
I & II here. We will describe them in lectures later in the course.)
-Class 5, page 1-
NBIO 401
Fall 2013
Cranial Nerve Nuclei Overview
SOMATIC CRANIAL NERVE NUCLEI - Below is a line drawing representing the dorsal surface of the
brainstem. The midbrain is at the top and the caudal medulla is at the bottom. The cerebellum has
been removed so that we can view the brainstem clearly.
The left side of this diagram shows somatic sensory nuclei contributing to cranial nerves in the
General Somatic Afferent (GSA) and Special Somatic Afferent (SSA).
This type of diagram emphasizes the fact that nuclei of particular categories lay in longitudinal
columns running rostral - caudal in the brainstem. The other columns here, 2 on the right and 1 on
the left, are columns of other categories of cranial nerve nuclei that we will describe below. Note here
that the somatic motor column is the most medial of all of the columns and somatic sensory column is
the most lateral. Labels give the name of the nucleus and the number of the nerves to which it
contributes in Roman numerals. Text in parentheses provides a capsule summary of the function of
each nucleus.
-Class 5, page 2-
NBIO 401
Fall 2013
VISCERAL CRANIAL NERVE NUCLEI The left side of the drawing below shows the visceral sensory
nuclei that contribute to cranial nerves in the General Visceral Afferent (GSA) category. The right side
shows visceral motor nuclei contributing to cranial nerves in the General Visceral Efferent category.
Note that the GVA column is medial to the GSA column. The GVE column is lateral to the GSE
column.
-Class 5, page 3-
NBIO 401
Fall 2013
The left side of the diagram below shows the visceral sensory nuclei that contribute to cranial
nerves in the Special Visceral Afferent (SVA) category. In this scheme taste is “special”, just like
vision and hearing, because these senses are not represented in the body, as opposed to, say, touch
or proprioception. Thus, the rostral half of nucleus solitarius is in the SVA category because it
mediates taste and the caudal half is in the GVA category because it receives information from the
viscera necessary for autonomic control of the gut, lungs, and vascular systems.
The right side shows visceral motor nuclei contributing to cranial nerves in the Special Visceral
Efferent category. These nuclei contain motoneurons for striated muscles. Nonetheless, they are
called “special” in this nomenclature, as opposed to “general”, efferents because they innervate
muscles that develop from the branchial arches. Another name for these nuclei is Branchial Motor
Nuclei. “Branchial” refers to tissue developing from embryonic gill arches. Note that the SVA column
is medial to the GSA column. The GVE column is lateral to the GSE column.
-Class 5, page 4-
NBIO 401
Fall 2013
The positions of the cranial nerve nuclei columns on a section through the mid-medulla are
shown schematically on the insert in the lower right. Note that, moving laterally from the midline, the
columns are GSE, GVE, SVA/GVA, SSA and GSA. The SVE column is displaced ventrally and
laterally from the GSE column. During development these columns are initially near one another but
the SVE column migrates ventrally and laterally, trailing its developing axons behind it. We can see a
reflection of this in the curved
paths of axons leaving both
the facial nucleus and the
nucleus ambiguus (an
elongated nucleus in the
medulla containing
motoneurons for laryngeal
and pharyngeal muscles).
The drawing to the
right starts to move us from
viewing the positions of
cranial nerve nuclei in a line
drawing to seeing their
positions in the brain. This
drawing shows the positions
of somatic, visceral and
special (branchial) cranial
nerve motor nuclei as we
have seen them above and in
transparent drawings of the
brainstem.
-Class 5, page 5-
NBIO 401
Fall 2013
In a slightly
more realistic view we
see the somatic motor
nuclei here in a
computer reconstruction
of a human brainstem.
Here are the
general visceral
efferent nuclei in a
human brainstem.
-Class 5, page 6-
NBIO 401
Fall 2013
Cranial Nerve Nuclei III-IV
NUCLEI FOR THE OCULOMOTOR NERVE (III)
For the rest of this lecture
and all of the next we will
describe each of the nuclei
associated with each cranial
nerve in order, starting with the
most rostral. We will skip the
olfactory (cranial nerve I) and the
optic (cranial nerve II) for two
reasons. First, they do not
terminate in cranial nerve nuclei.
Second, we will hear about them
in detail in upcoming lectures on
To the right we see a
computer reconstruction of a
coronal section through the
brainstem. It is at the level of the
caudal midbrain, just rostral to
the pons. Protruding rostrally
out of the section are the
oculomotor nuclei (motoneurons
for four of the six eye musclesGeneral Somatic Efferent or
GSE). Dorsal to them are the
Edinger-Westphal nuclei
(neurons that, through a
parasympathetic ganglion contract the smooth muscles to constrict the pupil-General Visceral Efferent
or GVE). Axons from both nuclei come together to form the oculomotor nerve (IIIrd cranial nerve) that
exits through the bottom of the midbrain medial to the medial edge of the cerebral peduncles. Please
identify the red nucleus,
substantia nigra, medial
lemniscus, and cerebral
peduncles that are outlined but
not labeled in this section.
NUCLEI FOR THE
TROCHLEAR NERVE (IV)
To the right we see a
coronal section just caudal to
the one above but still rostral to
the pons. Protruding rostrally
from section are the trochlear
nuclei (motor neurons for one of
the six eye muscles-General
Somatic Efferent or GSE) and
the axons of cells in the
trochlear nucleus leaving the
nucleus and crossing the
midline to exit the dorsal surface
of the brainstem. The trochlear
nerve (IVth cranial nerve) is the only cranial nerve axon to exit on the dorsal surface of the brain and
the only one to cross the midline between its origin and the periphery.
-Class 5, page 7-
NBIO 401
Fall 2013
NUCLEI FOR THE TRIGEMINAL NERVE (V)
The picture to the right shows the special
visceral (branchial) efferents in a human
brainstem. The most rostral of these is the trigeminal
motor nucleus (motoneurons for the chewing
muscles). The axons of these motoneurons form part
of the trigeminal nerve (Vth cranial nerve, GSA,
SVE).
The diagram below is a computer rendering of
a transverse section through the pons at the level
where the trigeminal nerve enters the brainstem.
Rostral is up and to the right while caudal is down
and to the left.
This drawing shows the 4 nuclei associated
with the trigeminal nerve the:
1) Main sensory nucleus (receives touch
information from the ipsilateral side of the head)
2) Motor nucleus (motoneurons for ipsilateral chewing muscles)
3) Spinal trigeminal nucleus (receives pain information from ipsilateral side of the head)
4) Mesencephalic trigeminal nucleus (primary sensory neurons, like DRG in the spinal cord, for
chewing muscle proprioception).
In this
drawing these
nuclei project
caudal to the
plane of the
section.
These nuclei
have two things in
common. First, the
trigeminal nerve
carries their axons
(for the motor and
mesencephalic nuclei) or axons carrying information to them (for the main sensory and spinal nuclei).
Second, they all carry sensory or motor information about the ipsilateral side of the head.
-Class 5, page 8-
NBIO 401
Fall 2013
In the computer reconstruction above we are again looking at the brainstem rostrally, medially,
and slightly down from the left. Again the nuclei protrude caudally. In this view we see the 4 nuclei
associated with the trigeminal nerve from a different angle. Note that the motor trigeminal nucleus is
medial to the main sensory trigeminal nucleus. (To remember that the motor nucleus is media
remember that motor begins with m and so does medial.)
Also clear in the drawing above, is the position of the spinal trigeminal tract lateral to the
spinal trigeminal nucleus. We can also see the caudal ends of the mesencephalic tracts just lateral to
the caudal ends of the mesencephalic trigeminal nucleus.
-Class 5, page 9-
NBIO 401
Fall 2013
The drawing below again shows the brainstem from above with the cerebellum removed. This
image summarizes the positions of the 4 trigeminal nuclei on each side from this angle. In addition
we see the 4 types of axons, as defined by the information that they carry, of the trigeminal nerve on
the right (3 sensory and 1 motor).
Two of the sensory axon types have their cell bodies in the trigeminal ganglion. These are:
-1) the axons carrying conscious touch and proprioception signals from the ipsilateral
head
-2) the axons carrying pain information from the ipsilateral head
One of the sensory axon types has its cell body in the mesencephalic trigeminal nucleus.
These are the axons carrying unconscious proprioception from the jaw muscles and joints.
Remember that the mesencephalic trigeminal nucleus is the only place in which primary sensory
axons are inside the CNS (brain and spinal cord) and not in ganglia immediately outside the CNS (i.e.,
in dorsal root ganglia or the trigeminal ganglion).
-Class 5, page 10-
NBIO 401
Fall 2013
Below are several cross sections through the pons and medulla. Please learn the positions of
the different trigeminal nuclei in these sections.
In the section above through the caudal pons you can see, in the upper right, the positions of
the main sensory trigeminal nucleus (lateral to the motor nucleus), the motor trigeminal nucleus
(medial to the sensory nucleus), and the rostral end of the spinal trigeminal tract and nucleus.
Rostral to the last section, at the level of the exit of the trigeminal nerve, we can see the
rostral end of the motor trigeminal nucleus and the caudal end of the mesencephalic tract and
nucleus. We are now rostral to the spinal trigeminal tract and nuclei. Note also the trigeminal nerve
on the right side of the section.
-Class 5, page 11-
NBIO 401
Fall 2013
Even more rostral in the pons we can see that this section is rostral to the main sensory and
motor trigeminal nuclei, but the mesencephalic trigeminal tract and nuclei are still in the section.
More caudally, behind the exit of the trigeminal nerve in the caudal pons, we can see that this
section is caudal to the main sensory and motor trigeminal nuclei but the spinal trigeminal tract and
nuclei are in this section. The spinal trigeminal tract and nuclei are very elongated, extending all of
the way caudally as far as the caudal medulla where they are continuous with the dorsal (sensory)
horn of the spinal cord, hence the name “spinal trigeminal nucleus”.
-Class 5, page 12-
NBIO 401
Fall 2013
So, for example, here we see a section from nearly the most caudal level of the medulla.
Even this caudally we can see the spinal trigeminal nucleus and tract labeled in the upper right part of
the section.
-Class 5, page 13-