Medical Neurosciences
Brainstem 2 - Pons
Brainstem – Part 2: Pons
DAVID GRIESEMER, MD
Professor of Neurosciences and Pediatrics
Key Concepts:
1. On the ventral surface of the pons is the basis pontis and the basilar artery. The dorsal surface of
the pons forms the floor of the fourth ventricle, in which the bulges of CN VII nuclei are seen.
2. On the dorsal side overlying the pons is the cerebellum. Pontine nuclei in the basis pontis send
fibers across the midline to the cerebellum via the brachium pontus (middle cerebellar peduncle).
3. The basis pontis contains three descending pathways: cortico-pontocerebellar, corticospinal and
corticobulbar.
4. The locus ceruleus, a cluster of melanin-containing neurons which provides noradrenergic
innervation throughout the brain, is located in the rostral portion of the pontine tegmentum.
5. The parabrachial nucleus has an important role in autonomic regulation.
6. The pontine tegmentum contains the ascending lemniscal sensory system which includes: medial
lemniscus, trigeminal lemniscus and spinothalamic tract.
7. Vestibular nuclei receive input from the cerebral cortex, cerebellum, spinal cord, and vestibular
apparatus (semicircular canals, utricle, saccule). They provide output to numerous areas to assist
in maintaining postural stability. These include the cerebral (vestibular) cortex, thalamus,
cerebellum, spinal cord, brainstem nuclei that control eye movements, and vestibular apparatus.
8. The facial nerve (CN VII) and the trigeminal nerve (CN V) both contain sensory and motor
components, the motor efferents predominating for CN VII and sensory afferents predominanting
for CN V.
9. The sensory facial nuclei are the spinal trigeminal nucleus (exteroception) and the nucleus
solitarius (taste). The motor facial nuclei are the facial motor nucleus (skeletal muscles) and the
superior salivatory nucleus (glands).
10. The motor nucleus of the trigeminal nerve innervates muscles of mastication, the tensor tympani,
and small muscles of the palate and neck.
11. The sensory nuclei of the trigeminal nerve include: spinal nucleus (pain, temperature, touch),
main sensory nucleus (touch), and mesencephalic nucleus (prioprioception).
12. Sensory projections from the ventral trigeminothalamic tract synapse on several motor nuclei in
the brainstem and spinal cord, providing the afferent arc of many reflexes that are tested
clinically.
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Brainstem 2 - Pons
EXTERNAL VIEW OF THE PONS
From the ventral perspective, the most apparent feature of the pons is a broad protuberance of the basis
called the basis pontis. The basal artery ascends in a shallow sulcus in the midline. Sweeping around
each side to the dorsal aspect of the brainstem is the brachium pontis, or middle cerebellar peduncle.1
The fourth ventricle and cerebellum are dorsal to the pons. Four cranial nerves emerge from the pons:
 abducens nerve (CN VI) at the junction of the medulla and pons
 facial nerve (CN VII) and vestibule-cochlear nerve (CN VIII) from the angle between the
cerebellum and pons
 trigeminal nerve (CN V) from the ventral lateral aspect of the basis pontis at the mid-pons level
INTERNAL STRUCTURE OF THE PONS
The basis pontis contains the scattered pontine nuclei,2 which are estimated to contain 20 million
neurons in each half of the pons. These neurons receive descending ipsilateral input from the motor
cortex anterior to the Rolandic fissure (prerolandic cortex) and the sensory cortex posterior to the
Rolandic fissure (postrolandic cortex). Axons leaving pontine nuclei travel medially and cross to the
opposite side where they project to the cerebellum via the middle cerebellar peduncle. This corticoponto-cerebellar pathway is involved in rapid correction of movements. There is a somatotopic
organization of cortical projections, with the “arm” region of the sensorimotor cortex synapsing on dorsal
pontine nuclei and the “leg” region of the sensorimotor cortex synapsing on ventral pontine nuclei.
There are also descending cortical fibers from parietal and temporal association areas, as well as from
premotor and prefrontal association areas in the frontal lobe, which synapse on neurons in the pontine
nuclei. Disruption of these pathways are likely responsible for the impairment of fine movements with
disorders of cognition. Finally, there are fibers that descend from the cingulate gyrus to the pontine
nuclei. This pathway is likely responsible for the impairment of motor control with strong emotion.
Other descending pathways in the pons include: the central tegmental tract which projects from the basal
ganglia to the inferior olive; the rubrospinal tract; and the medial longitudinal fasciculus (MLF).
The pontocerebellar fibers run transversely across the pons. Other fibers that run in the same direction
are fibers of the trapezoid body, however these are located in the tegmentum and not in the basis.
These transverse fibers, which almost bisect the ventral half and the dorsal half of the pons at this level,
originate in the cochlear nuclei, travel through the tegmentum, and gather in the lateral pons to form the
lateral lemniscus.
While pontocerebellar fibers and fibers of the trapezoid body run transversely, there are also fibers in the
pons that run longitudinally, creating a “basket weave” of intersecting fibers:
 corticospinal fibers – scattered widely at the rostral pons, these fibers gradually coalescing as
they descend through the pons and approach the pyramids in the medulla
 corticobulbar fibers – some of these axons arising in cerebral cortex synapse directly on cranial
nerve nuclei (corticobulbar) and some synapse on intermediate reticular neurons (corticoreticulobulbar), providing both direct and indirect cortical innervation of cranial nerves.
1
There are three connections from the brainstem to the cerebellum: the brachium pontis (middle cerebellar
peduncle) is the major connection that arises from pons; below is the restiform body (inferior cerebellar peduncle)
arises from medulla; above is the brachium conjunctivum (superior cerebellar peduncle) arises from midbrain.
.
2
These nuclei continue inferiorly into the medulla as the arcuate nuclei on the ventral surface of the pyramids. Like
the pontine nuclei, the arcuate nuclei project to the cerebellum but their axons travel via the restiform body rather
than via the brachium pontus.
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Brainstem 2 - Pons
Throughout the length of the tegmentum of the pons is the reticular formation, which is positioned in
the dorsal portion of the tegmentum and is involved in maintenance of consciousness. Ascending in the
ventral portion of the tegmentum is the lemniscal sensory system, which intersect the transverse fibers of
the trapezoid body. Spanning the width of the pons, this system consists of:
 medial lemniscus – this is located medially and carries “dorsal column” sensory fibers from the
nucleus cuneatus and the nucleus gracilis
 trigeminal tract – this is located just lateral to the medial lemniscus and carries information
about pain, temperature, touch and position from the contralateral face
 spinothalamic tract – this is located just lateral to the trigeminal tract and carries information
about pain and temperature from the contralateral body
The lower pons contains nuclei of CN VI, VII, and VIII. The rhomboid fossa is covered superiorly by the
cerebellum, whose nuclei appear close to the 4th ventricle. Important tracts include the trapezoid body (a
relay station and crossing point in the auditory pathway) and the central tegmental tract (part of the motor
system).
The mid pons (next page) contains the motor and sensory nuclei of CN V. This section demonstrates the
anterior spinocerebellar tract immediately dorsal to the pons.
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Brainstem 2 - Pons
The upper pons contains the locus ceruleus (see below). The only cranial nerve nucleus in the section is
the lower part of CN V mesencephalic nucleus, which innervates muscles of chewing. The scattered
pontine nuclei are intermingled with the pyramidal tract. Among the tracts, the medial longitudinal
fasciculus, which serves as the “highway” connecting brainstem nuclei, is prominent3. Fibers exiting the
cerebellum via the superior cerebellar peduncle are also seen. The medial lemniscus has a more lateral
and horizontal orientation at this level.
3
The smaller dorsal longitudinal fasculus connects nuclei in the hypothalamus with parasympathetic cranial nerve
nuclei.
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Brainstem 2 - Pons
In addition to cranial nerve nuclei (discussed below), there are several vital nuclei in the pons:

locus ceruleus – this small nucleus is located in the rostral pons in the dorsal part of the
tegmentum; it measures about ½ inch in its vertical dimension and it is populated with melanincontaining neurons. It is the primary source of noradrenergic innervation4 in the brain. From
this nucleus are two efferent pathways: one which ascends to the cortex, hippocampus and
cerebellum, and one which descends to the brainstem and spinal cord. Loss of these neurons is
found in Alzheimer’s disease and Down syndrome and Parkinson’s disease.

parabrachial nucleus –Located in the dorsolateral pons, the neurons in this nucleus serve as a
relay station in the brainstem pathway for taste.

pedunculopontine nucleus -- also in the dorsolateral pons, this nucleus contains two
populations of neurons: one containing acetylcholine (cholinergic neurons) and another
containing glutamate (glutamatergic neurons). The efferent cholinergic fibers are widely
distributed in the brain. They are involved with motor learning and voluntary motor control. The
efferent glutamatergic fibers descend to the pontine and medullary reticular and are involved
with locomotion. Together the neurons coordinate arm and leg movement while walking.
Another function relates to control of saccadic eye movements.
CRANIAL NERVES OF THE PONS
The pons gives rise to CN V through VIII. Unlike cranial nerves arising from the medulla, those in the
pons are associated with only a single nucleus. Reviewing the cranial nerves from caudal to rostral:
Vestibulo-cochlear nerve (CN VIII). This nerve is the sole exception to the statement above; it has two
divisions which travel together from the inner ear to the pons, but each have distinctive end organ and
nuclear connections.

4
Vestibular nerve – the axons of this nerve originate from neuronal cell bodies in Scarpa’s
ganglion. They carry information from the semicircular canals (concerning angular acceleration)
and the utricle and saccule (concerning linear acceleration and gravity). The nerve enters the
lateral brainstem and travels in a dorsal-medial direction toward the vestibular nuclei, which lie
adjacent to the floor of the fourth ventricle. A few of the axons of the vestibular nerve do not
synapse in the vestibular nuclei but travel directly to the cerebellum in the juxtarestiform body,5
becoming mossy fibers in the flocculonodular lobe and uvula of the cerebellum. Most axons of
the vestibular node project to one of four vestibular nuclei:
o medial and inferior – which straddle the division between pons and medulla
o lateral and superior – which are located fully in the pons
These neurons release noradrenalin (or norepinephrine), which is one of the catecholamines. Neurons of the
locus ceruleus would therefore be also considered catecholamine neurons.
5
These are fibers on the medial side of the restiform body (inferior cerebellar peduncle) that also connect the
brainstem to the cerebellum.
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
6
Brainstem 2 - Pons
Vestibular nuclei – these nuclei receive input from a variety of sources in addition to the
vestibular nerve. These include cerebral cortex, cerebellum and spinal cord. Efferents from the
vestibular nuclei project widely, reflecting the many parts of the nervous system involved with
balance:
o Spinal cord
 Lateral vestibulospinal tract – arises from the lateral vestibular nucleus and
descends in the MLF 6 to facilitate neurons which control extensor muscles
 Medial vestibulospinal tract – arises from the medial vestibular nucleus and
descends in the MLF to facilitate neurons which control flexor muscles
o
Cerebellum
 Vestibulocerebellar fibers – arises from all but the lateral vestibular nucleus and
travel to the ipsilateral cerebellum via the juxtarestiform body. Along this
pathway there are fare more axons traveling from the cerebellum to the vestibular
nuclei than traveling to the cerebellum, where they terminate in the flocculonodular lobe, uvula and fastigial nucleus of the cerebellum.
o
Thalamus -- vestibulothalamic fibers arise from all but the inferior vestibular nucleus
and travel via several pathways to the thalamus
o
Nuclei of extraocular muscles – axons projecting to the nuclei of CN III, IV and VI
arise from all four vestibular nuclei and travel via the MLF. Thalamic fibers which cross
to the contralateral side have an excitatory effect, while those which remain ipsilateral
have an inhibitory effect on nuclei of extraocular muscles. Vestibular efferents have a
critical role in coordinating conjugate eye movements.
o
Vestibular cortex – output from vestibular nuclei reaches this region of the temporal
lobe via the thalamus
o
Semicircular canals, utricle and saccule – axons traveling in the vestibular nerve
(“against the stream” of incoming sensory information) provide bilateral excitatory input
to the hair cells of the vestibular end organs.
MLF = medial longitudinal fasciculus
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
Brainstem 2 - Pons
Cochlear nerve -- the axons of this nerve originate from neuronal cell bodies in the spiral
ganglion. They carry information from the auditory end organ, or organ of Corti. The nerve
enters the brainstem lateral to the axons of the vestibular nerve, and they travel to the two
cochlear nuclei. Whereas the vestibular nuclei are located medial to the restiform body, the
cochlear nuclei are located lateral to the restiform body. Axons carrying high frequency sound
signals travel to the dorsal cochlear nucleus, while axons carrying low frequency sound signals
travel to the ventral cochlear nucleus. “Second order” sensory neurons project from the cochlear
nuclei to the superior olivary complex.
Facial nerve (CN VII). The facial nerve is a mixed nerve with both sensory and motor components.
SENSORY COMPONENTS – these travel via a branch of the facial nerve called the nervus
intermedius
o
exteroceptive axons from the external ear – these arise from neurons in the geniculate
ganglion and they project to the spinal trigeminal nucleus (as do axons from the same
region carried by CN IX and X)
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o
Brainstem 2 - Pons
axons carrying taste information from the anterior tongue – these arise from neurons in
the geniculate ganglion and they project to the nucleus solitarius (as do axons from CN
IX and X, carrying taste information from the posterior tongue and epiglottis).
MOTOR COMPONENTS
o
secretomotor fibers to the glands – these are preganglionic fibers that arise from the
superior salivatory nucleus in the pontine tegmentum. This is located just dorsal to the
facial motor nucleus (see below). These secretomotor fibers travel with sensory
components of the facial nerve in the nervus intermedius until they branch off to ganglia
where they synapse7
 axons to the lacrimal gland synapse in the pterygopalatine ganglion
 axons to the submandibular and sublingual glands travel via the chorda
tympani to synapse in the submandibular ganglion
o
somatic motor fibers – these constitute the major portion of the nerve, and they provide
innervation to muscles of the face, the stapedius in the inner ear, and the stylohyoid and
posterior digastric muscles of the neck. These axons begin in the facial motor nucleus
but they follow a bizarre course: they travel medially, loop around the nucleus of the
abducens nerve (CN VI), and then travel laterally to exit the pons. The facial motor
nucleus receives input from several sources:





Cerebral cortex – corticobulbar fibers arise from the primary motor cortex, the
supplementary motor cortex, the premotor cortex, and the cingulated cortex.
Cortical input to the facial motor nucleus is bilateral to the region of the nucleus
that innervates muscles of the forehead. Cortical input is unilateral (and
contralateral) to the region that innervates muscles of the lower face.8
Basal ganglia – this is the source of innervation that allows facial muscles to
activate in response to emotion, even when they may be paralyzed as a result of
losing corticobulbar input
Superior olive – this is the source of innervation that facilitates grimacing of
facial muscles in response to loud noise
Trigeminal nerve – this is the source of innervation that provides the afferent
(sensory) arc to the blink reflex in response to stimulation of the cornea
Superior colliculus – this is the source of innervation, which travels the
tectobulbar tract, that facilitates closing eyelids in response to a visual threat or
bright light.
7
Proximal lesions of the facial nerve can lead to later confusion in regrowth of nerve axons. This may cause
crocodile tears, a phenomenon in which food in the mouth stimulates production of tears rather than saliva.
8
This pattern of corticobulbar input has major clinical implications. When examining a patient with paralysis of
facial muscles on one side, it is necessary to determine whether weakness is caused by a problem in the cerebral
cortex (e.g. stroke) or whether it is caused by a problem in the facial nerve (e.g. Bell’s palsy). Bilaterally cortical
input to the region of the facial nucleus that innervates muscles of the forehead means that these muscles continue to
work after an injury to one side of the cerebral cortex. These muscles cannot work if the facial nerve itself is
injured. Therefore, lower facial weakness with preserved ability to “wrinkle the forehead” on the same side must be
caused by a central lesion (cerebral cortex) rather than a peripheral lesion (facial nerve).
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Facial nerve injury – CN VII traverses the temporal
bone and may sustain injury with bone fractures.
Clinical symptoms vary depending upon the site of
the lesion.
1
2
3
4
5
Facial paralysis, vestibulocochlear nerve
dysfunction (deafness and dizziness).
Facial paralysis with disturbances of
tearing, taste and salivation.
Same as 2 with addition of hyperacusis
due to paralysis of stapedius muscle.
Facial paralysis with disturbances of
taste and salivation.
Facial paralysis only.
Abducens nerve (CN VI). The abducens nerve is a
pure motor nerve that innervates the lateral rectus muscle. It cooperates with CN III and IV in
controlling eye movements. The abducens nucleus is located close to the midline, adjacent to the floor
of the fourth ventricle, in the tegmentum of the pons. Within the nucleus are large motor neurons which
give rise to the abducens nerve and innervate the lateral rectus muscle. There are also small interneurons
which project via the contralateral MLF to the oculomotor nucleus, which innervates the medial rectus
muscle. Thus, there is both control and coordination of side-to-side eye movements of the eyes from the
abducens nucleus, which receives crossed and uncrossed innervation from descending corticobulbar
fibers.
Trigeminal nerve (CN V). The trigeminal nerve is the largest of all cranial nerves. It has both motor
and sensory components. It also has two roots, the smaller related to motor function and the large related
to sensory function. The motor root is a coalescence of a dozen small rootlets exiting the pons. It is
almost the “reciprocal” of the facial nerve (CN VII), as CN V has a small motor component and a large
sensory component, compared to CN VII which has a large motor component and a small sensory
component.
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Brainstem 2 - Pons
MOTOR COMPONENTS – efferent axons arise from the motor nucleus of V in the tegmentum of
the pons. This nucleus receives bilateral input from corticobulbar fibers and from the sensory nuclei
of V. Motor axons innervate the muscles of chewing, the tensor tympani,9 tensor palati,10 and
myelohyoid and anterior digastric in the neck.
SENSORY COMPONENTS
o proprioceptive fibers – sensations of pressure and movement from the teeth, gums, hard
palate, and temporomandibular joint (TMJ) travel to the mesencephalic nucleus of V,
which is located where the pons transitions to midbrain. This nucleus is homologous to a
dorsal root ganglion of the spinal cord, but is located deep within the brainstem.
o
exteroceptive fibers – sensations of pain, temperature and light touch from the face and
anterior head are carried by axons which travel in one of three division of the trigeminal
nerve to the gasserian (trigeminal) ganglion. These divisions are: ophthalmic division
(V1), maxillary division (V2), and mandibular division (V3).
Shown on the left are points where the three
branches of the trigeminal nerve exit from the
skull: V1 from the supraorbital notch, V2 from
the infraorbital foramen, and V3 from the
mental foramen.
During examination each exit point may be
tested for tenderness.
Sensory axons leave the gasserian ganglion and enter the lateral aspect of the pons.
From this point they follow different courses:

Light touch fibers divide into ascending and descending pathways upon entering
the pons.
 Ascending touch fibers project a short distance to the main sensory
nucleus of V, from which “second order” neurons travel to the ventral
posteriomedial nucleus (VPN) of the thalamus. These include crossed
fibers which ascend in the ventral trigeminothalamic tract and uncrossed
fibers which ascend in the dorsal trigeminothalamic tract
 Descending touch fibers follow the course of pain and temperature
fibers (discussed below)

Pain and temperature fibers descend within the spinal tract of V to synapse in
the spinal nucleus of V. “Second order” neurons then cross the midline and
ascend to the thalamus via the ventral trigeminothalamic tract.
9
The tensor tympani (CN V) and the stapedius (CN VII) are stimulated by loud noises to constrict. This has the
effect of stiffening the ossicles of the inner ear and reducing the amount of sound energy transmitted to the cochlea.
Also to protect the inner ear, these muscles constrict during vocalization. This in part explains why a person’s voice
sounds different when heard from a recorded source.
10
This small ribbon-like muscle tenses the soft palate.
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Brainstem 2 - Pons
TRIGEMINAL REFLEXES – as the ventral trigeminothalamic tract ascends to the thalamus, it
gives off collateral branches to several cranial nerve motor nuclei. This is the mechanism by
which the trigeminal nerve (CN V) participates in many reflexes:
REFLEX
Jaw jerk
Blink reflex
Corneal reflex
Sneeze reflex
Vomiting reflex
Salivation reflex
Tear formation
MOTOR NUCLEI
motor nucleus of V
facial motor nucleus (CN VII)
nucleus ambiguus, respiratory center, and anterior horn
cells for diaphragm and intercostal muscles
dorsal motor nucleus of X
inferior salivatory nucleus (CN IX)
superior salivatory nucleus (CN VII)
BLOOD SUPPLY TO THE PONS
The pons receives blood from the basilar artery. There are three groups of branches, each serving a
particular region:
 paramedian arteries – four to six small vessels penetrate the pons from the ventral side,
supplying the medial part of the basis pontis and the pontine tegmentum. This includes the
pontine nuclei, corticospinal tracts, and medial lemniscus.
 short circumferential arteries – supply the ventrolateral part of the basis pontis
 long circumferential arteries –
o anterior inferior cerebellar artery (AICA) – supplies lateral tegmentum of the lower pons
and the ventrolateral cerebellum
o internal auditory artery – supplies the auditory, vestibular, and facial cranial nerves
o superior cerebellar artery – supplies dorsolateral pons, dorsal reticular formation and the
middle and superior cerebellar peduncles.
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