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Brainstem 1 - Overview
Brainstem – Part 1: Overview
DAVID GRIESEMER, MD
Professor of Neurosciences and Pediatrics
Key Concepts:
1. The brainstem is divided into three regions in the rostral-caudal direction: midbrain, pons and
medulla.
2. The brainstem is divided into three regions in the ventral-dorsal direction: basis, tegmentum and
tectum.
3. The tegmentum at all levels contains the reticular formation, which is critical to the process of
arousal and consciousness.
4. Prominent structures on the ventral aspect of the brainstem include the cerebral peduncles
(midbrain), pontis basis (pons), and the pyramids and olives (medulla).
5. Prominent structures on the dorsal aspect of the brainstem include the cerebellum (pons).
6. Unlike spinal nerves which have mixed motor and sensory components, cranial nerves may
possess pure motor function or pure sensory function.
7. Cranial nerves originate from nuclei which are arranged in vertical columns according to their
function.
8. Like reflexes mediated by the spinal cord, the brainstem also has superficial, muscle stretch, and
visceral reflexes.
The brainstem from rostral to caudal consists of midbrain (mesencephalon), pons (metencephalon) and
the medulla (myelencephalon). Approximately the diameter of a person’s smallest finger, the brainstem
is a crucial part of the nervous system that contains ascending sensory and descending motor tracts,
cranial nerve nuclei, secondary motor and sensory nuclei, autonomic pathways and centers, and the
reticular formation.
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Brainstem 1 - Overview
Listed below are the major components of the brainstem; these will be more thoroughly described later:
MOTOR COMPONENTS
o Lower motor neurons
 Cranial nerves III, IV, VI, VII, XI and XII
 Motor component of CN V
 Nucleus ambiguous (CN IX, X, XI)
o
Upper motor neurons
 Red nucleus
 Superior and inferior colliculi
 Medial and lateral vestibular nuclei
 Medial pontine and medullary reticular formation
o
Descending motor pathways
 Corticospinal tract
 Rubrospinal tract
 Lateral and medial vestibulospinal tracts
 Pontine and medullary reticulospinal tracts
 Tectospinal tract
SENSORY COMPONENTS
o Secondary sensory nuclei
 Nuclei gracilis and cuneatus
 Nucleus solitarius
 Main sensory and descending nucleus of CN V
 Dorsal and ventral cochlear nuclei
 Accessory auditory nuclei
 Superior, inferior, medial and lateral vestibular nuclei
 Superior and inferior colliculi
o
Ascending sensory pathways
 Medial and lateral lemnisci
 Trigeminal and spinal lemnisci
 Solitario-thalamic tract
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Brainstem 1 - Overview
AUTONOMIC COMPONENTS
o Parasympathetic preganglionic nuclei
 Edinger-Westphal (CN III)
 Superior (CN VII) and inferior (CN IX) salivatory nuclei
 Dorsal motor nucleus of CN X
o Medullary autonomic “centers” – respiratory, pressor, depressor and vomiting center
o Descending autonomic pathways – from the hypothalamus and limbic system
CEREBELLAR CONNECTIONS
o Superior cerebellar peduncle (brachium conjunctivum) – connects the midbrain with
the cerebellum *?
o Middle cerebellar peduncle (brachium pontis) – connects the pons and the cerebellum
*?
o Inferior cerebellar peduncle (restiform body) – connects medulla with the cerebellum
BRAINSTEM DEVELOPMENT AND DESCRIPTION
As noted above, the brainstem is divided from rostral to caudal into three regions: midbrain, pons, and
medulla. Less apparent are three divisions from ventral to dorsal. A mid-sagittal cross-section view of
the midbrain reveals the central canal. The canal helps to define these three regions. The very most
ventral (anterior in human) region is called the basis. At the level of the midbrain, this region includes
the cerebral peduncles, between which are located the small mammillary bodies (part of the
hypothalamus). The anterior bulges of the pons and medulla are also part of the basis region. The
enlargement of the ventral pons runs transversely, wraps around the brainstem, and connects via the
middle cerebellar peduncles to the cerebellum on the dorsal side of the brainstem. The enlargement of
the ventral medulla runs longitudinally. At the level of the medulla there are prominent, symmetrical,
cylindrical elevations on both sides of the midline. These are called the pyramids, and they contain fibers
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Brainstem 1 - Overview
from the descending corticospinal tracts or pyramidal tracts1. Immediately lateral to the pyramids are
large symmetric bumps called the olives (inferior olive), which contain large pre-cerebellar nuclei.2
The central canal separates the other two
ventral-to-dorsal areas.
The area ventral
(anterior) to the central canal is called the
tegmentum. Within the tegmentum, at the
level of the midbrain are two symmetric,
clearly defined, landmark nuclei (which
unfortunately cannot be seen from an external
view of the brainstem). These are the red
nuclei, which help control voluntary
movement, and the substantia nigra, which
contains the brain’s largest concentration of
dopamine-producing neurons. At the level of
the pons and medulla, the reticular formation
runs the full length of the tegmentum,
extending up into the midbrain.
Cross-section of brainstem at the level of midbrain (a),
pons (b), and medulla (c).
The region dorsal (posterior) to the central
canal is called the tectum. At the level of
midbrain the tectum is most fully developed.
When viewed from the dorsal side, the tectum
shows four symmetric hillocks: two superior
colliculi and two inferior colliculi.3 Together
these are called the quadrigeminal plate. At
the level of pons, the area dorsal to the central
canal does not develop tectum but undergoes
intense cellular proliferation, producing
neurons and glia that form the cerebellum and
populate the inferior olivary nucleus..
While the central canal at the level of the midbrain changes minimally to become the cerebral aqueduct,
the central canal at the level of the pons and midbrain changes dramatically. This is best understood if
one considers the basal plate to function as a hinge and the alar plate to stretch like a rubber band. With
development, the “hinge” of basal plate opens up and flattens, stretching the alar plate and rotating the
sulcus limitans4 anteriorly. This model can be refined by also considering the sides of the neural tube to
be hinges, with the axis of rotation along the sulcus limitans. The opening of these hinges creates a broad
flat ventral floor for the fourth ventricle and further stretches the thinning “rubber band” of alar plate.
This has the effect of rotating sensory nuclei (that arise from the alar plate) to a position lateral to the
motor nuclei of the basal plate. The process creates a large rhomboid-shaped fourth ventricle and moves
cranial nerve nuclei to the anterior portion of the brainstem.
1
At the midbrain level, the corticospinal tracts descend in the cerebral peduncles. At the junction of the medulla
and the spinal cord, the corticospinal tracts decussate, or cross from one side to the other. This is the point at which
fibers from the left side of the brain cross over to control the right side of the body, and vice versa.
2
3
The inferior olivary nuclei are easily recognized because of their rippled-ribbon configuration.
The superior colliculi help coordinate eye and head movements with shifting gaze. The inferior colliculi help
process auditory signals.
4
This is the longitudinal groove along the side of the central canal. It separates the ventral basal plate from the
dorsal alar plate.
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Brainstem 1 - Overview
RETICULAR FORMATION
The reticular formation is a phylogenetically ancient system that forms the core of the entire brainstem.
It is responsible for maintaining consciousness, maintaining general muscle tone and posture, processing
noxious (painful) stimuli, and regulating major visceral functions. The medial portion of the reticular
formation has a predominantly motor function, and the lateral portion has mainly sensory function.
Autonomic function is not precisely localized and is scattered through the entire reticular formation. The
various components can be summarized in this manner:
Motor components – maintenance of
muscle tone and posture by means of:
 reticulospinal tract (in the
pons) – which innervates
extensor muscles
 reticulospinal tract (in the
medulla) – which innervates
flexor muscles
Sensory components
 spinoreticular tract – which
conveys slow pain
 Maintenance of consciousness
– with the reticular formation
“activating”
the
cerebral
cortex in response to noxious
stimulation
Autonomic components – centers in the
medulla to control
 Blood pressure
 Respirations
 Cardiac function
 Gastrointestinal function
CRANIAL NUCLEI AND CRANIAL NERVES
Because most cranial nerve nuclei are anteriorly located in the brainstem, most cranial nerves exit
ventrally. Unlike sensory nerve roots in the spinal cord, even cranial nerves with sensory components
emerge from the anterior side of the brainstem. Cranial nerves further differ from spinal nerves in that
not all have mixed sensory and motor function like spinal nerve roots. Some cranial nerves have pure
motor functions, some have pure sensory functions, and some are mixed.
There is a strong correlation between function of cranial nerve nuclei and their position. Nuclei having
similar functions are found in the same vertical columns. The columns are not continuous, however, and
their vertical nature is somewhat compromised by distortions in the shape of the mature brainstem. There
are three distinct columns of motor nuclei and three distinct columns of sensory nuclei.
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Brainstem 1 - Overview
MOTOR NUCLEI
LOCATION
SOMATIC
MOTOR
BRANCHIAL
MOTOR
SENSORY NUCLEI
VISCERAL
MOTOR
GENERAL
SENSORY
SPECIAL
SENSORY
VISCERAL
SENSORY
Horizontal Level
Midbrain
Oculomotor
(III)
EdingerWestphal
(III)
Trochlear (IV)
Pons
Abducens (VI)
Trigeminal
(V)
Facial (VII)
Medulla
Hypoglossal
(XII)
N. ambiguus
(IX, X)
Spinal
accessory (XI)
Superior
salivatory
(VII)
Inferior
Salivatory
(IX)
Dorsal mot.
nucleus of
vagus (X)
Trigeminal:
mesencephalic
nucleus (V,
VII, IX, X)
Trigeminal:
principal
nucleus (V,
VII, IX, X)
Trigeminal:
spinal nucleus
(V, VII, IX, X)
Vestibular
(VIII)
Cochlear
(VIII)
N. of the solitary
tract (VII, IX, X)
Cranial nerves visible from the ventral (anterior) side of the brainstem and spinal cord all arise from the
same column of nuclei located within the neuraxis. Nerves include:
 pure motor (most medial in position)
o oculomotor (III)
o trochlear (IV) – which actually exits from the dorsal side of the brainstem
o abducens (VI)
o hypoglossal nerve (XII)
o spinal nerve rootlets
 mixed motor and sensory (intermediate in position)
o trigeminal nerve (V)
o facial nerve (VII)
o glossopharyngeal nerve (IX)
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
Brainstem 1 - Overview
o vagus nerve (X)
o accessory nerve (XI) – although located in this column, it is pure motor
pure sensory (most lateral in position)
o vestibular nerve (VIII)
o cochlear nerve (VIII)
Dorsal aspect of the brainstem
Cranial nuclei envisioned from the dorsal side of the brainstem (with cerebellum removed) demonstrate
the columnar organization of the brainstem nuclei. Motor nuclei are shown on the right half of the figure:
 somatic motor nuclei (most medial in position)
o oculomotor (III)
o trochlear (IV)
o abducens (VI)
o hypoglossal (XII)
o motor neurons of spinal cord (not shown)
 branchial motor nuclei (more lateral in position, more likely to also have sensory components)
o trigeminal motor (V)
o facial motor (VII)
o nucleus ambiguous (IX, X)
o spinal accessory (XI)
 visceral motor nuclei (variable in location)
o Edinger-Westfall (III) – actually dorsal to oculomotor nucleus, not medial as shown
o superior salivatory (VII) – slightly below and lateral to the facial nucleus
o inferior salivatory (IX) – below superior salivatory nucleus
o dorsal nucleus of the vagus (X) – just lateral to the hypoglossal nucleus
Sensory nuclei are shown on the left half of the figure. It is important to remember that sensory nuclei
originate in the alar (dorsal) plate. However, with creation of the fourth ventricle (which separates the
brainstem and cerebellum), the alar plate stretches at the midline and sensory nuclei rotate out to lay along
side the motor nuclei on the basal plate. As a result the most medial sensory nuclei developing in the alar
plate become the most lateral sensory nuclei of the mature brainstem, while the most lateral sensory
nuclei in the early alar plate rotate the least. These become the most medial sensory nuclei in the mature
Medical Neurosciences
Brainstem 1 - Overview
brainstem and they lie next to the most lateral motor nuclei.
 general sensory nuclei (medial in the alar plate, but more lateral in the mature brainstem)
o trigeminal sensory (V)
o spinal trigeminal (V) – continuous column with primary trigeminal sensory nucleus
 visceral sensory (lateral in the alar plate, but more medial in the mature brainstem)
o nucleus of the solitary tract (VII, IX, and X)
 special sensory (variable in location)
o vestibular (VIII)
o cochlear (VIII)
A review of this columnar organization of cranial nerve nuclei reveals the simple observation that some
cranial nerves are purely motor or sensory and other cranial nerves are mixed:
CRANIAL
NERVE
III
IV
V
VI
VII
VIII
IX, X
XI
XII
MOTOR
somatic, visceral
Somatic
Branchial
Somatic
branchial, visceral
branchial, visceral
Branchial
Somatic
COMPONENTS
SENSORY
general
general, visceral (SVA)
special
general, visceral (GVA & SVA)
At the end of the notes for this lecture is a comprehensive table summarizing cranial nerve classification,
components, connections and targets. This summaries information that you will learn later as different
levels of brainstem and functional systems are studied.
REFLEXES
Adding to established knowledge about reflexes at the spinal cord level, information about brainstem
reflexes completes the clinician’s knowledge of testable reflexes:
REFLEX
AFFERENT NERVE
SYNAPSE
EFFERENT NERVE
Superficial reflexes
Corneal
CN V
Pons
CN VII
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Sneeze (nasal)
Gag (pharyngeal)
Upper abdominal
Lower abdominal
Cremasteric
Plantar
Anal
Brainstem 1 - Overview
CN V
CN IX
T7-10
T10-12
Femoral
Tibial
Pudendal
Brain stem /
upper cervical cord
Medulla
T7-10
T10-12
L1
S1-2
S4-5
CN V, VII, IX, X and
C3-5
CN X
T7-10
T10-12
Genitofemoral
Tibial
Pudendal
Muscle stretch reflexes
Jaw
Biceps
Triceps
Brachioradialis
Knee (patellar)
Ankle (Achilles)
CN V
Musculocutaneous
Radial
Radial
Femoral
Tibial
Pons
C5-6
C7-8
C5-6
L3-4
S1-2
CN V
Musculocutaneous
Radial
Radial
Femoral
Tibial
Visceral reflexes
Light
Accommodation
Ciliospinal
CN II
CN II
Midbrain
Occipital cortex
T1-2
CN III
CN III
Cervical sympathetics
Oculocardiac
Carotid sinus
Bulbocavernosus
CN V
CN IX
Pudendal
Medulla
Medulla
S2-4
CN X
CN X
Pelvic autonomic
Bladder and rectal
Pudendal
S2-4
Pudendal and
autonomics
Pathologic reflexes
Extensor plantar
(Babinski)
Plantar
L3-S1
Extensor hallucis
longus
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COLUMN
Somatic
motor
OLD
SYSTEM
MODALITY
SE
CENTRAL
CONNECTIONS
Visceral
motor
General
sensory
LMN branchial
arches
SVE
GVE
Parasympathetic
GSA
Facial sensation
GVA
Taste
Sensory
SVA
Visceral
sensory
BRAINSTEM
NUCLEI
CRANIAL
NERVES
III, IV, VI
III, IV, VI
XII
XII
V
V
VII
VII
Facial muscles
Nucleus ambiguus
IX, X, XI
Pharynx, larynx
Edinger-Westphal
III
Motor cortex,
corticobulbar tract
TARGET
Extraocular muscles
Chewing muscles
Ciliary
Pupil constrictor
Pterygopalatine
Lacrimal glands
Submandibular &
sublingual glands
VII
Submandibular
Inferior salivatory
IX
Otic
Dorsal mot n of X
X
Intramural
Heart, lung, gut
Parotid gland
Main sensory V
V1, 2, 3
Trigeminal
Epicritic -- face
VPM - trigeminal
lemniscus
Spinal nucleus V
V1, 2, 3
Trigeminal
Protopathic -- face,
pharynx, larynx
VII
Geniculate
Anterior 2/3 tongue
VPM via Solitariothalamic tract
Nucleus solitarius:
rostral portion
IX
Petrosal
Posterior 1/3 tongue
X
Nodose
Epiglottis
IX
Petrosal
X
Nodose
Carotid, aortic body,
sinus, baroreceptors,
chemoreceptors, gut
sensation
Hypothalamus
Nucleus solitarius:
caudal portion
Visceral sensory
Hearing
Lateral lemniscus,
MGB
Dorsal & ventral
cochlear
Balance
Cortex, MLF,
cerebellum, VST,
reticular formation
Superior, inferior,
lateral & medial
vestibular
SSA
Tongue
Superior salivatory
Hypothalamus
Parabrachial nuclei
Special
sensory
GANGLIA
LMN myotomes
Motor
Branchial
motor
Brainstem 1 - Overview
Spiral
Cochlea
Scarpa
Vestibular apparatus
VIII