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of the spinal cord | Main Anatomy Index | The reticular formation
Last updated 30 March 2006
The brainstem consists of the medulla, the pons
and the midbrain.
The activities of the brainstem may be divided into 3 general types:
- Conduit functions;
- Cranial nerve functions;
- And integrative functions.
- The conduit functions
are apparent, since the only way for ascending tracts to
reach the thalamus or cerebellum (or for descending tracts to reach the spinal
cord) is through the brainstem.
- Many of these tracts, however, are not straight-through affairs,
and relay nuclei in the brainstem are frequently
- The cranial
- The head's equivalent of spinal nerve fibres;
- And those fibres involved in the special senses of olfaction, sight, hearing, equilibrium, and taste.
- The olfactory and optic nerves
project directly to the telecephalon and diencephalon, respectively.
- The others project to or emerge from the brainstem.
- A wide assortment of sensory
and motor nuclei related to cranial
nerve function can be found at various brainstem levels.
- A number of integrative
functions are organised at the level of the brainstem.
- These include complex motor patterns, aspects of respiratory and cardiovascular activity,
and even some regulation of the level
- Much of this is accomplished by the reticular
- These 3 general types of activity are far from mutually exclusive.
Internal Structure of the Brainstem
At any given brainstem level rostral to the obex, 3 general areas can be
identified in cross section.
- The area posterior to the ventricular space;
- The area anterior to the ventricular space;
- And large structures "appended" to the
anterior surface of the brainstem.
- In the caudal medulla the central
canal is surrounded by structures, including
some that will be anterior to the ventricular
space at more rostral levels.
Posterior to the Ventricular Space
The only place where this part contains a substantial amount of neural tissue is the midbrain.
Here this region is called the tectum (L. roof).
It consists of the superior and inferior
- In the pons and rostral medulla,
the 4th ventricle is covered
posteriorly by the superior and inferior medullary vela.
- Posterior to these, of course, is the cerebellum.
Anterior to the Ventricular Space
This part is called the tegmentum (L. covering).
The tegmentum contains most of the structures: the reticular
formation, cranial nerve nuclei and tracts, ascending
pathways from the spinal cord, and some descending pathways.
Structures Appended to the
These structures contain fibres descending from the cerebral
- The spinal cord;
- Certain cranial nerve nuclei;
- Or the pontine nuclei, which in turn project to the cerebellum.
Gross Anatomy of the Medulla
The medulla is vaguely scoop-shaped.
The "handle" corresponds to the caudal or closed portion
containing a central canal continuous with that of the spinal cord.
The open portion corresponds to the rostral
or open medulla, in which the central
canal expands as the fourth ventricle.
- The apex of the V-shaped caudal
fourth ventricle, where it narrows into the central canal, is called the obex.
Grooves of the Medulla
of the surface of the spinal cord continue
into the medulla.
They divide the surface of the medulla into a series of columns.
- The anterior median fissure is briefly
interrupted by the pyramidal decussation at
the junction between spinal
cord and brainstem.
- It continues, however, rostrally to the edge of the pons and
separates the two pyramids.
- The anterolateral sulcus marks the other side of the pyramid.
- The rootlets of the hypoglossal
nerve (CN XII) emerge from this sulcus.
- The rootlets of the glossopharyngeal
nerve (CN IX) and vagus nerve (CN X) emerge
from the same shallow lateral groove as the spinal part of the accessory nerve
- In the rostral medulla, the column
between the hypoglossal rootlets and the vagal and glossopharyngeal rootlets is enlarged
to form the olive.
- The posterolateral sulcus also continues into the
- The area between it and the line of rootlets of CN IX and CN X is referred to
as the tuberculum cinereum.
- Beneath the tuberculum cinereum is the spinal tract of the trigeminal nerve.
- This is the head's equivalent of Lissauer's
- The fasciculus cuneatus extends rostrally to a small swelling called the cuneate
- This marks the site of the nucleus cuneatus.
- The fasciculus gracilis extends rostrally to the gracile tubercle, and this marks the site of the nucleus gracilis.
The Floor of the Fourth Ventricle
Here the are also various grooves and elevations that signify the presence
of underlying nuclei.
- The sulcus limitans can be followed
rostrally along the floor of the ventricle into the pons.
- It is a line of separation between motor
nuclei (medial to it) and sensory nuclei
(lateral to it).
- The portion of the medulla and pons immediately beneath the floor of the ventricle, and lateral
to the sulcus limitans, is mostly occupied by vestibular nuclei.
- It is referred to as the vestibular area.
- The area medial to the sulcus
limitans overlies a series of motor nuclei.
- In the medulla, the hypoglossal nucleus and the dorsal motor nucleus of the vagus make small
- They thus called the hypoglossal and vagal
- Farther rostrally, in the pons, is
another elevation called the facial
- This elevation is not caused by an underlying motor
nucleus of the facial nerve.
- It is the location of the abducens
- Fibres destined for the facial nerve loop over at this
location on their way out of the brainstem.
Internal Structure of the Caudal
This looks somewhat like the spinal cord.
Part of the anterior horn is still present caudally, as are structures
similar to Lissauer's tract and part of
the posterior horn.
The latter two are actually the spinal tract and spinal nucleus of the trigeminal nerve.
These are the head's equivalent of Lissauer's tract and the substantia gelatinosa.
They deal with pain, temperature,
and some tactile information.
- The spinothalamic tract maintains
more or less the same location during its passage through
the brainstem, at least until it reaches the rostral midbrain.
- The pyramids and their decussation
are located most anteriorly in the caudal medulla.
- Each pyramid consists of corticospinal
fibres that originated in the ipsilateral
cerebral cortex and are (mostly) bound for the
contralateral anterior horn.
- Most of the area traversed by internal arcuate fibres
is reticular formation.
- The reticular formation
looks like the uniform neural tissue.
- In fact, however, the reticular formation is organised on a
The Gracile and Cuneate Nuclei
The fasciculi gracilis and cuneatus
continue into the caudal medulla but are gradually replaced by the posterior
column nuclei, i.e., the nucleus gracilis and nucleus cuneatus.
- The nucleus cuneatus begins and ends
a bit rostral to the nucleus
- Postsynaptic fibres leave these two nuclei
in a ventral direction and arch
across the midline.
- They form the contralateral medial lemniscus.
- They are vertically oriented bands of fibres.
- These decussating fibres are part of the internal arcuate fibres
and are sometimes called the sensory decussation.
- The medial lemniscus is organised (somatotopic organisation) so that
fibres representing cervical segments are most posterior.
The Lateral Cuneate Nucleus
Adjacent to the nucleus cuneatus and embedded in the fasciculus cuneatus is the lateral
(or external) cuneate nucleus.
This is the forelimb equivalent of nucleus
dorsalis (Clarke's nucleus).
The axons of these cells join the posterior spinocerebellar tract
in the inferior cerebellar peduncle at a slightly more rostral level.
Internal Structure of the Rostral
The rostral medulla no longer looks much like the
The walls of the embryonic neural
tube have been pushed outward to form the floor of the fourth ventricle.
The Inferior Olivary Nucleus
The caudal boundary (the obex) is approximately
coincident with the caudal edge of the inferior olivary nucleus.
This is a prominent structure that is responsible for the surface swelling of the olive.
- Fibres can be seen leaving the medially facing mouth
(or hilus) of the inferior olivary nucleus.
- They arch across the midline, and join the contralateral inferior cerebellar peduncle.
- These are also internal arcuate fibres.
- They are added progressively in a rostral direction increasing the size of the peduncle.
- The inferior cerebellar peduncle is located dorsolaterally at these levels.
- It grows progressively larger as it continues rostrally.
Structures around the Medial Lemniscus
to the inferior olivary nucleus is the medial
lemniscus, which still has the shape of a flattened band with
a dorsal-ventral axis.
Anterior to the medial lemniscus is the pyramid.
Fascicles of the hypoglossal nerve (CN XII) emerge lateral to the pyramid
in the groove between it and the inferior
- Posterior to the medial lemniscus, near the floor of the fourth ventricle, is a small but distinctive bundle of fibres.
- This is the medial longitudinal fasciculus (MLF), which
is involved in vestibular functions and eye movements.
Gross Anatomy of the Pons
The pons (L. bridge) is dominated by the massive, transversely oriented structure on its
This part of the pons is called the basal pons and
looks like a bridge interconnecting the two cerebellar hemispheres.
- It does not, however, interconnect them.
- Rather, many of the fibres descending in a cerebral peduncle synapse
in scattered nuclei of the ipsilateral
half of the basal pons.
- These nuclei in turn project
their fibres across the midline.
- They then funnel into the middle cerebellar peduncle
and finally enter the cerebellum.
The Entry and Exit of Cranial Nerves
around the Pons
The trigeminal nerve (CN V) enters
the brainstem at the midpons.
Three other cranial nerves enter (or leave) along the groove
between the basal pons and the medulla;
- The abducens nerve (CN VI) is the smallest
and most medially located of these three, exiting where
the pyramid emerges from the basal
- The facial nerve (CN VII) is farther
lateral and consists of two parts: a larger and more medial motor root
and a smaller sensory root (sometimes referred to as the intermediate
- The vestibulocochlear nerve (CN VIII) is slightly lateral to the facial nerve
and also has two parts: a vestibular
division and a more lateral cochlear division.
The Superior Cerebellar Peduncle
This forms much of the roof of the fourth
ventricle in the pons.
It emerges from the cerebellum and moves
toward the midline and the brainstem.
It enters the latter near the junction
between the pons and midbrain.
At this same junction, the trochlear nerve (CN IV)
emerges from the dorsal surface of the brainstem.
- The superior cerebellar peduncle is covered in the rostral pons by a flattened band of
fibres called the lateral lemniscus.
- This forms part of the ascending auditory system and
terminates in the inferior colliculus.
Internal Structure of the Caudal Pons
The caudal pons extends from the rostral wall of the lateral recess of the fourth ventricle
to the point of attachment of the trigeminal
- In the caudal pons the inferior olivary nucleus ends,
and the inferior cerebellar peduncle bends posteriorly and enters the cerebellum.
- As the inferior olivary nucleus ends, the medial lemniscus assumes a more oval
shape, as though it had previously been held upright against the midline.
The Fibres of the Basal Pons
The pyramidal tract becomes dispersed
in the basal pons.
This contains bundles of longitudinally oriented fibres,
bundles of transversely oriented fibres, and pontine nuclei scattered among these bundles.
- Some of the longitudinally oriented fibres are those of
the pyramidal tract.
- Most of the others are corticopontine fibres.
- These fibres originate in many areas of the cerebral cortex and terminate in ipsilateral
- Fibres arising in the pontine nuclei cross the midline and form the massive middle
Internal Structure of the Rostral Pons
The rostral pons extends from the attachment point of
the trigeminal nerve (CN V) to the beginning of the cerebral aqueduct.
This is about at the point of emergence of the trochlear nerve (CN IV).
- The MLF is visible throughout the rostral pons.
- The fourth ventricle narrows as the cerebral
aqueduct is approached.
- The superior cerebellar peduncle becomes
apparent in the wall of the ventricle.
- This is the major outflow from the cerebellum
and it projects to the thalamus
and to other structures.
- The medial lemniscus gradually takes on a more flattened profile, now with a medial-lateral
- It also assumes a transverse orientation at the junction between the basal pons
and the pontine tegmentum.
- As the medial lemniscus moves laterally,
it approaches the spinothalamic tract.
- From here through the midbrain, the two are adjacent.
The Locus Ceruleus
This is near the floor of the fourth ventricle.
It is a collection of pigmented cells that contain neuromelanin and appear blue-black
in unstained brain tissue.
- The locus ceruleus (L. blue spot) is a long thin nucleus.
- Cells of the locus ceruleus contain the
neurotransmitter norepinephrine and innervate
virtually the entire CNS, from the spinal cord
to cerebral cortex.
- Some of these cells send their axons to one particular site.
- Others have widely branched axons, e.g., a single neuron of the locus ceruleus
can have synaptic terminals in the diencephalon,
spinal cord, and cerebral cortex.
- The function of the locus ceruleus is not known with certainty.
- It has, however, been surmised from the widespread connections
of this nucleus that it has some sort of overall biasing effect
on the CNS.
- Consistent with this notion is the finding that the discharge rate
of locus ceruleus neurons increases as an animal's state
of arousal increases, decreases
with drowsiness, and is nearly
abolished in sleep.
Gross Anatomy of the Midbrain
The midbrain is characterised by:
- The paired superior and inferior
colliculi on its posterior surface;
- And the large cerebral peduncles on its anterior surface.
- The broad low ridge extending rostrally
from the inferior colliculus is the inferior
- This is a continuation of the ascending
auditory pathway, projecting from the inferior colliculus
to the medial geniculate nucleus (thalamic relay nucleus
Internal Structure of the Caudal
The caudal midbrain is the part that contains the inferior
It extends from the point of emergence of the trochlear nerve (CN IV) to the intercollicular
- The fourth ventricle has narrowed
into the cerebral aqueduct.
- The superior cerebellar peduncles sink deeper into the midbrain tegmentum
and begin to decussate.
- The MLF continues on its usual
- The basal pons protrudes rostrally
under the caudal midbrain tegmentum.
- The inferior colliculus is a prominent
- Ventromedial to it, encircling the aqueduct,
is a particularly pale-staining region of grey matter called the periaqueductal
- The medial lemniscus is still a flattened
band of fibres.
- Here, it curves a bit dorsally.
- The spinothalamic tract is dorsal to it at the surface of the
- At slightly more rostral levels, the spinothalamic
fibres are arranged in a band just beneath the brachium of the inferior
Internal Structure of the Rostral
This part contains the superior colliculi.
It extends from the intercollicular groove to the posterior commissure.
- At this level the MLF is ending.
- The decussation of the superior cerebellar peduncles is complete.
- In their place the large red nucleus becomes visible on each side.
- Some fibres from the contralateral half of the cerebellum end here.
- Most, however, continue on to the thalamus.
- Several parts of the diencephalon (the pineal gland and some thalamic nuclei)
hang back over and alongside the rostral midbrain.
- At rostral midbrain levels, the medial lemniscus and
the spinothalamic tract form a continuous curved band of fibres.
- Spinomesencephalic fibres (or spinotectal tract) that have accompanied the spinothalamic tract through the brainstem
terminate in the periaqueductal grey, adjacent
regions of the reticular formation and
certain portions of the superior colliculus.
The Substantia Nigra
is the substantia nigra
(dark in unstained or cell-stained
These neurons use dopamine for their neurotransmitter,
ending profusely on neurons of the putamen and caudate nucleus.
Malfunction of this system results in Parkinson's disease.
- Anterior to the red nucleus
- Ventral to the substantia nigra is a massive
bundle of fibres referred to as the cerebral peduncle.
The Cerebral Peduncle
This bundle consists principally of descending
corticopontine and corticospinal fibres.
The oculomotor nerve (CN III) emerges from the space between the cerebral peduncles
(the interpeduncular fossa).
Blood Supply of the Brainstem
This is chiefly through the vertebral-basilar system.
Blood Supply of the Medulla
The caudal medulla has a supply much like that of the spinal cord.
Anterior and lateral portions are supplied
by the anterior spinal artery and/or small
branches of the vertebral artery.
- Posterior portions are supplied by the posterior spinal
artery and/or small branches of the posterior inferior cerebellar artery (PICA).
- The rostral medulla receives a varying
- Anterior and medial structures, such as the pyramid and the medial lemniscus,
depend on vertebral branches and the anterior
- Lateral and posterior structures, such as
the spinothalamic tract and the inferior cerebellar peduncle, depend on the PICA
and the posterior spinal artery.
Blood Supply of the Pons
This is mainly by unnamed paramedian and circumferential branches of the basilar
The anterior inferior cerebellar artery (AICA) and the superior cerebellar artery contribute branches
to the middle and superior
cerebellar peduncles and to dorsal and lateral portions of the pontine tegmentum.
Blood Supply of the Midbrain
This is chiefly from the posterior cerebral artery,
with some contribution from the superior cerebellar artery caudally.
In addition, the anterior choroidal artery and the posterior communicating artery may send branches
to the cerebral peduncle.