BRAIN STEM
Brainstem
 Performs spinal cord-like functions for the head
 Contains LMN for head muscles
 Initial processing of general afferent information
concerning head
Conduit function
 Ascending tracts reach thalamus and cerebellum
 Descending tracts reach spinal cord
 Many tracts do not run through straight- frequent
involvement of relay nuclei
Cranial nerve functions
 Cranial nerves are head’s equivalent of spinal nerves
 Involved in olfaction, sight, hearing, equilibrium and
taste
 Olfactory → telencephalon
 Optic → diencephalon
 The rest project to or emerge from brainstem
 Collection of sensory and motor nuclei related to
cranial nerve function at various levels in brainstem
Integrative function
 Complex motor patterns
 Regulation of cardiovascular and respiratory activity
 Some regulation of consciousness [function of
reticular formation in central core of brainstem]
 Much of this is accomplished by thereticular
formation, which forms the central core of the
brainstem
Medulla
 The medulla is vaguely scoop The "handle"
corresponds to the caudal or closed portion,
containing a central canal continuous with that of
the spinal cord.
 The open portion of the scoop corresponds to
the rostral or open medulla, in which the central
canal expands into the fourth ventricle.
 The apex of the V-shaped caudal fourth ventricle,
where it narrows into the central canal, is called
the obex
 The longitudinal grooves on the surface of the spinal
cord continue into the medulla, more clearly at some
levels than at others.
 They divide the surface of the caudal medulla and
part of the rostral medulla into a series of columns
that completely encircle it
 The anterior median fissure is briefly interrupted by
the pyramidal decussation at the junction
between spinal cord and brainstem, but then it
continues rostrally to the edge of the pons,
separating the two pyramids
The pons
 Pons is the Latin word for "bridge," and this portion
of it (called the basal pons) looks like a bridge
interconnecting the two cerebellar hemispheres.
 It is not, however, a direct interconnection.
 Rather, many of the fibers descending in each
cerebral peduncle synapse in scattered nuclei of the
ipsilateral half of the basal pons.
 These nuclei in turn project their fibers across the
midline, after which they funnel into the middle
cerebellar peduncle (brachium pontis) and
finally enter the cerebellum
Transverse planes defining the subdivisions of the brainstem
The Internal Structure of the Brainstem Reflects Surface
Features and the Position of Long Tracts
 The corticospinal and spinothalamic tracts have
consistent locations throughout the brainstem
 The three major longitudinal pathways
(corticospinal tract, poterior columns, and
spinothalamic tract) can be followed systematically
through the brainstem
 Two of the three stay in more or less the same
location throughout the brainstem.
 Corticospinal fibers travel in the most ventral part of
the brainstem, traversing the cerebral peduncle,
basal pons, and medullary pyramid.
 At the spinomedullary junction, most of the fibers in
the pyramids decussate and form the lateral
corticospinal tracts
 The spinothalamic tract at all levels of the brainstem
is in or near the anterolateral corner of the
tegmentum, similar to its position in the spinal cord.
 The posterior columns terminate in the posterior
column nuclei (nucleus gracilis and nucleus
cuneatus) of the medulla
Locations of the corticospinal tract, medial lemniscus, spinothalamic tract, and
reticular formation in the caudal and rostral medulla (A, B), caudal pons and
midpons(C, D) and caudal and rostral midbrain (E, F)
 Efferent fibers from these nuclei decussate in the
medulla to form the medial lemniscus, which reaches
the thalamus
Caudal/ “closed” medulla
 Extends from caudal edge of pyramidal decussation
[spinomedullary junction] to obex [caudal end of 4th
ventricle]
 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 (i.e., they deal with
pain, temperature, and some tactile information,
 Fasciculi gracilis and cuneatus continue into the
caudal medulla but are gradually replaced by the
posterior column nuclei (nucleus gracilis and nucleus
cuneatus).
 Nucleus cuneatus begins and ends a bit rostral to
nucleus gracilis
 Postsynaptic fibers leave these two nuclei in a ventral
direction and arch across the midline to form the
contralateral medial lemniscus, a vertically oriented
band of fibers
 These decussating fibers are part of the collection of
internal arcuate fibers and are sometimes called
the sensory decussation.
Caudal medulla –spinomedullary junction- level of pyramidal decussation
 Throughout the medulla, the medial lemniscus is
organized so that fibers representing cervical
segments are most posterior
 Adjacent to nucleus cuneatus and embedded in
fasciculus cuneatus is the lateral (or external)
cuneate nucleus
 This is the upper extremity equivalent of Clarke's
nucleus, and the axons of these cells join the
posterior spinocerebellar tract in the inferior
cerebellar peduncle at a slightly more rostral level.
Caudal medulla, just caudal to the obex..
Level of sensory decussation
 The prominent pyramids and their decussation are
located most anteriorly in the caudal medulla.
 Each pyramid consists of corticospinal fibers that
originated in ipsilateral cerebral cortex and are
(mostly) bound for the contralateral anterior horn.
Rostral Medulla
 The rostral (open) medulla, as defined here, extends
from the obex to the rostral wall of the lateral recess,
where the inferior cerebellar peduncle turns
posteriorly to enter the cerebellum.
 The rostral medulla no longer looks much like the
spinal cord,
 The caudal boundary (the obex) is approximately
coincident with the caudal edge of the inferior
olivary nucleus, a prominent structure that is
responsible for the appearance of the olive as a
surface swelling
 Fibers leave the medially facing mouth (or hilus) of
the inferior olivary nucleus, arching across the
midline, and joining the contralateral inferior
cerebellar peduncle.
 Medial 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 (XII) nerve emerge
lateral to the pyramid in the groove between it and
the inferior olivary nucleus.
 Posterior to the medial lemniscus, near the floor of
the fourth ventricle, is a small but distinctive bundle
of fibers that can be followed all the way to the
midbrain
Rostral medulla, just rostral to the obex
 This is the medial longitudinal fasciculus
(MLF), which is involved in coordinating head and
eye movements.
 The spinothalamic tract remains in the anterolateral
portion of the tegmentum, just above the inferior
olivary nucleus, as does the anterior spinocerebellar
tract.
 The posterior spinocerebellar tract moves posteriorly
and joins the inferior cerebellar peduncle
Level of ‘sensory’ decussation
Level of pyramidal decussation
The Caudal Pons
 the inferior olivary nucleus ends, and the inferior
cerebellar peduncle bends posteriorly and enters the
cerebellum
 The MLF is in the same relative position as it was
previously, adjacent to the midline and the floor of
the fourth ventricle.
Caudal pons
 The pyramidal tract becomes dispersed in the basal
pons, which contains bundles of longitudinally
oriented fibers, bundles of transversely oriented
fibers, and pontine nuclei scattered among these
bundles.
 Some of the longitudinally oriented fibers are those
of the pyramidal tract
 Fibers arising in the pontine nuclei cross the midline
and form the massive middle cerebellar peduncle
(brachium pontis).
 The spinothalamic tract and the anterior
spinocerebellar tract remain in the anterolateral
portion of the tegmentum..
 Spinoreticular fibers related to the spinothalamic
system terminate medial to the direct spinothalamic
fibers in the reticular formation throughout the
brainstem, as do collaterals of direct spinothalamic
fibers
Mid pons
Rostral pons
 extends from the rostral edge of the middle
cerebellar peduncle to the beginning of the cerebral
aqueduct
 it includes parts of the basal pons and fourth
ventricle but has no physical connection with the
cerebellum.
Rostral pons- near pons-midbrain junction
 The trigeminal nerve (V) is attached to the brainstem
at a midpontine level
 the trochlear nerve (IV) emerges at the ponsmidbrain junction
 The MLF is visible throughout the rostral pons, as is
the basal pons
 The fourth ventricle narrows as the plane of section
approaches the cerebral aqueduct, and the superior
cerebellar peduncle (brachium conjunctivum)
becomes apparent in the wall of the ventricle.
 This is the major outflow from the cerebellum,
projecting to the thalamus and to other structures
 As the medial lemniscus moves laterally, it
approaches the spinothalamic tract; from here
through the midbrain, the two are adjacent.
 The corticospinal tract travels through the rostral
pons as a series of longitudinally oriented bundles of
fibers, accompanied by more numerous
corticopontine bundles.
 The anterior spinocerebellar tract moves posteriorly
onto the surface of the superior cerebellar peduncle
 From here it turns caudally and enters the
cerebellum, traveling "backward" along the
peduncle.
Caudal midbrain
 essentially the part that contains the inferior
colliculi.
 extends from the point of emergence of the trochlear
nerve to the groove between the inferior and superior
colliculi.
 The fourth ventricle has narrowed into the cerebral
aqueduct
 the superior cerebellar peduncles sink deeper into
the midbrain tegmentum and begin to decussate,
and the MLF continues on its usual course
 The basal pons protrudes rostrally under the
tegmentum of the caudal midbrain.
 The inferior colliculus, a major component of the
ascending auditory pathway is (literally) a prominent
nuclear mass.
 Medial to it, encircling the aqueduct, is a particularly
pale-staining region of gray matter called,
appropriately enough, the periaqueductal gray.
 The periaqueductal gray is part of an important
descending pain-control system discussed later in
this chapter
 In the caudal midbrain, the basal pons gives way to
a cerebral peduncle on each side, through which
corticospinal and corticopontine fibers travel.
Caudal midbrain- level of inferior colliculus
Rostral Midbrain
 The rostral midbrain contains the superior colliculi
 At this level the MLF is ending, decussation of the
superior cerebellar peduncles is complete, and in
their place a large red nucleus becomes visible on
each side.
 Some fibers from the contralateral half of the
cerebellum end here, but most continue on to the
thalamus. Anterior to the red nucleus is
the substantia nigra
 Anterior to the red nucleus is the substantia nigra
 The pigmented cells characteristic of the dorsal part
of the substantia nigra use dopamine for their
neurotransmitter
 Anterior to the substantia nigra is a massive bundle
of fibers commonly referred to as the cerebral
peduncle.
Rostral midbrain- level of superior colliculus
 This bundle consists principally of descending
corticopontine and corticospinal fibers.
 The oculomotor nerve (III) emerges into the space
between the cerebral peduncles (the interpeduncular
fossa).
 the medial lemniscus and the spinothalamic tract
form a continuous curved band of fibers.
 Spinomesencephalic fibers (sometimes referred to as
the spinotectal tract) that have accompanied the
spinothalamic tract through the brainstem terminate
in the periaqueductal gray, adjacent regions of the
reticular formation, and certain portions of the
superior colliculus
Components of inferior cerebellar peduncle