Nolte Chapter 11 – Organization of the Brainstem
The reticular formation forms the central core of the brainstem
The medulla’s pyrmaids decussate at the junction between the spinal cord and the brainstem
The hypoglossal nerve(XII) emerges between the pyramids and the olive.
Moving laterally around the olive, we see the glossopharyngeal(IX) and the vagus(X).
The trigeminal nerve V is attached to the brainstem at the midpontine level.
The portion of the pons and medulla in the floor of the ventricle, lateral to the sulcus limitans is mostly
occupied by vestibular nuclei and is referred to as the vestibular area.
In the pons there is a dorsal elevation known as the facial colliculus which is a result of the facial nerve
wrapping around the abducens nucleus.
Fibers coming from the cerebral peduncle (corticospinal / corticopontine) cross the midline on their way
to the cerebellum by way of the middle cerebellar peduncle.
The trigeminal nerve enters the brainstem at the midpontine level
The abducens nerve exits very medially where the pyramid emerges from below the basal pons.
The facial nerve is more lateral to abducens and has a motor and sensory component.
Vestibulocochlear is yet more lateral.
Trochlear nerve is the only to emerge from the dorsal surface and its at the pons/midbrain junction.
The superior cerebellar peduncle is covered in the rostral pons by a flattened band of fibers called the
lateral lemniscus, which forms part of the ascending auditory system and terminates in the inferior
colliculus.
Occulomotor nerve III emerges from the interpeduncular fossa between the peduncles.
The brachium of the inferior colliculus extends rostrally from the inferior colliculus and is an extension of
the ascending motor pathway on its way to MGN.
The area anterior to the ventricular space is called the tegmentum.
Corticospinal fibers travel in the most ventral part of the brainstem, traversing the cerebral peduncle,
basal pons, and medullary pyrmids. They decussate at the spinomedullary junction andform the lateral
corticopspinal tract(the part that doesn’t cross is the medial corticospinal)
The spinothalamic is situated by its other name: anterolateral corner of the tegmentum.
The posterior columns terminate in the gracile and cuneate nuclei in the caudal medulla. The efferent
fibers decussate(in what is called the internal arcuate(which is actually in the reticular formation)) and
form the medial lemniscus, which starts near the midline then rotates 180 degrees as it proceeds
through the brainstem.
The caudal medulla contains the spinal tract and the spinal nucleus of the trigeminal nerve.
The lateral cuneate in the caudal medulla and serves as the clarke’s nucleus (which sends fibers to
cerebellum for the lower extremities) before they leave as the cuneocerebellar and join the posterior
spinocerbellar
The spinothalamic remains in the anterolateral portion until it reaches the midbrain.
In the rostral medulla, we find the inferior olivary nucleus which sends its fibers across the midline(also
as internal arcuate fibers in the reticular formation) into the inferior cerebellar peduncle to become the
climbing fibers that synapse on purkinje cells in the cerebellum.
Posterior to the medial lemniscus is the medial longitudinal fasciculus, which is involved in coordinating
head and eye movements.
Caudal Pons shows the pyramidal (corticospinal) tract all dispersed because it is intertwined with the
pontine nuclei. These pontine nuclei are mainly corticopontine fibers that are decussating, aimed for the
middle cerebellar peduncle.
In the rostral pons, we see that the medial lemniscus has a transverse orientation and approaches the
spinothalamic tract and run adjacently into the midbrain.
In the rostral pons we also see that the anterior spinocerebellar tract move posteriorly onto the superior
cerebellar peduncle where it decussares, turns caudally and enters the cerebellum (travelling backward
along the superior peduncle which is primarily output). Remember that this tract is related to anticipate
movement and already crossed at the level of the spinal cord.
The superior cerebellar peduncles decussate in the caudal midbrain.
encircling the aqueduct in the midbrain is whats known as the periaqueductal gray, which is a
descending pain-control system.
In the midbrain we see the cerebral peduncles which house the corticospinal and corticopontine fibers.
In the rostral midbrain is where we see the medial longitudinal fasciculus ending, and in their place the
red nucleus. The red nucleus is the end of the line for some of contralateral cerebellum.
Just anterior to the red nucleus is the substantia nigra. anterior to substantia nigra are the cerebral
peduncles.
The occulomotor nerve emerges into the space between the cerebral peduncles.
At the rostral midbrain level the medial lemniscus and the spinothalamic tract form a continuous curved
band of fibers. The spinotectal tract that has accompanied the spinothalamic then synapses in the
periaqueductal gray, reticular formation, and superior colliculus.
The periaqueductal gray can project to the raphe nuclei in the reticular formation, which then project to
the superficial laminae of the posterior horn and can suppress the transmission of pain information that
is coming in from the spinothalamic neurons.
The reticular contains the raphe nuclei which are immediately adjacent to the agittal plane. The medial
zone of the reticular is known for its gigantocellular nucleus with big cells. The lateral zone is where we
find most of the cranial nerve reflexes. The reticulospinal tracts arise from the medial zone and are used
to carry descending motor commands that are generated within the reticular formation.