Each concept is accompanied by vivid diagrams and illustrations, making it easier to visualize and understand the brain's intricate structures. Complex topics are broken down into digestible segments, making it easier to grasp difficult concepts and retain information longer. Organized logically, w...
The brainstem is located upon the basal portion of the occipital bone and is connected to cerebellum. Caudally the medulla is continuous with the
spinal cord, and rostrally the midbrain is continuous with diencephalon of the forebrain. Brainstem receives number of cranial nerves that terminate
in the cranial nerve nuclei in the brainstem. Brainstem also contains complex matrix of neurons – reticular formation. Reticular formation is involved
in controlling the level of consciousness, the perception of pain and in regulation of the cardiovascular and respiratory systems. Reticular formation
has extensive connections with cranial neve nuclei, cerebellum, and brainstem-spinal motor mechanisms. The brainstem also contains cells of origin
of dopaminergic, adrenergic, and serotonergic projections of the brain, that are involved in sensory, motor, autonomic, and cognitive functions.
• External features of the brainstem
Dorsal surface of the brainstem - Figure 1
The dorsal can be viewed, when the overlying cerebellum is removed by
cutting nerve fiber bundles – peduncles. The midline is marked by a
dorsal median sulcus which is continuous with that of spinal cord. In the
caudal part of the medulla the dorsal columns (fasciculus gracilis and
cuneatus) continue from the spinal cord to their side of termination in
the nucleus gracilis and cuneatus, their location is marked by two
elevations, the gracile and cuneate tubercule.
The caudal 2/3 contain rostral continuation of the central spinal canal.
Central canal progresses into rostral medulla where it opens into the
fourth ventricle. The caudal 1/3 of thee fourth ventricle floor is made of
dorsal medulla and rostral 2/3 o fourth ventricle floor is made of dorsal
part of the pons. The fourth ventricle is widest at the level of the
pontomedullary junction, where lateral recess extends to the lateral
margins of the brainstem. At this point lateral aperture – foramen of
Luschka provides passage of CSF within the fourth ventricle into the
subarachnoid space around the brain. The lateral walls of the rostral part
Figure 1. Dorsal aspect of the brainstem.
of the fourth ventricles are made by superior and inferior cerebellar
peduncle, which connect
peduncle, which connect thethebrainstem
brainstemtotocerebellum.
cerebellum.The The fourth
fourth ventricle
ventricle continuous
become become continuous
with thewith the cerebral
cerebral aqueduct aqueduct
which which
passespasses
through the
through the whole
whole length of thelength of the brainstem.
brainstem.
The dorsal part of the midbrainisismarked
The dorsal part of the midbrain markedbybyfourfourelevations,
elevations,thethe
superior
superior and inferior colliculi, which are part of the visual
and inferior colliculi, which are part of the visual (superior) (superior)
andandauditory
auditory
(inferior) (inferior)
systems. systems. The trochlear
The trochlear nerve
nerve (IV) (IV) emerges
emerges caudalcaudal
to thetoinferior
the inferior colliculi.
colliculi.
Ventral surface of the brainstem – Figure 2
Medulla has longitudinal columns/pyramids which contain pyramidal or
corticospinal tract. Fibers of this tract cross over n decussation of the pyramids.
Lateral to the pyramids, lies the elongated elevation, the olive within which
located inferior olivary nucleus. Olivary nucleus contains connection with the
cerebellum and is involved in the control of movement.
The ventral part of pons contain transverse system of fibers (the transverse
pontine fibers/pontocerebellar fibers) that originate from pontine nuclei and
pass though the contralateral middle cerebellar peduncle to the cerebellum.
The pontine nuclei receives corticopontine fibers from the cerebral cortex. They
Figure 2. Ventral aspect of the brainstem.
create connections between cerebral cortex and cerebellum that are involved
ininthe
thecontrol of movement.
The surface of the midbrain contains columns of descending fibers, the crus cerebri/basis peduncli. The two crus cerebri are separated by a
depression – interpeduncular fossa. Rostrally, the crus cerebri are continuous with internal capsule of the cerebral hemisphere and are made of
corticobulbar and corticospinal fibers, that carry motor function.
• Internal structure of the brainstem
Caudal medulla – Figure 3
There is transition of shape and size of white and grey matter between the spinal cord and medulla (Figure 3). The ventral horn becomes attenuated,
and the dorsal horn is replaced by caudal part of the trigeminal sensory nucleus (nucleus of the spinal tracts of the trigeminal nerve). It receives and
conveys sensory info from the head, which enter the brainstem in the trigeminal nerve (V), then extends through the whole brainstem into the upper
tact of spinal cord. Caudal parts of the trigeminal nerve is associated with modalities of pain and temp. the trigeminal nerve attaches to pons, and its
fibers travel in the spinal tracts of trigeminal to the spinal cord, which lies superficial to the nucleus.
In the ventral medulla majority of fibers decussate and pass laterally, dorsally and caudally to form corticospinal tract.
Mid-medulla – Figure 4
On the ventral surface of the mid-medulla there are pyramids and on the dorsal the ascending fibers of the dorsal column reach their termination in
, the gracile and cuneate nuclei, which are located
beneath the respective tract (Figure 4). The dorsal
column is made of first order neurons which
synapse onto second order neurons, second order
neurons pass ventrally and medially as internal
arcuate fibers and decussate in the midline. They
then turn rostrally decussate and form medial
lemniscus which passes through the rostral
medulla, pons and midbrain into the thalamus.
Rostral medulla – Figure 5
Dorsal to the medial part of the medulla contain
ascending fibers of the medial lemniscus on each
Figure 3. Transverse section through the caudal medulla at the level of decussation of the pyramids. side of the medulla.
In the midline in the medullary part of the
brainstem there is raphe nuclear complex at this
level represented by raphe magnus, it is the major
origin of the serotonergic neurons.
Dorsolateral to the pyramid and lateral to the
medial lemniscus is the inferior olivary nucleus
which is involved in the control of movement and
receives afferent from the motor and sensory
cortices from hemispheres and from the red
nucleus of the midbrain. It has efferent
connections with cerebellum, it sends climbing
fibers and heave excitatory synapse in the dentate
gyrus onto Purkinje cells.
Dorsal to the inferior olivary nucleus and lateral
to the medial lemniscus there is second-order
Figure 4. Transverse section through the mid-medulla at the level of the great sensory decussation. fibers ascending to the ventral posterior thalamus
nucleus (the trigeminothalamac tract/trigeminal
lemniscus).
The dorsal surface of the rostral medulla is part
of the fourth ventricle floor, and it contains
number of cranial nuclei.
Beneath the ventricular floor, lateral to the
midline there is hypoglossal nucleus, which
contains MNs innervating the muscles of tongue
via hypoglossal nerve (XII).
Lateral to hypoglossal nucleus there is soral
motor nucleus of the vagus containing
preganglionic parasympathetic neurons that pass
through the vagus nerve (X).
The caudal part of ventricular floor is called –
area postrema. At which blood-brain barrier
Figure 5. Transverse section through the rostral medulla at the level of the inferior olivary nucleus.
disappears. This region is involved in the cation of
disappears. This region is involved in the cation of substances that cause vomiting. substances that cause vomiting.
The lateral part of the fourth ventricle floor contains vestibular nuclei. It receives primary afferent fibers from the vestibular nerve (XIII).
Ventromedial to the hypoglossal nucleus close to the midline there is medial longitudinal fasciculus. It contains ascending and descending fibers.
It links the vestibula nuclei with the nuclei supplying the extraocular muscles and is partly involved in the control of head and eye movement.
The dorsolateral part of the rostral medulla contains inferior cerebral peduncle or restiform body. They made of fibers passing between medulla
and the cerebellum, such as olivocerebellar fibers that connect vestibular nuclei and cerebellum; and fibers of spinocerebellar tracts which transfer
proprioceptive info from lower limbs.
Dorsal and lateral parts of inferior cerebellar peduncle are located close to dorsal and ventral cochlear nuclei. It received afferent fibers from the
cochlear nerve (XIII).
Medial to the inferior cerebellar peduncle and ventral to the vestibular nuclei there is the nucleus solitaruis which receives visceral afferent fibers
from facial (XII), glossopharyngeal (IX) and vagus (X) nerve.
Beneath the ventricular floor, ventral to the nucleus solitaruis and dorsal to inferior olivary nucleus, there is nucleus ambiguous. It sends motor
fibers into the glossopharyngeal (XI) and vagus (X) nerves and cranial roots of the accessory (XI) nerve, which innervate muscles of pharynx and larynx.
Pons – Figure 6, 7 and 8
The pons are divided into ventral/basal and dorsal portions – tegmentum. The ventral portion is marked by the transversely orientated fascicles pf
pontocerebellar fibers that originate from the pontine nuclei and pass into the contralateral side of the cerebellum which travel through the middle
cerebellar peduncle (branchium pontis).
The ascending fibers of the medial lemniscus are separated from the pyramids and are displaced dorsally, together with spinal lemniscus and
trigeminothalamic tract, by innervating transverse pontocerebellar fibers.
The medial lemniscus rotates 90 degrees and lies mostly horizontally, marking the border between ventral and tegmental portions of the pons.
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