Brainstem anatomy
INTRODUCTION:
 Brainstem consist of midbrain, pons, medulla oblongata.
 The archaic (old) term of brainstem is bulb (e.g. ‘corticobulbar’ refers to axons that arise
in the cerebral cortex and terminate in the brainstem)
 The brainstem lies upon the basal portion of the occipital bone (clivus) and is connected
to, and largely covered by, the
cerebellum.
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Caudally, the medulla is continuous with the spinal cord just below the foramen
magnum. Rostrally, the midbrain is continuous with the
diencephalon (thalamus + hypothalamus) of the forebrain.
The brainstem contains numerous ascending and
descending fibers tracts. Some of these passes throughout
its whole length, having their origin in the spinal cord or
cerebral hemisphere, respectively; others have their origin
or termination within brainstem nuclei. Certain of these
brainstem nuclei receive fibers rom, or send fibers into,
cranial nerves, 10 pairs of which (III–XII) attach to the
surface of the brainstem. These are known as the cranial
nerve nuclei.
Also, the brainstem contains a complex and heterogeneous
matrix of neurons known as the reticular formation, within
which a number of individually identified nuclei exist. The
reticular formation has several important functions:
o control over the level of consciousness
o the perception of pain
o regulation of the cardiovascular and respiratory systems.
The brainstem contains the cells of origin of monoaminergic (dopamine, noradrenaline,
serotonin) neurons that have widespread projections throughout the CNS and are
important in sensory, motor, autonomic and cognitive functions.
External features of brainstem:
 Dorsal surface of the brainstem: it can be viewed if the overlying cerebellum is
removed by cutting the three pairs of nerve fibers bundles, or peduncles, by which it is
attached on each side
 On the dorsal surface of the medulla, the midline is marked by a dorsal median sulcus,
continuous with that of the spinal cord. In the caudal part of the medulla, the dorsal
columns (fasciculi gracilis and cuneatus, containing first-order sensory neurons)
continue rostrally from the spinal cord to their termination in the nuclei gracilis and
cuneatus, the locations of which are marked by two small elevations, the gracile and
cuneate tubercles.
 The caudal two-thirds of the medulla  contain
continuation of the central canal of the spinal cord
and is, therefore, sometimes referred to as the
‘closed’ portion of the medulla. in the rostral medulla
 the central canal moves progressively more
dorsally until; it opens out into the fourth ventricle.
This portion is sometimes referred to as the ‘open’
medulla.
 The floor of the fourth ventricle forms a shallow,
rhomboid depression on the dorsal surface of the
rostral medulla and the pons
 Caudal third of floor of 4th ventricle  rostral medulla (dorsal aspect of it)
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Rostral 2/3 of floor of 4th ventricle  made
up of dorsal aspect of pons.
The 4th ventricle is widest at the level of the pontomedullary junction, where a lateral
recess extends to the lateral margin of the
brainstem.
A small lateral aperture (foramen of Luschka)
provides passage for CSF within the 4th ventricle
to reach the subarachnoid space surrounding the
brain.
 Internal structures of brainstem:
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Caudal medulla
 At the transition from spinal cord to medulla, the
pattern of grey and white matter undergoes
considerable rearrangement  the ventral horn
become attenuated, dorsal horn  is replaced by
trigeminal sensory nucleus
 caudal part of the trigeminal nucleus is particularly
associated with the modalities of pain and
temperature.
 Btw the trigeminal nerve attaches to the pons & its
fibers will descend in a tract (spinal tract of trigeminal)
to terminate on caudal part of trigeminal nucleus.
 In the ventral medulla  the majority of fibers of
pyramid undergo decussation & then pass laterally,
dorsally & caudally to form the lateral corticospinal
tract.
Mid-medulla
 On the ventral sur ace o
the mid-medulla the
pyramids are prominent,
above their decussation.
On the dorsal sur ace, the
ascending fibres o the
dorsal columns reach
their termination in the
gracile and cuneate
nuclei, which appear
beneath their respective
tracts.
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Rostral medulla