mesencephalon is the most superior part of the brainstem. It
is divided into an anterior and a posterior section by the
Aqueduct of Sylvius which connects the third and fourth
ventricles. Motor tracts, including the fibers of the
pyramidal system, pass downward on the midbrain's anterior
surface. Sensory axons, including those of the spinothalamic
tract also ascend, along the front of the midbrain behind
the motor tracts.
corpora quadrigemina, which is located on the
posterior surface of the midbrain, is composed of two
two inferior colliculi. The superior colliculi
are part of the visual system, relaying input from the optic
tract to the lateral geniculate bodies of
the thalamus. The inferior colliculi are part of the
auditory pathway and send information to the medial
geniculate bodies of the thalamus.
nuclei are located in the midbrain, including the
nuclei, the substantia nigra, and the
of cranial nerves III and IV.
nuclei connect the midbrain to the
cerebellum and to the inner ear. It is also an important part of the extra pyramidal tract. The cerebellum compares input from muscles and joints with motor output from the cortex and relays subsequent adjustments to the cortex through the red nucleus and thalamus.
substantia nigra is a group of
dark-colored, dopaminergic cells. Lesions here cause Parkinson's disease
is the oculomotor nerve.
Both of these
cranial nerves provide innervation for motor movements of
peduncles (cruz cerebri) are two very large bundles of axons
which are a continuation of the efferent projections within
the internal capsule. They begin and end within the brain stem. The efferent projections include cortico bulbar (cortex to brain stem) and cortico spinal (cortex to spinal cord) axons. Webb, Adler and Love, 2008.
pons, which is also part of the brain stem, is inferior to
the midbrain and superior to the medulla. Its posterior
border is separated from the cerebellum by the aqueduct of
Sylvius, and more inferiorly, by the fourth ventricle. Motor
and sensory tracts traverse the anterior surface of the
pons. The sensory fibers are located behind the motor
The nuclei of
cranial nerves V and VI are located in the pons.
the trigeminal, sends motor messages to
the jaw and receives sensory messages from the teeth,
tongue, and parts of the face. CN VI, or the
abducens, provides motor
innervation to the eye.
The motor nucleus
of cranial nerve
the facial nerve, is located on the border of the pons and medulla.
The upper part of the nerve innervates the muscles of facial
expression including the eye lids, forehead and the
lips The lower part innervates the voluntary muscles of the face below the eyelids. Cortico-bulbar (pyramidal) fibers provide contralateral and ipsa innervation (bilateral) to the muscles of the upper face but only contralateral (unilateral) innervation to the lower face. Additionally, facial paralysis due to a pyramidal lesion will not permit voluntary control of the muscles but these paralyzed muscles will respond to emotional expression. This is due to extrapyramidal nonvolitional control. An example is the natural smile created by the basal ganglia, and a smile for the camera, or Biden's smile during the debate with Palin. I can't decide whether Palin's wink was deliberate or involuntary.
medulla is the most inferior part
of the brain stem. The cell bodies of several cranial nerves are found there. Tthe cell body with it's nucleus, and the axon and axonal branches are all part of the lower motor neuron.
Neuron Cell Bodies
the glossopharyngeal nerve
CN X, the vagus
CN XI, the spinal
CN XII, the
Because the nuclei
of the vagus nerve are found in the medulla, it is
a center for circulation and respiration.
It is also quite important to swallowing. It controls
muscles of the pharynx, larynx and velum.
(Note: Most of the cranial nerves
important for speech and swallowing are located in the
reticular formation is a set of interconnected nuclei that
are located throughout the brain stem. Its
tegmental nuclei are in the midbrain while its
nuclei are in the pons and its
central and inferior nuclei are found in the
formation has two components:
formation is also called the
system. It is responsible for the
sleep-wake cycle, thus mediating various levels of
alertness. This part of the reticular system projects to the
mid-line group of the thalamus, which also plays a role in
wakefulness. From there, information is sent to the
formation is involved in posture and
equilibrium as well as autonomic nervous system activity. It
receives information from the hypothalamus. The descending
reticular formation also plays a role in motor
the reticular formation receive some of the cortico-bulbar
fibers from the motor cortex. It is those fibers that
innervate the three cranial nerves involved in eye movement.
Other cortico-bulbar fibers innervate cranial nerves
directly. The descending reticular nuclei in the brain are
involved in reflexive behavior such as coughing, chewing,
swallowing and vomiting.