3. INTRODUCTION
■ The brain stem is the part of the brain between the spinal cord and the
diencephalon.
■ It consists of 3 structures :
1. Medulla oblongata
2. Pons, and
3. Midbrain
■ It forms the pathway for ascending and descending tracts of nerve fibers
between the brain and spinal cord.
■ Location of important centres for the regulation of vital functions in the
body.
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4. MEDULLA OBLONGATA
GROSS ANATOMY
■ Forms the inferior part of the brain stem.
■ Begins at the foramen magnum and extends to the inferior border of the
pons.
■ White matter contains all sensory tracts and motor tracts that extend
between the spinal cord and other parts of brain.
■ Some of the white matter forms bulges on the anterior aspect of medulla
and these are called pyramids.
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6. Contd.
■ Pyramids are formed by the large
corticospinal tracts which control the
voluntary movements of the limbs and
trunk.
■ Pyramids taper below and have most
of the descending fibres which cross
to the opposite side, forming
decussation of pyramids.
■ Oval-shaped swelling called olive is
present just lateral to each pyramid.
■ Within the olive is the inferior olivary
nucleus which receives input from
cerebral cortex, red nucleus of
midbrain and spinal cord.
Fig : Internal anatomy of the medulla
oblongata28-10-2017 6
7. Contd.
■ Cerebellar peduncles, which connect medulla to cerebellum are present behind
the olives.
■ Gracile nucleus and cuneate fasciculus, nuclei associated with sensations of
touch, pressure and vibration are located in the posterior part of the medulla.
■ Cranial nerves, VIII, IX, X, XI and XII emerge from the surface of medulla.
■ Medulla also contains several nuclei which control vital body functions.
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8. FUNCTIONS
1. Pathway for ascending and descending tracts of the spinal cord.
2. House of vital centres.
Respiratory centres – control normal rhythmic respiration.
Vasomotor and cardiac centres – regulates rate and force of
heartbeat and the diameter of blood vessels.
Deglutition centre – promotes swallowing of food moved from oral
cavity of mouth into pharynx.
Vomiting centre – induces vomiting in disorders of GI tract.
Superior and inferior salivary nuclei – control the salivary secretion
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9. Contd.
3. Cranial nerve nuclei in the medulla control following functions :
1. Vestibulocochlear (VIII) nerves – convey impulses related to hearing.
2. Glossopharyngeal (IX) nerves – relay sensory and motor impulses related
to taste, swallowing and salivation.
3. Vagus (X) nerves – receive sensory impulses from and provide motor
impulses to pharynx, larynx, thoracic and abdominal viscera.
4. Accessory (XI) nerves – control swallowing.
5. Hypoglossal (XII) nerves – control tongue movements during speech and
swallowing.
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10. PONS
■ Lies directly superior to the medulla
and anterior to the cerebellum.
■ The pons is a bridge that connects
parts of the brain with one another.
These connections are provided by
bundles of axons.
■ The pons has two major structural
components: a ventral region and a
dorsal region.
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11. Contd.
■ The ventral region of the pons forms a large synaptic relay station
consisting of scattered gray centres called the pontine nuclei.
■ Dorsal region contains ascending and descending tracts along with
the nuclei of cranial nerves.
■ Other nuclei located in the pons are the pneumotaxic area and the
apneustic area. They help control breathing.
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12. FUNCTIONS
1. Connecting pathway between cerebral cortex and cerebellum.
2. Pathway for ascending and descending tracts of spinal cord and medulla
oblongata.
3. Contains pneumotaxic and apneustic centres for regulation of respiration.
4. Joining station for medial lemniscus with fibres of V, VII, IX and X cranial
nerves.
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13. Contd.
5. Houses the nuclei of V, VI, VII, and VIII cranial nerves.
Trigeminal (V) nerves - receive sensory impulses for somatic
sensations from the head and face and provide motor impulses
that govern chewing.
Abducens (VI) nerves - provide motor impulses that control
eyeball movement.
Facial (VII) nerves - receive sensory impulses for taste and
provide motor impulses to regulate secretion of saliva and tears
and contraction of muscles of facial expression.
Vestibulocochlear (VIII) nerves - convey impulses related to
balance and equilibrium.
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14. MIDBRAIN
■ Extends from the pons to the diencephalon.
■ Cerebral aqueduct of the midbrain passes through the midbrain,
connecting the third ventricle above with the fourth ventricle below.
■ The anterior part of the midbrain contains paired bundles of axons known
as the cerebral peduncles.
■ The posterior part of the midbrain, called the tectum, contains four
rounded elevations.
■ The two superior elevations, nuclei known as the superior colliculi, serve
as reflex centres for visual activities.
■ The superior colliculi are also responsible for reflexes that govern
movements of the head, eyes, and trunk in response to visual stimuli.
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16. ■ The two inferior elevations, the inferior colliculi, are part of the auditory pathway.
■ These two nuclei are also reflex centres for the startle reflex, sudden
movements of the head, eyes, and trunk.
■ Cerebral peduncle includes : Basis pedunculi, Substantia nigra and tegmentum
which includes red nuclei.
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17. Contd.
■ Basis pedunculi – consists of large mass of vertically running descending
fibres from cerebral cortex.
■ Substantia nigra – mass of pigmented gray matter. Part of basal ganglia.
■ Tegmentum
Consists of red nucleus which is involved in regulation of posture, control
muscular movements, muscle tone, movements of eyeball, etc.
Nuclei of III and IV cranial nerves.
Reticular formation - small clusters of neuronal cell bodies (gray matter)
interspersed among small bundles of myelinated axons (white matter).
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20. REFERENCES
■ Anatomy and Physiology – Gerard J. Tortora and Bryan Derrickson, 2014,
Wiley.
■ Essentials Of Medical Physiology – K Sembulingam and Prema
Sembulingam, 4th Ed, 2006, Jaypee Brothers Medical Publishers Ltd.
■ Textbook Of Medical Physiology – Indu Khurana, 1st Ed,2006, Elsevier.
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