The Brain: Pons, Medulla, and Cerebellum

The Brain

Pons

• The pons is the largest part of the brain stem.
• Location: Above the medulla oblongata and below the midbrain.
• Size: Approximately 2.5 cm in adults.
• Etymology: "Pons" means "bridge."
• Position: Wedged between the midbrain and medulla.
• Development: Develops from the embryonic metencephalon (part of the hindbrain) along with the cerebellum.
• Anatomical Relations:
  • Posteriorly: The cerebellum, separated by the fourth ventricle.
  • Inferiorly: The medulla oblongata.
  • Superiorly: The midbrain lies immediately above the pons.

Contents of the Pons

1. Sensory and Motor Nuclei of 4 Cranial Nerves
   • Trigeminal (5th cranial nerve)
   • Abducens (6th cranial nerve)
   • Facial (7th cranial nerve)
   • Auditory/Vestibular (8th cranial nerve)
2. Respiratory Nuclei
   • Apneustic center: Excitatory respiratory center.
   • Pneumotaxic center: Inhibitory respiratory center.
   • Function: Regulate inspiratory volume and respiratory rate; work with the medulla to maintain respiratory rhythm (integration center).
3. Nuclei and Tracts
   • Process and relay information to/from the cerebellum.
4. Ascending, Descending, and Transverse Tracts
   • Interconnect other portions of the CNS.

Anterior Surface of the Pons

• Convex from side to side.
• Bulge formed by second-order neuron fibers of the cortico-ponto-cerebellar tracts.
• These fibers cross to the other side of the pons.
• They run within the middle cerebellar peduncles, reaching the contralateral cerebellum.

Basilar Groove/Sulcus

• Demarcates the midline of the ventral surface.
• Location of the basilar artery.

Pontomedullary Junction

• Important anatomical landmark.
• Defined by the angle between the lower border of the pons and the superior border of the medulla.

Cranial Nerve Origins from Ventral Surface

• Several cranial nerves originate from the ventral surface of the pons: CN V, VI, VII, and VIII.
  • Cranial nerve V (trigeminal): Originates from the lateral aspect of the mid pons.
  • Cranial nerve VI (abducens): Originates from the pontomedullary junction, close to the midline.
  • Cranial nerve VII (facial): Originates from the cerebellopontine angle, the more lateral aspect of the pontomedullary junction.
  • Cranial nerve VIII (vestibulocochlear): Originates laterally to the facial nerve.

Posterior Surface of the Pons

• Intimately related to the cerebellum.
• Connected to the cerebellum by the middle cerebellar peduncles.
• Removal of the cerebellum reveals the underlying fourth ventricle.
• Floor of the fourth ventricle is composed of the dorsal surface of the pons and the medulla.

Anatomical Landmarks on Posterior Surface

• Medial eminence: Marks the midline of the floor (vestibular complex).
• Facial colliculus: Bulging formed by the fibers of the facial nerve looping around the abducens nucleus.
• Stria medullaris of the fourth ventricle: Bundle of nerve fibers crossing transversely from the lateral aspect into the midline; marks the posterior border between the pons and the medulla.
• Sulcus limitans: Separates the cranial nerve motor nuclei from the sensory nuclei.
• Cerebellopontine angle: Angle formed at the junction of the pons and cerebellum.

Internal Anatomy of the Pons

• Comprised of two major components:
  1. Ventral pons
     • Contains the pontine nuclei, responsible for coordinating movement.
     • Fibers from the pontine nuclei cross the midline and form the middle cerebellar peduncles on their way to the cerebellum.
  2. Tegmentum
     • Forms part of the reticular formation – a set of nuclei found throughout the brainstem responsible for arousal and attentiveness.

Tracts Passing Through the Pons

• Descending corticospinal tracts: Responsible for voluntary motor control of the body.
• Descending corticobulbar tracts: Responsible for voluntary motor control of the face, head, and neck.
• Ascending medial lemniscus tracts: Responsible for fine touch, vibration, and proprioception.
• Ascending spinothalamic tracts: Responsible for pain and temperature sensation.

Medulla Oblongata

• Part of the brainstem between the pons and spinal cord.
• Length: Approximately 3 cm.
• Extends from the ponto-medullary junction through the foramen magnum to the level of the atlas (0.5 cm from F.M.).
• Above the foramen magnum, it is embraced posteriorly by the cerebellar hemispheres.
• Two parts:
  1. Open medullary part: Upper end of the floor of the ventricle.
  2. Closed medullary part: Lower end.

External Features of the Medulla Oblongata

• Anteriorly:
  • Upper part is deeply grooved in the midline with broad convexities on either side, the pyramids (motor fibers of the corticospinal and corticobulbar tracts).
  • Lateral to the pyramids is another convexity, the olives, due to the underlying inferior olivary nucleus (ascending fibers to cerebellum).
• Laterally:
  • Lateral to the olives are the inferior cerebellar peduncles, connecting the medulla oblongata to the cerebellum.
• Cranial Nerves Originating from the Medulla:
  • CN IX: Between pyramids and olives.
  • CN X, XI, XII: Between olives and ICP.

Dorsal Features of the Medulla Oblongata

• The lower part of the floor of the fourth ventricle is formed by the upper part of the medulla.
• Lower part has hypoglossal and vagal trigone.
• Lowest closed end has gracile and cuneate tubercles.

Internal Features of the Medulla Oblongata

• Majorly contains nuclei for the 4 cranial nerves, olives, and pyramids.

Cerebellum

• Means "little brain."
• Develops from the metencephalon division.
• Location: Back of the brain, immediately inferior to the occipital and temporal lobes, within the posterior cranial fossa.
• Separated from these lobes by the tentorium cerebelli, a tough layer of dura mater.
• Lies at the same level as and posterior to the pons, from which it is separated by the fourth ventricle.

Anatomical Structure and Divisions of the Cerebellum

• Consists of two hemispheres connected by the vermis, a narrow midline area.
• Composed of grey matter and white matter:
  • Grey matter: Located on the surface, forming the cerebellar cortex (which is tightly folded).
  • White matter: Located underneath the cerebellar cortex, containing the four cerebellar nuclei (dentate, emboliform, globose, and fastigi nuclei).

Cerebellar Nuclei

• Four cerebellar nuclei embedded in the white matter in its center.
• Most output fibers of the cerebellum originate from these nuclei.

Lobes of the Cerebellum

• Three anatomical lobes:
  • Anterior lobe.
  • Posterior lobe.
  • Flocculonodular lobe.
• Divided by two fissures: primary fissure and posterolateral fissure.

Cerebellar Peduncles

• Joined to midbrain by superior cerebellar peduncle.
• Joined to pons by middle cerebellar peduncle.
• Joined to medulla oblongata by inferior cerebellar peduncle.

Functional Divisions of the Cerebellum

1. Cerebro-cerebellum
   • Largest division, formed by the lateral hemispheres.
   • Involved in planning movements and motor learning.
   • Receives inputs from the cerebral cortex and pontine nuclei, and sends outputs to the thalamus and red nucleus.
   • Regulates coordination of muscle activation and is important in visually guided movements.
2. Spinocerebellum
   • Comprised of the vermis and intermediate zone of the cerebellar hemispheres.
   • Involved in regulating body movements by allowing for error correction.
   • Receives proprioceptive information.
3. Vestibulocerebellum
   • Functional equivalent to the flocculonodular lobe.
   • Involved in controlling balance and ocular reflexes, mainly fixation on a target.
   • Receives inputs from the vestibular system and sends outputs back to the vestibular nuclei.

Difference Between Cerebellum and Cerebrum

Histology of the Cerebellum

• Cerebellar cortex is responsible for motor coordination and balance.
• Characterized by three distinct layers:
  • Molecular layer.
  • Purkinje cell layer.
  • Granular cell layer.