Brainstem: Midbrain, Pons, and Medulla Oblongata

Brainstem Overview

• The brainstem is located between the cerebrum and the spinal cord.
• It serves as a pathway for neural tracts between higher and lower brain centers.
• It consists of the midbrain, pons, and medulla oblongata.
• Each region is about an inch in length.
• Microscopically, it consists of deep gray matter surrounded by white matter fiber tracts.
• The brainstem produces automatic behaviors necessary for survival.

Midbrain

• The midbrain connects the pons and cerebrum with the forebrain.
• It is the shortest part of the brainstem, around 2 cm long.
• It is located in the posterior cranial fossa.
• For descriptive purposes, it is divided into the dorsal tectum and right and left cerebral peduncles.
• Each cerebral peduncle divides further into the ventral Crus.
• The cerebral peduncles contain descending fibers that go to the cerebellum via the pons and descending pyramidal tracts.
• The cerebral aqueduct runs through the midbrain, connecting the 3rd and 4th ventricles.

Pons

• The pons exhibits a convex anterior surface with prominent transverse running fibers.
• The fibers collect to form bundles, which are the middle cerebellar peduncles.
• The trigeminal nerve emerges from the anterior surface at the junction between the pons and the middle cerebellar peduncle.

Medulla Oblongata

• The medulla oblongata is the most inferior region of the brain stem.
• It becomes the spinal cord at the level of the foramen magnum.
• The medulla joins with the pons above and is continuous with the spinal cord below.
• It is about 3cm long and 2cm wide at its upper end.
• The surface shows a series of fissures including the anterior median fissure and posterior median fissure,
• The ventral surface of the medulla oblongata contains the pyramid and the olive.
  • Pyramid - elevation between the anterior median and anterolateral sulcus, which is formed due to the decussation of corticospinal fibers.
  • Olive - oval swelling between anterolateral and posterolateral sulcus, approximately half an inch long, which is produced by a large mass of gray matter called the inferior olivary nucleus.

Cerebellum Location

• It is the largest part of the hindbrain and occupies most of the posterior cranial fossa.
• It is separated from the occipital lobes of the cerebral hemispheres by the tentorium cerebelli.
• It lies dorsal to the pons and medulla, separated by the fourth ventricle.
• The cerebellum is joined to the brain stem by three bilaterally paired cerebellar peduncles that contain afferent and efferent fibers.

Cerebellum - External Features

• The cerebellum consists of two cerebellar hemispheres united to each other through the median vermis.
• It has two surfaces: superior and inferior.
• The superior surface is convex.
• The inferior surface has a deep median notch called the vallecula, which separates the right and left hemispheres.
• The anterior aspect of the cerebellum is marked by a deep notch occupied by the pons and medulla.
• The posterior aspect of the cerebellum has a narrow deep notch occupied by the falx cerebelli.
• Each hemisphere is divided into three lobes: anterior, middle (posterior), and flocculonodular lobe.

Cerebellum - Lobes

• Anterior lobe - located on the anterior part of the superior surface and is separated from the middle (posterior) lobe.
• Posterior (middle) lobe - largest lobe limited in the front by the primary fissure (on the superior surface) and the posterolateral fissure.
• Flocculonodular lobe - smallest lobe that lays on the inferior surface in front of the posterolateral fissure.
• The fissures subdivide the lobes.
• The horizontal fissure separates the superior from the inferior surface.
• The primary fissure separates anterior lobe from middle lobe on superior surface.
• The posterolateral fissure separates middle lobe from flocculonodular lobe on inferior surface.

Superior Parts of Cerebellum

• Lingula
• Central lobule
• Culmen
• Declive
• Folium.

Inferior Parts of Cerebellum

• Tuber
• Pyramid
• Uvula
• Nodule

Subdivision of Cerebellar

• The SUPERIORLY:
  • Simple lobule
  • Superior semilunar lobule
  • Inferior semilunar lobule.
• The INFERIORLY:
  • Ala
  • Quadrangular lobule
  • Biventral lobule
  • Tonsil
  • Flocculus.

Morphological & Functional Areas of the Cerebellum

• Made up of Flocculonodular, chiefly Vestibular in connections.

  • ARCHICEREBELLUM,
  • Controls Axial musculature & bilateral movements used for locomotion & maintenance of equilibrium.

• Made up of Anterior lobe, Pyramid and Uvula; connections chiefly are spinocerebellar.

  • PALEOCEREBELLUM

• Made up of Middle (largest part of cerebellum) minus pyramid & uvula of inferior vermis, chiefly cortico-cerebellar in its connections.

  • NEOCEREBELLUM,
  • Concerned with regulation of fine movements of body.

Cerebellar Connections

• Fibers entering or leaving the cerebellum are grouped to form three peduncles through which the cerebellum communicates.
  • Superior cerebellar peduncle:
    • Afferent Pathway is anterior spinocerebellar (uncrossed) pathway.
    • Efferent Pathway from dentate nucleus (crossed) to: thalamus, cerebral cortex, & red nucleus.
  • Middle cerebellar peduncle:
    • Afferent Pathway is Pontocerebellar (crossed) relays from cerebral cortex via pontine nuclei.
    • There is no Efferent Pathway in the middle cerebellar peduncle.
  • Inferior cerebellar peduncle:
    • Afferent Pathways are: Vestibulocerebellar (uncrossed), Posterior spinocerebellar (uncrossed), & Olivocerebellar (crossed) function unknown.
    • Efferent pathway is from cerebellar cortex and nucleus to vestibular nuclei.

Cerebellar Grey Matter

• Consists of cerebellar cortex and 4 pairs of nuclei:
  • Dentate nucleus
  • Globose
  • Emboliform .
  • Nucleus

Cerebellar Functions

• The Cerebellum controls the same side of the body (ipsilateral).
• It coordinates voluntary movements to ensure smoothness, balance, and accuracy, primarily done by the neocerebellum.
• Dysfunction leads to incoordination of movement, loss of posture, and imbalance; this is chiefly governed by the archicerebellum and paleocerebellum.
• Cerebellar lesions result in hypotonia and disturbances of equilibrium.