The Brainstem

The Brainstem Overview

Brainstem Breakdown

• Functional Centers:

  • Houses most functional centers of cranial nerves.

  • Controls fundamental reflexes crucial for survival.

• Impulse Transmission:

  • Transmits somatosensory impulses from the body to the forebrain (ascending).

  • Receives motor impulses for voluntary movements from the forebrain to the body (descending).

• Implications of Damage to the Brainstem:

  • Damage can result in sensory, motor, or both dysfunctions.

  • The higher the lesion is located, the more fatal the consequences may be.

  • Impacts cranial nerves significantly.

• Cranial Nerve Example:

  • If Cranial Nerve VII (facial nerve) is affected, patients may exhibit physical symptoms such as drooping of the eye and mouth.

  • This condition exemplifies Bell's Palsy.

Cranial Nerve VII Lesion

• Lesion Description:

  • Specifically a lesion around the synapse of cell bodies in the brainstem.

• Clinical Effects:

  • Physical symptoms include drooping of the eye and mouth.

  • Motor signaling to muscles is hindered because the signaling pathway is obstructed by the lesion.

  • Characterized as Bell's Palsy.

The Structure of the Brainstem

• Components of the Brainstem:

  • Midbrain

  • Pons

  • Medulla

• Connections:

  • The brainstem is continuous with the midbrain and the spinal cord, merging with the medulla.

Anatomical Structures of the Brainstem

Medulla Oblongata

• Location:

  • Found between the Pons and Spinal Cord (SC), with the cerebellum positioned posterior.

  • Connected to the cerebellum via inferior cerebellar peduncles.

  • The 4th ventricle lies between the medulla and the cerebellum.

• Cranial Nerve Nuclei Present:

  • Vestibulocochlear CN VIII

  • Glossopharyngeal CN IX

  • Vagus CN X

  • Accessory CN XI

  • Hypoglossal CN XII

Pons

• Location:

  • Positioned between the midbrain and medulla, connected to the 4th ventricle posteriorly.

  • Attached to the cerebellum via middle cerebellar peduncles.

• Cranial Nerve Nuclei Present:

  • Trigeminal CN V

  • Abducens CN VI

  • Facial CN VII

• Functions Managed:

  • Controls mastication, eye movements, facial expressions, blinking, and salivation.

Midbrain

• Location:

  • Sits at the tentorial notch, between the forebrain and pons.

• Key Structures:

  • Contains the Cerebral Aqueduct, a narrow passage through which cerebrospinal fluid (CSF) flows from the forebrain's ventricles to the 4th ventricle.

• Cranial Nerve Nuclei Present:

  • Oculomotor CN III

  • Trochlear CN IV

• Functionality:

  • Contains centers for auditory, visual, and pupillary reflexes.

Anterior Brainstem Anatomical Structures

Anterior Medulla

• Key Features:

  • Pyramids (medullary pyramids): Elongated elevations located on each side of the Anterior Median Fissure.

  • The Olive: Located lateral to the pyramids bilaterally.

  • Preolivary Sulcus: Rootlets for Hypoglossal CN XII emerge.

  • Postolivary Sulcus: Rootlets for Glossopharyngeal CN IX & Vagus CN X emerge, and Accessory CN XI emerge inferiorly.

Anterior Pons

• Key Features:

  • Basilar Sulcus: Contains the Basilar Artery.

  • Pontomedullary Sulcus: Where Abducens CN VI and Facial CN VII emerge.

  • Anterior Lateral Pons: Trigeminal CN V emerges.

Anterior Midbrain

• Key Features:

  • Formed by the cerebral peduncles.

  • Interpeduncular Fossa: Area where Oculomotor CN III emerges.

Posterior Brainstem Anatomical Structures

Posterior Medulla

• Key Features:

  • Posterior Median Sulcus: A demarcation line on the posterior side.

  • Gracile Tubercle: Positioned lateral to the posterior median sulcus.

  • Cuneate Tubercle: Located lateral to the Gracile Tubercle.

  • Floor of the 4th ventricle lies alongside the posterior rostral surface of the medulla.

4th Ventricle

• Description:

  • The inferior tip of the 4th ventricle, termed the Obex, is situated between the gracile tubercles.

  • Pons and medulla are separated by an imaginary line at the lateral recess.

• Subdivisions:

  • Superior/Inferior Foveae: These divide into medial and lateral areas.

  • Medial Area (Motor Structures): Hypoglossal Trigone, Vagal Trigone.

  • Lateral Area (Sensory Structures): Vestibular Area, Acoustic Tubercle.

Cerebellar Peduncles

• Description:

  • Located at lateral aspects and roof of the 4th ventricle, comprising:

  • Superior Cerebellar Peduncle

  • Middle Cerebellar Peduncle

  • Inferior Cerebellar Peduncle (restiform body)

Posterior Midbrain

• Posterior Surface:

  • Referred to as the Tectum.

  • Contains four areas:

  • Left/Right Superior Colliculi

  • Left/Right Inferior Colliculi

  • Trochlear CN IV emerges below the inferior colliculi.

Reticular Formation

• Overview:

  • Runs through the Medulla, Pons, and Midbrain.

  • Composed of ascending/descending nerve tracts and cranial nerve nuclei.

• Functionality:

  • Receives input from all parts of the nervous system.

  • Exerts widespread influences on CNS function.

  • More details covered in Chapter 20 of the source material.