Dr. Farah Nabil Abbas MBChB, MSc, PhD

Brain Stem

Consists of three regions:

midbrain

Pons

medulla oblongata

Similar to spinal cord – deep gray matter surrounded by white tracts but contains embedded nuclei in the white matter.

Brain Stem

Controls automatic behaviors necessary for survival (breathing, digestion, heart rate, blood pressure)

Provides the pathway for tracts between higher and lower brain centers

Associated with 10 of the 12 pairs of cranial nerves.

Midbrain

Located between diencephalon and pons

Midbrain structures include:

Cerebral peduncles – two structures that contain descending pyramidal motor tracts.

Cerebral aqueduct – hollow tube that connects the third and fourth ventricles

Various nuclei

Midbrain Nuclei

Nuclei that control cranial nerves III (oculomotor) and IV (trochlear).

Corpora quadrigemina - 4 domelike protrusions of dorsal midbrain. 2 Superior colliculi - visual reflex centers; coordinate

head and eye movement when we follow a moving object (does not have to be a conscious decision).

2 Inferior colliculi - auditory relay centers between hearing receptors to sensory cortex. ○ Reflexive responses to sound – turn head toward sound.

Midbrain Nuclei

Substantia nigra – functionally linked to basal nuclei, contains melanin pigment (precursor of dopamine - NT).

Red nucleus – largest nucleus (rich blood supply) of the reticular formation; relay nuclei for some descending motor pathways.

Pons

Location: between midbrain and medulla oblongata

Forms part of anterior wall of 4th ventricle

Fibers of the pons: Connect higher brain centers and spinal cord

Relay impulses between motor cortex and cerebellum

Origin of cranial nerves V (trigeminal), VI (abducens), and VII (facial)

Contains nuclei of the reticular formation.

Lateral Gaze Palsy

Medulla Oblongata

Most inferior part of brain stem.

With pons forms ventral wall of 4th ventricle.

Contains choroid plexus of 4th ventricle.

Pyramids – two longitudinal ridges formed by corticospinal tracts descending from motor cortex.

Medulla Oblongata

Before entering spinal cord corticospinal tracts crossover.

The point in which crossover occur is called decussation of the pyramids.

Results in controlling of each cerebral hemisphere in movements of opposite side.

Medulla Nuclei

Inferior olivary nuclei - gray matter that relays sensory information regarding stretch of muscle and joints

Cranial nerves IX (glosopharyngeal, X (vagus), XI (accessory; neck muscle), and XII (hypoglossal; tongue).

Vestibular nuclear complex - synapses that mediate and maintain equilibrium

Ascending sensory tract nuclei, including nucleus cuneatus and nucleus gracilis Serve as relay station for general somatic sensation.

Medulla Nuclei

Medulla has an important role in autonomic reflex center that maintain homeostasis:

Cardiovascular control center – adjusts force and rate of heart contraction

Respiratory centers – control rate and depth of breathing

Additional centers – regulate vomiting, hiccupping, swallowing, coughing, and sneezing

There is an overlap between medulla and hypothalamus that uses medullary centers to carry out instructions.

Reticular formation

Network of nuclei and nerve fibers in different areas of brainstem with projection to cerebrum.

Functions as reticular activating system (RAS)

Involved in attention and in transition from sleep to wakefulness

RAS sends impulses to cortex to arouse it from sleep.

RAS and Motor Function

In addition to its RAS functions, reticular formation:

Helps control unrefined motor movements - maintain tone and posture

Relay signals from eyes and ears to the cerebellum so it can integrate these signals into its motor coordination

Autonomic centers regulate visceral motor functions such as cardiovascular control (centers found in the medulla).