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Brainstem. Assoc. Prof. Stefan Sivkov, Ph.D. Dept. of Anatomy, Histology and Embryology. Development. Ventricles in brainstem. Mesencephalon  cerebral aqueduct Metencephalon  4 th ventricle Mylencephalon  4 th ventricle. Posterior commissure. Corpus callosum. Fornix.

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Assoc. Prof. Stefan Sivkov, Ph.D.

Dept. of Anatomy, Histology and Embryology

ventricles in brainstem
Ventricles in brainstem
  • Mesencephalon  cerebral aqueduct
  • Metencephalon  4th ventricle
  • Mylencephalon  4th ventricle

Posterior commissure

Corpus callosum


Occipital Lobe


Anterior commissure

Quadrigeminal cistern



Optic nerve

4th ventricle

Mammillary body



basal ganglia

internal capsule

optic chiasm

optic nerve

optic tract


mammillary body

cerebral peduncle


interpeduncular fossa


inferior olivary nuclear complex

cerebellar tonsil


pyramidal decussation


Anterior view of brainstem




optic tract

trigeminal nerve

optic nerve

middle cerebellar peduncle

optic chiasm

vestibulocochlear nerve



cuneate tubercle


inferior olivary nuclear complex


anterior median fissure

Lateral view of brainstem


Posterior view of brainstem

Superior colliculus

Cerebral peduncle

Inferior colliculus

Superior cerebellar peduncle

4th ventricle

Middle cerebellar peduncle

Inferior cerebellar peduncle


components of the brainstem
Components of the brainstem
  • Sensory ascending pathways (dorsal):
    • Relay nuclei, tracts
  • Motor descending pathways (ventral)
    • Tracts, motor nuclei brainstem
  • Cerebellar pathways
    • Tracts, cerebellar afferent and efferent nuclei
  • Cranial nerve sensory and motor tracts
    • Cranial nerve nuclei, nerve entry and exit points
  • CPGs: rhythmic chewing, respiration, cardiovascular regulation & gain adjustments for reflexes
  • Modulatory systems: locus coeruleus, raphe & substantia nigra
    • Chemically coded nuclei
brain stem
Brain Stem
  • Located btwn the cerebrum and the SC
    • Provides a pathway for tracts running btwn higher and lower neural centers.
  • Consists of themidbrain, pons, andmedullaoblongata.
    • Each region is about an inch in length.
  • Microscopically, it consists of deep gray matter surrounded by white matter fiber tracts.
  • Produce automatic behaviors necessary for survival.
brainstem 3 major divisions
Brainstem: 3 major divisions
  • Midbrain
  • Pons
  • Medulla
  • Located btwn diencephalon and pons.
    • 2 bulgingcerebral peduncleson the ventral side. These contain:
      • Descending fibers that go to the cerebellum via the pons
      • Descending pyramidal tracts
    • Running thru the midbrain is the hollowcerebral aqueductwhich connects the 3rd and 4th ventricles of the brain.
    • The roof of the aqueduct (thetectum) contains thecorpora quadrigemina
      • 2superior colliculithat control reflex movements of the eyes, head and neck in response to visual stimuli
      • 2inferior colliculithat control reflex movements of the head, neck, and trunk in response to auditory stimuli

Cranial nerves 3&4 (oculomotor and trochlear) exit from the midbrain

  • Midbrain also contains the headquarters of the reticular activating system
  • On each side, the midbrain contains ared nucleusand asubstantia nigra
    • Red nucleus contains numerous blood vessels and receives info from the cerebrum and cerebellum and issues subconscious motor commands concerned w/ muscle tone & posture
    • Lateral to the red nucleus is the melanin-containing substantia nigra which secretes dopamine to inhibit the excitatoryneurons of the basal nuclei.
      • Damage to the substantia nigra would cause what?
  • Literally means “bridge”
  • Wedged btwn the midbrain & medulla.
  • Contains:
    • Sensory and motor nuclei for 4 cranial nerves
      • Trigeminal (5), Abducens (6), Facial (7), and Auditory/Vestibular (8)
    • Respiratory nuclei:
      • Apneustic&pneumotaxic centerswork w/ the medulla to maintain respiratory rhythm
    • Nuclei & tracts that process and relay info to/from the cerebellum
    • Ascending, descending, and transverse tracts that interconnect other portions of the CNS
medulla oblongata
Medulla Oblongata
  • Most inferior region of the brain stem.
  • Becomes the spinal cord at the level of the foramen magnum.
  • Ventrally, 2 ridges (themedullary pyramids)are visible.
    • These are formed by the large motorcorticospinal tracts.
    • Right above the medulla-SC junction, most of these fibers cross-over (decussate).
medulla oblongata1
Medulla Oblongata
  • Nuclei in the medulla are associated w/ autonomic control, cranial nerves, and motor/sensory relay.
  • Autonomic nuclei:
    • Cardiovascular centers
      • Alter the rate and force of cardiac contractions
      • Alter the tone of vascular smooth muscle
    • Respiratory rhythmicity centers
      • Receive input from the pons
    • Additional Centers
      • Emesis, deglutition,coughing, hiccupping, and sneezing
medulla oblongata2
Medulla Oblongata
  • Sensory & motor nuclei of 5 cranial nerves:
    • Auditory/Vestibular (8), Glossopharyngeal (9), Vagus (10), Accessory (11), and Hypoglossal (12)
  • Relay nuclei
    • Nucleus gracilisandnucleus cuneatuspass somatic sensory information to the thalamus
    • Olivary nucleirelay info from the spinal cord, cerebral cortex, and the brainstem to the cerebellar cortex.
ascending sensory pathways
Ascending sensory pathways

Fine discriminitive touch, conscious proprioception

  • Fasciculus gracilis: Terminates in the nucleus gracilis (medulla)
  • Fasciculus cuneatus: Terminates (medulla) in the cuneate and accessory cuneate nuclei

Sensations of pain and temperature

  • Lateral Spinothalamic Tract
    • origin dorsal horn cells of the gray matter
    • Fibers cross contralaterally through the anterior commissure and ascend to the VPL nucleus

Transmits sensations of touch

  • Ventral Spinothalamic Tract
    • origin cells of the posterior horn
    • Fibers cross to the opposite side in the anterior commissure
descending motor pathways
Descending motor pathways

Voluntary movement

  • Lateral Corticospinal Tract
    • Originates in large pyramidal cells (precentral gyrus)
    • cross to the opposite side of the cord at the pyramidal decussation & terminate in the dorsal horn cells
  • Ventral Corticospinal Tract
    • Originates in the pyramidal cells (motor area of the cortex)

Impulses related to equilibrium and antigravity reflexes

  • Vestibulospinal Tract
    • Fibers originate in the vestibular nuclei of the medulla and terminate at level of the sacral spinal nerves

Connects vestibular complex and head and eye movement coordination center in medulla

  • Medial Longitudinal Fasciculus
    • Contains both ascending and descending fibers

Motor Hierarchy

  • Lateral group (extremities; fine motor control)
    • Corticospinal tract
    • Rubrospinal tract
  • Medial group (axial musculature; rhythmic and postural movements)
    • Vestibulospinal tract
    • Tectospinal tract
    • Reticulospinal tract
  • “Final common path”: motor pool
reticular formation
Reticular Formation
  • Extensive network of neurons that runs thru the medulla and projects to thalamic nuclei that influence large areas of the cerebral cortex.
    • Midbrain portion of RAS most likely is its center
  • Functions as a net or filter for sensory input.
    • Filter out repetitive stimuli. Such as?
    • Allows passage of infrequent or important stimuli to reach the cerebral cortex.
    • Unless inhibited by other brain regions, it activates the cerebral cortex – keeping it alert and awake.

How might the “sleep centers” of your brain work? Why does alcohol make you tired?

reticular formation1
Reticular Formation
  • “Core” of brainstem (midbrain, pons and medulla) composed of loosely organized neurons, outside of the major nuclear groups of the brainstem.
  • Medial-to-lateral: raphe nuclei, gigantocellular region, small cell region
  • Participate in widespread connections
  • Rostral continuation of interneuronal network found in spinal cord
dorsal column medial lemniscal system
Dorsal Column/Medial Lemniscal system
  • Secondary neuron is in brainstem: nucleus gracilis and cuneatus=dorsal column nuclei
  • Output of dorsal column nuclei crosses midline and forms recognizable bundle: medial lemniscus
  • Medial lemniscus fibers synapse in the thalamus in the ventroposterior nuclei
  • Thalamic axons synapse in primary somatosensory cortex in several somatotopic maps with some segregation of submodalities