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Brain Stem II
Basic Neuroscience
James H. Baños, Ph.D.
Today…
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Brain Stem Reticular Formation
Corticobulbar tract
Cranial nerves and their nuclei
Major Brain Stem Activities
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Conduit
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Ascending and descending pathways
Integrative functions
Complex motor patterns
 Respiratory and cardiovascular activity
 Regulation of arousal and level of
consciousness
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Cranial Nerve functions
Integrative Functions:
Brain Stem Reticular
Formation
Brain Stem Reticular Formation
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Reticular = “netlike”
Loosely defined nuclei and tracts
Extends through the central part of the medulla, pons,
and midbrain
Intimately associated with
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Ascending/descending pathways
Cranial nerves/nuclei
Input and output to virtually all parts of the CNS
Brain Stem Reticular Formation
Brain Stem Reticular Formation
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Can be roughly divided into three
longitudinal zones
Midline - Raphe Nuclei
 Medial Zone - Long ascending and
descending projections
 Lateral Zone - Cranial nerve reflexes and
visceral functions
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Brain Stem Reticular Formation
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Connectivity is extremely complex
Many different types of neurons
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Innervate multiple levels of the spinal cord
Numerous ascending and descending collaterals
Some have bifurcating collaterals that do both
Many have large dendritic fields that traverse multiple
levels of the brain stem
Brain Stem Reticular Formation
Reticular Formation Functions
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I. Participates in control of movement through
connections with both the spinal cord and cerebellum
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Two reticulospinal tracts originate in the rostral pontine and
medullary reticular formation
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Major alternate route by which spinal neurons are controlled
Regulate sensitivity of spinal reflex arcs
Tonic inhibition of flexor reflexes
Mediates some complex “behavioral” reflexes
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Yawning
Stretching
Babies suckling
Some interconnectivity with cerebellar motor control circuitry
Clinical Correlation
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Pseudobulbar affect (as seen in Amyotrphic Lateral
Sclerosis)
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Degeneration of descending motor pathways from the cortex to
the brainstem
“Release” of some of complex motor behaviors such as laughing
and crying
Usually uncontrollable, not consistent with mood
May laugh when angry, cry at sad things, etc
Conceptually analogous to upper motor neuron hyperreflexia
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Disinhibited spinal reflexes are very simple
Disinhibited brainstem reflexes are very complex
Clinical Correlation
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The Terri Schiavo case
Reticular Formation Functions
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II. Modulates transmission of information in pain
pathways
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Spinomesencephalic fibers bring information about noxious
stimuli to the periaqueductal grey
Periaqueductal grey also receives input from the hypothalamus
and cortex about behavioral and drive states
Efferents from the periaqueductal grey project to one of the
raphe nuclei and medullay reticular formation
These project to the spinal cord and can suppress transmission
of pain information in the spinothalamic tract
Reticular Formation Functions
Cortex
Thalamus
Spinothalamic
Tract
Hypothal
Periaqueductal Grey
Raphe
Spinal Cord Level
Clinical Correlation
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Pain Management
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Periaqueductal grey has high concentration of opiate
receptors
Natural pain modulation relies on endogenous opiates
Exogenous opiates are used for pain management
Pause for contemplation!
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Major recurring theme: LOOPS
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Many brain functions are represented in
loops (usually with a modulatory influence)
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Muscle tone
Reflex loops
Pain modulation
Pathology and treatment of pathology are
often related to modulating these loops
Many of the basic pathways are
supplemented by more complex pathways
that complete this modulated loop
architecture
Pause for contemplation!
Cortex
Thalamus
Spinothalamic
Tract
Hypothal
Periaqueductal Grey
Raphe
Basic
Pathway
Spinal Cord Level
Modulatory
circuitry
…meanwhile, back at the reticular formation…
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III. Autonomic reflex circuitry
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Reticular formation receives diverse input related to
environmental changes
Also receives input from hypothalamus related to autonomic
regulation
Output to
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cranial nerve nuclei
Intermediolateral cell column of the spinal cord
Involved in
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Breathing
Heart rate
Blood pressure
Etc.
Clinical Correlation
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Damage to the medulla often kills you
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Horner’s Syndrome
Interruption of descending pathways to the
intermediolateral cell column
 Ipsilateral Miosis (small pupil)
 Ipsilateral Ptosis (drooping eyelid)
 Ipsilateral Flushing/lack of sweating
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Reticular Formation Functions
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IV. Involved in control of arousal and consciousness
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Input from multiple modalities (including pain)
Ascending pathways from RF project to thalamus, cortex, and
other structures.
Thalamus is important in maintaining arousal and “cortical tone”
This system is loosely defined, but referred to as the Ascending
Reticular Activating System (ARAS)
ARAS is a functional system, not an anatomically distinct
structure
Clinical Correlation
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Normal functions
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Loss of Consciousness
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Traumatic brain injury
Smelling salts, sternal rubs, and the ARAS
Coma
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Sleep/wakefulness
Can result from extensive damage to cortex
More focal damage to ARAS
Coma vs Minimally Conscious State
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Intact sleep/wake patterns in brain activity
The Corticobulbar
Tract
The Corticobulbar Tract
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Corticospinal tract
Descending motor pathways to ventral horn of
the spinal cord
 Includes only fibers for torso, arms, legs (i.e.,
headless HAL)
 Decussates at a single point in the pyramids
of the medulla (pyramidal decussation)
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The Corticobulbar Tract
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Corticobulbar tract
Descending motor pathways to cranial nerve
nuclei
 Includes descending fibers for HAL’s head
 Fibers for each CN nucleus decussate at the
level of that nucleus (i.e., multiple points of
decussation)
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Cranial Nerves and
Their Nuclei
A word about organization…
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Sensory and motor spinal nerves can be
divided into
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Sensory (dorsal)
Somatic - pain, temperature, mechanical stimuli
 Visceral - from receptive endings
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Motor (ventral)
Somatic - Innervate skeletal muscle
 Visceral - To visceral autonomic ganglia
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A word about organization…
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Cranial Nerves also include:
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Special Sensory fibers
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Hearing, equilibrium, etc
Special motor fibers
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Branchial motor
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Muscles of the head and face
Different embryologic origin and location
Otherwise, structurally and functionally the same as other
muscle
Autonomic fibers
A word about organization…
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All of these fiber types organize
predictably around the sulcus limitans
A word about organization…
Starting from the top…CN I
Starting from the top…CN I - Olfactory
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Fiber types:
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Special Sensory -- Smell
The olfactory bulb and tract aren’t really CNI
The fibers of CNI originate in the olfactory
mucosa of the nasal cavity, pass through the
cribiform plate, and synapse onto the olfactory
bulb
Note that there is no brain stem nucleus for CNI
Cribiform plate
Olfactory bulb
CN I
Clinical Correlation
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Olfactory nerve dysfunction is often
reported as altered taste and smell
Conditions affecting CNI include:
Upper respiratory tract infection
 Traumatic Brain Injury (TBI)
 Subfrontal meningioma
 Dementia
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Clinical Correlation
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Anosmia - Total loss of smell
Hyposmia - Partial loss of smell
Hyperosmia - Exaggerated sense of smell
Dysomia - Distorted sense of smell
Olfactory hallucinations - Associated with
seizures
CN II - Optic
CN II - Optic
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Fiber Types
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Special Sensory -- Vision
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Retinal ganglion cells to:
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Thalamus (lateral geniculate nucleus) -- Primary visual
pathway
Superior colliculus -- Reflexes involving vision and light
Hypothalmus -- Light-dependent behavioral cycles
Does not have a specific nucleus in the brain
stem
CN III - Oculomotor
CN III - Oculomotor
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Somatic Motor - Eye movement
Superior, inferior, medial recti
 Inferior oblique
 Levator palpebrae superioris
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Autonomic - Pupillary constriction
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Edinger-Westphal nucleus to pupillary
sphincter
CN III - Oculomotor
Nucleus of III
Edinger-Westphal
Spinothalamic
Medial Lemniscus
Corticospinal
CN III - Oculomotor
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Eye movement
Superior rectus - elevation
 Inferior rectus - depression
 Medial rectus - adduction
 Inferior Oblique - extorsion/elevation
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Levator muscle of the upper eyelid
CN III - Oculomotor
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III
7
CN III Oculomotor
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“Pillars” that hold the
eye open
CN VII Facial
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“Hook” that pulls the
eye closed
CN III - Oculomotor
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Edinger-Westphal nucleus
Receives bilateral projections from superior
colliculi (which had received unilateral
projections from CN II)
 This is the efferent component of the pupilary
light reflex
 Also involved in pupilary accommodation
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Clinical Correlation
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Damage to CN III or nucleus of III
“Down and out” eyeball
 Diplopia
 Ptosis
 Dilated and fixed pupil
 Paralysis of pupillary accommodation
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Can be cause by…
Uncal/transtentorial herniation
 Aneurysm
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Clinical Correlation
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Pupillary light reflex
Direct
 Consensual
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II - left
III - left
II - right
III - right
Clinical Correlation
II - left
III - left
II - right
III - right
Clinical Correlation
II - left
III - left
II - right
III - right
Clinical Correlation
II - left
III - left
II - right
III - right
CN IV - Trochlear
CN IV - Trochlear
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Somatic Motor
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Superior Oblique - Intorts, depressed, adducts
the eye
CN IV - Trochlear
Nucleus of IV
CN VI - Abducens
CN VI - Abducens
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Somatic Motor
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Lateral Rectus
CN VI - Abducens
III
III
IV IV
VI
VI
Finally, lets add a pathway
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What muscles are being used when we
look left or right?
What cranial nerves?
Is the same thing happening on each
side?
Finally, lets add a pathway
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During horizontal conjugate eye
movements, each eye is doing the
opposite of the other
Adduction (CN III) on one side
 Abduction (CN VI) on the other side
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This is accomplished by “cross wiring” the
nuclei via the medial longitudinal
fasciculus (MLF)
Finally, lets add a pathway
III
III
IV IV
VI
VI
Learn More…
University of California -- Davis Eye Simulation Website:
http://cim.ucdavis.edu/eyes/version15/eyesim.html
Coming Up…
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More cranial nerves
Diencephalon