Brain Stem II

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Brain Stem II

• Basic Neuroscience
• James H. Baños, Ph.D.

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Today

• Brain Stem Reticular Formation
• Corticobulbar tract
• Cranial nerves and their nuclei

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Major Brain Stem Activities

• Conduit
• Ascending and descending pathways
• Integrative functions
• Complex motor patterns
• Respiratory and cardiovascular activity
• Regulation of arousal and level of consciousness
• Cranial Nerve functions

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Integrative FunctionsBrain Stem Reticular
Formation
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Brain Stem Reticular Formation

• Reticular netlike
• Loosely defined nuclei and tracts
• Extends through the central part of the medulla,
  pons, and midbrain
• Intimately associated with
• Ascending/descending pathways
• Cranial nerves/nuclei
• Input and output to virtually all parts of the CNS

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Brain Stem Reticular Formation
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Brain Stem Reticular Formation

• 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

• Connectivity is extremely complex
• Many different types of neurons
• 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

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Brain Stem Reticular Formation
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Reticular Formation Functions

• I. Participates in control of movement through
  connections with both the spinal cord and
  cerebellum
• Two reticulospinal tracts originate in the
  rostral pontine and medullary reticular formation
• 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
• Yawning
• Stretching
• Babies suckling
• Some interconnectivity with cerebellar motor
  control circuitry

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Clinical Correlation

• Pseudobulbar affect (as seen in Amyotrphic
  Lateral Sclerosis)
• 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
• Disinhibited spinal reflexes are very simple
• Disinhibited brainstem reflexes are very complex

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Clinical Correlation

• The Terri Schiavo case

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Reticular Formation Functions

• II. Modulates transmission of information in pain
  pathways
• 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

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Reticular Formation Functions
Cortex
Hypothal
Thalamus
Periaqueductal Grey
Spinothalamic Tract
Raphe
Spinal Cord Level
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Clinical Correlation

• Pain Management
• Periaqueductal grey has high concentration of
  opiate receptors
• Natural pain modulation relies on endogenous
  opiates
• Exogenous opiates are used for pain management

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Pause for contemplation!

• Major recurring theme LOOPS
• Many brain functions are represented in loops
  (usually with a modulatory influence)
• 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

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meanwhile, back at the reticular formation

• III. Autonomic reflex circuitry
• Reticular formation receives diverse input
  related to environmental changes
• Also receives input from hypothalamus related to
  autonomic regulation
• Output to
• cranial nerve nuclei
• Intermediolateral cell column of the spinal cord
• Involved in
• Breathing
• Heart rate
• Blood pressure
• Etc.

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Clinical Correlation

• Damage to the medulla often kills you
• Horners 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

• IV. Involved in control of arousal and
  consciousness
• 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

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Clinical Correlation

• Normal functions
• Sleep/wakefulness
• Loss of Consciousness
• Traumatic brain injury
• Smelling salts, sternal rubs, and the ARAS
• Coma
• Can result from extensive damage to cortex
• More focal damage to ARAS
• Coma vs Minimally Conscious State
• Intact sleep/wake patterns in brain activity

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The Corticobulbar Tract
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The Corticobulbar Tract

• 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

• Corticobulbar tract
• Descending motor pathways to cranial nerve nuclei
• Includes descending fibers for HALs 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
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A word about organization

• Sensory and motor spinal nerves can be divided
  into
• Sensory (dorsal)
• Somatic - pain, temperature, mechanical stimuli
• Visceral - from receptive endings
• Motor (ventral)
• Somatic - Innervate skeletal muscle
• Visceral - To visceral autonomic ganglia

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A word about organization

• Cranial Nerves also include
• Special Sensory fibers
• Hearing, equilibrium, etc
• Special motor fibers
• Branchial motor
• Muscles of the head and face
• Different embryologic origin and location
• Otherwise, structurally and functionally the same
  as other muscle
• Autonomic fibers

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A word about organization

• All of these fiber types organize predictably
  around the sulcus limitans

See p. 292
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A word about organization
See p. 294, 296
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Starting from the topCN I
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Starting from the topCN I - Olfactory

• Fiber types
• Special Sensory -- Smell
• The olfactory bulb and tract arent 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

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Cribiform plate
Olfactory bulb
CN I
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Clinical Correlation

• 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

• 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

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CN II - Optic
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CN II - Optic

• Fiber Types
• Special Sensory -- Vision
• Retinal ganglion cells to
• 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

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CN III - Oculomotor
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CN III - Oculomotor

• Somatic Motor - Eye movement
• Superior, inferior, medial recti
• Inferior oblique
• Levator palpebrae superioris
• Autonomic - Pupillary constriction
• Edinger-Westphal nucleus to pupillary sphincter

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CN III - Oculomotor
Edinger-Westphal
Nucleus of III
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CN III - Oculomotor

• Eye movement
• Superior rectus - elevation
• Inferior rectus - depression
• Medial rectus - adduction
• Inferior Oblique - extorsion/elevation
• Levator palpebrae superioris?

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CN III - Oculomotor

• CN III Oculomotor
• Pillars that hold the eye open
• CN VII Facial
• Hook that pulls the eye closed

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III
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CN III - Oculomotor

• Edinger-Westphal nucleus
• Receives bilateral projections from superior
  colliculi (which had received unilateral
  projections from CN II)
• This is the efferent component of the pupillary
  light reflex
• Also involved in pupillary accomodation

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Clinical Correlation

• Damage to CN III or nucleus of III
• Down and out eyeball
• Diplopia
• Ptosis
• Dilated and fixed pupil
• Paralysis of pupillary accommodation
• Can be cause by
• Uncal/transtentorial herniation
• Aneurysm

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Clinical Correlation

• Pupillary light reflex
• Direct
• Consensual

II - left
III - left
II - right
III - right
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Clinical Correlation
II - left
III - left
II - right
III - right
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Clinical Correlation
II - left
III - left
II - right
III - right
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Clinical Correlation
II - left
III - left
II - right
III - right
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CN IV - Trochlear
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CN IV - Trochlear

• Somatic Motor
• Superior Oblique - Intorts, depressed, adducts
  the eye

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CN IV - Trochlear
Nucleus of IV
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CN VI - Abducens
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CN VI - Abducens

• Somatic Motor
• Lateral Rectus

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CN VI - Abducens
III
III
IV
IV
VI
VI
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Finally, lets add a pathway

• What muscles are being used when we look left or
  right?
• What cranial nerves?
• Is the same thing happening on each side?

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Finally, lets add a pathway

• 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
• This is accomplished by cross wiring the nuclei
  via the medial longitudinal fasciculus (MLF)

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Finally, lets add a pathway
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Learn More
University of California -- Davis Eye Simulation
Website
http//cim.ucdavis.edu/eyes/version15/eyesim.html
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Coming Up

• Tomorrow
• More cranial nerves
• Thursday
• Diencephalon