Pons Anatomy, Pathology Flashcards
What structure in the metencephalon is closely related to the pons?
The cerebellum
What cells cause the enlargement of the pons?
Pontine nuclei
What region of the pons is phylogenetically newer?
The basis
What 3 types of fibers are found in the basis?
- Corticospinal fibers
- Corticobulbar/ corticonuclear fibers (only in pons)
- Corticopontine fibers (only in pons)
What tract is related to the corticospinal fibers found in the basis of the pons?
The pyramidal tract
What are the function of the corticobulbar/ corticonuclear fibers of the pons?
- Fractionalize movement on cranial nerve nuclei
What cranial nerve nuclei are involved in the corticobulbar/ corticonuclear systems?
CN V CN VII CN IX CN X CN XI CN XII
What do the path do the axons of the pontine nuclei take after being synapsed on by corticopontine fibers?
The pontine nuclei project ventrally,and then move contralaterally and dorsally to the contralateral cerebellum
What tract/ structure is formed by the axons of the pontine nuclei?
- The middle cerebellar peduncle
- Pontocerebellar tract
What is the name for the combination of the corticopontine and pontocerebellar tracts?
Corticopontocerebellar
What nuclei lie in the lower pons?
- CN VI/ Abducens nucleus
- Spinal nucleus of V/ trigmeninal nucleus with associated tract
- CN VII/ Facial nerve proper nucleus
- CN VIII/ Vestibulocochlear nucleus
- Pontine nuclei
How does the facial nerve proper exit the lower pons?
- Winds posteriorly between the MLF and CN VI nuclei and exits the pons laterally
How does the abducens nerve exit the lower pons?
- Travels directly anteriorly
What is the SVE of the facial nerve proper?
Muscles of facial expression and stapedius
- Parotid gland (?)
What muscle is innervated by CN VI?
Lateral rectus of the eye
Where does the medial lemniscus become somatopically arranged?
From lower to mid pons.
What nuclei are found in the mid-pons?
- Chief/ primary nucleus of CN V
- Motor nucleus of CN V
What tract arises in the mid pons?
The ventral trigemonothalamic tract
What sensory information travels along the VTTT?
- Pain
- Thermal
- Touch
from face
What are the GSA and SVE components of CN V?
GSA: Sensory information from face and anterior 2/3rds of tongue
SVE: Muscles of mastication
What tracts are present in the mid pons?
- VTTT
- Medial lemniscus
- Medial longitudinal fasciculus
- Spinothalamic tract
- Corticospinal tract
- Cotricobulbar/ corticonuclear fibers
- Corticopontocerebellar tract/ middle cerebellar peduncle
What nucleus arises in the upper pons?
- Mesencephalic nucleus of CN V
What sensory information is processed by the mesencephalic nucleus of CN V?
- Proprioception
What structure related to higher centers arises in the upper pons?
- Central tegmental bundle
What system is the central tegmental bundle involved in?
The ascending reticular activating system
What structures does the central tegmental bundle terminate onto?
- Thalamus, and then cortex
What is the function of the ascending reticular activating system?
- Arousal
What is the shape of the medial lemniscus in the upper pons?
Curving from medioanterior to lateroposterior
Describe the path of the saccade reflex.
- Superior colliculus sends axon to CN VI motor nuclei in low pons
- CN VI sends an axon to the ipsilateral lateral rectus
- CN VI sends a collateral off to an interneuron which crosses sides in the low-pons, and ascends up to CN III nuclei, which activates the contralateral medial rectus
What are the 5 types of eye movement?
- Saccades
- Smooth pursuit movements
- Vergence
- Vestibulo-ocular movements
- Optokinetic movements
What is a saccade?
Rapid shift of fovea to target in periphery
How fast can a saccade move the eye?
900 degrees/second
What is a smooth pursuit movement?
Keeps image of a moving target on the fovea
How fast does a smooth pursuit movement move?
100 degrees/second
What is vergence?
Moves the eyes in opposite directions to position eye on both fovae (crossing of the eyes)
What are vestibulo-ocular movements?
Hold images still on the retina during brief head movements
What are optokinetic movements?
Hold images on retina during sustained head rotation
What drives optokinetic movements?
Visual stimuli
What tendency of movement causes optokinetic nystagmus?
Eyes tend to follow or track the motion of one element at a time of a steadily moving display. As a tracked element leaves the field of vision, the eyes will “snap back” to a new object
When is optokinetic nystagmus present after birth?
5 days after
What system is thought to control optokinetic nystagmus?
Tectopulvinar system
How can optokinetic nystagmus be used as an evaluative tool?
The display can be altered such as a fineness of grating and contrast to determine if a patient perceives the different grating as single or multiple objects. This determines visual acuity or ability to determine contrast
What muscle is innervated by the abducens?
Lateral rectus of the eye
What muscle is innervated by the trochlear nerve?
Superior oblique of the eye
What 5 muscles are innervated by the oculomotor nerve?
- Superior rectus
- Inferior rectus
- Medial rectus
- Inferior oblique
- Levator palpebrae superiorus (eyelid opening muscle)
What cranial nerve closed the eye? What is the name of the muscle?
- CN VII
- Obiqularis occuli
What is the parasympathetic component of CN III?
Innervates iris and muscles that control lens
Which extraoccular eye muscle is found in the lower pons?
The abducens
What orientation of eye movements is the paramedian reticular formation responsible for?
Horizontal
What orientation of eye movements is the mesencephalic reticular formation responsible for?
Vertical
Describe the circuitry of a horizontal gaze.
- At rest, the OMNIPAUSE center in the nucleus of the DORSAL MEDIAN RAPHE inhibits the activations centers in the PONTINE RETICULAR FORMATION
- When something enters the visual field, the excitatory nuclei of the pontine formation become disinhibited, and superior colliculus also provides excitation
- Cells project to ipsilateral CN VI nucleus and contralateral CN III nucleus
- CN VI goes to the ipsilateral lateral rectus
- CN III goest to the contralateral medial rectus
What are the 2 signals that control the motor circuit for horizontal saccades?
- Velocity signal
- Position signal
Which signal controls the saccade?
Velocity
What 2 nuclei are responsible for the position/ step signal?
- Medial vestibular nuclei
- Nucleus prepositus hypoglossi
What is the function of the positional signal?
Maintains position of the eye through control of extraocular eye muscles
During which signal is the omnipause neuron not active?
Velocity signal
During what eye position is the abducens motor neuron most active?
Maintenance of lateral position
When is the abducens motor neuron least active?
When eye is near midline
What results from a lesion to the rootlet of CN VI?
- LMN paralysis of ipsilateral rectus muscle
- Diploplia
- Eye can’t move directly laterally or inferior laterally
- As eye tracks towards ipsilateral side, ipsilateral side will stop at midline, while the other continues to track
What results from a lesion to the nucleus of CN VI?
- LMN paralysis of ipsilateral rectus muscle
- No reflex to contralateral eye (so it will not move with rectus)
Can the eyes move to the contralateral side with a CN VI lesion?
Yes
What results from a lesion to the medial longitudinal fasciculus?
- Contralateral eye will not move because the interneuron is lost
- Ipsilateral rectus will move normally
What results from a lesion to CN III?
Eye can only move laterally
What results from a lesioning of the corticobulbar tracts?
- Perimedial reticular formation of contralateral side lesioned
- Contralateral CN VI affected
- Ipsilateral CN III affected
What is an alternative name of the a medial longitudinal fasciculus lesion?
Internuclear opthalmoplegia
What is one and a half syndrome? What are the symptoms?
Lesion to medial longitudinal fasciculus and paramedian pontine reticular formation on one side OR lesion to abducens nucleus and medial longitudinal fasciculus on one side.
Lesions:
- Ipsilateral abducens lesioned
- Contralteral oculomotor effectively lesioned
- Ipsilateral oculomotor effectively lesioned
Symptoms:
- When gazing to contralateral side
- Contralateral abducens activates, but interneuron connection is lost, so ipsilateral eye does not move
- When gazing to ipsilateral side
- Drive to abducens, and interneuron is lost so neither eye moves
Which direction will an individual with a CVA look with a cortical lesion? With a brainstem lesion?
Cortical: Away from lesion
Brain stem: Towards lesion
What provides the blood supply to the pons?
Basilar artery, and its following 3 branches:
- Paramedian
- Short circumferential
- Long circumferential
What condition may result from a lesion to the basis of the pons?
- Alternating Abducens Hemiplegia
- Contraltaeral UMN of body
- Ipsilateral LMN of lateral rectus
(If medial lemniscus is involved–> contralateral loss of disciminitive sensation to body
What is strabismus?
Eyes are not properly aligned (cross-eyed)
What structures are potentially affected by a unilateral lesion to the tegmentum of the pons?
LSTT: Contralteral P&T to body
CN V: Ipsilateral P&T to body
Med Lem: UE & LE discriminitive touch loss to body
CN VII rootlet: Ipsilateral LMN of facial expression
What is the term for a lesion to the rootlet of CN VII? What are the symptoms?
Bell’s Palsy
- LMN to facial expression muscles
What is Millard-Gubler syndrome?
Lesion to ipsilateral:
CN VI
CN VII
Corticospinal tracts
What syndrome involving the medial longitudinal fasciculus and median pontine reticular formation occurs in the tegmentum of the pons?
One and a half syndrome
What results from a lesion to the pontine reticular formation?
Same effect as a lesion to CN VI and MLF ipsilaterally/ one and a half syndrome
Which portion of the CN VII nuclei SVE is bilateral, which is contralateral?
Upper: Bilateral
Lower: Contralateral
How is CN VII tested?
- Pt smiles, moves face
- Corneal reflex
What nerves does the corneal reflex assess?
CN V and VII
How is the corneal reflex assessed?
Lightly cotton swab the eye
Describe the pathway of the corneal reflex.
- Discriminitive touch of CN V via grade II fibers and interneurons projects to CN VII ipsilaterally
- CN VII goes to face and closes the eyelid
- CN V also crosses over
What is the CN V portion of the corneal reflex termed?
Consensual corneal reflex
What structures may be lesioned in the mid-pons? What are the resultant symptoms of each?
- CN V: Discriminitive sensation loss to ipsilateral face. (motor nucleus) LMNL to ipsilateral muscles of mastication (Motor nucleus)
- STT: Contralateral P&T of body
- VTTT: Contralateral P&T of face
- MLF: Internuclear opthalmoplegia (above medulla)
What syndrome will result from a bilateral lesion to the basil portion of the upper pons or midbrain?
Locked-in syndrome
- Bilateral loss of all volitional movement except the eyes
- No sensation or cognition loss
- All CN nuclei except 3,4, and 6 are lesioned
What syndrome results from a lesion to the superior cerebellar peduncle?
Neocerebellar syndrome:
- Dysnergia
- Ataxia
- Dysmetria (inability to measure distance)
- Dysdiadochokinesia (inability to execute alternate movements rapidly)
- Dysarthria
- Etc…
What will result from a lesion to the basil portion of the midbrain?
Lesioning of corticospinal/ corticobulbar tracts?
UMNL to body and CN
What structures may be affected by a lesion to the tegmentum of the midbrain?
- Medial lemniscus: (contralateral disciminitive touch, proprioception, P&T to body)
- VTTT: Contralateral discriminitive sensaiton, proprioception, P&T to contralateral face (possibly body? Look this up.)
- Bilateral CTB: Coma
- Bilateral Inferior colliculus: Hearing loss
- CN IV: Contralateral superior oblique lesions (can’t look down and out)
What is the nervus intermedius?
Portion of the facial nerve (from the pons) that provides:
SVA (taste from anterior 2/3rds of tongue)
GVE (parasympathetic motor to submandibular, sublingual and lacrimal glands)
