Paramedian pontine reticular formation

Paramedian pontine reticular formation
Paramedian pontine reticular formation
	Axial section of the pons at the level of the facial colliculus (PPRF not labeled, but region is visible, near abducens nucleus)
Details
Part of	Brain stem
Artery	Pontine arteries
Vein	Transverse and lateral pontine veins
Identifiers
Latin	formatio reticularis pontis paramediana
NeuroNames	1399
	Anatomical terms of neuroanatomy [ edit on Wikidata]

Last updated August 26, 2024

The paramedian pontine reticular formation (PPRF) (also known as paraabducens nucleus) is a subset of neurons of the oral and caudal pontine reticular nuclei mediating horizontal gaze.^[1] It is situated in the pons adjacent to the abducens nucleus.^[2] It projects to the ipsilateral abducens (cranial nerve VI) nucleus, and contralateral oculomotor (cranial nerve III) nucleus ^{[note 1]} to mediate conjugate horizontal eye movements and saccades.

Anatomy

The PPRF is situated in the pons just^[3] ventralmedial to the abducens nucleus.^[2] It is located anterior and lateral to the medial longitudinal fasciculus.^{[ citation needed ]} It is continuous caudally with the nucleus prepositus hypoglossi.^[4]

The PPRF (and adjacent regions of the pons) are traversed by fibers projecting to the abducens nucleus that mediate smooth pursuit, vestibular reflexes, and gaze holding.^[5]^: 498

Afferents

The PPRF receives afferents from:

contralateral frontal eye field of the middle frontal gyrus of the frontal lobe (via frontopontine fibers ^[1]). The frontal eye field meanwhile receives afferents from the visual cortex.^[6]
superior colliculus ^[1]
vestibular nuclei ^[1]
other parts of the reticular formation.^[1]

Efferents

The PPRF mediates horizontal conjugate gaze (i.e. simultaneous horizontal movement of both eyes) by projecting to both:^[1]^[6]

the ipsilateral abducens (CN VI) nucleus (which controls the ipsilateral lateral rectus muscle),
(through the medial longitudinal fasciculus) the contralateral oculomotor (CN III) nucleus (specifically the population of its neurons that innervate the contralateral medial rectus muscle).

The pararaphal nucleus - one of distinct neuron population in the PPRF - projects to the flocculus of the cerebellum.^[5]^: 498

Function

The PPRF mediates horizontal conjugate eye movements.^[3] It is important in mediating saccadic eye movements.^[2] It is probably not involved in smooth pursuit.^[2]

The PPRF generates excitatory bursts that are delivered to the ipsilateral abduecens nucleus to drive ipsilateral saccades (inhibitory saccadic stimuli are meanwhile delivered to the abducens nucleus from the contralateral medulla oblongata).^[5]^: 499

Pathophysiology

Destructive lesions of the PPRF cause ipsilateral horizontal conjugate gaze palsy and mostly impair ipsilateral horizontal saccades, however, other horizontal and vertical eye movements may also be affected as the PPRF contains multiple distinct populations of neurons important in saccade generation, as well as being traversed by nerve fibers involved in eye movements that elsewhere; dysfunction of horizontal saccades will additionally also indirectly disrupt (slow and misdirect) vertical saccades^[5]^: 498-499 (though slowing of all saccades may also be accounted for by destruction of adjacent omnipause neurons of the interposited raphe nucleus^[5]^: 221).

In the short-term, unilateral lesions of the PPRF may be characterised clinically by contralateral deviation of the eyes; looking contralaterally induces nystagmus characterised by quick twitches directed contralaterally whereas ipsilateral twitches are slow and do not move beyond the midline. More extensive lesions will also affect inhibition of antagonists, abolishing ipsilateral saccades.^[5]^: 499

Clinical significance

Lesions of the medial pontine regions are relatively common. Due to the small size of the arteries in the area, the most common cause of a local lesion is an infarction due to lipohyalinosis and hypertension. Like other small arteries of the brain, these vessels are vulnerable to microemboli, especially those generated due to turbulence or low-flow states in those with artificial heart valves or arrhythmias, respectively.^[7] Unilateral lesions of the PPRF produce characteristic findings:

Loss of horizontal saccades directed towards the side of the lesion, no matter the current position of gaze
Contralateral gaze deviation (acute lesions, such as early stroke, only)
Gaze-evoked lateral nystagmus on looking away from the side of the lesion
Bilateral lesions produce horizontal gaze palsy and slowing of vertical saccades

Note

↑ These two cranial nerve nuclei in turn control the ipsilateral lateral rectus muscle, and contralateral medial rectus muscle, respectively - their silmuntaneous contraction will thus cause both eyes to move ipsilaterally (i.e. towards the side of the PPRF in question).

Related Research Articles

The abducens nerve or abducent nerve, also known as the sixth cranial nerve, cranial nerve VI, or simply CN VI, is a cranial nerve in humans and various other animals that controls the movement of the lateral rectus muscle, one of the extraocular muscles responsible for outward gaze. It is a somatic efferent nerve.

The vestibulo-ocular reflex (VOR) is a reflex that acts to stabilize gaze during head movement, with eye movement due to activation of the vestibular system, it is also known as the Cervico-ocular reflex. The reflex acts to stabilize images on the retinas of the eye during head movement. Gaze is held steadily on a location by producing eye movements in the direction opposite that of head movement. For example, when the head moves to the right, the eyes move to the left, meaning the image a person sees stays the same even though the head has turned. Since slight head movement is present all the time, VOR is necessary for stabilizing vision: people with an impaired reflex find it difficult to read using print, because the eyes do not stabilise during small head tremors, and also because damage to reflex can cause nystagmus.

<span class="mw-page-title-main">Medial longitudinal fasciculus</span> Nerve tracts in the brainstem

The medial longitudinal fasciculus (MLF) is a prominent bundle of nerve fibres which pass within the ventral/anterior portion of periaqueductal gray of the mesencephalon (midbrain). It contains the interstitial nucleus of Cajal, responsible for oculomotor control, head posture, and vertical eye movement.

Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms to fixate, inspect and track visual objects of interests. A special type of eye movement, rapid eye movement, occurs during REM sleep.

<span class="mw-page-title-main">Lateral rectus muscle</span> Muscle on lateral side of the eye

The lateral rectus muscle is a muscle on the lateral side of the eye in the orbit. It is one of six extraocular muscles that control the movements of the eye. The lateral rectus muscle is responsible for lateral movement of the eyeball, specifically abduction. Abduction describes the movement of the eye away from the midline, allowing the eyeball to move horizontally in the lateral direction, bringing the pupil away from the midline of the body.

<span class="mw-page-title-main">Abducens nucleus</span>

The abducens nucleus is the originating nucleus from which the abducens nerve (VI) emerges—a cranial nerve nucleus. This nucleus is located beneath the fourth ventricle in the caudal portion of the pons near the midline, medial to the sulcus limitans.

Internuclear ophthalmoplegia (INO) is a disorder of conjugate lateral gaze in which the affected eye shows impairment of adduction. When an attempt is made to gaze contralaterally, the affected eye adducts minimally, if at all. The contralateral eye abducts, however with nystagmus. Additionally, the divergence of the eyes leads to horizontal diplopia. That is if the right eye is affected the patient will "see double" when looking to the left, seeing two images side-by-side. Convergence is generally preserved.

<span class="mw-page-title-main">Sixth nerve palsy</span> Medical condition

Sixth nerve palsy, or abducens nerve palsy, is a disorder associated with dysfunction of cranial nerve VI, which is responsible for causing contraction of the lateral rectus muscle to abduct the eye. The inability of an eye to turn outward, results in a convergent strabismus or esotropia of which the primary symptom is diplopia in which the two images appear side-by-side. Thus, the diplopia is horizontal and worse in the distance. Diplopia is also increased on looking to the affected side and is partly caused by overaction of the medial rectus on the unaffected side as it tries to provide the extra innervation to the affected lateral rectus. These two muscles are synergists or "yoke muscles" as both attempt to move the eye over to the left or right. The condition is commonly unilateral but can also occur bilaterally.

The one and a half syndrome is a rare weakness in eye movement affecting both eyes, in which one cannot move laterally at all, and the other can move only in outward direction. More formally, it is characterized by "a conjugate horizontal gaze palsy in one direction and an internuclear ophthalmoplegia in the other". Nystagmus is also present when the eye on the opposite side of the lesion is abducted. Convergence is classically spared as cranial nerve III and its nucleus is spared bilaterally.

The frontopontine fibers or frontopontine tract are corticopontine fibers projecting from the cortex of the frontal lobe to the pons. In the internal capsule, the fibers descend predominately in the anterior limb, passing inferior to the thalamus to reach the mesencephalon (midbrain) where they descend in the medial portion of base of the cerebral peduncles. In the pons, the fibers flare out between the pontine nuclei.

The paramedian reticular nucleus sends its connections to the spinal cord in a mostly ipsilateral manner, although there is some decussation.

Foville's syndrome is caused by the blockage of the perforating branches of the basilar artery in the region of the brainstem known as the pons. It is most frequently caused by lesions such as vascular disease and tumors involving the dorsal pons.

Conjugate gaze palsies are neurological disorders affecting the ability to move both eyes in the same direction. These palsies can affect gaze in a horizontal, upward, or downward direction. These entities overlap with ophthalmoparesis and ophthalmoplegia.

<span class="mw-page-title-main">Rostral interstitial nucleus of medial longitudinal fasciculus</span>

The rostral interstitial nucleus of medial longitudinal fasciculus (riMLF) is a collection of neurons in the mesencephalon (midbrain) responsible for mediating vertical conjugate eye movements and vertical saccades. It mostly projects efferents to the ipsilateral oculomotor and trochlear nuclei.

A horizontal gaze palsy is a subtype of gaze palsy in which conjugate, horizontal eye movements are limited by neurologic deficits. Horizontal gaze palsies typically result from an ipsilateral pontine lesion or a contralateral frontal lobe lesion.

The term gaze is frequently used in physiology to describe coordinated motion of the eyes and neck. The lateral gaze is controlled by the paramedian pontine reticular formation (PPRF). The vertical gaze is controlled by the rostral interstitial nucleus of medial longitudinal fasciculus and the interstitial nucleus of Cajal.

The nucleus prepositus or nucleus prepositus hypoglossi is one of the largest of the three perihypoglossal nuclei. It is situated in the caudal pons and rostral medulla oblongata. It contributes to several aspects of gaze control including the horizontal gaze holding system.

Conjugate eye movement refers to motor coordination of the eyes that allows for bilateral fixation on a single object. A conjugate eye movement is a movement of both eyes in the same direction to maintain binocular gaze. This is in contrast to vergence eye movement, where binocular gaze is maintained by moving eyes in opposite directions, such as going “cross eyed” to view an object moving towards the face. Conjugate eye movements can be in any direction, and can accompany both saccadic eye movements and smooth pursuit eye movements.

In neuroanatomy, corticomesencephalic tract is a descending nerve tract that originates in the frontal eye field and terminate in the midbrain. Its fibers mediate conjugate eye movement.

The interstitial nucleus of Cajal is a collection of neurons in the mesencephalon (midbrain) which are involved in integrating eye position-velocity information in order to coordinate head-eye movements - especially those related to vertical and torsional conjugate eye movements (gaze). It also mediates vertical gaze holding.

References

1 2 3 4 5 6 Patestas, Maria A.; Gartner, Leslie P. (2016). A Textbook of Neuroanatomy (2nd ed.). Hoboken, New Jersey: Wiley-Blackwell. p. 310. ISBN 978-1-118-67746-9.
1 2 3 4 Brazis, Paul W.; Masdeu, Joseph C.; Biller, José (2022). Localization in Clinical Neurology (8th ed.). Philadelphia: Wolters Kluwer Health. ISBN 978-1-9751-6024-1.
1 2 Loftus, Brian D.; Athni, Sudhir S.; Cherches, Igor M. (2010), "Clinical Neuroanatomy", Neurology Secrets, Elsevier, p. 42, doi:10.1016/b978-0-323-05712-7.00002-7, ISBN 978-0-323-05712-7 , retrieved 2024-07-17
↑ Kiernan, John A.; Rajakumar, Nagalingam (2013). Barr's The Human Nervous System: An Anatomical Viewpoint (10th ed.). Philadelphia: Wolters Kluwer Lippincott Williams & Wilkins. p. 156. ISBN 978-1-4511-7327-7.
1 2 3 4 5 6 Leigh, R. John; Zee, David S. (1999). The Neurology of Eye Movements. Contemporary Neurology Series (3rd ed.). New York: Oxford University Press. ISBN 978-0-19-512972-4.
1 2 Sinnatamby, Chummy S. (2011). Last's Anatomy (12th ed.). p. 404. ISBN 978-0-7295-3752-0.
↑ Blumenfeld, Hal (2021). Neuroanatomy through Clinical Cases (3rd ed.). New York: Oxford University Press. p. 661. ISBN 978-1-60535-962-5.

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[3] These two cranial nerve nuclei in turn control the ipsilateral lateral rectus muscle, and contralateral medial rectus muscle, respectively - their silmuntaneous contraction will thus cause both eyes to move ipsilaterally (i.e. towards the side of the PPRF in question).

[:4-1] 1 2 3 4 5 6 Patestas, Maria A.; Gartner, Leslie P. (2016). A Textbook of Neuroanatomy (2nd ed.). Hoboken, New Jersey: Wiley-Blackwell. p. 310. ISBN 978-1-118-67746-9.

[:2-2] 1 2 3 4 Brazis, Paul W.; Masdeu, Joseph C.; Biller, José (2022). Localization in Clinical Neurology (8th ed.). Philadelphia: Wolters Kluwer Health. ISBN 978-1-9751-6024-1.

[:1-4] 1 2 Loftus, Brian D.; Athni, Sudhir S.; Cherches, Igor M. (2010), "Clinical Neuroanatomy", Neurology Secrets, Elsevier, p. 42, doi:10.1016/b978-0-323-05712-7.00002-7, ISBN 978-0-323-05712-7 , retrieved 2024-07-17

[5] Kiernan, John A.; Rajakumar, Nagalingam (2013). Barr's The Human Nervous System: An Anatomical Viewpoint (10th ed.). Philadelphia: Wolters Kluwer Lippincott Williams & Wilkins. p. 156. ISBN 978-1-4511-7327-7.

[:3-6] 1 2 3 4 5 6 Leigh, R. John; Zee, David S. (1999). The Neurology of Eye Movements. Contemporary Neurology Series (3rd ed.). New York: Oxford University Press. ISBN 978-0-19-512972-4.

[:022-7] 1 2 Sinnatamby, Chummy S. (2011). Last's Anatomy (12th ed.). p. 404. ISBN 978-0-7295-3752-0.

[8] Blumenfeld, Hal (2021). Neuroanatomy through Clinical Cases (3rd ed.). New York: Oxford University Press. p. 661. ISBN 978-1-60535-962-5.

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