Paramedian pontine reticular formation
The paramedian pontine reticular formation is a subset of neurons of the oral and caudal pontine reticular nuclei. With the abducens nucleus it makes up the horizontal gaze centre. It is situated in the pons adjacent to the abducens nucleus. It projects to the ipsilateral abducens nucleus, and contralateral oculomotor (cranial nerve III) nucleus to mediate conjugate horizontal gaze and saccades.
Anatomy
The PPRF is situated in the pons just ventralmedial to the abducens nucleus. It is located anterior and lateral to the medial longitudinal fasciculus. It is continuous caudally with the nucleus prepositus hypoglossi.The PPRF are traversed by fibers projecting to the abducens nucleus that mediate smooth pursuit, vestibular reflexes, and gaze holding.
Afferents
The PPRF receives afferents from:- contralateral frontal eye field of the middle frontal gyrus of the frontal lobe (via frontopontine fibers The frontal eye field meanwhile receives afferents from the visual cortex.
- superior colliculus
- vestibular nuclei
- other parts of the reticular formation.
Efferents
The PPRF mediates horizontal conjugate gaze by projecting to both:- the ipsilateral abducens (CN VI) nucleus,
- the contralateral oculomotor (CN III) nucleus.
Function
The PPRF mediates horizontal conjugate eye movements. It is important in mediating saccadic eye movements. It is probably not involved in smooth pursuit.The PPRF generates excitatory bursts that are delivered to the ipsilateral abduecens nucleus to drive ipsilateral saccades.
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 vertical saccades.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.
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. 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
- Gaze-evoked lateral nystagmus on looking away from the side of the lesion
- Bilateral lesions produce horizontal gaze palsy and slowing of vertical saccades