The paramedian pontine reticular formation (PPRF) is a subset of neurons of the

oral The word oral may refer to: Relating to the mouth * Relating to the mouth, the first portion of the alimentary canal that primarily receives food and liquid **Oral administration of medicines ** Oral examination (also known as an oral exam or ora ...

and caudal pontine reticular nuclei. With the abducens nucleus it makes up the horizontal gaze centre. It is situated in the

pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other mammals, lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of ...

adjacent to the abducens nucleus. It projects to the ipsilateral abducens (cranial nerve VI) nucleus, and contralateral oculomotor (cranial nerve III) nucleus to mediate conjugate horizontal gaze and

saccade In vision science, a saccade ( ; ; ) is a quick, simultaneous movement of both Eye movement (sensory), eyes between two or more phases of focal points in the same direction. In contrast, in Smooth pursuit, smooth-pursuit movements, the eyes mov ...

Anatomy

The PPRF is situated in the

just ventralmedial to the abducens nucleus. It is located

anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

and lateral to the

medial longitudinal fasciculus 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 ...

. It is continuous caudally with the nucleus prepositus hypoglossi. 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.

Afferents

The PPRF receives afferents from: * contralateral frontal eye field of the

middle frontal gyrus The middle frontal gyrus makes up about one-third of the frontal lobe of the human brain. (A gyrus is one of the prominent "bumps" or "ridges" on the cerebral cortex The cerebral cortex, also known as the cerebral mantle, is the outer la ...

of the

frontal lobe The frontal lobe is the largest of the four major lobes of the brain in mammals, and is located at the front of each cerebral hemisphere (in front of the parietal lobe and the temporal lobe). It is parted from the parietal lobe by a Sulcus (neur ...

(via frontopontine fibers The frontal eye field meanwhile receives afferents from the

visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalam ...

. *

superior colliculus In neuroanatomy, the superior colliculus () is a structure lying on the tectum, roof of the mammalian midbrain. In non-mammalian vertebrates, the Homology (biology), homologous structure is known as the optic tectum or optic lobe. The adjective f ...

vestibular nuclei The vestibular nuclei (VN) are the cranial nuclei for the vestibular nerve located in the brainstem. In Terminologia Anatomica, they are grouped in both the pons and the medulla in the brainstem The brainstem (or brain stem) is the poste ...

* other parts of the reticular formation.

Efferents

The PPRF mediates horizontal conjugate gaze (i.e. simultaneous horizontal movement of both eyes) by projecting to both: * the ipsilateral abducens (CN VI) nucleus (which controls the ipsilateral

lateral rectus muscle 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, specif ...

), * (through the

) 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 The cerebellum (: cerebella or cerebellums; Latin for 'little brain') is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as it or eve ...

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 (inhibitory saccadic stimuli are meanwhile delivered to the abducens nucleus from the contralateral medulla oblongata).

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 (though slowing of all saccades may also be accounted for by destruction of adjacent omnipause neurons of the interposited raphe nucleus). 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

Lesion A lesion is any damage or abnormal change in the tissue of an organism, usually caused by injury or diseases. The term ''Lesion'' is derived from the Latin meaning "injury". Lesions may occur in both plants and animals. Types There is no de ...

s 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 Hypertension, also known as high blood pressure, is a Chronic condition, long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High blood pressure usually does not cause symptoms i ...

. Like other small arteries of the brain, these vessels are vulnerable to microemboli, especially those generated due to

turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...

or low-flow states in those with artificial heart valves or

arrhythmia Arrhythmias, also known as cardiac arrhythmias, are irregularities in the cardiac cycle, heartbeat, including when it is too fast or too slow. Essentially, this is anything but normal sinus rhythm. A resting heart rate that is too fast – ab ...

s, 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 (acute lesions, such as early

stroke Stroke is a medical condition in which poor cerebral circulation, blood flow to a part of the brain causes cell death. There are two main types of stroke: brain ischemia, ischemic, due to lack of blood flow, and intracranial hemorrhage, hemor ...

, 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

References

{{Visual system Pons