Pretectum (cerebral region): location, functions and pathologies.

This region of the midbrain is linked to the sense of vision.

The midbrain is a part of the brain essential for the homeostasis of our organism and for our survival.

Inside it we can find the pretectum, an area involved in unconscious visual processing and related to automatic processes such as the oculomotor reflex or REM sleep.

In this article we explain what the pretectum is, where it is located and how this brain region is structured.. In addition, the main functions it performs and an example of one of the most common disorders following injury to this area of the brain are presented.

Pretectum: definition, location and structure

The pretectal area or pretectum is a brain region located in the midbrain, a structure that links the brainstem to the diencephalon. This area is part of the subcortical visual system and has reciprocal connections with the retina. It is composed of several highly interconnected nuclei.

In the midbrain is located the tectum, a structure located in its posterior part, composed of two superior and two inferior colliculi. The pretectum is located in the anterior part of the superior colliculus and posterior to the thalamus (information relay center). (relay center of sensory information to the cerebral cortex), and over the periaqueductal gray matter and the nucleus of the posterior commissure.

Although it has not been possible to clearly delimit the seven nuclei of the pretectum, the seven nuclei of the pretectum are each named after their respective region; the five primary nuclei are the first and second nuclei of the pretectum.The five primary nuclei are: the olivary pretectal nucleus, the nucleus of the optic tract, the anterior nucleus, the medial nucleus and the posterior nucleus. In addition, two additional nuclei have been identified: the pretectal commissural area and the posterior limitans.

Although the latter two nuclei have not been studied in the same way as the five primary nuclei, research has shown that the two additional nuclei receive connections from the retina, suggesting that they also play a role in the processing of visual information. To clarify this, we will now look at the functions of the pretectal nuclei.

Functions

The pretectum is part of the subcortical visual system, and the neurons in this structure respond to varying intensities of illumination. The cells of the pretectal nuclei are primarily involved in mediating unconscious behavioral responses to acute changes in light.

In general, these responses include the initiation of certain optokinetic reflexes, although, as we will see later, the pretectum is also involved in other processes such as the regulation of nociception (the encoding and processing of potentially harmful or painful stimuli) or REM sleep.

1. The photomotor reflex

The photomotor reflex or pupillary light reflex occurs when the pupil of the eye responds to light stimuli by increasing or decreasing in diameter.The pupil of the eye responds to light stimuli by increasing or decreasing its diameter. This reflex is mediated by several of the nuclei of the pretectum, in particular the olivary pretectal nucleus, which receive light level information from the ipsilateral retina via the optic tract.

The pretectal nuclei gradually increase their activation in response to increasing levels of illumination, and this information is transmitted directly to the Edinger-Westphal nucleus, which is responsible for transmitting the nerve impulses and sending the signal to the pupillary sphincter, via the ciliary ganglion, for pupillary contraction to occur.

2. Tracking eye movements

The pretectal nuclei, and in particular the optic tract nucleus, are involved in the coordination of eye movements during slow eye tracking. These movements allow the eye to follow a moving object closely and catch up after an unexpected change in direction or speed.

Neurons in the retina that are sensitive to direction and are located in the nucleus of the optic tractThe retinal neurons, which are direction sensitive and located in the nucleus of the optic tract, provide horizontal displacement error information to the retina via the inferior olive. In daylight, this information is detected and transmitted by neurons possessing large receptive fields, while parafoveal neurons with small receptive fields do so in darkness or half-light.

It is in this way that the nucleus of the optic tract is able to send error information to the retina to guide eye movements. In addition to its role in maintaining such eye movements, the pretectum is activated during optokinetic nystagmus in which the eye returns to a central, forward-facing position after an object that was being tracked moves out of the field of vision.

3. Antinocyception

The anterior pretectal nucleus is involved in actively decreasing the perception of painful stimuli or antinociception.. Although the mechanism by which the pretectum alters the body's response to these stimuli is still unknown, research suggests that the activity of the ventral anterior pretectal nucleus involves cholinergic and serotonergic neurons.

These neurons activate descending pathways that synapse in the spinal cord and inhibit nociceptive cells in its dorsal horn. In addition to its direct antinociceptive mechanism, the anterior pretectal nucleus sends projections to brain regions that, through connections from the somatosensory cortex, regulate Pain perception. Two of these regions known to be projected by the pretectum are the zona incerta (a nucleus of the subthalamus) and the posterior nucleus of the thalamus.

Several studies have found that the dorsal anterior pretectal nucleus most significantly decreases the perception of brief pain, while its ventral part would do so in cases of chronic pain. Because of its role in reducing chronic pain, it has been suggested that abnormal activity of this pretectal nucleus may be involved in central neuropathic pain syndrome.

4. REM sleep

With regard to rapid eye movement sleep, or REM sleep, research suggests that multiple nuclei of the pretectal nucleus may be involved in central neuropathic pain syndrome, research suggests that multiple pretectal nuclei may be involved in the regulation of this type of sleep and other similar behaviors. and other similar behaviors. It has been suggested that the pretectum, along with the superior colliculus, may be responsible for causing non-circadian alterations in REM sleep-related behaviors.

Animal studies, specifically in albino rats, have shown that the pretectal nuclei that receive information from the retina, in particular the optic tract nucleus and the posterior pretectal nucleus, are partly responsible for initiating rapid eye movement sleep.

The discovery of the existence of projections from the pretectum to several thalamic nuclei involved in cortical activation during REM sleep, specifically to the suprachiasmatic nucleus which is part of a regulatory mechanism of this type of sleep, would support the latter hypothesis.

The pretectal syndrome

The pretectal syndrome, also called Parinaud's syndrome or dorsal mesencephalic syndrome, refers to a set of clinical signs and symptoms that include alterations in ocular motility and whose main cause is the involvement of the pretectum and other adjacent brain areas.

This syndrome causes alterations in ocular motility, both externally and internally. The most common signs are the following:

• Pupillary alterationsPupillary alterations: asymmetry of pupil size, dissociated reaction to light, accommodative paresis and pupillary areflexia.

• Paralysis of vertical gaze upward (supranuclear type).

• Retraction of the eyelid (Collier's sign).

• Lid lag signwhen the upper eyelid cannot maintain its position relative to the eyeball (when moving the eyes downward).

• Convergence-retraction nystagmus: when the person tries to look upward, the eyes return to their central position and the eyeballs retract.

Bibliographical references:

• Gamlin, P. D. (2006). The pretectum: connections and oculomotor-related roles. Progress in brain research, 151, 379-405.

• Keane, J. R. (1990). The pretectal syndrome: 206 patients. Neurology, 40(4), 684-684.

• Miller, A. M., Miller, R. B., Obermeyer, W. H., Behan, M., & Benca, R. M. (1999). The pretectum mediates rapid eye movement sleep regulation by light. Behavioral neuroscience, 113(4), 755.