The midbrain is an essential part of the brain for our body’s homeostasis and for our survival.
Inside, we can locate 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 tell you what the pretectum is, where it is, and how this brain region is structured.. In addition, the main functions that it performs are described and an example of one of the most common disorders after injury to this area of the brain.
Pretectum: definition, location and structure
The pretectal area or pretectum is a region of the brain located in the midbrain, a structure that connects the brainstem to the diencephalon. This area is part of the subcortical visual system and has reciprocal connections with the retina. It consists of several highly interconnected cores.
In the midbrain is the tectum, a structure located at the back, made up of two upper collicles and two lower collicles. The pretectum is located in the anterior part of the superior and posterior colliculus (Center of relief of sensory information which goes to the cerebral cortex), and on the periaqueductal gray matter and the nucleus of the posterior commissure.
Although they cannot be clearly demarcated, the seven nuclei of the pretectum are each given a name with their respective region; the five primary nuclei are: the pretectal olive nucleus, the nucleus of the optic tract, the anterior nucleus, the medial nucleus and the posterior nucleus. In addition, two other additional nuclei were identified: the pretectal commissural area and the posterior borders.
While these latter two nuclei have not been studied in the same way as the five primary nuclei, research has shown that the additional two nuclei receive retinal connections, suggesting that they may also play a role in processing visual information. . To clarify this, we will see below what functions perform the pretectal kernels.
The pretectum is part of the subcortical visual system, and the neurons in this structure respond to varying light intensities.. Cells in the pretectal nuclei are primarily involved in mediating unconscious behavioral responses to acute changes in light.
In general, these responses include the onset 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 coding and processing of potentially stimuli. harmful or painful) or REM sleep.
1. The photomotor reflex
The photomotor or pupil light reflex occurs when the pupil of the eye responds to light stimuli, Increase or decrease its diameter. This reflection is mediated by several pretectal nuclei, in particular the olivar pretectal nucleus, which receive information about the light level of the ipsilateral retina through 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 nerve impulses and sending the signal to the brain. ganglion, for pupillary contraction to occur.
2. Eye movement tracking
The pretectal nuclei, and in particular the nucleus of the optic tract, are involved in the coordination of eye movements during slow eye tracking. These movements allow the eye to closely follow a moving object and move after an unexpected change in direction or speed.
Direction-sensitive retinal neurons located in the nucleus of the optic tractThey provide information on horizontal displacement errors of the retina using the inferior olive. In daylight, this information is detected and transmitted by neurons that have large receptive fields, while parafoveal neurons with small receptive fields do so while there is darkness or darkness.
This is how the core of the optical tube is able to send error information to the retina to guide eye movements. In addition to its role in maintaining these eye movements, the pretectum is activated during optokinetic nystagmus in which the eye returns to a central forward-facing position after the exit of an object followed by the field of view.
The anterior pretectal nucleus participates in the active decrease in 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 includes cholinergic and serotonergic neurons.
These neurons activate descending pathways that synchronize in the spinal cord and inhibit nociceptive cells in their dorsal horn. In addition to its direct antinociceptive mechanism, the anterior pretectal nucleus sends projections to regions of the brain which, through connections of the somatosensory cortex, regulate the perception of pain. Two of these regions known to project the pretectum are the uncertain zone (a nucleus of the subthalamus) and the posterior nucleus of the thalamus.
Several studies have shown that the anterior pretectal nucleus significantly reduces the perception of brief pain, while its ventral part does in chronic pain. Due to 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.
As for the sleep of rapid eye movements or REM sleep, research suggests that several pretectal nuclei may be involved in the regulation of this type of sleep and other similar behaviors. It has been suggested that the pretectum, as well as the superior colliculus, may be responsible for non-circadian changes in REM sleep behaviors.
Animal studies, especially albino rats, have shown that the pretectal nuclei that receive information from the retina, especially the optic tract nucleus and the posterior pretectal nucleus, are partly responsible for the rapid initiation of sleep from eye movements.
The discovery of the existence of projections from the pretectum to various thalamic nuclei involved in cortical activation during REM sleep, in particular in the suprachiasmatic nucleus which is part of a regulatory mechanism of this type of sleep, would support this last hypothesis. .
Pretectal syndrome, also known as Parinaud’s syndrome or dorsal mesencephalic syndrome, refers to a set of clinical signs and symptoms that include alterations in ocular motility and the main cause is involvement of the pretectum and other adjacent areas of the brain. .
This syndrome causes alterations in ocular motility, both external and internal. The most common signs are:
pupillary alterations: Asymmetry in the size of the pupil, dissociated reaction to light, accommodative paresis and pupillary areflexia.
Vertical gaze palsy upwards (supranuclear type).
Eyelid retraction (Collar sign).
Sign the cover: When the upper eyelid cannot maintain its relative position to the eyeball (when moving the eyes downward).
Convergence-retraction nystagmus: When the person tries to look up, the eyes return to the central position and the eyeballs retract.
Gamlin, PD (2006). The pretext: connections and roles linked to the oculomotor. Advances in Brain Research, 151, 379-405.
Keane, JR (1990). Pretectal syndrome: 206 patients. Neurology, 40 (4), 684-684.
Miller, AM, Miller, RB, Obermeyer, WH, Behan, M. and Benca, RM (1999). The pretext mediates the regulation of sleep by rapid eye movement through light. Behavioral Neuroscience, 113 (4), 755.