Pretectum (brain region): location, functions and pathologies - psychology - 2023
- Pretectum: definition, location and structure
- 1. The photomotor reflex
- 2. Tracking eye movements
- 3. Antinociception
- 4. REM sleep
- Pretectal syndrome
The midbrain is an essential part of the brain for the homeostasis of our body 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 explain what the pretectum is, where it is located and how this brain region is structured. In addition, the main functions it performs are exposed, 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 brain region located in the midbrain, a structure that joins the brainstem with 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, the tectum is located, a structure located in its posterior part, composed of two upper and two lower colliculi. The pretectum is located in the anterior part of the superior colliculus and posterior to the thalamus (relay center for sensory information that is directed to the cerebral cortex), and on the periaqueductal gray matter and the nucleus of the posterior commissure.
Although they have not been able to clearly delimit, the seven nuclei of the pretectum each receive a name with their respective region; The five primary nuclei are: the olivar 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 limits.
Although these last 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 see what functions the pretectal nuclei perform.
The pretectum is part of the subcortical visual system and the neurons of this structure respond to variable intensities of illumination. 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 also participates in other processes such as the regulation of nociception (the coding and processing of potentially harmful or painful stimuli) or sleep. REM.
1. The photomotor reflex
The photomotor reflex or pupillary reflection of light occurs when the eye pupil responds to light stimuli, increasing or decreasing its diameter. This reflex is mediated by several of the pretectum nuclei, in particular the olivar pretectal nucleus, which receive information from 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 nucleus of Edinger-Westphal, which is responsible for transmitting nerve impulses and sending the signal to the pupillary sphincter, through the ganglion ciliary, so that pupillary contraction occurs.
2. Tracking eye movements
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 catch up with it after an unexpected change in direction or speed.
The neurons of the retina that are sensitive to direction and are located in the nucleus of the optic tract, provide information on horizontal displacement errors in the retina through the lower 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 when it is dark or dim.
It is in this way that the nucleus of the optic tract is able to send the information of errors in 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 view. .
The anterior pretectal nucleus participates in the active reduction of 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 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 regions of the brain that, through connections in the somatosensory cortex, regulate pain perception. Two of these regions that the pretectum is known to project are the uncertain zone (a nucleus of the subthalamus) and the posterior nucleus of the thalamus.
Several studies have found that the dorsal anterior pretectal nucleus decreases the perception of brief pain more significantly, while its ventral part would do so in cases of chronic pain. Due to its role in reducing chronic pain, it has been suggested that abnormal activity of this pretectal nucleus could be involved in central neuropathic pain syndrome.
4. REM sleep
When it comes to rapid eye movement sleep or REM sleep, Research suggests that multiple pretectal nuclei could be involved in the regulation of this type of sleep and other similar behaviors. It has been suggested that the pretectum, together with the superior colliculus, may be responsible for causing non-circadian disturbances in REM sleep-related behaviors.
Animal studies, specifically albino rats, have shown that the pretectal nuclei that receive information from the retina, in particular the nucleus of the optic tract and the posterior pretectal nucleus, are in part responsible for initiating rapid eye movement sleep.
The discovery of the existence of projections from the pretectum to various 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 this last hypothesis.
Pretectal syndrome, also called Parinaud syndrome or dorsal midbrain syndrome, refers to a set of clinical signs and symptoms that include ocular motility alterations and whose main cause is 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 alterations: pupil size asymmetry, dissociated reaction to light, accommodative paresis and pupillary areflexia.
Vertical gaze paralysis upward (supranuclear type).
Eyelid retraction (Collier's sign).
Lid lag sign: when the upper eyelid cannot maintain its position relative to the eyeball (by moving the eyes down).
Convergence-retraction nystagmus: When the person tries to look up, the eyes return to their central position and the eyeballs retract.