Pretectum (cerebral region): location, functions and pathologies

He midbrain It is an essential part of the brain for the homeostasis of our organism and for our survival.

Inside it 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.

The tectum is located in the midbrain, a structure located in its posterior part, composed of two superior and two inferior colliculi.

The pretectum is located anterior to the superior colliculus and posterior to the thalamus. (relay center for sensory information that goes to the cerebral cortex), and on the periaqueductal gray matter and the nucleus of the posterior commissure.

Although they could not be clearly defined, the seven nuclei of the pretectum each receive a name with their respective region; The five primary nuclei are: the olivary pretectal nucleus, the optic tract nucleus, 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 these last two nuclei have not been studied in the same way as the five primary nuclei, the investigations carried out have shown that the two additional nuclei receive connections from the retina, suggesting that they may also play a role in processing visual information. To clarify this, we will now see what functions the pretectal nuclei perform.

functions

The pretectum is part of the subcortical visual system, and neurons in this structure respond to varying intensities of illumination.. Cells in 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 sleep REM.

1. The photomotor reflex

The photomotor reflex or pupillary light reflex occurs when the pupil of the eye responds to light stimuli., increasing or decreasing its diameter. This reflex is mediated by several of the nuclei of the pretectum, in particular the pretectal nucleus. olivary, 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 nucleus of Edinger-Westphal, which is responsible for transmitting nerve impulses and sending the signal to the pupillary sphincter, through the ciliary ganglion, for contraction to occur pupillary

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.

Neurons in the retina that are sensitive to direction and are located in the nucleus of the optic tract, provide information on the horizontal displacement errors in the retina through 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 when it is dark or in gloom.

It is in this way that the nucleus of the optic tract is capable of sending information about errors in 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 The eye returns to a central, forward-facing position after an object being tracked moves out of the field of view. vision.

3. Antinociception

The anterior pretectal nucleus participates in the active decrease of the perception of painful stimuli or antinociception. Although the mechanism by which the pretectum alters the organism's response to these stimuli is still unknown, the Research suggests that activity in the ventral anterior pretectal nucleus includes cholinergic neurons and serotonergic.

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 the perception of pain. Two of these regions that are known to project from the pretectum are the uncertain zone (a nucleus of the subthalamus) and the posterior nucleus of the thalamus.

Various studies have found that the dorsal anterior pretectal nucleus decreases more the perception of brief pain is significant, while its ventral part would do so in cases of pain chronic. 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 may 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 in albino rats, have shown that the pretectal nuclei that receive information from the retina, in in particular the nucleus of the optic tract and the posterior pretectal nucleus, are partly responsible for initiating eye movement sleep rapid.

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 the latter. hypothesis.

pretectal syndrome

Pretectal syndrome, also called Parinaud's syndrome or dorsal midbrain syndrome, refers to a group of clinical signs and symptoms that include alterations in ocular motility and whose main cause is the involvement of the pretectum and others brain areas adjacent.

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

• pupil changes: asymmetry of the size of the pupils, dissociated reaction to light, accommodative paresis and pupillary areflexia.

• vertical gaze palsy upwards (supranuclear type).

• eyelid retraction (Collier's sign).

• lid lag sign: when the upper eyelid cannot maintain its relative position with respect to the eyeball (when moving the eyes downwards).

• Convergence-withdrawal nystagmus: When the person tries to look up, the eyes return to their central position and the eyeballs retract.

Bibliographic 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.