RETICULAR FORMATION: FUNCTIONS, ANATOMY AND DISEASES - NEUROPSYCHOLOGY - 2024

The reticular formation is a set of neurons that extend from the spinal cord to the thalamus. This structure allows the body to wake up after a long sleep, and stay alert during the day.

The complex network of neurons of the reticular formation participates in the maintenance of arousal and consciousness (sleep-wake cycle). In addition, it intervenes in the filtering of irrelevant stimuli so that we can focus on the relevant ones.

Reticular activation system

The reticular formation is made up of more than 100 small neural networks that are spread unevenly throughout the brain stem and medulla. Its nuclei influence cardiovascular control and motor control, as well as modulation of pain, sleep and habituation.

For the correct performance of the named functions, this structure maintains connections with the medulla oblongata, the midbrain, the pons and the diencephalon. On the other hand, it connects directly or indirectly with all levels of the nervous system. His unique position allows him to participate in these essential functions.

Generally, when there is some type of pathology or damage to the reticular formation, drowsiness or coma occurs. The main diseases that are associated with reticular formation are characterized by problems in the level of alertness or muscle control. For example, narcolepsy, Parkinson's, schizophrenia, sleep disorders, or attention deficit hyperactivity disorder.

Where is the reticular formation located?

It is very difficult to visualize the exact location of the reticular formation, since it involves groups of neurons that are found in different parts of the brainstem and spinal cord. Furthermore, locating it is further complicated by its numerous connections to multiple brain areas.

The reticular formation is found in different areas such as:

Spinal cord

At this point the cells are not in a group, but are inside the spinal cord. Specifically in the intermediate area of ​​the medullary gray matter. In this area there are tracts called "reticulospinal", which are both in the anterior cord and in the lateral cord.

Most of these tracts transmit stimuli in a descending way (from the medulla to the rest of the body), although some also do it in an ascending way (from the organism towards the nuclei of the brainstem).

The brainstem

In the brainstem it is the main place where the reticular formation is located. Studies have shown that your organization is not random. That is, according to their connections or functions, they have characteristics that allow it to be divided into three groups of reticular nuclei that are explained later.

The hypothalamus

There appears to be an area of ​​neurons in the reticular formation called the uncertain zone. This is located between the subthalamic nucleus and the thalamus, and has numerous connections with the reticular nuclei of the brainstem. (Latarjet & Ruiz Liard, 2012).

Nuclei or parts of the reticular formation

The reticular formation has different nuclei of neurons according to their functions, connections and structures. Three are distinguished:

Median group of nuclei

Also called raphe nuclei, they are located in the medial column of the brain stem. It is the main place where serotonin is synthesized, which has a fundamental role in regulating mood.

In turn, they can be divided into the dark core of the raphe and the great core of the raphe.

Central group of nuclei

They are divided according to their structure into medial or gigantocellular nuclei (large cells) and posterolateral nuclei (made up of groups of small cells called parvocellular).

Lateral group of nuclei

They are integrated into the reticular formation because they have a very peculiar structure. These are the reticular, lateral and paramedian nuclei at the level of the bulb, and the reticular nucleus of the pontic tegmentum.

The lateral group of the reticular formation has connections mainly with the cerebellum.

Reticular formation and neurotransmitters

Different groups of cells that produce neurotransmitters reside in the reticular formation. These cells (neurons) have many connections throughout the central nervous system. In addition, they intervene in the regulation of the activity of the entire brain.

One of the most important dopamine production zones is the ventral tegmental area and the substantia nigra, which is in the reticular formation. While the locus coeruleus is the main area that originates noradrenergic neurons (which release and capture norepinephrine and adrenaline).

As for serotonin, the main nucleus that secretes it is the nucleus of the raphe. It is located in the midline of the brainstem, in the reticular formation.

On the other hand, acetylcholine is produced in the midbrain of the reticular formation, specifically in the pedunculopontine and laterodorsal tegmental nuclei.

These neurotransmitters are produced in these areas and are then transmitted to the central nervous system to regulate sensory perception, motor activity, and other behaviors.

Features

The reticular formation has a great variety of basic functions, since from a phylogenetic point of view it is one of the oldest areas of the brain. Modulates the level of consciousness, sleep, pain, muscle control, etc.

Its functions are explained in more detail below:

Regulation of alertness

Reticular formation greatly influences arousal and consciousness. When we sleep, the level of consciousness is suppressed.

The reticular formation receives a multitude of fibers from sensory tracts, and sends these signals to the cerebral cortex. In this way, it allows us to be awake. Greater activity of the reticular formation translates into a more intense state of alertness.

This function is carried out through the activation lattice system (SAR), which is also known as the ascending excitation system. It plays an important role in attention and motivation. In this system, thoughts, internal sensations and external influences converge.

Information is transmitted through neurotransmitters such as acetylcholine and norepinephrine.

Injuries to the reticular activation system can seriously compromise consciousness. Severe damage to this area can lead to coma or a persistent vegetative state.

Postural control

There are descending projections from the reticular formation to certain motor neurons. This can facilitate or inhibit muscle movements. The main fibers responsible for motor control are found, above all, in the reticulospinal tract.

In addition, the reticular formation transmits visual, auditory, and vestibular signals to the cerebellum for integration into motor coordination.

This is essential in maintaining balance and posture. For example, it helps us to stand, stereotypical movements such as walking, and control of muscle tone.

Control of facial movements

Cranial nerves

The reticular formation establishes circuits with motor nuclei of the cranial nerves. In this way, they modulate the movements of the face and head.

This area contributes to orofacial motor responses, coordinating the activity of the trigeminal, facial and hypoglossal nerves. As a result, it allows us to carry out correct movements of the jaw, lips and tongue, to be able to chew and eat.

On the other hand, this structure also controls the functioning of the facial muscles that facilitate emotional expressions. Thus, we can make the correct movements to express emotions such as laughter or crying.

As it is found bilaterally in the brain, it provides motor control to both sides of the face in a symmetrical manner. It also allows coordination of eye movements.

Regulation of autonomic functions

The reticular formation exerts motor control of certain autonomic functions. For example, the functions of the visceral organs.

Neurons of the reticular formation contribute to motor activity related to the vagus nerve. Thanks to this activity, an adequate functioning of the gastrointestinal system, respiratory system and cardiovascular functions is achieved.

Therefore, the reticular formation is involved in swallowing or vomiting. As in sneezing, coughing, or breathing rhythm. While, in the cardiovascular plane, the reticular formation would maintain an ideal blood pressure.

Pain modulation

Through the reticular formation, pain signals are sent from the lower body to the cerebral cortex.

It is also the origin of the descending analgesic pathways. The nerve fibers in this area work on the spinal cord to block pain signals that reach the brain.

This is important because it allows us to alleviate pain in certain situations, for example during a very stressful or traumatic situation (gate theory). Pain has been seen to be suppressed if certain drugs are injected into these pathways or destroyed.

Habituation

It is a process by which the brain learns to ignore repetitive stimuli, which it considers irrelevant at the time. At the same time, it maintains sensitivity to stimuli of interest. Habituation is achieved through the aforementioned activation reticular system (SAR).

Impact on the endocrine system

The reticular formation indirectly regulates the endocrine nervous system, as it acts on the hypothalamus for hormonal release. This influences somatic modulation and visceral sensations. This is essential in regulating the perception of pain.

Reticular formation diseases

As the reticular formation is located in the back of the brain, it appears to be more vulnerable to injury or damage. Normally, when the reticular formation is affected, the patient goes into a coma. If the injury is bilateral and massive, it can lead to death.

Although also, the reticular formation can be affected by viruses, tumors, hernias, metabolic disorders, inflammation, poisonings, etc.

The most typical symptoms when there are problems in the reticular formation are drowsiness, stupor, alterations in breathing and heart rate.

Problems in sleep, wakefulness and level of consciousness

The reticular activating system (SAR) of the reticular formation is important in the level of alertness or arousal of the person. It seems that with age there is a general decrease in the activity of this system.

Therefore, it seems that when there is a malfunction in the reticular formation, it is possible that problems occur in the sleep and wake cycles, as well as in the level of consciousness.

For example, the reticular activator system sends signals to activate or block different areas of the cerebral cortex depending on whether it considers that new stimuli or familiar ones appear. This is important to know which elements to attend to and which to ignore.

In this way, some models that attempt to explain the origin of attention deficit hyperactivity disorder, affirm that this system could be insufficiently developed in these patients.

Problems in psychiatric illnesses

García-Rill (1997), affirms that there may be failures in the reticular activation system in neurological and psychiatric diseases such as Parkinson's disease, schizophrenia, post-traumatic stress disorder, REM sleep disorder, and narcolepsy.

It has been found in post-mortem studies in patients with Parkinson's disease, a degeneration of the pontine peduncle nucleus.

This area consists of a set of neurons that form the reticular formation. These are neurons that have many connections with structures involved in movement, such as the basal ganglia.

In Parkinson's disease there appears to be a significant decrease in the number of neurons that make up the locus coeruleus. This produces a disinhibition of the pontine peduncle nucleus, which also occurs in post-traumatic stress disorder and REM sleep disorder.

For this reason, there are authors who propose deep brain stimulation of the pedunculopontic nucleus of the reticular formation to treat Parkinson's disease.

Regarding schizophrenia, it has been observed that in some patients there is a significant increase in neurons in the pedunculopontine nucleus.

Regarding narcolepsy, there is excessive daytime sleepiness, which can be associated with damage to the nuclei of the reticular formation.

Cataplexy

On the other hand, cataplexy or cataplexy, which are sudden episodes of loss of muscle tone when awake, is associated with alterations in cells of the reticular formation. Specifically in the cells of the magnocellular nucleus, which regulate muscle relaxation in REM sleep.

Chronic Fatigue Syndrome

Furthermore, abnormal activity in reticular formation has been found in some investigations in patients with chronic fatigue syndrome.

References

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