The Reticular formation Is a set of neurons that extend from the spinal cord to the thalamus. The arrangement of the reticular formation resembles a lattice (reticular comes from the Latin"rete"which means"lattice").
This structure allows the body to wake up after prolonged sleep, and stay alert throughout the day. Thus, the complex network of Neurons Of the reticular formation participates in the maintenance of the excitation and consciousness (sleep cycle and wakefulness).
In addition, it intervenes in the filtering of irrelevant stimuli so that we can focus on the relevant ones.
The reticular formation is constituted by more than 100 small neural networks that are spread by the Brain stem And marrow in an uneven manner.
Their nuclei influence cardiovascular control and motor control. As well as the modulation of pain, sleep and habituation.
For the correct performance of the named functions, this structure maintains connections with the Medulla bulb , The midbrain, the pons and the Diencephalon .
On the other hand, it connects directly or indirectly with all levels of the nervous system. Its particular position allows you to participate in these essential functions.
Generally, somnolence or coma occurs when there is some type of pathology or damage in the reticular formation.
The main diseases associated with reticular formation are characterized by problems in the level of alertness or muscle control. For example, Narcolepsy , he Parkinson's , the schizophrenia , sleep disorders , or the Attention deficit and hyperactivity .
Where is the reticular formation?
It is very difficult to visualize the exact location of the reticular formation, since these are groups of neurons that are found in different parts of the brainstem and spinal cord. In addition, locating it is further complicated by its numerous connections with multiple brain areas.
The reticular formation is found in different areas such as:
Spinal cord
At this point the cells are not in the group, but are in the interior of the spinal cord. Specifically in the middle area of the medullary gray matter.
In this area there are so-called"reticulospinal"tracts, which are in both the anterior cord and the lateral cord.
Most of these tracts transmit the stimuli downward (from the marrow to the rest of the body). Although some also do so in an ascending way (from the organism to 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 given at random. That is, according to their connections or functions, they present characteristics that allow to divide it into three groups of reticular nuclei that are explained later.
The hypothalamus
There appears to be an area of reticular formation neurons called the uncertain zone. It lies 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
As mentioned, the reticular formation has different nuclei of neurons according to the functions, connections and structures of these. Three are distinguished:
Medium group of nuclei
Also called raphe nuclei, they are located in the medial spine of the brain stem. It is the main place in which the Serotonin Which plays a fundamental role in the regulation of mood.
In turn, they can be divided into the dark core of the rafe and the grand nucleus of the raphe.
Central core group
They are divided according to their structure into medial or gigantocellular nuclei (of large cells) and posterolateral nuclei (constituted by 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 height of the bulb. And the reticular nucleus of the pontic tegmento.
The lateral group of the reticular formation has connections mainly with the cerebellum .
Reticular formation and neurotransmitters
In the reticular formation reside different groups of cells that produce neurotransmitters. These cells (neurons) have many connections throughout the central nervous system. In addition, they intervene in the regulation of the activity of the whole brain .
One of the most important areas of dopamine production is the ventral tegmental area and the substantia nigra, which is in the reticular formation. While the locus coeruleus is the main zone that originates noradrenergic neurons (releasing and capturing Noradrenaline Y adrenalin ).
As for serotonin, the main nucleus that secretes it is the nucleus of raphe. It is located in the midline of the brainstem, in the reticular formation.
On the other hand, acetylcholine is produced in the middle brain of the reticular formation. Specifically, in the laterodorsal pedunculopontine and tegmental nuclei.
These neurotransmitters are produced in these areas and then transmitted to the central nervous system to regulate sensory perception, motor activity and other behaviors.
Functions
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. It modulates the level of consciousness, sleep, pain, muscle control, etc.
The following explains their functions in more detail:
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. Higher activity of the reticular formation results in a more intense alertness.
This function is carried out through the lattice activation system (SAR), which is also known as the excitation up system. It plays an important role in attention and motivation. In this system converge thoughts, internal sensations and external influences.
The information is transmitted through neurotransmitters such as Acetylcholine and the Noradrenaline .
Injuries to the reticular activation system can seriously endanger the consciousness. Severe damage in this area can lead to a coma or persistent vegetative state.
Postural control
There are downward projections from the reticular formation to Certain motor neurons . This can facilitate or inhibit muscle movements. The main fibers that are in charge of the motor control are, mainly, in the reticulo-spinal tract.
In addition, the reticular formation transmits visual, auditory and vestibular signals to the cerebellum so that they are integrated in the motor coordination.
This is critical in maintaining balance and posture. For example, it helps us to stand, stereotyped movements such as walking, and control of muscle tone.
Control of facial movements
The reticular formation establishes circuits with motor nuclei of the cranial nerves. In that way, they modulate the movements of the face and the head.
This area contributes to the orofacial motor responses, coordinating the activity of the trigeminal, facial and hypoglossal nerves. As a result, it allows us to perform correct movements of the jaw, lips and tongue, 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 right moves to express emotions such as laughter or crying.
As it is found bilaterally in the brain, it provides motor control on both sides of the face symmetrically. 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.
The 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 in the respiratory rhythm. Whereas, in the cardiovascular plane, the reticular formation would maintain a suitable arterial pressure.
Modulation of pain
Signs of pain from the lower part of the body are sent through the reticular formation to the cerebral cortex .
It is also the origin of the descending analgesia pathways. The nerve fibers in this area act in the spinal cord to block the 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 ). It has been seen that pain is 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 moment. At the same time it maintains sensitivity to the stimuli of interest. The habituation is achieved through said reticular activation system (SAR).
Impact on the endocrine system
The reticular formation regulates indirectly the endocrine nervous system, because it acts on the Hypothalamus For hormonal release. This influences somatic modulation and visceral sensations. This is fundamental in the regulation of pain perception.
Diseases of reticular formation
As the reticular formation is located in the back of the brain, it appears to be more vulnerable to any injury or damage. Usually when there is an involvement of the reticular formation the patient enters 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, intoxications, etc.
The most typical symptoms when there are problems in the reticular formation are drowsiness, stupor, alterations in breathing and in the heart rate.
The reticular activator 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 appears that when there is a malfunction in the reticular formation, it is possible that problems occur in the sleep and wake cycles. Just as on the level of consciousness.
For example, the reticular activator system sends signals to activate or block different areas of the cerebral cortex as new stimuli or familiar stimuli appear. This is important in order to know what elements to attend to and which to ignore.
Thus, some models that attempt to explain the origin of the attention deficit disorder and hyperactivity, affirm that this system could be insufficiently developed in these patients.
In fact, García-Rill (1997) states that there may be faults in the reticular activation system in neurological and psychiatric diseases such as Parkinson's disease , schizophrenia , he Posttraumatic stress disorder , REM sleep disorder, and Narcolepsy .
It has been found in post mortem studies performed in patients suffering from Parkinson's disease, a degeneration in the pontine peduncle nucleus.
This area consists of a set of neurons that form the reticular formation. These are neurons that have many connections to structures involved in movement, such as Basal ganglia .
In Parkinson's disease there seems 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.
Therefore, there are authors who propose deep brain stimulation of the pedunculoponic nucleus of the reticular formation to treat Parkinson's disease.
As for schizophrenia, it has been observed that in some patients there is a significant increase of neurons in the pedunculopontine nucleus.
Regarding narcolepsy, there is excessive daytime sleepiness, which may be associated with damage to nuclei of the reticular formation.
On the other hand, Cataplexy Or cataplexy, which are sudden episodes of loss of muscle tone when awake, is linked to alterations in cells of the reticular formation. Specifically in the cells of the magnocellular nucleus, which regulate muscle relaxation in the REM sleep .
In addition, abnormal activity in reticular formation has been found in some studies in patients with chronic fatigue syndrome.
References
- Ávila Álvarez, A.M., et al. (2013). Core pontine peduncle, and its relationship to the pathophysiology of Parkinson's Disease. Acta Neurológica Colombiana 29 (3): 180-190.
- Reticular Formation. (S.f.). Retrieved on January 28, 2017, from Boundless: boundless.com.
- García-Porrero, J.A., Hurlé, J.M., & Polo, L.E. (2015). Human neuroanatomy. Buenos Aires: Medical Editorial Panamericana.
- Garcia-Rill, E. (1997) Disorders of the reticular activating system; Med. Hypotheses, 49 (5): 379-387.
- Reticular Formation. (S.f.). Retrieved on January 28, 2017, from KENHUB: kenhub.com.
- Latarjet, M., & Liard, A. R. (2012). Human anatomy (Vol. 1). Buenos Aires: Medical Editorial Panamericana.
- López, L. P., Pérez, S. M., & de la Torre, M. M. (2008). Neuroanatomy. Buenos Aires: Medical Editorial Panamericana.
- Know your brain: Reticular formation. (25 July 2015). Obtained from Neuroscientifically Challenged: neuroscientificallychallenged.com.