Reticular Formation

The reticular formation is a complex network of brainstem nuclei and neurons that serve as a major integration and relay center for many vital brain systems to coordinate functions necessary for survival.

Structure

The reticular formation is made up of a net-like structure of various brainstem nuclei and neurons and covers an expansive portion of the brainstem. It begins in the mesencephalon, extending caudally through the medulla oblongata, and projects into the superior cervical spinal cord segments. It contains over 100 individual brainstem nuclei such as the red nucleus and the nucleus reticularis tegmenti pontis.

We can divide the retical formation into 2 parts presented in the pons and the medulla.

The lateral tegmental field contains mostly populations of interneurons, which is the major cell type presented throughout the entirety of the reticular formation. These interneurons influence many of the cranial nerve motor nuclei (trigeminal, facial, vagal, and hypoglossal), as well as form projections to various structures of the limbic system.

The medial tegmental field has the function of coordinating eye and head movements and integrating these movements with other somatosensory, vestibular, and proprioceptive stimuli through descending axonal tracts.

We can also devide it into 3 columns based on their neuronal structure and function. These three columns from medial to lateral are the raphe nuclei, the gigantocellular reticular nuclei and the parvocellular reticular nuclei.

• raphe nuclei

Located in the central ridge of the reticular formation. Play an important role in mood regulation and arousal through neurotransmission via serotonin and projections to the limbic regions.

• gigantocellular reticular nuclei

In the medial column. They contain larger neurons and coordinates motor movements.

• parvocellular nuclei

Reticular formation

They contain smaller neurons and are known to regulate respiratory function, specifically exhalation. They are in the most lateral part of the columns.

The Ascending tracts of the reticular formation

The ascending reticular activating system (ARAS) is the primary pathway that carries signals upward from the reticular formation to the thalamus and cerebral cortex, particularly the prefrontal cortices. This system is involved in regulating several vital functions, including:

- Alertness and Arousal: plays a central role in maintaining wakefulness and alertness.

- Consciousness: disruptions can potentially lead to coma or altered states of awareness.

- Sleep-Wake Cycles: The ARAS contributes to sleep regulation, particularly the transition between wakefulness and sleep, and is closely linked to circadian rhythms.

- Habituation: the ARAS allows the brain to filter out irrelevant, repetitive stimuli, thus focusing attention on more meaningful changes in the environment.

The ARAS is composed of a diverse set of neurons, including:

Dopaminergic, noradrenergic, serotonergic, histaminergic, cholinergic, and glutamatergic neurons. These neurons project to the thalamus and cortex, where they influence mood, attention, and cognitive functions. The lateral hypothalamus, which contains orexin neurons, serves as a major regulatory hub, coordinating alertness and the sleep-wake cycle. Damage to this region or the ARAS can lead to impaired consciousness and even coma. Severe bilateral damage, particularly at the midbrain level, may result in death.

The Descending tracts of the reticular formation

projecting to spinal cord motor neurons to control posture, movement, and autonomic functions. The reticulospinal tracts are divided into two main systems:

Lateral System: Involves the corticospinal and rubrospinal tracts, which are primarily responsible for modulating fine motor control and voluntary movements.

Medial System: Includes the reticulospinal and vestibulospinal tracts, which are vital for maintaining posture and coordinating body movements, especially during balance-related tasks. The medial system helps to integrate sensory information, including from the visual, auditory, vestibular, and proprioceptive systems.

The reticulospinal pathway divides further into:

Medial Pontine Reticulospinal Tract: This tract mainly controls extensor muscles, contributing to postural stability. Lateral Medullary Reticulospinal Tract: This tract helps to inhibit excitatory axial extensor muscles and regulate autonomic functions such as breathing.

Damage to the descending pathways, such as the reticulospinal or vestibulospinal tracts, can lead to various motor dysfunctions, including:

- Postural Instability and Ataxia: Difficulty in maintaining balance.

- Decerebrate Posturing: Caused by damage to the brainstem, where the arms and legs extend and rotate inward in response to pain, accompanied by hyperreflexia and increased muscle tone. This occurs due to disrupted signaling from the vestibular nuclei to the red nucleus.

- Decorticate Posturing: Damage above the red nucleus can lead to flexion of the arms towards the body and extension of the legs in response to pain.

- Spinal Shock: Damage below the medulla can result in flaccid paralysis, hypotonia, and loss of respiratory drive due to loss of tonic motor activity.

Used literature

• Incomplete citation of web. MANGOLD SA, – DAS JM,. StatPearls Publishing, ©2024. [cit. 14.12.2024]. <https://www.ncbi.nlm.nih.gov/books/NBK556102/>.

Mangold SA, Das JM. Neuroanatomy, Reticular Formation''. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK556102/