Reticular formation

Template:Infobox Brain Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

The reticular formation is a part of the brain which is involved in stereotypical actions, such as walking, sleeping, and lying down. It is essential for governing some of the basic functions of higher organisms, and phylogenetically one of the oldest portions of the brain.

Location and relations

It is a poorly-differentiated area of the brain stem, centered roughly in the pons. The ascending reticular activating system connects to areas in the thalamus, hypothalamus, and cortex, while the descending reticular activating system connects to the cerebellum and sensory nerves.

Functions

The reticular formation appears to not only control physical behaviors such as sleep, but also has been shown to play a major role in alertness, fatigue, and motivation to perform various activities. Some researchers have speculated that the reticular formation controls approximately 25 specific behaviors, including sleeping, walking, eating, urination, defecation, and sexual activity.

The reticular formation has also been traced as one of the sources for the introversion and extroversion character traits. Introverted people have been found to have a more easily stimulated reticular formation, resulting in a diminished desire to seek out stimulus. Extroverted people, however, have a less easily stimulated reticular formation, resulting in the need for more stimulation to maintain brain activity.

Pathology

Lesions in the reticular formation have been found in the brains of people who have post-polio syndrome, and some imaging studies have shown abnormal activity in the area in people with chronic fatigue syndrome, indicating a high likelihood that damage to the reticular formation is responsible for the fatigue experienced with these syndromes.

History and etymology

The term "reticular formation" was coined in the late 19th century, coinciding with Ramon y Cajal’s "neuron doctrine". Allan Hobson states in his book The Reticular Formation revisited that he thought the name was an etymological vestige from the fallen era of the aggregate field theory in the neural sciences. The term reticulum means a “netlike structure”, which is what the Reticular Formation appears to be at first glance. It has been described as being either too complex to study or an undifferentiated part of the brain with no organization at all. Eric Kandel even describes the reticular formation as being organized in a similar manner to the intermediate gray matter of the spinal cord. This chaotic, loose and intricate form of organization is what has turned off many researchers from looking farther into this mysterious area of the brain which seems to be at the crux of our basic neurological and behavioral functions. The cells lack clear ganglionic boundaries, but do have clear functional organizations and distinct cell types.

The term 'reticular formation' is seldom used anymore except to speak in generalities. Modern anatomy, or neuroscience articles usually refer to the individual nuclei which comprise the reticular formation.

Structure

Taken from "The Brainstem Reticular Formation and its Significance for Autonomic and Affective Behavior". The above diagram illustrates the reticular nuclei in the brainstem in a tiered fashion

The reticular formation has been functionally cleaved both sagittally and coronally.

The original functional differentiation was a division of caudal and rostral, this was based upon the observation that the lesioning of the rostral reticular formation induced a hypersomnia in the cat brain. Conversely, lesioning of the more caudal portion of the reticular formation produced insomnia in cats. This study has led to the idea that the caudal portion inhibits the rostral portion of the reticular formation.

Sagittal division reveals more morphological distinctions. The raphe nuclei form a ridge in the middle of the reticular formation and directly to its periphery there is a division called the medial reticular formation. The medial RF is large and has long ascending and descending fibers, and is surrounded by the lateral reticular formation. The lateral RF is close to the motor nuclei of the cranial nerves, and mostly mediates their function.

Medial and lateral reticular formation

The medial reticular formation and lateral reticular formation are two columns of neuronal nuclei with ill-defined boundaries which go up through from the medulla and into the mesencephalon. The nuclei can only be teased out by function, cell type and projections of efferent or afferent nature.

Additional images

Deep dissection of brain-stem. Ventral view.
The formatio reticularis of the medulla oblongata, shown by a transverse section passing through the middle of the olive.