Reticular formation

Reticular formation

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

Location and relations

The reticular formation is a poorly-differentiated area of the brain stem, centered roughly in the pons. The reticular formation is the core of the brainstem running through the mid-brain, pons and medulla. 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 is an important regulator in the autonomic nervous system for such processes as respiration rate, heart rate and gastrointestinal activity. It also plays an important role in sleep and consciousness as well as modulation of pain. The reticular formation not only appears to 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 .

Pathology

Mass lesions in the brain stem cause severe alterations in level of consciousness such as coma due to their effects on the reticular formation. Bilateral damage to the reticular formation of the midbrain may lead to a coma or death.

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 is 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 that seems to be at the crux of basic neurological and behavioral functions of the human being. The cells lack clear ganglionic boundaries, but do have clear functional organizations and distinct cell types.

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

Structure

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 induces a hypersomnia in the cat brain. In contrast, lesioning of the more caudal portion of the reticular formation produces 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 the medulla and into the mesencephalon. The nuclei can only be teased out by function, cell type, and projections of efferent or afferent nature.

References

See also

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