Reticular formation
Encyclopedia
The reticular formation is a part of the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 that is involved in actions such as awaking/sleeping cycle, and filtering incoming stimuli to discriminate irrelevant background stimuli. It is essential for governing some of the basic functions of higher organisms, and is one of the phylogenetically
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...

 oldest portions of the brain.

Functions

The reticular formation consists of more than 100 small neural networks, with varied functions including the following:

1. Somatic motor
Motor neuron
In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...

 control - Some motor neurons send their axons to the reticular formation nuclei, giving rise to the reticulospinal tracts of the spinal cord. These tracts function in maintaining tone, balance, and posture--especially during body movements. The reticular formation also relays eye and ear signals to the cerebellum so that the cerebellum can integrate visual, auditory, and vestibular
Vestibular
The Vestibular is a competitive examination and is the primary and widespread system used by Brazilian universities to select their students. The Vestibular usually takes place from November to January, right before the start of school year in February or March, although certain universities hold...

 stimuli in motor coordination. Other motor nuclei include gaze centers, which enable the eyes to track and fixate objects, and central pattern generator
Central pattern generator
Central pattern generators are neural networks that produce rhythmic patterned outputs without sensory feedback. CPGs have been shown to produce rhythmic outputs resembling normal "rhythmic motor pattern production" even in isolation from motor and sensory feedback from limbs and other muscle...

s, which produce rhythmic signals to the muscles of breathing and swallowing.

2. Cardiovascular control - The reticular formation includes the cardiac and vasomotor
Vasomotor
Vasomotor refers to actions upon a blood vessel which alter its diameter. More specifically, it can refer to vasodilator action and vasoconstrictor action....

 centers of the medulla oblongata
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...

.

3. Pain modulation - The reticular formation is one means by which pain signals from the lower body reach the cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

. It is also the origin of the descending analgesic pathways. The nerve fibers in these pathways act in the spinal cord to block the transmission of some pain signals to the brain.

4. Sleep and consciousness - The reticular formation has projections to the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...

 and cerebral cortex that allow it to exert some control over which sensory signals reach the cerebrum and come to our conscious attention
Attention
Attention is the cognitive process of paying attention to one aspect of the environment while ignoring others. Attention is one of the most intensely studied topics within psychology and cognitive neuroscience....

. It plays a central role in states of consciousness like alertness
Alertness
Alertness is the state of paying close and continuous attention, being watchful and prompt to meet danger or emergency, or being quick to perceive and act. It is related to psychology as well as to physiology...

 and sleep
Sleep
Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...

. Injury to the reticular formation can result in irreversible coma
Coma
In medicine, a coma is a state of unconsciousness, lasting more than 6 hours in which a person cannot be awakened, fails to respond normally to painful stimuli, light or sound, lacks a normal sleep-wake cycle and does not initiate voluntary actions. A person in a state of coma is described as...

.

5. Habituation - This is a process in which the brain learns to ignore repetitive, meaningless stimuli while remaining sensitive to others. A good example of this is when a person can sleep through loud traffic in a large city, but is awakened promptly due to the sound of an alarm or crying baby. Reticular formation nuclei that modulate activity of the cerebral cortex are called the reticular activating system
Reticular activating system
The reticular activating system is an area of the brain responsible for regulating arousal and sleep-wake transitions.- History and Etymology :...

 or extrathalamic control modulatory system.

Pathology

Mass lesions in the brain stem
Brain stem
In vertebrate anatomy the brainstem is the posterior part of the brain, adjoining and structurally continuous with the spinal cord. The brain stem provides the main motor and sensory innervation to the face and neck via the cranial nerves...

 cause severe alterations in level of consciousness such as coma
Coma
In medicine, a coma is a state of unconsciousness, lasting more than 6 hours in which a person cannot be awakened, fails to respond normally to painful stimuli, light or sound, lacks a normal sleep-wake cycle and does not initiate voluntary actions. A person in a state of coma is described as...

 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
Post-polio syndrome
Post-polio syndrome is a condition that affects approximately 25–50% of people who have previously contracted poliomyelitis—a viral infection of the nervous system—after the initial infection. Typically the symptoms appear 15–30 years after recovery from the original paralytic attack, at an age of...

, and some imaging studies have shown abnormal activity in the area in people with chronic fatigue syndrome
Chronic fatigue syndrome
Chronic fatigue syndrome is the most common name used to designate a significantly debilitating medical disorder or group of disorders generally defined by persistent fatigue accompanied by other specific symptoms for a minimum of six months, not due to ongoing exertion, not substantially...

, 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
Neuron doctrine
The neuron doctrine is a descriptive term for the fundamental concept that the nervous system is made up of discrete individual cells, a discovery due to decisive neuro-anatomical work of Santiago Ramon y Cajal and later presented, among others, by H. Waldeyer-Hartz...

. Allan Hobson
Allan Hobson
John Allan Hobson, M.D. is an American psychiatrist and dream researcher.He is known for his research on Rapid eye movement sleep. He is Professor of Psychiatry, Emeritus, Harvard Medical School,...

 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
    Hypersomnia
    Hypersomnia is a disorder characterized by excessive amounts of sleepiness.There are two main categories of hypersomnia: primary hypersomnia and recurrent hypersomnia...

     in the cat brain. In contrast, lesioning of the more caudal portion of the reticular formation produces insomnia
    Insomnia
    Insomnia is most often defined by an individual's report of sleeping difficulties. While the term is sometimes used in sleep literature to describe a disorder demonstrated by polysomnographic evidence of disturbed sleep, insomnia is often defined as a positive response to either of two questions:...

     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
    Raphe nuclei
    The raphe nuclei are a moderate-size cluster of nuclei found in the brain stem. Their main function is to release serotonin to the rest of the brain...

     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
Lateral reticular formation
Moving caudally from the rostral midbrain, at the site of the rostral pons and the midbrain, the medial RF becomes less prominent, and the lateral RF becomes more prominent....

 are two columns of neuronal nuclei with ill-defined boundaries, which go up through the medulla
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...

 and into the mesencephalon
Mesencephalon
The midbrain or mesencephalon is a portion of the central nervous system associated with vision, hearing, motor control, sleep/wake, arousal , and temperature regulation....

 (midbrain). The nuclei can only be teased out by function, cell type, and projections of efferent or afferent nature.

See also

  • Raphe nuclei
    Raphe nuclei
    The raphe nuclei are a moderate-size cluster of nuclei found in the brain stem. Their main function is to release serotonin to the rest of the brain...

  • Locus coeruleus
  • Pedunculopontine nucleus
    Pedunculopontine nucleus
    The pedunculopontine nucleus is located in the brainstem, caudal to the substantia nigra and adjacent to the superior cerebellar peduncle. It has two divisions, one containing cholinergic neurons, the pars compacta, and one containing mostly glutamatergic neurons, the pars dissipata...

  • reticular activating system
    Reticular activating system
    The reticular activating system is an area of the brain responsible for regulating arousal and sleep-wake transitions.- History and Etymology :...

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