The
reticular formation is a part of the
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
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
phylogeneticallyIn biology, phylogenetics is the study of evolutionary relatedness among various groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
oldest portions of the brain.
Location and relations
The reticular formation is a poorly-differentiated area of the
brain stemThe brainstem is the lower 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...
, centered roughly in the
ponsThe pons is a structure located on the brain stem. It is cranial to the medulla oblongata, caudal to the midbrain, and ventral to the cerebellum...
. The reticular formation is the core of the brainstem running through the mid-brain, pons and medulla. The ascending
reticular activating systemThe reticular activating system is the name given to the part of the brain believed to be the center of arousal and motivation in mammals .-Functions:...
connects to areas in the
thalamusThe thalamus is a midline paired symmetrical structure within the brain of vertebrate animals, including humans. It is between the cerebral cortex and the midbrain, both in terms of its location and its neurological connections...
,
hypothalamusThe hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland ....
, and
cortexThe cerebral cortex is a structure within the brain that plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It constitutes the outermost layer of the cerebrum. In preserved brains, it has a grey color, hence the name "grey matter"...
, while the descending reticular activating system connects to the
cerebellumThe cerebellum is a region of the brain that plays an important role in the integration of sensory perception, coordination and motor control...
and
sensory nerveSensory nerves are nerves that receive sensory stimuli, such as how something feels and if it is painful.They are made up of nerve fibers, called sensory fibers .Sensory neurons are neurons that are activated by sensory input ,...
s.
Functions
The reticular formation controls approximately 25 specific behaviors, including sleeping,
walkingWalking is the main form of animal locomotion on land, distinguished from running and crawling. When carried out in shallow waters, it is usually described as wading and when performed over a steeply rising object or an obstacle it becomes scrambling or climbing...
,
eatingEating is the process of ingesting food to provide for an animal's nutritional needs, particularly for energy and growth. All animals must eat organisms in order to survive: carnivores eat other animals, herbivores eat plants, and omnivores consume a mixture of both; see feeding.-Eating...
,
urinationUrination, also known as micturition, voiding, peeing, and more rarely, emiction, is the process of disposing of urine from the urinary bladder through the urethra to the outside of the body. In healthy humans, the process of urination is under voluntary control; in infants and individuals with...
,
defecationDefecation is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material from the digestive tract via the anus. Humans usually defecate from three times a week, up to three times a day...
, and sexual activity .
Pathology
Mass lesions in the
brain stemThe brainstem is the lower 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 consciousnessLevel of consciousness is a measurement of a person's arousability and responsiveness to stimuli from the environment. A mildly depressed level of consciousness may be classed as lethargy; someone in this state can be aroused with little difficulty. People who are obtunded have a more depressed...
such as
comaIn medicine, a coma is a profound state of unconsciousness. A comatose person cannot be awakened, fails to respond normally to pain or light, does not have sleep-wake cycles, and does not take voluntary actions....
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 syndromePost-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 recovery from the initial paralytic attack. Typically the symptoms appear 15-30 years after the original infection, at an age...
, and some imaging studies have shown abnormal activity in the area in people with
chronic fatigue syndromeChronic fatigue syndrome is the most common name given to a variably debilitating disorder or disorders characterized by persistent fatigue unrelated to exertion and not substantially relieved by rest...
, 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 doctrineThe neuron doctrine is the now fundamental idea that the nervous system is made up of individual and discrete cells and was proposed by Heinrich Wilhelm Gottfried von Waldeyer-Hartz in 1891. Waldeyer-Hartz coined the term "neuron" to describe the individual cells. The "neuron doctrine" served to...
.
Allan HobsonJohn Allan Hobson, M.D. is an American psychiatrist and dream researcher.He is known for his research on the 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
coronalCoronal may refer to:* anything relating to a corona* Coronal view, an anatomical term of location.* Coronal loop* In linguistics, coronals refer to coronal consonants.* In zoology, the coronal plane is an anatomical term of location...
ly.
- The original functional differentiation was a division of caudal and rostral
Rostral can refer to:* Anatomical location or direction, top or cephaled, as opposed to caudal or down* Rostral bone* Rostral column, a monumental scaled column, frequently decorated with ship bows* Rostral scale, in scaled reptiles* Rostral organ...
, this was based upon the observation that the lesioning of the rostral reticular formation induces a hypersomniaHypersomnia is a disorder characterized by excessive amounts of sleepiness.From the website of the U.S. National Institute of Neurological Disorders and Stroke :-Causes:...
in the cat brain. In contrast, lesioning of the more caudal portion of the reticular formation produces insomniaInsomnia is a symptom of any of several sleep disorders, characterized by persistent difficulty falling asleep or staying asleep despite the opportunity. Insomnia is a symptom, not a stand-alone diagnosis or a disease. By definition, insomnia is "difficulty initiating or maintaining sleep, or both"...
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
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 formationMoving 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
medullaThe 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
mesencephalonIn biological anatomy, the mesencephalon comprises the tectum , tegmentum, the ventricular mesocoelia , and the cerebral peduncles, as well as several nuclei and fasciculi...
(midbrain). The nuclei can only be teased out by function, cell type, and projections of efferent or afferent nature.