UNIT 10 - Reticular Formation
A. Location and Function
The reticular formation forms the central core of the brainstem. It is comprised of a complex network of both gray and white matter that is located in the tegmentum of the brainstem from the mesencephalon through the myelencephalon.
The reticular formation represents a smooth transition between ascending and descending sensory and motor systems. It has been described as being formed by cells that were “left over” after the sensory and motor systems were formed and it is thought to be one of the more primitive systems within the C.N.S. It can be considered to be a continuation of the internuncial pool and propriospinal fibers of the spinal cord.
The reticular formation with its intricate network of cell fibers contributes to a variety of functions within the C.N.S.
It:
1.) regulates cortical activity (via the Reticular Activating System)
2.) contributes to the facilitation and inhibition of motor activity
3.) controls many vital autonomic functions (i.e. respiration, cardiovascular activity, vomiting, and deglutition),
4.) helps regulate limbic activity in regard to our emotional response to visceral activity, and
5.) modulates the control of pain.
Fig. 1 –
B. Structures
The reticular formation is a loosely knit network of cells within the brainstem, as opposed to other structures within the C.N.S., which are formed by tight, compact groups of cells. This has made histologic analysis of the reticular formation difficult. However, multiple groups of nuclei have been identified throughout the reticular formation and are functionally divided into four major groups of cells. These reticular nuclei are: the precerebellar nuclei, the Raphe nuclei, the lateral reticular nuclei, and the central reticular nuclei. The subdivisions of the four groups of these reticular nuclei will not be discussed here for purposes of simplicity.
C. Sensory Input
1. Ascending Influences
The reticular formation in total receives ascending collaterals from most afferent pathways on their way to the thalamus, as well as input from surrounding supraspinal structures (i.e. olives, cerebellum, and cranial nerves I, II, V, VII, VIII, and X). It is now generally agreed that the medial lemniscal fibers do not send collaterals directly to the reticular formation and that much of its fibrous input is received by what has been designated the spinoreticular tract. Intermingled with spinothalamic fibers in the anterolateral funiculus, these fibers have been found to terminate in the caudal and lateral portion of the medullary reticular formation.
2. Descending Influences
Corticoreticular fibers, originating primarily from the sensorimotor areas (but also from frontal, superior temporal, occipital, and cingulate cortex), are received by the reticular formation and are thought to help control the formation’s level of excitability. This control mechanism is important in perception and learning, for it allows man to modulate his own activity. In summary, it can be said the reticular formation is dependent on information from all cortical, sub-cortical and brain stem structures as well as information from all major afferent pathways.
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D. Function of Reticular Formation
1. Precerebellar Nuclei
These nuclei depend heavily on information received from areas 4-6-8 of the cerebral cortex, the vestibular nuclei, and the spinoreticular tracts. This information is then projected to the cerebellum. The connections between the precerebellar nuclei and the cerebellum are reciprocal in nature. The input received by the cerebellum from these nuclei is used to control and monitor posture and movement. This allows the cerebellum to alter and adjust our posture and movement according to our needs.
2. Raphe Nuclei
These nuclei depend heavily on information received from the limbic system and the hypothalamus. The major function of these nuclei is to produce peptides in response to injury or disease. These peptides modulate, or control, the transmission of pain impulses in a manner similar to that of morphine. The Raphe nuclei also produce the neurotransmitter serotonin, which facilitates cortical activity thus playing a part in the Reticular Activating System.
3 & 4 Lateral and Central Reticular Nuclei
These two groups of nuclei are very closely interrelated from a functional standpoint. They both depend upon information received from all cortical, sub-cortical and brainstem structures along with information from all major afferent pathways. Functionally, both groups of nuclei are responsible for controlling: 1.) the ascending Reticular Activating System, 2.) motorically, they are involved in relaying information from the motor cortices (4-6) and the cerebellum to the spinal cord through the medial and lateral reticulospinal tract, and to the cranial nerves via the reticulobulbar tract, 3.) autonomically, these nuclei are responsible for cardiovascular and respiratory control, as well as controlling our swallowing and vomiting mechanisms, 4.) in addition, these nuclei send fibers to the hypothalamus and septal nuclei, which help regulate limbic activity in regard to our emotional responses to visceral activity.
Also, these nuclei are part of the ascending pathway to the parietal lobe for the sensation of taste via the ventral medial nucleus of the thalamus.
5. Miscellaneous Reticular Nuclei
a. Area Postrema
This region deals with the control of vomiting.
b. Interpedicular Reticular Nuclei
These nuclei play a role in the limbic system, affecting autonomic functioning that correspond to the emotions gerenated by the limbic system.
c. Perihypoglossal and Accessory Occulomotor Reticular Nuclei
These two groups of nuclei deal with occular control.
d. Locus Ceruleus
This nucleus plays a role in the production of the neurotransmitter, noradrenalin which is used to facilitate our motivational centers.
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E. Ascending Reticular Activating System
The component of the Reticular Activating System (R.A.S.) consists of the Raphe, central, and lateral nuclei. These nuclei receive information from all afferent pathways, as well as most supraspinal structures, cranial nerves, and all areas of the cerebral cortex. Thus the R.A.S. is aware, at all times, of the internal and external stimuli affecting us. Therefore, based upon integration of this input and the commands, or instructions coming from the frontal lobe a message is sent to all areas of the cerebral cortex which can deal with any of the following: 1.) the sleep/wake cycle, 2.) the maintenance of our level of conscious and attention level, and 3.) the ability to tune in or tune out specific stimuli within our environment. Once the appropriate decision has been made as to the implementation of the command, this information leaves the Raphe, central, and lateral nuclei via the reticulothalamic tract and is sent to the intralaminar nuclei of the thalamus. This message is then relayed to all areas of the cerebral cortex. The R.A.S. also has the ability to learn so as to focus our attention immediately to a particular stimuli which we have deemed of importance to us.
F. Motor Function Summary
The central and lateral reticular nuclei of the reticular formation also deal with motor activity. Specifically, they are concerned with: 1.) innate muscle tone, 2.) associated reactions which are adjustive movements of our body musculature, 3.) alterations of movements we are performing, 4.) the relay of information to the spinal cord from higher motor centers regarding mobility skills and background postural stability, 5.) proactive control of our body musculature to maintain our balance or stability, 6.) reactive control over our non-antigravity muscles to regain our balance, 7.) to influence non-antigravity musculature in response to vestibular input during high risk activities to maintain optimal stability, 8) the relay of information to various cranial nerves from area 4 dealing with automatic control of these nerves.
The relay of efferent information down the spinal cord occurs via Reticulospinal tracts and down the cranial nerves via the Reticulobulbar tracts. The reticulospinal tract is split into the lateral reticulospinal tract, or medullary reticulospinal tract, and the medial reticulospinal tract, or pontine reticulospinal tract.
G. Visceral Function Summary
The reticular formation acts as a part of the limbic system by providing information which coordinates establishment of our emotional responses in regard to visceral activity. It also processes information dealing with the control of our cardiovascular, respiratory, swallowing, and vomiting mechanisms, as well as relaying information dealing with the sensation of taste. These functions are the responsibility of the central and lateral reticular nuclei.
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