In this article we will discuss about:- 1. Meaning of Reticular Formation 2. Connections of Reticular Formation 3. Functions.
Meaning of Reticular Formation:
Reticular formation is phylogenetically oldest part of brain developed very well in the case of higher animals. It is not an anatomical structure but more of a physiological entity.
The reticular formation extends throughout the brainstem. It gets extended on cephalic side to thalamus and hypothalamus and on caudal side it can extend up to cervical segments of spinal cord as well.
In the brainstem reticular formation, nerve cell bodies are scattered within the meshwork of nerve fibers/reticulum. Many of these cell bodies are grouped together as nuclei, and have definite functions. The reticular formation extending in neuraxis/brainstem is divided into medial division and lateral divisions. These divisions have bilateral symmetrical arrangement.
Connections of Reticular Formation:
(Figs 9.44 and 9.45)
Afferent Connections:
i. All sensory pathway (special or general senses), along their way to cerebral cortex, give out a number of branches that is collaterals to reticular formation. So afferent collaterals from many ascending tracts including that from special sensations go to reticular formation and keep feeding afferent information to this area.
ii. Motor and premotor cortices also have afferent connections to reticular formation.
iii. Afferent impulse also comes from certain nuclei of basal ganglia.
iv. Afferent impulses come from nuclei of cerebellum
v. Afferent impulses also come from vestibular nucleus in brainstem.
Efferent Connections:
i. Reticulospinal tracts that take origin from medullary and pontine reticular formation carry impulses to anterior horn cells present in spinal cord.
ii. Another reticulospinal tract connects brainstem reticular formation with lateral horn cells of spinal cord present in thoracolumbar region. These lateral horn cells give rise to sympathetic nerves responsible for vasomotor tone, venomotor tone, etc.
iii. Ascending reticular activating system: Reticular formation sends efferent impulses to almost all areas of cerebral cortex through ARAS.
In the diagram (Fig. 9.45) descending tracts numbered in black color are reticulospinal tracts which would descend down into spinal cord.
And numerals indicate:
1. Cerebral cortex (motor and premotor cortex)
2. Basal ganglia
3. Cerebellum
4. Inhibitory area of reticular formation
5. Facilitatory area of reticular formation
6. Vestibular nucleus
Functions of Reticular Formation:
1. Visceromotor function:
There are many centers present in brainstem, like vasomotor center; deglutition center, respiratory center, etc. which have influence over functioning of autonomic nerves in body. Thereby they have role to play in regulation of heart rate, blood pressure, GI tract motility and secretion, etc. The impulses from these centers reach autonomic nerves present in spinal cord through reticulospinal tract.
2. Somatomotor function:
Reticular formation present in brainstem is very important for this function. The pontine reticular formation sends excitatory impulses to gamma motor neurons controlling muscle tone whereas influence from reticular formation of medulla oblongata on same motor neurons will be inhibitory.
The impulses from regions of pons and medulla reach anterior horn cells present in spinal cord through reticulospinal tracts. Under normal conditions, activity of medullary regions is dominant because inhibitory area of medulla is constantly stimulated by impulses coming from cerebral cortex, basal ganglia, cerebellum, etc.
Hence, inhibitory influence has dominant role to play. If there is a mid-collicular section (transection between superior and inferior colliculi), it will cut off majority of the inhibitory inputs exerted (from cerebral cortex and basal ganglia) over the reticular formation in medulla oblongata.
In addition to this, now pontine facilitatory reticular formation influence becomes unopposed. Because of this, it results in a condition know as decerebrate rigidity. In this condition, there will be severe increase in muscle tone throughout the body, especially so in muscles present in extensor compartment.
3. Somatosensory:
Modulation of all sensations. All sensory pathways from general sensations or special sense organs reach reticular formation either through collaterals or through multisynaptic pathways. The reticular formation in turn is connected to most of the parts of cerebral cortex. These sensory inputs when processed and sent to cerebral cortex would result in alteration of behavior of individual.
It is because of this, for same type of stimulus, reaction of individual varies depending on factors, like:
a. Place of stimulation
b. Time of stimulation
c. Mental state of individual at the given time, etc. Because of above reasons, the way the person reacts to situations varies.
4. Ascending reticular activating system (ARAS) has role to play in arousal, alertness, and sleep state (AR-arouse, A-alert, S-sleep). Impulses discharged from reticular formation reach thalamus; hypothalamus before reaching cerebral cortical region. The pathway involved is ARAS.
The frequency of impulse discharge in this pathway determines state of activity of brain. ARAS activity would bring about alterations in the pattern of EEG recordings. The activity of ARAS can be influenced by substances, like anesthetics, tranquilizers, sedatives, etc.
5. Modulation of pain:
From nucleus raphe magnus present in reticular formation of brainstem, dorsolateral funiculus takes origin. This tract has ability to alter activity of the gate that is involved in modulation of pain sensation. The neurotransmitter liberated by this tract is 5-hydroxy- tryptamine (refer to gate control theory of pain and descending analgesic system).