The reticular formation is a complex network of nuclei (a group of neurons) which are connected with each other throughout the brain stem. As it is a complex system of the network so it is not easy to locate and define appropriately.
The reticular formation network present in different parts i.e. in medulla oblongata and midbrain. Ascending pathways of the reticular formation leading towards the cortex and descending pathways leading towards the spinal cord. Neurons of the ascending reticular activating system, have an important impact in controlling behavioural arousal and consciousness.
The reticular formation classifies in three main regions: raphe nuclei, gigantocellular reticular nuclei and parvocellular. The raphe nuclei are the site of neurotransmitter synthesis. The gigantocellular nuclei control motor coordination. The parvocellular nuclei regulate respiration. Most of the basic functions of the brain controlled by reticular formation.
The reticular formation is a very complex network of nuclei along with there ascending and descending fibres (Gerhard, 2013; Kandel ER, 2015). Due to high complexity and networking system it is difficult to mark between nuclei present within the reticular formation and overall reticular formation system, seems to be an unorganized structure (Gerhard, 2013; Kandel ER, 2015).
The reticular formation studied by physiological and experimental methods (ROSSI and BRODAL, 1956; Valverde, 1962). Data obtained from these experiments confirm that there is a connection between reticular formation and spinal cord, cerebellum and tectum of the midbrain and cerebral cortex (Valverde, 1962; Gerhard, 2013).
Certain confine areas referred to as ‘centres’ have been recognized involved in inhibition and induction of different brain parts (ROSSI and BRODAL, 1956). These centres of reticular formation receive stimuli from different brain parts including the cerebral cortex, cerebellum and the spinal cord (ROSSI and BRODAL, 1956; Valverde, 1962; Gerhard, 2013).
Respiratory and cardiovascular systems also have a link with these parts of the brain (ROSSI and BRODAL, 1956). The ‘reticular activating system’ present within the reticular formation of brain stem cells makes it further complex in order to understand its functional and organizational capacities (ROSSI and BRODAL, 1956; Jones, 2008).
Reticular fibres there origin and insertion
Several functions controlled by reticulation formation of the brain are affected by the different regions of cerebral cortex connected by fibres carry electrical impulses between these two parts of the brain, but the extent of transmission still a point of discussion (ROSSI and BRODAL, 1956).
Some the major parts involved with reticular formation carry neurotransmitter production, cranial nerve association (Jones, 2008; Kandel ER, 2015). Descending reticular fibres tracts involve in sensory and motor functions while ascending tracts involve in arousal and consciousness (Jones, 2008; Kandel ER, 2015).
Rosi and Brodal in 1956 studied that fibres originate from the cerebral cortex and end descends to the brain stem and spinal cords and they classify them on the basis of the origin of cortico-reticular fibres from the four cerebral lobes having a frontal lobe, parietal lobe, temporal lobe and occipital lobe.
The main part of cortico-reticular fibres originates from the frontal lobe, especially from the motor areas (ROSSI and BRODAL, 1956; Valverde, 1962) although basal areas, medial and occipital also have reticular fibre projections (Valverde, 1962).
Reticular formation in vertebrates
The vertebrate reticular formation takes place within brainstem as well as midbrain (Purnell and Helmuth, 2002). Reticular part of the brain begins anteriorly from spinal cord’s junction with the medulla, passes via the pons and ends at the thalamic border (Purnell and Helmuth, 2002).
The reticular formation is consist of a complex system of nuclei and its projections (Purnell and Helmuth, 2002). Each of which has a specific functional role.
In an article by Warren McCulloch and colleagues proposed the reticular system is a region of controlling the organism’s overall behavioural state (Purnell and Helmuth, 2002).
Despite a number of modern functional evaluation techniques a complete understanding of anatomical structure and physiology of this system still not be understood completely (Purnell and Helmuth, 2002).
The medial reticular formation has a medium to large cell body neurons distinguished by their long bifurcating axons running rostrocaudally (Purnell and Helmuth, 2002). The long fibre branch directed towards the spinal cord or forebrain (Purnell and Helmuth, 2002).
Figure: (a) Cat brain’s sagittal view depicting the relative location and of the reticular formation of the brain along with the medial core. Abbreviations are CPu, Caudate putamen; SC, Superior colliculus; SN, Substantia nigra. Source: (Purnell and Helmuth, 2002)
An overview of the experimental study on reticular formation:
In a study done by Purnell and his colleagues, they analyze the connectivity of vertebral brain at the level of neural network by using graph-theoretic analysis for which they select medial reticular formation structure of brainstem as it has a complex of motor and sensory connections (Purnell and Helmuth, 2002).
The projection neurons of the reticular formation act as a single synaptic junction or pathway between afferent (sensory neurons) and efferent neurons (motor neurons) of the spinal cord (Purnell and Helmuth, 2002).
Study of Rossi and Brodal
they did their experiment to check the origin as well as the termination point of cortico-reticular fibres to check their organizational function (ROSSI and BRODAL, 1956). To achieve this target they followed a method of fibre degeneration as a consequence of cortical lesions, which permit the tracing of fibres to their respective terminal point (ROSSI and BRODAL, 1956).
Their study not only helped in determining the origin and insertion of fibres in reticular formation areas but also to observe that whether fibres from variable origins have different termination ends within the reticular formation (ROSSI and BRODAL, 1956). For this purpose, they took the pontine and medullary reticular formation under study (ROSSI and BRODAL, 1956).
The cerebral projecting reticular formation fibres were excluded from their study (ROSSI and BRODAL, 1956). They observed that the pontine and medullary fibres of reticular formation have an origin in all regions of the cerebral cortex which help them in conclusion.
That the cortical reticular fibres organizations system is involved in transmission to the reticular formation of impulses mediating cortical effects on the somatomotor activity as well as respiratory and cardiovascular functions. These fibres also affect the “ascending activating system” od reticular formation (ROSSI and BRODAL, 1956).
Valverde in an experiment tried to find out the following points regarding reticular formation in a rat: Region from which fibres descend to the reticular formation(Valverde, 1962); Variable reticular nuclei have variable cortical projections from different brain parts (Valverde, 1962) ; Cranial nerves of motor nuclei have corticofugal projections (Valverde, 1962).
They studied it by using morphological methods in combination with experimental ones (Valverde, 1962). The cortical projections originated from the sensory and motor cortices are terminated from both sides with a contralateral predominance (Valverde, 1962).
They did not find any degeneration of cranial nerves (Valverde, 1962). Therefore they reached a conclusion that between cerebral cortex and neurons the motor nuclei of cranial nerves at least a single reticular cell exist (Valverde, 1962).
The reticular formation is although a complex but vital part of brain stem having an association with many brain parts in the form of ascending activating system and descending activating system. These ascending and descending activating system form a connection between the reticular formation system and other parts of the brain by reticular fibres.
Origin and insertion of the fibres help in a determination that which part of the reticular formation dealing with what another part of the brain. Still, a lot of mysteries needs to be open to further explore this enigma. Many of the functionalities associated with that part of the brain will reveal many new pathways in the near future.
Authors: Zohaib Saleem1, Muhammad Fakhar I Adil2, Hafeez Arshad1
- Institute of Pharmacy, Physiology and Pharmacology, FVS, UAF.
- Department of Anatomy, FVS, UAF