The Reticular Formation (RF) is the central arousal modulating system of the CNS and the structural core of the vertebrate nervous system.
The RF is the only neural structure absolutely required for the sustenance of vertebrate life functions (lesions to other structures do not necessarily lead to death, provided all other structures are intact and the environmental conditions are tightly controlled with respect to the lesion).
Physical destruction of the reticular core at the midbrain-pontine boundary, or as it is sometimes called - the"wasp's waist", destroys the function of the entire reticular formation and leads to inevitable and unavoidable death.
The RF is a system composed of a highly inter-connected diffusely projecting non-nuclear cell network that lies adjacent to the seven major cell columns (ie. the GSA, GSE, GVA, GVE, SSA, SVA, and SVE cell columns).
It is the convergence of the these brainstem columns with the reticular core - from the level of the midbrain to the posterior end of the brainstem - that forms the antomical and sturctural basis for the pharyngeal-hindbrain complex and the means for integrating the dual organsim.
Also, embedded within the RF are the major global neurotransmitter cell groups (5HT, ACh, NE, and DA) and their respective tracts (to be discussed in next chapter).
The primary role of the RF is cooridinating and integrating the somatic body with the visceral body in response to internal hedonic needs and externally contingent events.
The RF runs from the entire length of the Pharyngeal-Hindbrain Complex extending from the lower end of the medullary brainstem to the upper end of the mesencephalic midbrain and on into the diencephalon as the thalamic reticular nucleus (NRT).
A similiar pattern of may also pertain to the hypothalamus and the intraneuronal network of the spinal cord.
At the level of the Pharyngeal-Hindbrain Complex, the RF may be divided into several divisions (Mesencephalic and Medullary), levels (Midbrain, Pontine, and Medullary) and, zones/columns (Median, Paramedian, Lateral, and Intermediate).
Classically, the RF may be divided into two functional divisions; the Mesencephalic, which possesses excititory ascending efferents, and the Medullary which possesses inhibitory efferent outflow.
Lesion studies suggest that the medullary RF is largely responsible for inhibiting the mesencephalic RF.
The midbrain-pontine levels of RF comprise the Mesencephalic RF divsion, and send excititory projections to the intralaminar nucleus of the thalamus (IN) and hypothalamus via the central tegmental tract as well as to the spine via the medial reticulospinal tract (mRST).
The mesencephalic RF possesses three reticular zones or columns of functional significance: the Median RF which is the major site of origin for RF outflow, the Paramedian RF which projects to the cerebellum, and the Lateral RF which appears to function as an afferent association area.
The midbrain level RF is responsible for cooridinating the sensory integration centers of the tectum with the phylogenetically sterotyped motor reflex centers of the tegmentum.
The pontine RF serves as a somatic sensori-motor integration center responsible for cooridinating the posture of the organism in the context of vestibular and midbrain activity.
The medullary level and medullary division are one and the same, and send inhibitory projections to the mesencephalic RF and the interneuronal network of the spine.
The medullary reticular formation is also responsible for cooridinating autonomic responses in the context of activity in the midbrain and pons.
The medullary division possesses a fourth zone/column, the Intermediate zone, which participates in the regulation of visceral functions such as respiration, heart rate and blood pressure via the autonomic systems.
Reticular Formation Dendrites and Axons
The thalamic reticular nucleus (NRT) is a diffuse intra-thalamic projection nucleus responsible for the modulation of thalamic activity.
Primitively, the NRT would have received inputs from the intralaminar nucleus of the thalamus and the brainstem reticular formation.
At gnathostomes, the ventral nuclei come to provide affernt input to the NRT
With the transition of mammals the dorsal thalamic nuclear group and cerebral cortex would come to contribute afferent input to the NRT.
The RF resembles the interneuronal network of the spinal column, except that it is correspondingly more complex because it incorporates the source nuclei of the major CNS neurotransmitter systems as well as facilitating the integration of the special and general columns.
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The Reticular Formation is thought to be involved in the following:
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"The reticular formation is a set of interconnected nuclei that are located throughout the brain stem. Its dorsal tegmental nuclei are in the midbrain while its central tegmental nuclei are in the pons and its central and inferior nuclei are found in the medulla.
The reticular formation has two components:
Interneurons of the reticular formation receive some of the cortico-bulbar fibers from the motor cortex. It is those fibers that innervate the three cranial nerves involved in eye movement. Other cortico-bulbar fibers innervate cranial nerves directly. The descending reticular nuclei in the brain are involved in reflexive behavior such as coughing, chewing, swallowing and vomiting."
