(from Latin, reticulum: net) The reticular formation refers to the net-like structure in the central core of the BRAINSTEM. It is composed of nerve cell bodies lying among fascicles of myelinated nerve fibres which course longitudinally, as well as transversely, through the MEDULLA, PONS and MID-BRAIN. The reticular neurons have long smooth dendrites (see DENDRITE) that radiate out from the cell body and through the passing fibres. They can thus receive input from multiple sources. Neurons in the lateral portion of the reticular formation are medium sized (15–25 m in diameter) and project predominantly to other brainstem sites. They are important for sensorimotor reflexes involving the cranial sensory and motor nerves. Neurons in the medial portion of the reticular formation are large (>25 μm) or giant (>50 μm) in size and give rise to bifurcating axons of which one major branch extends to the SPINAL CORD or to the FOREBRAIN. The large and giant reticular neurons concentrated in the caudal pons and medulla give rise to the major reticulo-spinal projections that are important in the control of POSTURE and movement (see LOCOMOTION). Medium and large reticular neurons concentrated in the ORAL PONS and midbrain give rise to the ascending projections which pass to the THALAMUS (and innervate the midline, medial and intralaminar nuclei) along a dorsal pathway, and to the HYPOTHALAMUS and BASAL FOREBRAIN, along a ventral pathway. These neurons comprise the ASCENDING RETICULAR ACTIVATING SYSTEM which via the dorsal relay to the non-specific thalamo-cortical projection system and the ventral relay to the hypothalamo- and basalo-cortical projection systems, stimulate and maintain cortical activation. Through both its descending and ascending components, the reticular formation thus plays a critical role in the generation and maintenance of behavioural and cortical responsiveness and arousal that characterize wakefulness.
Particular reticular neurons are also important for the central activation that occurs during the state of REM SLEEP. Extensive lesions of the pontine and mesencephalic reticular formation in animals and humans can result in COMA, in which a loss of responsiveness and cortical activation occurs.
The reticular formation is made up of neurons containing different chemical neurotransmitters. The major population of cells contains the excitatory amino acid GLUTAMATE. Although pharmacological studies have not yet specifically proven the importance of these glutamatergic neurons in mechanisms of activation, they must be presumed to be the most important component of the ascending reticular activating system. In addition, however, there are collections of cells containing other neurotransmitters. A large group of cells in the PONTOMESENCEPHALIC TEGMENTUM (see PEDUNCULOPONTINE TEGMENTAL NUCLEUS and LATERODORSAL TEGMENTAL NUCLEUS) contain ACETYLCHOLINE and project in parallel with other reticular neurons in this region to the thalamus and less so to the hypothalamus and basal forebrain. These cholinergic cells play an important role in mechanisms of both waking and REM sleep. Clustered in a small nucleus (LOCUS COERULEUS) dorsal to the reticular core in the pons, NORADRENERGIC neurons also serve to enhance wakefulness and cortical activation, although these cells prevent the occurrence of REM sleep. Ventral to the reticular core in the midbrain, DOPAMINERGIC cells (of the VENTRAL TEGMENTAL AREA and SUBSTANTIA NIGRA) play an important role in behavioural arousal and responsiveness of wakefulness. Finally, the inhibitory neurotransmitter GABA is contained in small (<15 μm) to medium-sized cells that are distributed among the glutamatergic, cholinergic and catecholaminergic cells and give rise predominantly to local projections. These GABAergic neurons would have the capacity to inhibit and thus turn off the excitatory projection neurons of the reticular activating system, which maintain wakefulness. They may thus be important for the onset and maintenance of SLOW-WAVE SLEEP.
BARBARA E.JONES
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