The Reticular Formation

The brainstem | Main Anatomy Index | Cranial nerves and nuclei I

• The reticular formation is an apparently (but not actually) diffusely organised area that forms the central core of the brainstem.
• The reason it appears to be diffusely organised is twofold:

1. Its pattern of connectivity is characterised by a great deal of convergence and divergence, so that a single cell may respond to several different sensory modalities or to stimuli applied practically anywhere on the body;
2. Although it is involved in several quite separate functions, the areas involved in these functions overlap considerably.

• With the reticular formation, it is as though several nuclei had been scrambled together and dispersed along the brainstem, while their constituent cells retained their original connections.

• At most levels of the brainstem, the reticular formation can be divided into 3 longitudinal zones arranged in a medial to lateral sequence:

1. The raphe nuclei;
2. The medial zone;
3. And the lateral zone.

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• These are thin plates of cells in and immediately adjacent to the sagittal plane.
• Like the cells of the locus ceruleus, raphe neurons (G. raphe, seam) have exceedingly far-flung connections.

• Serotonin is used as the neurotransmitter.

• This, alongside the midline raphe nuclei, contains a mixture of large and small neurons.
• It is the source of most of the long ascending and descending projections from the reticular formation.

• Some of the neurons in the medial zone of the rostral medullary reticular formation are so large that this area is referred to as the gigantocellular reticular nucleus.

• This is particularly prominent in the rostral medulla and caudal pons.
• It is primarily concerned with cranial nerve reflexes and visceral functions.

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• Many of these have extensive and complex axonal projections.
• They may innervate multiple levels of the spinal cord or send numerous collaterals to the brainstem and diencephalon.
• The generally do not send axons directly to the cerebral cortex, however.

• A few may even have bifurcating axons that give rise to both ascending and descending connections.

• Reticular neurons have large fields of dendrites.
• The sometimes spread out in a plane perpendicular to the long axis of the brainstem.
• This allows them to receive synaptic inputs from ascending sensory pathways, descending cortical axons, and a variety of other sources.

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• The reticular formation is involved in 4 general types of function:

1. Motor control;
2. Sensory control;
3. Visceral control;
4. And control of consciousness.

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• Motor control has several different aspects.

1. Certain reticular regions are closely related to the cerebellum and its motor control functions.
• A fairly discrete collection of cells in the medullary reticular formation, the lateral reticular nucleus is resolved adjacent to the spinothalamic tract.
• It receives direct spinoreticular fibres, collaterals of spinothalamic fibres, and projects them to the cerebellum.
• It also receives input from the red nucleus, so it is more than a straightforward somatosensory relay to the cerebellum.
• Collections of reticular neurons near the medullary midline, the paramedian reticular nucleus, also project to the cerebellum.
• Afferents to the paramedian nucleus arise in the cerebellum and in other locations, including the cerebral cortex.
• The reticular tegmental nucleus, located between the medial lemnisci in the rostral pons, receives inputs from the cerebral cortex and other sites.
• It also projects to the cerebellum.

2. There are 2 reticulospinal tracts arising from the medial zone of the pontine and the rostral medullary reticular formation.
• Fibres from the pons descend with the ipsilateral MLF and travel through the ventral funiculus in the spinal cord, as the medial reticulospinal tract.
• Fibres from the medulla descend bilaterally in the ventral part of the lateral funiculus, as the lateral reticulospinal tract.
• These tracts are a major alternate route to the corticospinal tract.
• These reticular neurons receive projections from many areas, including the basal ganglia, red nucleus, and substantia nigra.
• Input from widespread areas of the cerebral cortex, particularly the somatosensory and motor cortex, seems to be especially important.
• Most of these descending fibres travel to their reticular terminations in the central tegmental tract.

3. The reticulospinal tracts also carry descending motor commands generated within the reticular formation itself.
• The reticular formation contains the neural machinery for considerably complex patterns of movement.
• A cat whose brainstem has been surgically separated from its diencephalon can, after a recovery period, assume a variety of complex tasks such as walking and running, and righting itself if tipped over.

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• Reticular neurons exert some control over activity in spinal reflex arcs.
• They can control over the access of sensory information to ascending pathways.

• Tonic inhibition of flexor reflexes originates in the reticular formation.
• The result that only noxious stimuli can normally evoke such a reflex.

• In addition, stimulation of certain regions of the medullary reticular formation causes inhibition of some sensory interneurons and tract cells in the spinal cord.
• This seems to be important in the regulation of pain perception.

• Centres controlling inspiration, expiration, and the normal rhythm of breathing have been identified physiologically in the medulla and pons.
• Other centres controlling heart rate and blood pressure have been identified in the medullary reticular formation.

• Ascending projections from the reticular formation terminate in the thalamus, subthalamus, hypothalamus, and basal ganglia.
• The functions of most of these are poorly understood, but those to the thalamus seem to be particularly important.
• They terminate in the intralaminar nuclei, which in turn project to widespread areas of the cortex.

• Activity in this pathway is essential for the maintenance of a normal state of consciousness.
• Bilateral damage to these fibres as they traverse or originate in the midbrain reticular formation results in prolonged coma.

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