The Reticular Formation
Advertisements help pay for this website. Thank you for
The brainstem | Main Anatomy Index | Cranial nerves and nuclei I
Last updated 30 March 2006
The Reticular Formation
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:
- 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;
- 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
- At most levels of the brainstem, the reticular formation can be divided
into 3 longitudinal zones arranged in a medial to lateral sequence:
- The raphe nuclei;
- The medial zone;
- And the lateral zone.
The Raphe Nuclei
These are thin plates of cells in and immediately adjacent to the sagittal
Like the cells of the locus ceruleus, raphe neurons (G. raphe, seam) have exceedingly
- Serotonin is used as the neurotransmitter.
The Medial Zone
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
The Lateral Zone
This is particularly prominent in the rostral medulla and caudal pons.
It is primarily concerned with cranial
nerve reflexes and visceral functions.
The Reticular Neurons
Many of these have extensive and complex
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
- Reticular neurons have large fields of dendrites.
- The sometimes spread out in a plane
perpendicular to the long axis of the
- This allows them to receive synaptic inputs from ascending sensory pathways, descending
cortical axons, and a variety of other sources.
Functions of the Reticular Formation
The reticular formation is involved in 4 general types
- Motor control;
- Sensory control;
- Visceral control;
- And control of consciousness.
Motor control has several different aspects.
- Certain reticular regions are closely related to the cerebellum and its motor control
- 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.
- There are 2 reticulospinal
tracts arising from the medial zone of the pontine and the rostral medullary
- 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
- Most of these descending fibres travel to their reticular terminations in the central
- The reticulospinal tracts also carry
descending motor commands generated within the reticular
- 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.
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.
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
- Centres controlling inspiration
Control of Consciousness
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