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Physiology of the Reticular Formation

*Formatio reticularis*

For medical students2 min readUpdated 2026-10-10

The reticular formation is an extensive network of neurons and nerve fibers located within the core of the brainstem. It gathers sensory information from all modalities, acting as a primary "energy center" that maintains cerebral cortical tone while regulating spinal reflexes and autonomic functions.

Synaptic densityUp to 40,000 synapses per single neuron

Signal delayCortical excitation is delayed by 20–30 ms relative to specific pathways

Polysensory natureResponds to any sensory and humoral stimuli

ChemosensitivityUrethane preserves cortical activation, whereas nembutal completely blocks it

Neuronal Organization and Pathways

The structures of the reticular formation (RF) occupy a significant portion of the brainstem, including the medulla oblongata, pons, and midbrain. Anatomically, it is represented by numerous clusters of cells, among which are the tegmental nuclei (Bechterew's nuclei), gigantocellular, parvocellular, paramedian nuclei, as well as the oral and caudal pontine nuclei.

Reticular formation cells possess unique properties:

Polysensory properties: they are capable of processing signals from diverse sensory modalities.
Presence of spontaneous activity and high excitability.
Chemical heterogeneity: cholinergic, adrenergic, and dopaminergic neurons are localized here.
Sensitivity to humoral factors (for example, elevated carbon dioxide levels or direct administration of epinephrine stimulate a pronounced EEG response).

Connections with other parts of the central nervous system are maintained via neural pathways. Afferent (incoming) information arrives from the spinal cord (pain and temperature receptors), cerebellum, as well as the cerebral cortex and subcortical nuclei. Efferent (outgoing) signals are directed back to the cortex, spinal cord, and cerebellum.

Ascending Activating Influences on the Cortex

The primary somatic function of the RF is maintaining the tone of the cerebral cortex. This effect is generalized in nature, encompassing virtually the entire brain.

If the brainstem of a sleeping animal is stimulated via implanted electrodes, it wakes up instantly, and an activation response is recorded on the EEG. Conversely, bilateral destruction of these structures leads to a state of deep coma.

How the activating influence is formed:

Signals from receptors travel along specific lemniscal pathways to cortical projection areas, eliciting a primary response—the evoked potential.
Concurrently, excitation diverges via numerous branches (collaterals) into the structures of the RF.
Approximately 20–30 milliseconds after the primary response, a generalized wave of activation from the brainstem reaches the cortex a second time.

Tonic (continuous) excitation is sustained by a continuous influx of receptor information, as well as intrinsic mechanisms: reverberation (circulation of impulses through closed neuronal loops) and signal multiplication.

Despite the non-specific nature of these reactions, P.K. Anokhin formulated the theory of specificity. According to this theory, brainstem influences depend on the biological quality of the reaction, involving the limbic system and hypothalamus. For example, the drug chlorpromazine selectively blocks excitation during defensive pain reactions while leaving feeding reflexes unaffected.

Descending Influences on the Spinal Cord

The reticular formation controls the segmental apparatus of the spinal cord via extrapyramidal tracts (reticulospinal, rubrospinal, and vestibulospinal tracts).

This influence can take two forms:

Facilitating influence: originates primarily from the structures of the pons and mesencephalic regions (midbrain). Their stimulation enhances the amplitude of muscle contractions triggered by motor cortex stimulation.
Inhibitory influence: associated with the bulbar reticular area (medulla oblongata). Stimulation of this region activates spinal interneurons, leading to the suppression of motor activity. A classic example of central reflex inhibition is Sechenov inhibition.

Additionally, rubrospinal influences act reciprocally: they stimulate flexor motor neurons while simultaneously inhibiting corresponding extensor muscles. These mechanisms are critical for maintaining postural tone and executing phase movements.

Regulation of Autonomic Functions

Beyond controlling skeletal musculature and higher nervous activity, the RF serves as the localization site for vital autonomic centers.

Respiratory center: located in the medulla oblongata and includes inspiratory and expiratory neurons. Its activity is controlled by the pneumotaxic center situated in the pontine reticular formation.
Vasomotor center: also located in the medulla oblongata. Its electrical stimulation evokes pronounced vascular reactions that regulate blood circulation.

Mnemonic

To remember the descending influences of the brainstem: "Pons and Midbrain = Maximum, Medulla = Pause". The Pons and Mesencephalon (midbrain) stimulate (maximum), whereas the Medulla (oblongata) inhibits (pause) spinal reflexes.

Frequently asked questions

In which nuclei of the reticular formation are serotonergic neurons localized?

Serotonergic neurons and structures mentioned in the literature correspond to the following brainstem formations:

Nucleus raphe magnus (Raphe nuclei) — associated with serotonergic pain-suppression mechanisms; hyperalgesia during hemodynamic fluctuations is mediated via serotonergic structures of the nucleus raphe magnus.
Medullary nuclei and lower midbrain regions — harbor serotonergic neurons of the monoaminergic system.

What is the methodology of I.M. Sechenov's classic experiment demonstrating central inhibition?

Sechenov's classic experiment involves chemically irritating the brain structures of a frog to assess the suppression of spinal reflex responses.

The experiment proceeds through the following steps:

Preparation — a transection is made in the region of the optic thalami, removing the cerebral hemispheres.
Baseline measurement — determine the latency of the acid-induced withdrawal reflex by immersing the paw in a 0.25% hydrochloric acid solution.
Irritation — a small crystal of sodium chloride (NaCl) is placed onto the transection surface.
Result evaluation — the reflex latency is measured again, showing an increase (the reaction is delayed or absent).

Why does excitation from the reticular formation reach the cortex with a delay?

The 20–30 ms delay occurs because impulses enter the brainstem by a detour route—via numerous collaterals from specific lemniscal tracts—after which they require time for processing within synaptic networks.

What happens if the reticular formation of the brainstem is completely destroyed?

According to experiments by D. Lindsley, bilateral destruction of these structures completely deprives the cortex of tonic activating inputs, causing the animal to fall into an unarousable sleep (coma).

What is the core of P.K. Anokhin's theory of specificity?

The theory states that brainstem influences on the cortex depend on the biological quality of the stimulus (pain, food). This is evidenced by the fact that certain drugs (e.g., the alpha-blocker chlorpromazine) can selectively suppress defensive reactions while sparing feeding behaviors.

Go deeper

Mechanisms of impulse reverberation and multiplication in neuronal traps.
Interaction of the reticular formation with the hypothalamus and limbic system.
Detailed anatomy of extrapyramidal tracts (rubrospinal and vestibulospinal tracts).
The phenomenon of Sechenov inhibition: experimental methodology in the frog.
Chemical heterogeneity of brain structures (adrenergic, dopaminergic, and cholinergic systems).