Neural 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 consists of numerous cell clusters, among which are the tegmental nuclei (Bechterew's nuclei), gigantocellular, parvocellular, paramedian, as well as oral and caudal pontine nuclei.
Cells of the reticular formation possess unique properties:
- Polysensory properties: they are capable of processing signals from various analyzers.
- 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 adrenaline stimulate a pronounced EEG response).
Connections with other parts of the central nervous system are maintained via neural pathways. Afferent (incoming) information is received from the spinal cord (pain and temperature receptors), cerebellum, as well as the cortex and subcortical nuclei. Efferent (outgoing) signals are sent back to the cortex, spinal cord, and cerebellum.
Ascending activating influences on the cortex
The main somatic function of the RF is the maintenance of cerebral cortical tone. This effect is generalized in nature, encompassing virtually the entire brain.
If the brainstem of a sleeping animal is stimulated via implanted electrodes, it instantly wakes up, and an activation pattern is registered 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 via specific lemniscal pathways to the projection areas of the cortex, eliciting a primary response—the evoked potential.
- Simultaneously, excitation branches out via numerous collaterals into the structures of the RF.
- 20–30 milliseconds after the primary response, a generalized activation wave from the brainstem reaches the cortex for a second time.
Tonic (constant) excitation is sustained by a continuous influx of information from receptors, as well as internal mechanisms: reverberation (circulation of impulses through closed networks) and signal multiplication.
Despite the non-specific nature of the responses, P.K. Anokhin formulated the theory of specificity. According to this theory, brainstem influence depends on the biological quality of the response, mediated by the limbic system and hypothalamus. For instance, 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).
The influence can be of two types:
- Facilitating: originates predominantly from structures of the pons and mesencephalic regions (midbrain). Their stimulation increases the amplitude of muscle contractions elicited by motor cortex stimulation.
- Inhibitory: associated with the bulbar reticular region (medulla oblongata). Stimulation of this area activates spinal interneuronal inhibitory units, leading to a suppression of motor activity. A classic example of central reflex inhibition is Sechenov inhibition.
Rubrospinal influences also warrant mention, acting 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
In addition to controlling skeletal musculature and higher nervous activity, the RF is the site where vital autonomic centers are localized.
- Respiratory center: located in the medulla oblongata and includes inspiratory and expiratory neurons. Its activity is controlled by the pneumotaxic center located in the pontine reticular formation.
- Vasomotor center: also located in the medulla oblongata. Its electrical stimulation produces pronounced vascular reactions that regulate blood circulation.
Briefly: answers to topical questions
In which nuclei of the reticular formation are serotonergic neurons localized?
Serotonergic neurons/structures in the cited sources are indicated for the following brainstem formations:
- Nucleus raphe magnus (Nucleus raphe magnus / Raphe nuclei) — associated with the serotonergic mechanism of pain suppression; hyperalgesia during hemodynamic fluctuations is realized through the serotonergic structures of the nucleus raphe magnus.
- Nuclei of the medulla oblongata and lower midbrain — house the serotonergic neurons of the monoaminergic system.
What is the methodology of performing I.M. Sechenov's classic experiment to demonstrate central inhibition?
The methodology of I.M. Sechenov's classic experiment involves chemical stimulation of frog brain structures to evaluate the suppression of spinal reflexes.
The experimental procedure includes the following steps:
- Preparation — making a transverse cut in the region of the optic thalami, removing the cerebral hemispheres.
- Baseline measurement — determining the latency of the acid withdrawal reflex upon immersing the paw in a 0.25% hydrochloric acid solution.
- Stimulation — placing a small crystal of sodium chloride (NaCl) onto the cut surface.
- Outcome evaluation — re-measuring the reflex latency, which ultimately increases (the response is slowed or absent).
To remember descending brainstem influences: "MM — maximum, PM — pause". Mons (pons) and mesencephalon (midbrain) stimulate (maximum), while the medulla oblongata inhibits (pause) spinal reflexes.
Class and exam questions
Why does excitation from the reticular formation reach the cortex with a delay?
The 20–30 ms delay occurs because impulses reach the brainstem via a detour—through numerous collaterals from specific lemniscal tracts—after which they require time for processing within synaptic networks.
What happens upon complete destruction of the reticular formation of the brainstem?
According to experiments by H. Magoun and R. Rhines (or D. Lindsley), bilateral destruction of these structures completely deprives the cortex of tonic activating influences, causing the animal to fall into a state of unrousable sleep (coma).
What is the essence of P.K. Anokhin's theory of specificity?
The theory states that brainstem influence on the cortex depends on the biological quality of the stimulus (pain, food). This is proven by the fact that various drugs (e.g., the alpha-blocker chlorpromazine) can selectively suppress defensive reactions while sparing feeding ones.
What else to study on this topic
- Mechanisms of impulse reverberation and multiplication in neuronal traps.
- Interaction of the reticular formation with the hypothalamus and limbic system.
- Detailed anatomy of extrapyramidal pathways (rubrospinal, vestibulospinal tracts).
- The Sechenov inhibition phenomenon: experimental methodology in the frog.
- Chemical heterogeneity of brain structures (adrenergic, dopaminergic, and cholinergic systems).
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