Anatomy and Basic Functions
The anatomical composition of the extrapyramidal system (EPS) includes the basal nuclei of the telencephalon, cerebellar structures, and midbrain formations—the red nucleus and substantia nigra.
The primary physiological objective of this system is to provide prolonged tonic excitation of musculature. The EPS can synchronously activate or inhibit extensive muscle groups, controlling primarily the axial trunk muscles and proximal limb segments. Several pathways transmit efferent signals to the periphery: rubro-tegmental-vestibulo-reticular, bulboreticulospinal, and monoaminergic tracts.
Reticulospinal Tracts: Posture Maintenance and Relaxation
Two key motor pathways originating in the brainstem reticular formation perform opposite tasks.
- Medial (pontine) reticulospinal tract originates from the medial part of the pontine reticular formation. It descends ipsilaterally in the anterior funiculus of the spinal cord and terminates in the medial part of the anterior horns. This is a postural maintenance system: the tract excites $\alpha$- and $\gamma$-motoneurons of extensors, ensuring tone in anti-gravity muscles. A classic example is the rigid limbs of an animal during the standing reflex.
- Lateral (medullary) reticulospinal tract originates from the gigantocellular nucleus of the medulla oblongata. It descends bilaterally (though predominantly ipsilaterally) in the lateral funiculus. Its function is relaxation and preparation for movement. The tract suppresses extensor motoneuron activity and can facilitate flexor function.
Fine Motor Control and Reflexes
Specialized fiber bundles are recruited for more complex and precise motor acts:
- Rubrospinal tract (tractus rubrospinalis). Originates in the midbrain red nucleus, immediately undergoes the decussation of Forel, and runs in the lateral funiculus. Terminates on the lateral group of motoneurons. By exciting flexors and inhibiting extensors, it controls precise movements of distal parts (hands, feet)—for example, ensuring active, goal-directed limb flexion.
- Tectospinal tract. Arises in response to visual, auditory, and tactile stimuli. Regulates contralateral head movements, providing the orientation reflex.
- Monoaminergic pathways. Originate from the locus coeruleus in the brainstem. By releasing norepinephrine, they exert an inhibitory influence on spinal motoneurons and interneurons.
Medial and Lateral Motor Systems
Globally, descending tracts can be divided into two functional groups that support different phases of stepping and body posture.
Lateral System (Flexor Control) Tracts of this system are localized in the lateral funiculi (corticospinal, rubrospinal, and medullary reticulospinal). They act on flexor muscles—for example, the iliopsoas muscle (m. iliopsoas). The system is maximally active during the swing phase of a limb, when it is necessary to lift and swing the leg forward.
Medial System (Extensor Control) These pathways run in the anterior funiculi (e.g., the vestibulospinal tract). They activate anti-gravity muscles: the quadriceps femoris (m. quadriceps femoris) and gastrocnemius (m. gastrocnemius), which functions as an extensor during standing. The peak activity of this system occurs during the stance phase, when it is necessary to support body weight and push off the surface.