Anatomy and Basic Functions
The anatomical composition of the extrapyramidal system (EPS) includes the basal nuclei of the telencephalon, cerebellar structures, and midbrain formations—specifically the red nucleus and substantia nigra.
The primary physiological role of this system is to maintain sustained tonic excitation of the musculature. The EPS is capable of synchronously activating or inhibiting extensive muscle groups, predominantly controlling the axial muscles of the trunk and the proximal segments of the limbs. Several pathways are utilized to 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 opposing functions.
- The medial (pontine) reticulospinal tract originates in the medial part of the pontine reticular formation. It descends ipsilaterally in the anterior funiculus of the spinal cord and terminates in the medial portion of the anterior horns. This is a postural support system: the tract excites $\alpha$- and $\gamma$-motor neurons of extensors, maintaining the tone of antigravity muscles. A classic example is the rigid limbs of an animal exhibiting the standing reflex.
- The 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 to promote relaxation and prepare for movement. The tract suppresses extensor motor neuron activity and can facilitate flexors.
Fine motor control and reflexes
Specialized fiber bundles are recruited for more complex and precise motor acts:
- The rubrospinal tract (tractus rubrospinalis). It originates in the midbrain red nucleus, immediately undergoes Forel's decussation, and travels within the lateral funiculus. It terminates on the lateral group of motor neurons. By exciting flexors and inhibiting extensors, it controls precise movements of the distal extremities (hands, feet)—for instance, mediating active, purposeful limb flexion.
- The tectospinal tract. It arises in response to visual, auditory, and tactile stimuli. It regulates contralateral head movements, mediating the orienting reflex.
- Monoaminergic pathways. They originate from the locus coeruleus of the brainstem. By releasing norepinephrine, they exert an inhibitory influence on spinal motor neurons and interneurons.
Medial and lateral motor systems
Globally, descending tracts can be divided into two functional groups responsible for different phases of stepping and body positioning.
Lateral system (flexor control) The 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 the limb, when it is necessary to lift and advance the leg.
Medial system (extensor control) These pathways run in the anterior funiculi (e.g., the vestibulospinal tract). They activate antigravity muscles: the quadriceps femoris (m. quadriceps femoris) and the gastrocnemius (m. gastrocnemius), which acts as an extensor during standing. Peak activity of this system occurs during the stance phase, when it is necessary to support body weight and push off from the surface.
Briefly: answers to topical questions
From which specific brain structures does the tectospinal tract originate?
The tectospinal tract originates from midbrain structures. The specific source of this pathway is the nuclei of the superior colliculi (tectum). From the collicular plate, fibers project to the motor nuclei of the brainstem and spinal motor neurons, traveling within the anterior funiculi of the white matter. This pathway is involved in executing unconscious automatic movements, as well as visual and auditory startle reflexes.
Which tracts make up the medial motor system?
The vestibulospinal tract is a component of the medial motor system.
- The vestibulospinal tract (Tr. vestibulospinalis) ensures balance maintenance and the tone of antigravity musculature.
This group of descending pathways is localized predominantly in the anterior funiculi of the spinal cord. The medial system is responsible for activating extensors, maintaining posture, supporting body weight, and facilitating the stance phase of gait.
Where do the vestibulospinal tracts originate?
The vestibulospinal tracts originate from the vestibular nuclei of the brainstem. There are two such pathways:
- The lateral vestibulospinal tract originates from the lateral vestibular nucleus (Deiters' nucleus).
- The medial vestibulospinal tract originates from the medial vestibular nucleus.
These tracts project downward to spinal motor neurons, regulating extensor reflexes, balance, and neck muscle tone in response to changes in head position.
Pathways in the lateral funiculi (lateral system) cause the limb to FLEX for stepping, whereas pathways in the anterior funiculi (medial system) help it EXTEND for weight-bearing and counteracting gravity.
Class and exam questions
What is the difference between the lateral and medial motor systems?
The lateral system controls flexors and operates during the swing phase of the limb (advancing the leg forward). The medial system controls extensors and operates during the stance phase, supporting body weight and maintaining posture.
What is the function of the rubrospinal tract?
It is responsible for the fine motor control of distal limb segments (hands, feet). The pathway excites flexor motor neurons, ensuring active, purposeful actions, in contrast to coarse postural control.
What is the role of monoaminergic pathways?
They descend from the locus coeruleus of the brainstem and, using norepinephrine, induce inhibition of spinal interneurons and motor neurons, modulating their excitability.
What else to study on this topic
- Topography of tracts in the spinal cord funiculi
- Forel's decussation and the course of the rubrospinal tract
- Mechanisms of the orienting reflex (tectospinal tract)
- Biomechanics of walking: swing and stance phases
- Interaction of alpha and gamma motor neurons in postural maintenance
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