Anatomical basis of the extrapyramidal system
Centers that regulate unconscious motor activity are arranged in a strict hierarchy—from the cerebral cortex down to the spinal cord:
- Cerebral cortex: transmits information to subcortical nuclei and the cerebellum. The premotor cortex and area 8 play an important role; stimulation of the latter causes conjugate deviation of the gaze in the direction opposite to the stimulation.
- Basal ganglia and diencephalon: striatum (head of the caudate nucleus and putamen), globus pallidus, and thalamus (the collector of afferent pathways).
- Brainstem: contains key nuclei that act as generators for descending pathways. In the midbrain, these are the red nucleus (Nucleus ruber), substantia nigra (Substantia nigra), and tectal nuclei. In the pons and medulla oblongata, these are the reticular formation, the lateral vestibular nucleus (Deiters' nucleus), and the inferior olivary nucleus.
- Cerebellum: coordinates voluntary movements by connecting with the brainstem, including via the fastigial nucleus (Nucleus fastigii).
Main descending tracts
Brainstem centers process nerve impulses and transmit them to the spinal cord motor neurons via extrapyramidal pathways. Most of them terminate on anterior horn cells not directly, but through a system of interneurons.
- Rubrospinal tract (tractus rubrospinalis): runs in the lateral funiculi. Controls muscle tone and coordination, primarily activating flexor muscles.
- Vestibulospinal tract (tractus vestibulospinalis): regulates extensor tone, and is responsible for posture and balance maintenance.
- Reticulospinal tract (tractus reticulospinalis): participates in the regulation of muscle tone and autonomic functions.
- Tectospinal tract (tractus tectospinalis): provides protective motor reflexes in response to sudden visual and auditory stimuli.
- Olivospinal tract (tractus olivospinalis): originates from the inferior olivary nucleus.
Lateral and medial motor systems
At the spinal cord level, descending motor pathways are anatomically and functionally divided into two large groups:
- Lateral system: runs in the lateral funiculi (includes the rubrospinal and corticospinal tracts). Innervates primarily the distal parts of the limbs (especially the hands) and is responsible for smooth, precise voluntary movements.
- Medial system: runs in the anterior funiculi (includes vestibulospinal, reticulospinal, and tectospinal fibers). Innervates medially located motor neurons responsible for trunk and leg muscles. Its main task is to ensure standing and walking.
Role in the development of spastic paresis
Large $\alpha$-motor neurons (innervating contractile extrafusal fibers) and small $\gamma$-motor neurons (innervating receptor intrafusal fibers) are located in the anterior horns of the spinal cord. Extrapyramidal pathways form synapses with both cell types.
Acute lesions of central motor pathways initially cause a shock phase with flaccid paresis and hyporeflexia. After a few days or weeks, the paresis transforms into spastic paresis. It is important to note that isolated damage to the cerebral cortex alone causes flaccid paresis. Spasticity is always the result of combined damage to the pyramidal tract and additional extrapyramidal structures.
The mechanism of hyperreflexia lies in the loss of descending inhibitory control from the brain over $\gamma$-motor neurons. Because of this, intrafusal muscle fibers (inside muscle spindles) remain constantly active, "as if pre-stretched." They react excessively fast to stretch, especially in arm flexors and leg extensors. Clinically, this manifests as increased tone, clonus, and the appearance of pathological signs (e.g., Babinski sign — tonic extension of the great toe).
Briefly: answers to topical questions
What symptoms occur with lesions of the globus pallidus and substantia nigra?
A lesion of the substantia nigra leads to akinetic-rigid syndrome, whereas specific symptoms of isolated damage to the globus pallidus are not described in sources. The main manifestations of substantia nigra damage include:
- Akinesia — slowness of movement (hypokinesia).
- Rigidity — increase in muscle tone.
- Rest tremor — shaking that occurs at rest.
- Postural disturbances — specific changes in posture.
What afferent pathways project to the extrapyramidal system?
Afferent fibers from the cerebral cortex and brainstem nuclei project to the structures of the extrapyramidal system. The main pathways include:
- Corticostriatal pathway — glutamatergic fibers from motor areas of the cortex to the striatum.
- Cortical projections — fibers from the cortex to the substantia nigra, red nucleus (nucleus ruber), and subthalamic nucleus (nucleus subthalamicus).
- Striopallidal pathway — transmits information from the striatum to the globus pallidus (globus pallidus), which has no direct connections with the cortex.
- Pathways from the raphe nuclei (nuclei raphes) — serotonergic fibers.
Which neurotransmitters ensure signal transmission in the basal ganglia?
Signal transmission in the basal ganglia is provided by excitatory, inhibitory, and modulatory neurotransmitters, as well as cotransmitters.
- Glutamate — the excitatory neurotransmitter of the corticostriatal pathway.
- GABA is an inhibitory neurotransmitter of the direct and indirect pathways.
- Dopamine — the modulatory neurotransmitter of the nigrostriatal pathway.
- Serotonin is the neurotransmitter of afferent fibers from the raphe nuclei.
- Substance P — a cotransmitter of combined action in the direct pathway.
- Enkephalin — participates in the transmission of signals in the indirect pathway.
To memorize the functions of the main extrapyramidal tracts, use mnemonics based on the first letters: Rubrospinal — Rest (flexor tone), Vestibulospinal — Vertical/Posture (extensor tone, posture), Tectospinal — Threat/Startle (protective startle reflexes to vision and hearing).
Class and exam questions
Why doesn't spasticity occur when only the cerebral cortex is damaged?
Spastic elevation of muscle tone is largely due to damage to additional (extrapyramidal) pathways. With isolated cortical damage, these pathways remain intact, so the paresis is flaccid, and weakness does not reach the degree of complete plegia.
How does the extrapyramidal system interact with the spinal cord?
Descending extrapyramidal tracts terminate on $\alpha$- and $\gamma$-motor neurons of the anterior horns. In most cases, impulse transmission occurs not directly, but through a system of small interneurons that integrate and modulate the signal.
What is the function of the medial motor system of the spinal cord?
It includes the anterior funiculus tracts (reticulo-, vestibulo-, and tectospinal). This system forms long propriospinal connections and innervates trunk and leg muscles, providing basic functions of posture maintenance, standing, and walking.
What else to study on this topic
- Topical diagnosis of brainstem lesions: Weber, Millard-Gubler, and Jackson alternating syndromes.
- Pathogenesis of central spastic paresis: detailed role of $\gamma$-motor neurons and muscle spindles.
- Somatotopic organization of motor pathways at the level of the internal capsule and brainstem.
- Peripheral part of the motor system: from anterior horn motor neurons to neuromuscular junctions.
- Spinal cord afferent fibers (Ia and Ib) and their participation in the formation of reflex arcs.
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