Extrapyramidal system

Anatomical organization

The extrapyramidal system includes a number of interconnected nuclei located at various levels of the brain. Key structures involved in motor coordination:

• Subcortical nuclei of the cerebral hemispheres:
• Striatum (corpus striatum): serves as the entry gate of the system. It consists of the caudate nucleus and the putamen.
• Globus pallidus (globus pallidus): divided into internal and external segments.
• Nuclei of the diencephalon:
• Thalamus: its ventral nuclei act as a specific filter before returning the signal to the cortex.
• Subthalamic nucleus.
• Nuclei of the midbrain:
• Substantia nigra (substantia nigra): divided into the pars compacta (source of dopamine) and pars reticulata.
• Nuclei of the brainstem: participate in the coordination of unconscious movements.

The axons of associative neurons in this system form a decussation at the level of the medulla oblongata. The crossed fibers form the medial lemniscus, which heads toward the thalamus. Unlike sensory pathways that ascend to the postcentral gyrus, this pathway terminates in the precentral gyrus.

Functional cycle of movement control

The logic of signal transmission in the system represents a sequential exchange of information packets between the cortex and subcortical structures.

1. Sensory information from sensory organs enters the thalamus, is relayed, and travels to the sensory and motor areas of the cortex.
2. The cortex analyzes the data, makes a decision to move, and projects a control signal to the extrapyramidal system—specifically to the striatum (input signal).
3. Within the basal ganglia, the signal is processed taking into account existing motor experience. The generated corrective impulse goes to the output nuclei, returns to the thalamus, and from there back to the cortex.
4. This corrected signal modulates the activity of the giant pyramidal cells of the cortex. The final executive command descends via the pyramidal tracts to the muscles, ensuring precise movement.

Direct and indirect pathways of signal transmission

The circulation of impulses between the cortex, striatum, and thalamus occurs via two main pathways that use different neurotransmitters (excitatory glutamate and inhibitory GABA). The presence of pathways with opposing effects increases the precision of the motor response.

Direct pathway (movement initiation)

Produces a positive reaction by facilitating the execution of the motor program. The cortex excites the neurons of the striatum. These neurons send an inhibitory signal to the output nuclei of the system—the internal segment of the globus pallidus and the reticular part of the substantia nigra. At rest, these nuclei continuously inhibit the thalamus, blocking unnecessary information. The signal from the striatum suppresses their activity ("releases the brake"). The disinhibited thalamus becomes activated and sends an excitatory impulse back to the motor cortex, permitting movement. A mathematical principle applies here: an even number of inhibitory links (minus times minus) results in overall excitation.

Indirect pathway (movement suppression)

Produces a negative reaction by blocking unnecessary or competing movements. The circuit includes three inhibitory links (an odd number results in overall inhibition). The cortex excites other neurons in the striatum, which inhibit the external globus pallidus. Normally, the external segment inhibits the subthalamic nucleus. Since the external globus pallidus is suppressed, the subthalamic nucleus becomes activated. It sends an excitatory signal to the internal globus pallidus, which strongly inhibits the thalamus. Information does not pass to the cortex, and movement is blocked.

Role of dopaminergic modulation

The balance between the direct and indirect pathways is maintained by dopamine, secreted by neurons of the pars compacta of the substantia nigra. The axons of these neurons project to the striatum.

The effect of dopamine depends on the type of receptors on the striatal neurons. It has an excitatory effect on the direct pathway (promoting movement initiation) and an inhibitory effect on the indirect pathway (preventing blockade). As a result, the signals of both pathways are positively summated, reinforcing each other. Dopamine shifts the balance toward the facilitation of motor activity.

A sharp decrease in dopamine production in the substantia nigra underlies Parkinson's disease. A dissonance arises between the signals of the direct and indirect pathways. Signals enter the thalamus out of phase and conflict with each other, causing the cortex to receive weakened or distorted impulses. This manifests as impaired coordination of voluntary movements, rigidity, and paralytic tremor.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Cytoarchitectonics of the precentral gyrus
• Types of dopamine receptors in the striatum
• Connection of the extrapyramidal system with the cerebellum
• Topography and composition of the medial lemniscus
• Descending pyramidal tracts of the spinal cord

Other topics in the section «Nervous system: brain»

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