Anatomical Organization
The extrapyramidal system includes a series of interconnected nuclei located at various levels of the brain. Key structures involved in motor coordination:
- Subcortical nuclei of the cerebral hemispheres:
- Corpus striatum: serves as the entry gate of the system. It consists of the caudate nucleus and the putamen.
- Globus pallidus: divided into internal and external segments.
- Diencephalic nuclei:
- Thalamus: its ventral nuclei act as a filter before signals return to the cortex.
- Subthalamic nucleus.
- Midbrain nuclei:
- Substantia nigra: divided into the pars compacta (source of dopamine) and the pars reticulata.
- Brainstem nuclei: participate in the coordination of involuntary movements.
The axons of associative neurons in this system decussate at the level of the medulla oblongata. The crossed fibers form the medial lemniscus, which heads toward the thalamus. Unlike sensory pathways that project to the postcentral gyrus, this pathway terminates in the precentral gyrus.
Functional Motor Control Loop
The logic of signal transmission in the system is a sequential exchange of information packets between the cortex and subcortical structures.
- Sensory information from sensory organs arrives at the thalamus, is relayed, and projects to the sensory and motor areas of the cortex.
- The cortex analyzes the data, makes a decision about movement, and projects a control signal to the extrapyramidal system — specifically to the striatum (input signal).
- Within the basal ganglia, the signal is processed using existing motor experience. The generated corrective impulse is sent to the output nuclei, returns to the thalamus, and from there goes back to the cortex.
- This corrected signal modulates the activity of giant pyramidal cells in the cortex. The final executive command descends via 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 utilize different neurotransmitters (excitatory glutamate and inhibitory GABA). The presence of pathways with opposing effects increases the precision of the motor response.
Direct Pathway (Initiation of Movement)
Provides a positive response, facilitating the execution of a motor program. The cortex excites striatal neurons. These neurons send an inhibitory signal to the system's output nuclei — the internal globus pallidus and the pars reticulata 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 active and sends an excitatory impulse back to the motor cortex, permitting movement. Here, a mathematical principle applies: an even number of inhibitory links (minus times minus) results in net excitation.
Indirect Pathway (Suppression of Movement)
Provides a negative response, blocking unnecessary or competing movements. The circuit includes three inhibitory links (an odd number results in net inhibition). The cortex excites other striatal neurons, 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 active. It sends an excitatory signal to the internal globus pallidus, which powerfully inhibits the thalamus. Information does not reach the cortex, and movement is blocked.
Role of Dopaminergic Modulation
The balance between the direct and indirect pathways is maintained by dopamine, released by neurons in 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 striatal neurons. It exerts an excitatory effect on the direct pathway (facilitating 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 arrive at 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 resting tremor.