Morphophysiology and neurochemistry of the extrapyramidal system
The extrapyramidal system (EPS) includes all motor pathways except the pyramidal tract and regulates muscle tone, smoothness, and automatism of movements. It exerts a regulating influence on the spinal cord, brainstem, and cerebellum.
Deep within the cerebral hemispheres lie subcortical nuclei (basal ganglia):
- Caudate nucleus (nucleus caudatus);
- Putamen (putamen);
- Globus pallidus (globus pallidus);
- Subthalamic nucleus (nucleus subthalamicus).
The combination of the caudate nucleus, putamen, and globus pallidus is called the corpus striatum, or striatum (striatum). The brainstem includes the substantia nigra (substantia nigra), red nucleus (nucleus ruber), and reticular formation (formatio reticularis).
The influence on the spinal cord is transmitted via descending pathways: tectospinal tract (tractus tectospinalis), rubrospinal tract (tractus rubrospinalis), reticulospinal tract (tractus reticulospinalis), and vestibulospinal tract (tractus vestibulospinalis). Regulation is carried out by dopamine (substantia nigra $\rightarrow$ striatal D1 and D2 receptors), the inhibitory neurotransmitter GABA, and the excitatory glutamate.
Akinetic-rigid syndrome
The pathogenesis of the syndrome is associated with dopamine deficiency due to the death of pigmented cells of the substantia nigra or decreased sensitivity of striatal neurons, which leads to reduced activation of the motor cortex.
Manifestations of the syndrome:
- Hypokinesia — difficulty initiating movements, reduction in their amplitude and speed. Slow gait, hypomimia (mask-like face), and micrographia (reduction in letter size when writing) are observed.
- Rigidity — increased muscle tone with a predominance in the flexors of the trunk and limbs.
To detect mild hypokinesia, functional tests are used: rhythmic fist-clenching, rapid pronation-supination movements of the hands, opposition of the thumb and index finger, as well as heel-tapping on the floor.
Tone changes and differential diagnosis
Plastic increase in tone in EPS damage differs from spasticity in central paresis and disorders in functional disturbances.
| Parameter | Plastic tone (Rigidity) | Spastic tone / Functional paresis |
|---|---|---|
| Nature of resistance | Felt throughout the entire passive movement | Resistance occurs only at the beginning of the movement (spasticity) |
| Phenomena | «Lead-pipe» and «cogwheel» rigidity | Absent |
| Muscle strength and reflexes | Strength is preserved, pathological reflexes are absent | In central paresis, pathological reflexes are present; in functional disorders, there is no hypotrophy |
Hyperkinetic disorders: tremor and tics
Hyperkinesias occur due to the death of inhibitory GABAergic neurons of the striatum or excessive activation of dopaminergic pathways.
Tremor — involuntary rhythmic oscillations due to contraction of antagonist muscles:
- Rest tremor: a characteristic sign of Parkinson's disease; increases with emotions, decreases with voluntary movement of the affected limb.
- Action tremor: includes postural (when holding a static posture) and kinetic (during movements) tremor.
Tics — rapid, involuntary, stereotyped muscle contractions:
- Motor: simple (blinking, head twitching) and complex (gestures).
- Vocal: simple (throat clearing, sniffling) and complex (repeating or shouting out phrases).
In Tourette syndrome, a chronic course (more than 1 year) is observed with a combination of generalized motor and complex vocal tics, often associated with attention deficit hyperactivity disorder.
Briefly: answers to topical questions
What types of ataxia exist depending on the lesion localization?
Depending on the lesion localization, the following forms of ataxia are distinguished:
- Cerebellar ataxia — occurs during a pathological process in the cerebellum and its pathways.
- Cortical (frontal) ataxia — caused by damage to the cortex of the frontal, temporal, or temporo-occipital regions of the brain.
- Sensory (posterior column) ataxia — develops when the posterior columns or posterior roots of the spinal cord, the thalamus, and peripheral nerves are damaged.
- Vestibular ataxia — observed with damage to the vestibular apparatus, vestibular nuclei, brainstem, or the IV ventricle region.
What clinical tests are used to diagnose cerebellar ataxia?
A set of special clinical tests is used to diagnose cerebellar ataxia and coordination disorders:
- Gait tests — assessment of gait during normal, tandem, and flanged walking.
- Romberg test — examination of stability in a standing position; in cerebellar ataxia, instability is observed with open eyes, and when eyes are closed, swaying increases only slightly.
- Babinski's test — detection of asynergia when trying to sit up from a lying position without using hands.
- Finger-to-nose test — detection of movement accuracy impairment and intention tremor.
- Heel-to-shin test — detection of impairment in accuracy, smoothness, and proportionality of movements in the lower limbs.
- Diadokokinesis tests — detection of impaired coordination of rapid alternating movements.
What types of extrapyramidal hyperkinesia exist besides tremor and tics?
Besides tremor and tics, the sources mention:
- Chorea — a manifestation of hyperkinetic syndrome in Huntington's disease.
- Choreoathetosis — a manifestation of hyperkinetic syndrome in Huntington's disease.
Remember the key triad of EPS neurotransmitters: Glutamate — Excites, GABA — Inhibits, Dopamine — Modulates (via D1 and D2 receptors).
Class and exam questions
How does plastic tone in rigidity differ from spastic tone?
With plastic tone, muscle resistance is felt evenly throughout the passive movement (the "lead-pipe" phenomenon), muscle strength is preserved, and pathological reflexes are absent. With spastic tone, resistance increases only at the beginning of muscle stretch.
What is the pathogenetic mechanism of hypokinesia development?
Hypokinesia develops due to dopamine deficiency resulting from the death of substantia nigra neurons or decreased sensitivity of striatal receptors, which leads to inadequate activation of the motor areas of the frontal cortex.
What is the difference between postural and kinetic tremor?
Postural tremor occurs during static muscle tension to maintain posture (e.g., with outstretched arms), whereas kinetic tremor manifests directly during the execution of a voluntary movement.
What are the diagnostic criteria for Tourette syndrome?
Tourette syndrome is characterized by a chronic course of tics (more than 1 year) with the obligatory combination of generalized motor and complex vocal tics.
What else to study on this topic
- Direct and indirect pathways of impulse conduction in the basal ganglia
- Methodology for performing pronation-supination and finger-to-nose tests
- Neurochemistry of thalamocortical and corticostriatal pathways
- Differential criteria for functional and organic paresis
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