Involuntary movements in central paresis
A lesion of the upper motor neuron (corticomuscular pathway) leads to spastic paresis or paralysis. The loss of inhibitory cortical control over lower structures results in specific involuntary motor acts.
- Pathological synkinesias are associated involuntary movements that accompany a targeted voluntary act. For example, when a patient attempts to squeeze a paralyzed hand, the arm may involuntarily elevate at the shoulder joint. Synkinesias can also occur reflexively during yawning, laughing, or sneezing.
- Defensive reflexes — the appearance of involuntary movements in paretic limbs in response to intense stimuli (such as deep pain or a skin prick).
Such disorders are most commonly seen in central hemiparesis, which occurs due to a lesion in the contralateral cerebral hemisphere (in the area of the corona radiata or the posterior limb of the internal capsule), as well as in brainstem or lateral spinal cord tract lesions. Additionally, central paresis is accompanied by spastic increases in muscle tone, hyperactive deep reflexes, and the appearance of pathological hand and foot signs (e.g., Hoffmann's reflex).
Muscle twitching in peripheral lesions
If the pathological process affects the lower motor neuron, flaccid (atrophic) paralysis develops. Their characteristic feature is local involuntary contractions of muscle fibers occurring against the background of muscle weakness and hypotonia:
- Fasciculations are visible subcutaneous twitches of muscle fiber bundles. This is a crucial topical sign of damage to motor neurons of the anterior horns of the spinal cord (anterior horn syndrome) or anterior roots.
- Fibrillations — detected instrumentally during electroneuromyography (ENMG) and represent contractions of individual muscle fibers as a result of their denervation.
In addition to involuntary contractions, peripheral paresis is characterized by rapidly developing muscle hypotrophy. They appear in 2–3 weeks due to the loss of the trophic factor normally supplied to the muscle via the neuromuscular synapse from the axon. In mixed paresis, fasciculations and pronounced atrophy may be combined with central signs — hyperreflexia and pathological reflexes.
The role of the cerebellum in movement coordination and inhibition
The cerebellum plays a key role in integrating sensory signals and modulating motor output to all motor areas of the brain. It gathers information from the vestibular apparatus, proprioceptors, visual, and auditory systems, preventing chaotic and excessive movements.
The cerebellar cortex has a strict three-layered structure (molecular, ganglionic, and granular layers), with Purkinje cells acting as the main efferent elements. Their densely branching dendrites are located in the molecular layer, while their axons project to the deep nuclei — the dentate nucleus (nucleus dentatus), emboliform, globose, and fastigial nucleus (nucleus fastigii). Purkinje cells generate inhibitory signals using the neurotransmitter gamma-aminobutyric acid (GABA).
At the same time, afferent pathways (mossy and climbing fibers) use excitatory neurotransmitters — glutamate and aspartate. Mossy fibers transmit signals to granule cells, whose axons (parallel fibers) excite the dendrites of Purkinje cells. Inhibitory interneurons (basket cells and Golgi cells) further regulate this process. It is precisely the complex balance of powerful glutamatergic excitation and GABAergic inhibition that ensures the smoothness of voluntary movements.
Briefly: answers to topical questions
What pathological plantar signs are elicited in central paresis?
When the upper motor neuron is affected in the lower limb, pathological reflexes are elicited, which are subdivided into extensor and flexor reflexes. Extensor pathological reflexes include the Babinski reflex, as well as Oppenheim and Gordon symptoms, manifested by reflex extension of the big toe. A flexor pathological reflex is the Rossolimo reflex, characterized by rapid plantar flexion of the toes. Bechterew's reflex is also examined. Extensor pathological reflexes are one of the early and consistent signs of pyramidal tract lesions.
What types of pathological synkinesias are distinguished in neurology?
In neurology, when the upper motor neuron is affected, pathological synkinesias are identified as involuntary associated movements accompanying a voluntary act. Synkinesias that occur during an attempted movement in a paretic limb are distinguished, such as elevation of the arm at the shoulder joint when attempting to squeeze the hand. Pathological synkinesias can also occur during other physiological acts, specifically during sneezing, laughing, or yawning.
What symptoms are included in Charcot's cerebellar triad?
The provided study materials do not contain information about the specific Charcot's cerebellar triad. Instead, the texts describe classic manifestations of cerebellar lesions and its pathways. The main manifestations include impaired coordination of movements in the limbs, balance and gait disorders, as well as muscle hypotonia, dysarthria, and nystagmus.
To remember cerebellar neurotransmitters: Glutamate — excites (stimulates granule cells), while GABA — inhibits (suppresses impulses from Purkinje cells).
Class and exam questions
What is the difference between synkinesias and fasciculations?
Synkinesias are associated involuntary movements of an entire limb when attempting to perform a voluntary action, which is a sign of central paresis. Fasciculations are local twitchings of muscle bundles indicating a lower motor neuron lesion (e.g., anterior horns of the spinal cord).
Why should glutamate antagonists not be used in cerebellar lesions?
Glutamate provides the primary transmission of excitatory signals to Purkinje cells via mossy fibers and granule cells. Blockade of these receptors inevitably exacerbates cerebellar motor dysfunction.
In which disease do fasciculations and hyperreflexia occur simultaneously?
Such a clinical presentation is characteristic of mixed paresis, which occurs with combined lesions of the upper and lower motor neurons. A classic clinical example of such a disease is amyotrophic lateral sclerosis.
What else to study on this topic
- Topical diagnosis of central hemiparesis and monoparesis.
- Clinical and instrumental signs of neuromuscular junction lesions.
- Myopathic syndrome and typical localization of muscle weakness.
- Detailed anatomy of cerebellar afferent and efferent pathways.
- Integration of sensory information in the deep dentate nucleus (nucleus dentatus).
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