Structure and Neurochemistry
Anomatically, the basal nuclei are located within the white matter of the telencephalon. They consist of several functionally connected structures:
- Striatum (striatum). This is the main afferent center of the system, which combines the caudate nucleus (nucleus caudatus) and the putamen (putamen). This is where the majority of signals from the cerebellum arrive.
- Globus pallidus (globus pallidus).
- Amygdala (amygdala).
Efferent pathways connect these nuclei to the thalamus and the roof of the midbrain. The normal functioning of this complex network is ensured by a number of neurotransmitters. The neurochemistry of the basal nuclei involves glutamate, dopamine, GABA (gamma-aminobutyric acid), and Substance P (Substance P).
Physiology: Striopallidal System
A crucial feature of the basal nuclei is that their direct stimulation does not cause a specific movement. Their function is modulating; they alter the excitability of motor neurons at various levels of the central nervous system.
The interaction between the cortex and the basal nuclei forms the so-called striopallidal system. It includes the caudate nucleus, putamen, globus pallidus, as well as the subthalamic nucleus and substantia nigra. The signal moves along the loop: Cortex → Striatum → Globus pallidus → Thalamus → Cortex.
This level is responsible for executing complex learned motor patterns (e.g., writing or cutting paper with scissors). The system ensures:
- Initiation and termination of movements.
- Control of the amplitude and speed of contractions.
- Suppression of extra, involuntary movements.
- Cognitive control over motor activity.
Different nuclei play different roles in activating motor function. Neurons of the caudate nucleus generate potentials 60 milliseconds before action onset. Neurons of the putamen are activated prior to slow arm movements, but remain "silent" during fast movements or slow movements in the opposite direction.
Pathophysiology and Parkinson's Disease
Damage to the basal nuclei leads to profound disorders exclusively in the motor sphere. Involuntary movements occur, and the kinematics of movement initiation and termination are disrupted.
A classic example of such pathology is Parkinson's disease. Its etiology is based on the destruction of dopaminergic pathways running from the substantia nigra (substantia nigra) to the striatum (striatum). The disease manifests with a characteristic clinical triad, which is often accompanied by a mask-like, immobile face.
Symptom Triad:
- Akinesia. The patient finds it difficult to initiate or complete a movement. Pathophysiologically, this is viewed as a disruption in the generation of nerve impulses within the basal nuclei.
- Rigidity. There is a plastic increase in muscle tone (the "lead-pipe rigidity" phenomenon).
- Resting tremor. Tremor of the head and limbs occurs at a frequency of 4–7 Hz or higher, which disappears upon attempting a purposeful movement.
Rigidity and tremor are a direct consequence of hyperactivity of the nuclei. Due to the loss of normal inhibitory control, the system becomes disinhibited, leading to motor deficit.
Briefly: answers to topical questions
From which structures does the striatum receive afferent fibers?
The striatum receives incoming fibers from the cerebral cortex, diencephalon, midbrain, and cerebellum.
- Cerebral cortex — afferents arrive from all cortical areas (the frontal cortex releases the excitatory neurotransmitter glutamate).
- Thalamus — from nonspecific nuclei and the centromedian nucleus, which transmits impulses from the cerebellum and reticular formation.
- Substantia nigra (Substantia nigra) — dopaminergic fibers of the nigrostriatal pathway.
- Raphe nuclei (Nuclei raphes) — serotonergic fibers.
- Cerebellum — supplies the majority of primary afferent signals.
What efferent pathways originate from the globus pallidus?
Efferent signals from the globus pallidus propagate via ascending and descending pathways.
- Thalamus — the main ascending output inhibitory pathway from the internal segment, which relays impulses back to the cerebral cortex.
- Subthalamic nucleus — an inhibitory pathway from the external segment, releasing GABA.
- Brainstem complex — descending pathways to the reticular formation and red nucleus for the regulation of muscle tone and posture.
- Inferior olivary nucleus — an efferent pathway proceeding further to cerebellar structures.
To memorize the classic clinical triad of basal nuclei lesions in Parkinson's disease, use the mnemonic ART: Akinesia, Rigidity, Tremor at rest.
Class and exam questions
Is it possible to induce muscle contraction by applying current to the basal ganglia?
No. Stimulation of these structures does not cause a specific movement, as they perform a modulating function, merely altering the excitability of motor neurons.
Where do the basal nuclei receive most of their afferent information from?
Most afferent signals come from the cerebellum directly into the striatum.
Why does muscle rigidity occur in parkinsonism?
Rigidity and resting tremor are consequences of basal nuclei disinhibition. Due to the death of dopaminergic pathways, inhibitory control is lost, leading to hyperactivity of the structures and a plastic increase in tone.
What else to study on this topic
- Functional differences between the caudate nucleus and the putamen
- The role of GABA, glutamate, and Substance P in the striopallidal system
- The mechanism of cognitive control over motor activity
- Connections of the basal nuclei with the thalamus and the roof of the midbrain
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