Frontal and parietal lobes: motor and sensory functions
The frontal lobe contains crucial motor and associative centers. According to Kleist's modification, area 11 is associated with individual and collective consciousness, area 10 provides the sequence of actions, and area 9 drives motivation. Area 4 (paracentral lobule) localizes the cortical representation of individual movements, including the innervation of the foot, urinary bladder, and rectum. Premotor zones such as area 6aβ are responsible for trunk rotation, while area 6aα is responsible for complex motor skills. Stimulation of area 8 elicits a falling reaction and eye deviation to the opposite side, whereas area 6b controls chewing, swallowing, and phonation.
The parietal lobe is the core of the somatosensory system. Areas 3, 1, and 2 form the primary sensory area, where tactile and proprioceptive impulses are processed. Area 7 acts as the sensory area for action, and at the border of areas 31 and 7, the leg projection is located. Stimulation of the superior parietal lobule (area 5a) induces complex synergistic movements of the contralateral limbs followed by head and eye turning.
Temporal, occipital lobes, and the limbic system
The temporal lobe houses the centers of hearing and complex gnostic perception. Area 20 provides the understanding of sounds and music, while area 37 is associated with the comprehension of names. Electrical stimulation of the superior temporal gyrus (area 22) elicits not only auditory sensations but also a conjugate turning of the head and trunk. The lower part of area 19 is involved in object recognition and color perception.
The limbic system integrates autonomic and olfactory functions. The cingulate gyrus (areas 33, 24, 23, 31) is responsible for body awareness. The olfactory analyzer is represented by area 34 (objective odor perception) and area 28 (sniffing). The uncus and parahippocampal gyrus region generates autonomic responses to odors.
The occipital lobe is entirely dedicated to visual perception. The cortical end of the visual analyzer is localized in areas 17, 18, and 19. The upper lip of the calcarine sulcus (cuneus) receives impulses from the lower quadrant of the visual field, while the lower lip (lingual gyrus) receives them from the upper quadrant. The occipital pole serves as the projection site for the macula (macula lutea).
Somatotopic organization: homunculus and plasticity
Motor and sensory projections in the cortex are organized according to somatotopy, classically described as the "homunculus". In the precentral gyrus, the human body is projected "upside down": the toes and foot are represented in the parasagittal zone, followed inferiorly by the trunk and hand (with a massive area for the hand and fingers), and in the lowest part by the face, tongue, pharynx, and larynx. The postcentral gyrus entirely duplicates this inverted scheme for sensation.
A crucial principle is the disproportion between cortical areas and the physical dimensions of body parts. The representation area depends exclusively on receptor density. The tongue, lips, and hand occupy gigantic areas of the cortex, whereas the back or thigh has a minimal projection.
Neuroplasticity allows these maps to change. With intensive tactile training, the zone of active fingers expands. Following amputation, reorganization occurs: the cortical area deprived of afferentation begins to process impulses from neighboring zones (e.g., from the face), which may underlie the pathogenesis of phantom pain.
Structural and functional organization of the primary visual cortex
The primary visual cortex (area 17) possesses a unique modular microstructure that ensures binocular vision and detailed stimulus analysis. Afferent fibers of the optic radiation arrive here in strict retinotopic order, obeying the rule of decussation: the right hemisphere sees the left half of the visual field.
The basic functional unit is the vertical cortical column, 30–100 microns wide. The complex architecture includes:
- Ocular dominance columns: alternating stripes responding strictly to stimuli from the ipsilateral or contralateral eye.
- Pinwheels (pinwheels): groups of columns converging radially toward a center, where each segment is tuned to a specific angle of orientation of a visual stimulus.
- Blobs (blobs): cylindrical zones within columns specialized for color perception.
A similar columnar organization is characteristic of the somatosensory cortex, where neurons at different depths of a single column simultaneously analyze proprioceptive, pain, temperature, and tactile sensitivity from a single narrow skin area.
Briefly: answers to topical questions
What functions do the basal nuclei perform in the extrapyramidal system?
The basal nuclei act as the highest subcortical regulatory and coordination center for organizing movements. They perform the following functions:
- Initiation of movement — starting and facilitating voluntary movements.
- Coordination of movement — ensuring coherence, smoothness, and efficiency by suppressing unwanted influences.
- Regulation of muscle tone — automatically coordinating the strength and duration of various muscles.
- Control and correction of movement — comparing the initial motor program with the actually executed movement through a feedback mechanism.
They also regulate phasic motor activity.
What structures make up the basal nuclei of the brain?
The basal nuclei comprise clusters of gray matter located deep within the white matter of the telencephalon. The main anatomical structures are:
- Caudate nucleus (nucleus caudatus) — together with the putamen, forms the striatum (striatum).
- Putamen (putamen) — the outer part of the lentiform nucleus.
- Globus pallidus (globus pallidus) — inner and outer segments, part of the lentiform nucleus.
- Claustrum (claustrum) — belongs to the basal ganglia embryologically.
- Amygdala — functionally belongs to the limbic system.
Associated nuclei also include the subthalamic nucleus, substantia nigra (substantia nigra), and red nucleus (nucleus ruber).
What clinical syndromes develop upon damage to the striopallidal system?
Damage to the striopallidal system leads to complex motor and cognitive disorders manifested by an excess or deficiency of motor impulses. The main clinical syndromes include:
- Parkinsonism — includes idiopathic Parkinson's disease, symptomatic and secondary parkinsonism (manifested by hypokinesia, rigidity, and tremor).
- Parkinson-plus syndrome — a combination of parkinsonism with other neurological disorders.
- Chorea — involuntary hyperkinesias (includes Huntington's disease and Sydenham's chorea).
- Ballism — large-amplitude flinging movements (usually hemiballism).
- Dystonia — pathological change in muscle tone with the formation of bizarre postures.
Combined damage is also possible, for example, in Wilson's disease.
What are the main differences in symptoms between damage to the basal nuclei and the pyramidal tract?
The main differences lie in the nature of motor disturbances and changes in muscle tone.
| Sign | Pyramidal tract lesion | Basal nuclei lesion |
|---|---|---|
| Nature of movements | Development of paresis or paralysis. | Hypokinesia or hyperkinesias: chorea, athetosis, ballism. |
| Muscle tone | In the acute phase, a decrease in tone (flaccid paresis) may occur, subsequently transitioning into spasticity. | Rigidity and dystonia, i.e., pathological changes in muscle tone, may occur. |
| Associated phenomena | Syncinesias may occur in spastic paralysis. | Tremor, postural instability, athetosis, and ballism are possible. |
What role does the substantia nigra play in the regulation of muscle tone?
The substantia nigra (substantia nigra) is involved in the regulation of muscle tone via the dopaminergic nigrostriatal pathway, which connects it to the striatum (striatum). It maintains a reciprocal relationship with the striatum, supplying it with dopamine. Normal functioning of these synapses is essential for maintaining adequate muscle tone and smooth movements. The degeneration of melanin-containing neurons in the pars compacta of the substantia nigra results in dopamine deficiency, leading to a compensatory increase in the activity of GABAergic neurons in the striatum. Clinically, this manifests as pathological alterations in muscle tone of the rigidity type, as well as akinesia and tremor.
What types of hyperkinesia are characteristic of basal ganglia pathology?
Basal ganglia pathology is characterized by excessive involuntary movements. The main types include:
- Chorea — a type of hyperkinesia characteristic, in particular, of Huntington's disease and Sydenham's chorea.
- Athetosis — a type of hyperkinesia listed among the main movement disorders in basal ganglia lesions.
- Ballism (most commonly hemiballism) — wide-amplitude, large-scale, flinging movements, predominantly affecting the proximal joints of the limbs.
- Dystonia — prolonged involuntary muscle contractions resulting in abnormal postures; blepharospasm and spasmodic torticollis may occur.
- Tremor — one of the cardinal symptoms of Parkinson's disease.
Where are the basal ganglia anatomically located within the hemispheres of the telencephalon?
The basal ganglia are anatomically situated within the subcortical white matter of the telencephalon (telencephalon). They represent clusters of gray matter deep within the cerebral hemispheres and lie in close proximity to the diencephalon and the ventricular system.
- The caudate nucleus has a curved shape: its head forms the lateral wall of the lateral ventricle, while its tail is located in the temporal lobe, forming the roof over the inferior horn of the lateral ventricle.
- The putamen overlies the head of the caudate nucleus laterally (rostrally).
- The globus pallidus and putamen (lenticular nucleus) are separated from the caudate nucleus and thalamus by the internal capsule, and from the claustrum by the external capsule.
Which neurotransmitters are involved in synaptic transmission within the basal ganglia?
Synaptic transmission within the basal ganglia involves the following neurotransmitters:
- Glutamate — the neurotransmitter of the corticostriatal pathway and fibers from the subthalamic nucleus.
- Dopamine — the neurotransmitter of the nigrostriatal pathway from the substantia nigra to the striatum.
- GABA — the neurotransmitter of the direct and indirect pathways originating from the striatum.
- Serotonin — the neurotransmitter of fibers from the raphe nuclei.
- Substance P — a co-neurotransmitter of the direct pathway.
- Enkephalin — a neurotransmitter of the indirect pathway.
Class and exam questions
What happens when the primary motor cortex (area 4) is damaged?
A contralateral flaccid paresis develops. If the adjacent premotor area containing extrapyramidal fibers is involved, the paresis becomes spastic.
How does irritation of the somatosensory cortex manifest?
It causes sensory Jacksonian seizures — localized paresthesias or dysesthesias on the contralateral side of the body, occasionally accompanied by focal motor phenomena.
Where does the right visual cortex receive its information from?
From the temporal half of the right retina and the nasal half of the left retina, which together comprise the left half of the visual field.
What else to study on this topic
- Brodmann Cytoarchitectonic Areas
- Jacksonian epilepsy symptom complex
- Morphology of ocular dominance columns
- Principles of cortical neuroplasticity
- Visual pathway tracts
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