Functional anatomy of the frontal lobe
The frontal lobe (lobus frontalis) plays a crucial role in regulating higher mental functions and organizing complex forms of human behavior.
Several functionally significant zones are distinguished within the structure of the frontal lobe:
- Broca's area: serves as the motor speech center. Topographically, it is localized in the inferior frontal gyrus of the dominant hemisphere.
- Prefrontal area: neurons in this region specialize in reflecting the dynamics of an ongoing behavioral act. This zone is inextricably linked with the mechanisms of working memory and attention, and is activated at the critical moment of goal selection.
- Anterior region of the frontal lobe: this area is responsible for the highest manifestations of nervous activity. It takes direct part in shaping individual personality traits and ensures the execution of creative processes.
Organization of the parietal lobe
The complex functionality of the parietal lobe (lobus parietalis) is determined by the presence of two key structural regions: the somatosensory area and the parietal association area.
Afferent connections of the somatosensory cortex are provided by the inflow of information from the thalamic structures. These signals are divided into two types:
- Impulses from specific thalamic nuclei carrying precise sensory information.
- Influences from nonspecific thalamic nuclei, whose function is to determine the level of the active state of the cerebral cortex (maintaining its tone).
The neuronal architecture of the parietal lobe is distinguished by the presence of a vast array of specific neurons. Their main feature is the ability to respond to external or internal stimuli of a strictly defined sensory modality. Within this group, projection neurons are particularly prominent, characterized by an unambiguous connection with a specific projective receptive peripheral field located on the human body.
Physiology of neurons in sensory cortical areas
Cellular elements of primary sensory areas are typically classified based on the types of their responses to incoming stimuli:
- Specific projection neurons exhibit reactions distinguished by a short and stable latent (hidden) period. Any such reaction is predictably accompanied by the subsequent process of activation or inhibition of the nerve cell.
- Nonspecific (as well as non-projection) neurons are capable of responding to both specific and nonspecific types of stimuli. They are characterized by a prolonged and highly variable latent period. Moreover, the duration of the neuronal response itself in most cases significantly exceeds the actual duration of the stimulus acting on the receptors.
The most important physiological phenomenon in specific projection areas is sensory-biological convergence. The essence of this phenomenon is that the same cortical neurons simultaneously receive two different types of excitation: primary sensory impulses and impulses from powerful motivogenic centers, which include the hypothalamus and various limbic structures.
Principle of localization of functions in the cortex
Modern neurophysiological understanding of cerebral hemisphere function categorically rejects two outdated paradigms: the narrow center localization theory (where each function was assigned a rigidly fixed point) and the equipotentiality theory (which postulated complete equivalence and interchangeability of all cortical areas).
Currently, the concept of dynamic localization of functions is considered fundamental. This concept is based on the constant and extremely close interaction of excitation flows propagating via both vertical and horizontal connection types between neurons. This very dynamic principle underlies the physiological mechanisms of compensation—the ability of the nervous system to restore impaired functions in case of local damage to cortical structures.
Briefly: answers to topical questions
In which gyrus is the primary somatosensory cortex of the parietal lobe located?
The primary somatosensory cortex is located in the posterior postcentral gyrus of the parietal lobe. This region contains the SI and SII somatosensory areas, where thalamocortical pathways originating from the contralateral half of the spinal cord terminate. The SI area has a clearly defined somatotopic organization, where the representation area of body parts is proportional to the number of cutaneous receptors in that region.
Where is the sensory speech center (Wernicke's area) localized?
The sensory speech center is localized in the posterior parts of the first (superior) temporal gyrus of the dominant hemisphere, usually the left. Anatomically, this region corresponds to the planum temporale, the size of which correlates with hemisphere dominance. The center is responsible for understanding spoken language and creating the semantic structure of an utterance.
What symptoms occur in lesions of the parietal association cortex?
Lesions of the parietal association cortex result in complex perceptual disorders while preserving sensitivity and motor function. The main symptoms include:
- Astereognosis — the loss of the ability to recognize objects by touch.
- Autotopagnosia — impaired perception of one's own body schema.
- Anosognosia — failure to recognize one's own deficit.
- Apraxia — impairment in performing automated practical skills.
Depending on the side of the lesion, agraphia, acalculia, alexia (in dominant hemisphere lesions) or emotional unconstraint and impaired time perception (in nondominant hemisphere lesions) may also be observed.
What cytoarchitectonic layers is the cerebral cortex divided into?
The neocortex (isocortex) is subdivided into six layers according to cyto- and myeloarchitectonics (from outside to inside):
- I. Molecular layer (Lamina molecularis) — contains few neurons and tangentially running nerve fibers.
- II. External granular layer (Lamina granularis externa) — includes small cells (pyramidal, stellate, inhibitory).
- III. External pyramidal layer (Lamina pyramidalis externa) — medium-sized pyramidal neurons predominate.
- IV. Internal granular layer (Lamina granularis interna) — consists predominantly of small stellate neurons.
- V. Internal pyramidal layer (Lamina pyramidalis interna).
The sixth layer is not explicitly named in the presented macro- and microscopic description data, but their total number is six.
To remember the functions of the frontal lobe, use the acronym "LVP": Personality (anterior region), Goal selection (prefrontal area), Speech (Broca's area).
Class and exam questions
Where exactly is the motor speech center located?
The motor speech center (the so-called Broca's area) is localized in the inferior frontal gyrus of the dominant hemisphere of the cerebral cortex.
What is the functional difference between projection and non-projection neurons of the sensory cortex?
Projection neurons are connected to a specific receptive field and react quickly and stably. Non-projection neurons respond to various stimuli, have a long variable latent period, and their reaction lasts longer than the exposure itself.
What is meant by sensory-biological convergence?
This phenomenon in cortical projection areas, where sensory impulses from receptors and signals from motivogenic centers (hypothalamus, limbic structures) simultaneously converge on the same neurons.
How does modern physiology explain the localization of functions in the cortex?
Instead of theories of narrow localization or complete equivalence of areas, the concept of dynamic localization of functions has been adopted. It relies on complex vertical and horizontal interactions of excitations, which provides opportunities for compensation following injuries.
What else to study on this topic
- Features of the formation of motivogenic influences from the hypothalamus and limbic system
- Mechanisms of compensation for impaired functions of the cerebral cortex
- Neurophysiological bases of working memory and attention
- Role of non-specific thalamic nuclei in the regulation of cortical tone
- Interaction of horizontal and vertical connections in the concept of dynamic localization
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