Primary sensory areas and their afferent pathways
Primary cortical areas are responsible for the basic analysis of incoming information. Their function is strictly tied to projection pathways (white matter), through which impulses reach the cortex.
Primary visual cortex (area 17) Its basic structural unit is the hypercolumn with an area of about 1 mm². Hypercolumns form a regular pattern and are interconnected by horizontal cells. This area is responsible for elementary stimulus analysis (form, color). Electrical stimulation of this area produces simple sensations: flashes, bright lines. Pathology: Unilateral lesion of area 17 leads to contralateral hemianopia. Partial damage causes quadrantanopia. Central vision is preserved if the posterior end of the calcarine sulcus is spared.
Primary auditory cortex (area 41) Located in Heschl's transverse gyri (superior temporal gyrus, depth of the lateral sulcus). Impulses arrive here via a complex trajectory: from the spiral organ through the lateral lemniscus to the medial geniculate body (MGB), and from there to the cortex. A key feature: the cortex of each hemisphere receives a bilateral projection (from both ears). There is a strict tonotopic organization: low frequencies are analyzed rostrolaterally, high frequencies caudomedially. The cortex has a columnar organization with alternating EE-neurons (excited by binaural signals) and EI-neurons (inhibited by them).
Primary gustatory and vestibular cortex Taste impulses travel from the rostral nucleus of the solitary tract via the central tegmental tract to the thalamus (ventral posteromedial nucleus). Further, through the posterior limb of the internal capsule, they reach area 43 (inferior frontal gyrus). Vestibular information travels from the brainstem nuclei to the thalamus and then to area 2v (base of the intraparietal sulcus). Neurons in area 2v integrate visual, proprioceptive, and vestibular information.
Associative areas: secondary and tertiary fields
Association fibers of the white matter connect primary areas with higher analysis zones.
Secondary fields Located adjacent to primary fields. Their role is to interpret sensory impulses and compare them with existing experience. Unlike primary areas, they do not receive afferent information directly from the thalamus (lack direct connections). These include:
- Areas 18 and 19 (visual association cortex): higher-level visual analysis.
- Area 5 (somatosensory association cortex).
- Area 22 (auditory association cortex).
Tertiary fields These are multifunctional zones not restricted to a single analyzer. They form numerous connections with various brain regions. It is here that motor and speech concepts independent of incoming sensory information originate ("draft version" of a thought).
- Frontal associative area: the largest, accounting for up to 20% of the neocortex.
- Posterior part of the parietal lobe: integrates visual and somatosensory information for complex purposeful movements.
Speech centers and motor cortex
White matter plays a critical role in supporting speech function by connecting sensory and motor centers.
Speech centers (in the left hemisphere in right-handers):
- Wernicke's center (sensory): area 22b (as well as 39, 40) in the superior temporal gyrus. Responsible for speech comprehension. The junction zone of the parietal, occipital, and temporal lobes is located in the angular gyrus (gyrus angularis).
- Broca's center (motor): area 44 in the inferior frontal gyrus. Responsible for the motor organization of speech.
These two centers are directly connected by an association pathway — the arcuate fasciculus (fasciculus arcuatus). Impulses pass from secondary areas to tertiary areas, and from there to premotor cortical fields, ensuring speech production and muscle innervation.
Frontal lobe and motor function:
- Primary motor cortex (area 4): planning and control of movements.
- Premotor cortex (area 6): higher center for the selection of motor programs. Stores "motor templates" in conjunction with the cerebellum. Movement of a single hand activates it in both hemispheres.
- Frontal eye field (area 8): center for voluntary gaze. Unilateral stimulation causes eye deviation to the opposite side. In stroke (lesion of area 8), the healthy hemisphere dominates, causing the patient's eyes to deviate toward the lesion (the patient "looks at the lesion" and away from the paralysis).
Briefly: answers to topical questions
Into what three main groups are the nerve fibers of the white matter of the cerebral hemispheres classified?
The nerve fibers of the white matter of the cerebral hemispheres are classified into three main groups:
- Projection fibers — connect various parts of the CNS located at a great distance from each other.
- Association fibers — connect different areas of the cortex within the same hemisphere.
- Commissural fibers — connect identical (symmetrical) regions of the two hemispheres.
Which pathway tracts pass through the anterior limb of the internal capsule?
The anterior limb of the internal capsule transmits pathways connecting the frontal lobe cortex with underlying structures. These include:
- Frontothalamic tract — formed by frontothalamic fibers providing connections between the frontal lobe and the thalamus.
- Frontopontine tract — formed by frontopontine fibers running from the cortex to the pontine nuclei.
The blood supply to the anterior limb of the internal capsule and the tracts passing through it is provided by the anterior cerebral artery.
Which structures form the commissural fibers of the hemispheric white matter?
The commissural fibers of the hemispheric white matter form two main structures:
- Anterior commissure — connects symmetrical areas of the hemispheres.
- Corpus callosum (corpus callosum) — a densely packed bundle of fibers connecting extensive cortical areas. Its anterior end (rostrum and genu) connects the frontal poles, forming the forceps minor (forceps minor). The posterior end (splenium) connects the occipital poles, forming the forceps major (forceps major).
Which descending projection pathways pass through the genu and posterior limb of the internal capsule?
The genu and posterior limb of the internal capsule transmit descending projection pathways that form the pyramidal tract.
- Corticonuclear tract (tractus corticonuclearis) — passes through the genu of the internal capsule and carries fibers to the face (motor cranial nerve nuclei).
- Corticospinal tract (tractus corticospinalis) — passes through the anterior two-thirds of the posterior limb (posterior crus). The fibers are somatotopically arranged from anterior to posterior: arm, then trunk, then leg.
Clinical rule for a frontal lobe stroke (area 8): "The patient looks toward the lesion and away from the paralyzed limbs."
Class and exam questions
Why is hearing only slightly impaired in unilateral lesions of the primary auditory cortex?
This is because the primary auditory cortex of each hemisphere has bilateral projections (receives impulses from the organ of Corti of both its own and the contralateral side).
What happens in case of a partial lesion of area 17?
Quadrant hemianopia will develop, in which the visual field defect strictly corresponds to the localization of the lesion.
How do secondary areas functionally differ from primary ones?
Secondary areas do not receive direct connections from the thalamus. Their task is to interpret signals from the primary areas and compare them with memory.
What else to study on this topic
- Organization of visual hypercolumns and horizontal connections
- Tonotopic organization of the primary auditory cortex (frequencies and isofrequency bands)
- Interaction of the premotor cortex with the basal ganglia and cerebellum
- Integrative function of area 2v and area 3a
- Organization of the gustatory pathway
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