The Cerebral Cortex

Figure1. Crude approximation of cerebral anatomical landmarks. Note: Extra-pyramidal motor cortex is widely distributed throughout cerebrum, though majority of contributions are from parietal and frontal lobes. Some "lobes" (e.g. limbic) and other landmarks (e.g. subcortical region of basal ganglia) are not seen from this view.

The cerebral cortex may be anatomically defined as the higher order center of the central nervous system. It is composed of embyrological divisions, called lobes (with gyri and sulci) which are functionally interdependent, though organized and localized with respect to primary attributes, such as motor, sensory, emotion, and special senses. There are intimate associations of these regions with high order subcortical regions as well as the brainstem and spinal cord

Cortical (Pyramidal) Motor Cortex:

    This system arises primarily from the pyramidal cortex (Parietal and Frontal lobes) and influences the excitability of spinal motor neurons either mono- or polysynaptically (via cord interneurons). However a portion of this derives from the somatosensory cortex; projections from these affect sensory transmission (i.e. they modulate reflex nocioception). Possible visceral reflexes (e.g. micturition???) are modulated by this system as well. The anatomical arrangements of cortical nuclei and their relationships to other areas of the brain and spinal cord will be discussed in the separate discussion on the subject of tracts.

Extrapyramidal Motor Cortex:

    This system encompasses a broad and vast interconnected network of cortical and subcortical nuclei. Cortical origins are primarily from the parietal and frontal lobes, but there are projections with origins from all cerebral cortices, Motor control via extrapyramidal nuclei is via effects on brainstem (diencephalon, midbrain, pons, medulla), cerebellar and basal ganglia motor nuclei.. Again...a more descriptive appreciation for the anatomical functionality of the motor system will evolve with the discussion of tracts, specifically the motor system tracts.

Somatosensory Cortex

    Located primarily in the parietal lobe there are essentially four submodalities of somatic sensation: Pain, Thermal, Touch-Pressure and Position-Kenesia (i.e. limb position and sense of joint movement). Somatosensory cortex is highly organized by input source---and the body regions capable of the greatest sensitivity occupy the greatest amount of somatosensory cortex. Pathways which carry somatosensory information from the body and from the head---the dorsal column-medial lemniscal system, the spinothalamic system, and the trigeminalthalamic system & trigeminoreticular system, respectively---will be discussed under the subject of tracts (to come). Some projections associated with pain are also associated with the Limbic cortex (possibly explaining the emotional response(s) (expletives(??)) to painful stimuli).

Visual Cortex

    The visual pathways will be described elsewhere. The visual cortex of the occipital lobe is divided into layers which are involved in a "division of labor'..each receiving visual input and processing it somewhat differently, via projections to other cortical and subcortical regions. These visual association areas are, thus, organized to serve specialized functionality. From the primary projection area, information is relayed to "visual association areas" located at the borders of the striate cortex and in the temporal lobe.

Auditory Cortex

    to come

Olfactory Cortex

    to come

The Limbic System

    It actually is a region (loosely, an "artificial" lobe) consisting of the most medial margins of the frontal, parietal and temporal lobes. This "lobe" contains the subcallosal, cingulate and parahippocampal gyri and the "hippocampal formation"... a primitive cortical derivative, the. The Limbic System is concerned primarily with behaviors governed by emotion: fear, aggression,anger/rage, affection. The hypothalamus is intimately related via numerous neural circuits between these structures and is considered central in the input-reponses of the system. The nature, magnitude and intensity of certain emotions, such as aggression/attack and fear are effected. Non-hypothalamic-limbic structures can modulate emotion, however. Neocortical inputs are mainly from the frontal lobe, which modulates initiative, imagination and social awareness to effects on emotional behaviors, enhances anxiety and increases the level of aggressive displays. Concurrent sympathetic discharges (directed via the hypothalamus, in part) and adrenocortical secretions (also, in part directed via hypothalamic release of CRH and amagdyla stimulation of ACTH release ) frequently occur during expressions of emotion and are somehow involved in the preparation for arousal and emotional stimulation. The amagdyloid nucleus (see basal ganglia) also appears to modulate certain responses such as fear, and dominance. The hippocampus is associated with reality-context awareness, as lesions here may result in hallucinations and psychomotor seizures. Whether this is related directly to effects on the Limbic System directly is not clear to this author.

Basal Ganglia

  • The basal ganglia are subcortical nuclei derived from the Telencephalon. They are concerned with complex aspects of motor control and consist of two major divisions: the 1) Corpus Striatum, and 2) Amagdyloid nuclear complex. Additionally, there are close relationships of these with the Substantia nigra and the Subthalamic region. In brief...
    • Corpus Striatum:
      • consists of two distinct parts...neostriatum (Caudate nucleus and Putamen) and paleostiatum (globus pallidus)
      • the Neostriatum receives afferent stimulation from telencephalon, thalamus and portions of the amagdyla, primarily
      • the Paleostriatum ..the globus pallidus, provides efferent to thalamic and cortical (motor and other) and brainstem centers.
      • there are no direct efferents to the spinal cord.
    • Amagdyloid:
      • appears to have intimate association with the limbic system and olefaction; also receives dopaminergic input from substantia nigra; stimulation provides a wide variety of somatic, visceral, endocrine and behavioral effects. A direct role in the coordination of motor activity is not clear to this author.
    • Substantia nigra:
      • lies dorsal to the crus cerebri, ventral to the tegmentum and lies throughout the course of the mesencephalon.
      • is believed to provide source of dopamine to the basal ganglia and is believed to underly some metabolic disturbances manifest as Parkinson's Disease.
      • there are many efferent projections from the substantia nigra..this discussion will not elaborate on them
 
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