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Lecture 10
Cerebral Hemisphere and Cortex
Cerebral Hemisphere
Right & left cerebral hemispheres are derived from the embryonic telencephalon. They are composed of gray and white matter.
Gray Matter: Cerebral Cortex — layer of gray matter at the surface of the cerebral hemisphere. Three phylogenetic categories of cerebral cortex are:
- archicortex — (hippocampus) oldest, composed of two layers
- paleocortex — (piriform lobe) old, three layers, olfaction related
- neocortex — new, six layers, detailed perception, learning, intelligence
Basal Nuclei — gray matter nuclei located deep within the white matter of the cerebral hemisphere. Basal nuclei include: caudate nucleus, putamen, pallidum, claustrum.
White Matter: Myelinated axons which connect cerebral cortex with other brain regions. Three cat- egories of white matter fibers are recognized:
Projection Fibers — fibers that leave the cerebral white matter. Projection fibers form the internal capsule. Two categories of projection fibers are: 1] corticofugal: terminate in the basal nuclei, brainstem, or spinal cord; 2] corticopedal: typically originate in thalamus & terminate in cerebral cortex.
Commissural Fibers—fibers that connect cortices of right and left cerebral hemispheres. The largest bundle forms the corpus callosum.
Association Fibers—fibers that connect regions of the cerebral cortex within one hemisphere. Two categories are recognized: short association fibers connect adjacent gyri; long association fibers connect distant gyri (different lobes);
Note : The ventromedial portion of each cerebral hemisphere is designated rhinencephalon because it is association with olfaction, the most primitive sensory modality.
Cerebral Cortical (Neocortex)
Neocortex , the phylogenetically most recent cortex, is only found in mammals. It is organized horizontally into six layers and varies in thickness among different regions of the hemisphere. Neocortex is involved in detailed sensory perception, in performing rapid sequences of fine- movements, and in learning and intelligent behavior. It is most abundant in the human brain. It forms about 85% of the dog cerebral cortex (the remaining 15% being archicortex and paleocortex).
Lecture 10
Cerebral Hemisphere and Cortex
Cerebral Hemisphere
Right & left cerebral hemispheres are derived from the embryonic telencephalon. They are composed of gray and white matter.
Gray Matter: Cerebral Cortex — layer of gray matter at the surface of the cerebral hemisphere. Three phylogenetic categories of cerebral cortex are:
- archicortex — (hippocampus) oldest, composed of two layers
- paleocortex — (piriform lobe) old, three layers, olfaction related
- neocortex — new, six layers, detailed perception, learning, intelligence
Basal Nuclei — gray matter nuclei located deep within the white matter of the cerebral hemisphere. Basal nuclei include: caudate nucleus, putamen, pallidum, claustrum.
White Matter: Myelinated axons which connect cerebral cortex with other brain regions. Three cat- egories of white matter fibers are recognized:
Projection Fibers — fibers that leave the cerebral white matter. Projection fibers form the internal capsule. Two categories of projection fibers are: 1] corticofugal: terminate in the basal nuclei, brainstem, or spinal cord; 2] corticopedal: typically originate in thalamus & terminate in cerebral cortex.
Commissural Fibers—fibers that connect cortices of right and left cerebral hemispheres. The largest bundle forms the corpus callosum.
Association Fibers—fibers that connect regions of the cerebral cortex within one hemisphere. Two categories are recognized: short association fibers connect adjacent gyri; long association fibers connect distant gyri (different lobes);
Note : The ventromedial portion of each cerebral hemisphere is designated rhinencephalon because it is association with olfaction, the most primitive sensory modality.
Cerebral Cortical (Neocortex)
Neocortex , the phylogenetically most recent cortex, is only found in mammals. It is organized horizontally into six layers and varies in thickness among different regions of the hemisphere. Neocortex is involved in detailed sensory perception, in performing rapid sequences of fine- movements, and in learning and intelligent behavior. It is most abundant in the human brain. It forms about 85% of the dog cerebral cortex (the remaining 15% being archicortex and paleocortex).
Lecture 10
Cerebral Hemisphere and Cortex
Cerebral Hemisphere
Right & left cerebral hemispheres are derived from the embryonic telencephalon. They are composed of gray and white matter.
Gray Matter: Cerebral Cortex — layer of gray matter at the surface of the cerebral hemisphere. Three phylogenetic categories of cerebral cortex are:
- archicortex — (hippocampus) oldest, composed of two layers
- paleocortex — (piriform lobe) old, three layers, olfaction related
- neocortex — new, six layers, detailed perception, learning, intelligence
Basal Nuclei — gray matter nuclei located deep within the white matter of the cerebral hemisphere. Basal nuclei include: caudate nucleus, putamen, pallidum, claustrum.
White Matter: Myelinated axons which connect cerebral cortex with other brain regions. Three cat- egories of white matter fibers are recognized:
Projection Fibers — fibers that leave the cerebral white matter. Projection fibers form the internal capsule. Two categories of projection fibers are: 1] corticofugal: terminate in the basal nuclei, brainstem, or spinal cord; 2] corticopedal: typically originate in thalamus & terminate in cerebral cortex.
Commissural Fibers—fibers that connect cortices of right and left cerebral hemispheres. The largest bundle forms the corpus callosum.
Association Fibers—fibers that connect regions of the cerebral cortex within one hemisphere. Two categories are recognized: short association fibers connect adjacent gyri; long association fibers connect distant gyri (different lobes);
Note : The ventromedial portion of each cerebral hemisphere is designated rhinencephalon because it is association with olfaction, the most primitive sensory modality.
Cerebral Cortical (Neocortex)
Neocortex , the phylogenetically most recent cortex, is only found in mammals. It is organized horizontally into six layers and varies in thickness among different regions of the hemisphere. Neocortex is involved in detailed sensory perception, in performing rapid sequences of fine- movements, and in learning and intelligent behavior. It is most abundant in the human brain. It forms about 85% of the dog cerebral cortex (the remaining 15% being archicortex and paleocortex).
Horizontal Layers of Cerebral Cortical
The cerebral cortex is organized into six horizontal layers (although layer boundaries are not very obvious in routine sections). The individual layers have different roles and vary in relative thick- ness among cortical regions (e.g., a sensory region has a thick internal granule layer; a motor area has a thick internal pyramidal cell layer). From superfi cial to deep, the six layers are: 1] Molecular layer — fi ber layer; apical dendrites & non-specific afferents; 2] Outer granule cell layer — interneurons for non-specific afferent input; 3] Outer pyramidal cell layer — small and medium cells; short association output 4] Inner granule cell layer — interneurons for specific afferent input 5] Inner pyramidal layer — large cells; projection & long association output 6] Multiform layer — variably shaped cells; projection & long association output
Cell Column (Vertical) Organization of Cerebral Cortical
The entire cerebral cortex is organized into functional units, each unit being a column (about 0.4 mm diameter) extending the entire thickness of the cortex (including all six layers). Each vertical column is a functional unit because all cells within an individual column are activated by the same particular feature of a stimulus. The vertical organization is the result of neuronal connections within a cortical column: — non-specifi c input to the column terminates in the molecular layer on distal dendrites of pyramidal cells, to provide background excitation to the column; — specifi c thalamic input terminates in the internal granule cell layer, exciting interneurons which excite other neurons of the column; — small pyramidal cells send their axons into the white matter to excite nearby cell columns; — large pyramidal cells (and multiform cells) send their axons into the white matter to excite distant sites via long association fibers, commissural fibers, and corticofugal projection fibers.
Fig. 2. Vertical cells columns constitute the functional units of cere- bral cortex. Usually the functional columns are not anatomically distinct, but in the case of the mas- sive sensory input from rat vibrissae, the cell columns per vibrissae are morphologically evident and give the impression of a “barrel”.
