Organization of Cerebral Cortex

1. Explain why the cerebral cortex is important.

It’s not that important and you don’t really need it. However, it is important for studying important clinical disorders and understanding major elements of human behavior. The human nervous system is specialized to sense the body and its surroundings, control motor responses of muscles and glands, and form perceptions, thoughts, memories and emotions that associate sensations and motor responses.

Neural discharges from a sensory pathway must activate neurons in appropriate sensory cortical areas before a sensory perception can occur, in appropriate motor cortical areas before motor pathways can activate muscle contractions, and in appropriate association cortical areas and subcortical brain structures to generate thoughts, memories and emotions.

2. Point out the locations and size differences of the neocortex and allocortex.

The neocortex has 6 layers of cells while the allocortex has fewer than 6 layers. Brain evolution has resulted in a dramatic increase in size of the neocortex and relatively little increase in allocortex. The human cerebral hemisphere consists of 90% neocortex and 10% allocortex.

3. Describe the arrangement of the cerebral cortex on the outside of the cerebral hemispheres and the arrangement of lobes, gyri, and sulci.

The neocortex is arranged as a grey colored rind of cells 2-5 mm thick, forming the outer layers of the cerebral hemisphere. Underlying the cortical grey is white matter. The cortex appears grey because it contains high densities of axon terminations, cell bodies with dendrites, and synapses. Brain tissue with a high density of grey matter is called a neurophil. All nuclei (collections of cell bodies) in the brain are neurophil areas.

White matter appears white because it consists of high densities of axons, many of which are myelinated. Tracts are collections of axons which make up white matter. White matter beneath the cortex contains axons that are transmitting information into or out of the grey cortex.

The cortex grey can be removed as 2 flat sheets with 2600 cm^3 surface area, 10-25 billion cells, and 500-20,000 synapses/neuron. These sheets fit into the skull by extensive folding, forming sulci (infoldings), fissures (large sulci), and gyri (cortical matter between infoldings) on the hemisphere surfaces.

4. Describe the morphology and functions of the types of neurons and glia in cortex.

The neocortex contains 2 types of cells: neurons and glia. Neurons are subdivided into pyramidal cells and granule cells.

Pyramidal cells are most numerous (75% of cortical neurons) and have a large pyramid cell body, apical dendrite extending radially to cortical surface, basal dendrites extending out horizontally. The main axons from the cell body project out into the subcortial white matter on its way to distant targets. Axon collaterals extend to make more local synaptic connections near the cell body. Pyramidal cells are output or efferent neurons of the cortex.

Granule or stellate cells are less numerous (25% of cortical neurons) and have a small star-shaped or spherical cell body, short dendrites that branch extensively around the cell body, and axons that branch and end locally around the cell body in the cortical grey matter. Granule cells are local projection neurons or interneurons.

Pyramidal cells are always pyramidal. Granule cells may have other names (basket, double bouquet, chandelier, bipolar, bitufted, fusiform, etc.)

Glial cells are specialized support cells of neurons and outnumber neurons 10-50:1. Adult human cortex has macroglia and microglia. Macroglia are astrocytes and oligodendrocytes. Astrocytes are star-shaped cells with numerous extensions that maintain ionic balance in the extracellular space. Oligodendrocytes function in production of myelin for axons of cortical neurons. Microglia are small cells that act as phagocytic cells that remove cellular debris due to cell component turnover, trauma, and inflammation.

5. Identify the names and arrangement of the 6 neocortical layers.

The neocortex is organized into 6 layers from surface to white matter, designated I through VI:

Layer I – Molecular layer
Layer II – External granular layer
Layer III – External pyramidal layer
Layer IV – Internal granular layer
Layer V – Internal granular layer
Layer VI – Multiform layer

Layer I has few neuronal cells and mostly axons, dendritic processes and glial cells. Layers II-V have mixtures of pyramidal and granule cell bodies and named according to the cell type most prevalently found. Layer VI has variably shaped pyramidal cells.

Supergranular layers are layers I-III, granular layer is layer IV, and infragranular layers are layer V-VI.

6. Describe structures that provide direct inputs (afferents) to neocortex, and targets of direct neocortical outputs (efferents).

Structures that provide direct input to the cortex are afferents. These structures are:

(1) cortical areas (ipsilateral or contralateral hemisphere)
(2) structures within the cerebral hemispheres such as the claustrum and nucleus basalis
(3) diencephalon structures such as the thalamus and tuberomammillary nucleus
(4) brainstem structures such as ventral tegmentum, locus ceruleus, and raphe complex

Direct inputs to cortex come from neurons located at or above upper brainstem levels of the nervous system. Inputs structures have different patters of axon termination in the cortex that can be limited to a small number of cortical areas or broadly distributed across numerous cortical areas. Inputs to different layers in the cortex also terminate at different densities to different laminae. However, even in the case where inputs are more uniformly distributed, different patters of distribution are seen in different cortical areas. In summary, the terminals of axons from different input structures are organized in particular patterns and density in different cortical areas and laminae resulting in different patterns of cortical activation.

Output neurons of the neocortex are pyramidal cells which project their axons to:

(1) other cortical areas in ipsilateral and contralateral hemisphere
(2) subcortical targets such as thalamus, basal ganglia
(3) brainstem such as tectum,pons, red nucleus, dorsal colum nuclei
(4) spinal cord

Pyramidal cells in different cortical laminae project their axons to different targets. Neurons in the supragranular layers typically project to other cortical layers. Neurons in infragranular layers typically project to subcortical areas as well as other cortical layers.

7. Explain the distinction between a cortical neurotransmitter and neuromodulator, and identify cortical components associated with amino acid transmitters and agents that function as transmitters or modulators.

Neurotransmitters produce fast onset, short duration, and (usually) strong changes in postsynaptic membrane potentials. Neuromodulators produce slow onset, prolonged, and weaker postsynaptic changes.

Examples of neurotransmitters are amino acids such as GABA, used by many cortical interneurons (inhibitory) and glutamate, used by pyramidal cells and some interneurons (excitatory). All thalamic inputs to cortex are excitatory and use glutamate.

Acetylcholine, amines, and peptides are important in neurotransmission but are unclear whther they act as neurotransmitters or neuromodulators. Acetylcholine is released by input fibers from the nucleus basalis and facilitates in cortical transmission. Amines such as serotonin, norepinephrine, dopamine, and histamine have excitatory or inhibitory effects on cortical neurons depending on receptor type. Peptides such as vasoactive intestinal polypeptide (VIP) cholecystokinin (CCK), somatostatin(SRIF or SS), neuropeptide Y (NPY), substance P, etc. serves as excitatory transmitters of some cortical interneurons.

Neurally active agents such as alcohol, estradiol, and glucocorticoids can also act on cortical neurons through the blood.

A given neurotransmitter or neuromodulator has multiple receptors that can be linked to ion channels or secondary messenger systems that act on altering the postsynaptic properties of neurons. Individual granule and pyramidal neurons have receptors for multiple neurotransmitters and neuromodulators.

Cortical chemical transmission may be abnormally balanced in psychiatric disorders. Drug treatments for these disorders act to reestablish balance by altering synthesis, release, receptor binding, or breakdown/reuptake of neurotransmitters or neuromodulators.

8. Summarize understanding of how many cortical areas exist in human brain, and the general arrangement of primary sensory, primary motor, and higher order cortical areas.

Traditional cytoarchitectonic maps use the relative appearance of the 6 cortical layers to differentiate between different cortical areas based on relative densities of granule and pyramidal cells. Brodmann’s map distinguishes between 50 cortical areas but the problem with these traditional maps is that they map different areas based on subtle differences that are usually hard to determine.

Current maps are based on cytoarchitectonics, connections and functional criteria. These maps use a number of criteria including input/output projections, functional response features, and histochemical procedures which allow visualization of clustering patterns of axons, terminations, and dendrites. The exact number of cortical areas is not known but evidence indicates at least 30+ areas.

9. Describe the basic organization of a cortical column as a cortical information processing unit.

Cortical columns are basic units of organization in cortical areas that are repeated. These columns of cells are aligned perpendicular to the cortical surface in a radial direction to form a dominant pattern of synaptic connections and information processing.

Input fibers terminate in particular cortical layers to make strong excitatory synapses on adjacent granule and pyramindal neurons in the same cortical column. Synaptic connections are strong and dense between granule and pyramidal neurons of a cortical column such that activation of inputs tends to activate the column of granule and pyramidal neurons to produce a corresponding output from the efferent neuron in that column.

The size and shape of cortical columns are best defined in the ocular dominance columns of the primary visual cortex where columns processing input from left and right eyes are arranged alternately as elongated bands.

10. Explain the concepts of cortical serial and parallel processing.

Information is processed serially and in parallel in the cortical columns of a single cortical area and in more wide-ranging circuits involving several cortical areas. Information processing within a cortical column is done serially – signals are successively integrated in sequence (input fibers, interneurons, output fibers). Simultaneous processing of the same information can occur in parallel across serial circuits in more than one column.

Example: inputs for the left eye are processed in serial with each left eye column but in parallel between many left eye columns.

11. Identify locations of allocortex components in the hemisphere.

The allocortex is composed of the paleocortex and the archicortex.

The paleocortex is located on the ventral surface of the frontal and temporal lobes. It includes the anterior olfactory nucleus, anterior perforated substance (olfactory tubercule), piriform cortex, entorhinal cortex, and other areas on the parahippocampal gyrus. The paleocortex contributes to sense of smell and limbic system function.

The archicortex is the hippocampal formation located within the temporal lobe. It includes the dentate gyrus, hippocampus, and subiculum. Unlike the neocortex, the archicortex consists of 4 layers. It functions in memory, emotional responses, etc.

12. Present a basic understanding of neurogenesis in adult human cortex.

The original dogma as that all cortical neurons were produced during fetal development but recent studies have shown evidence of a limited generation of new neurons (neurogenesis) occurring in some locations of the adult human cortex. New neurons are granule neurons; new pyramidal neurons have not been seen.

Cells are generated in the dentate gyrus of the hippocampal formation (archicortex) and from neuronal stem cells located in regions adjacent to ventricles. These stem cells survive into adulthood and can divide and differentiate into neurons when exposed to appropriate factors. Similar neurons in animals appear to be able to make synaptic connections and become functional. Neurogenesis slows with age but remains ongoing up until the 5-7 decade of life.