The Brain III
The main features of cerebral cortex organization
The cerebral cortex has a laminated organization (bonded layers) with most regions (95%) have at least six cell layers with varying distribution of cellular concentration. Layer I does not contain many neurons but is a cortical communication (cortico-cortico processing) transfer layer. Layers II, III, V and VI are output layers. The thalamus projects primarily to a thick input layer where they synapse on dendrites; layer IV for sensory input which is distributed to neurons in more superficial and deep layers and a thick layer V for motor cortex ouput which contains neurons that project to the spinal cord via the corticospinal tract. Association areas of the cerebral cortex, such as prefrontal and parietal-temporal-occiital association cortex, have a morphology that is intermediate between those of sensory cortex and motor cortex.
Pyramidal cells are the main source of long-distance projections and cortical output. Pyramidal cells are typically comprised of a large cell body, a myelinated axon and use glutamate, aspartate, or acetylcholine as a neurotransmitter; approximately 75% of cortical neurons can be characterized with this functionality. Stellate cells and other types of non-pyramidal cells (such as bipolar or basket cells) are the main source of local and intralaminar projections. They typically have a small cell body, an unmyelinated axon, and use GABA as a neurotransmitter.
The cerebral neocortex, of which the somatic sensory cortical areas are part, has six principal cell layers. Thalamic neurons that project to the cortex send their axons primarily to layer IV. This is the input layer of cortex. There they synapse on dendrites of layer IV neurons, as well as neurons whose cell bodies are located in other layers, but they have dendrites in layer IV. Neurons in layer IV distribute this These incoming information to neurons in other layers. Layers II, III, V, and VI are the output layers of cortex.
Most of the excitatory connections within a local area of cortex remain somewhat confined to a vertical slice of cortex, termed a cortical column which constitutes a function unit. Neurons within a column in the primary somatic sensory cortex, sparing all cortical layers, receive input from the same peripheral location on the body and from the same class of mechanoreceptor. Although originally conceived as cylindrical in shape, the cortical columns have a more irregular configuration.
The cortex contains three kinds of efferent projections from the output layers, mediated by three separate classes of pyramidal neurons: corticocortical association, callosal, and descending projections. The efferent projection neurons with different targets are located in different cortical layers; Corticocortical association neurons, located predominantly in layers II and III, project to cortical areas on the same side.
Pyramidal cells are the main source of long-distance projections and cortical output. Pyramidal cells are typically comprised of a large cell body, a myelinated axon and use glutamate, aspartate, or acetylcholine as a neurotransmitter; approximately 75% of cortical neurons can be characterized with this functionality. Stellate cells and other types of non-pyramidal cells (such as bipolar or basket cells) are the main source of local and intralaminar projections. They typically have a small cell body, an unmyelinated axon, and use GABA as a neurotransmitter.
Pyramidal neurons in these layers project to other cortical areas as well as to subcortical structures. layer I does not contain many neurons in the mature brain, only dendrites of neurons located in deeper layers and elsewhere. Each region of neocortex that subserves a different function has its own microscopic anatomy: the thickness of the six principal cell layers varies, as does the density of neurons in each layer. Areas that subserve sensation have a thick layer IV. This is the layer in which axons from most thalamic sensory neurons synapse. In contrast, the primary motor cortex has a thin layer IV and a thick layer V. Layer V contains the neurons that project to the spinal cord, via the corticospinal tract.
The spatial and temporal control of the excitability of cortical neurons is extremely precise. Specific connections have particular laminar targets. Synaptic input is spatially segregated onto postsynaptic cells; the laminar basis for connectional specificity. There are a variety of inputs onto the dendrite arbors of a pyramidal cell. Inputs from local circuits onto apical dendrites in layer I, non-specific thalamic input to dendritic segments in layer VI and specific thalamic input to dendritic segments in layer IV. The intrinsic connections with a layer provide for local feedback and feedforward loops. Local collaterals of pyramidal cell axons within layer V provide for intrinsic excitatory interactions.
There are interlaminar connections between different layers, projections from layer VI cells to layer IV cells in visual cortex, short, ipsilateral corticocortical connections within an area may provide further local processing. Facial cortiocortical projections connect area 17 with itself. Medium-length corticocortical projections link different areas within a modality, such connections link A1 with A2, S1 with S2 and area 17 with area 18. Long corticocortical projections allow for interactions between different modalities. Convergens of visual, auditory, and somatic sensory information in the superior temporal sulcus permits large-scale transfer of information across modalities and the comparison of results with expectations. Commissural projections may interconnect homo- or heterotypic cortical regions. Some cortical areas (for example area A1) have rich commissural connections, others have not while still others have intermedaite patterns.
Based primarily on differences in the thickness of cortical layers and on the sizes and shapes of neurons, the German anatomist Korbinian Brodmann identified over 50 divisions (now termed Brodmann's areas;) These divisions are based only on the neuronal architecture, or cytoarchitectrue, of the cortex, such as the size and shapes of neurons in the different laminae and their packing densities. It is remarkable that research on the functions of the cerebral cortex has shown that different functional areas of the cortex have a different cytoarchitecture. In humans, by noting the particular behavioral changes that follow discrete cortical lesions and using functional imaging approaches, such as positron emission tomography and functional MRI, we have gained some insight into the functions of most of the cytoarchitectonic divisions identified by Brodmann.
Callosal neurons are also located in layers II and III. they project their axons to the contralateral cortex via the corpus callosum
Descending projection neurons are separate classes of projection neurons whose axons descend to 1 the straiatum (the caudate nucleus and putamen) 2 the thalamus, 3 the brain stem or 4 the spinal cord. Descending projection neurons that terminate in the striatum brain stem, and spinal cord are found in layer V whereas those projecting to the thalamus are located in layer VI.
The cerebral cortex has a laminated organization (bonded layers) with most regions (95%) have at least six cell layers with varying distribution of cellular concentration. Layer I does not contain many neurons but is a cortical communication (cortico-cortico processing) transfer layer. Layers II, III, V and VI are output layers. The thalamus projects primarily to a thick input layer where they synapse on dendrites; layer IV for sensory input which is distributed to neurons in more superficial and deep layers and a thick layer V for motor cortex ouput which contains neurons that project to the spinal cord via the corticospinal tract. Association areas of the cerebral cortex, such as prefrontal and parietal-temporal-occiital association cortex, have a morphology that is intermediate between those of sensory cortex and motor cortex.
Pyramidal cells are the main source of long-distance projections and cortical output. Pyramidal cells are typically comprised of a large cell body, a myelinated axon and use glutamate, aspartate, or acetylcholine as a neurotransmitter; approximately 75% of cortical neurons can be characterized with this functionality. Stellate cells and other types of non-pyramidal cells (such as bipolar or basket cells) are the main source of local and intralaminar projections. They typically have a small cell body, an unmyelinated axon, and use GABA as a neurotransmitter.
The cerebral neocortex, of which the somatic sensory cortical areas are part, has six principal cell layers. Thalamic neurons that project to the cortex send their axons primarily to layer IV. This is the input layer of cortex. There they synapse on dendrites of layer IV neurons, as well as neurons whose cell bodies are located in other layers, but they have dendrites in layer IV. Neurons in layer IV distribute this These incoming information to neurons in other layers. Layers II, III, V, and VI are the output layers of cortex.
Most of the excitatory connections within a local area of cortex remain somewhat confined to a vertical slice of cortex, termed a cortical column which constitutes a function unit. Neurons within a column in the primary somatic sensory cortex, sparing all cortical layers, receive input from the same peripheral location on the body and from the same class of mechanoreceptor. Although originally conceived as cylindrical in shape, the cortical columns have a more irregular configuration.
The cortex contains three kinds of efferent projections from the output layers, mediated by three separate classes of pyramidal neurons: corticocortical association, callosal, and descending projections. The efferent projection neurons with different targets are located in different cortical layers; Corticocortical association neurons, located predominantly in layers II and III, project to cortical areas on the same side.
Pyramidal cells are the main source of long-distance projections and cortical output. Pyramidal cells are typically comprised of a large cell body, a myelinated axon and use glutamate, aspartate, or acetylcholine as a neurotransmitter; approximately 75% of cortical neurons can be characterized with this functionality. Stellate cells and other types of non-pyramidal cells (such as bipolar or basket cells) are the main source of local and intralaminar projections. They typically have a small cell body, an unmyelinated axon, and use GABA as a neurotransmitter.
Pyramidal neurons in these layers project to other cortical areas as well as to subcortical structures. layer I does not contain many neurons in the mature brain, only dendrites of neurons located in deeper layers and elsewhere. Each region of neocortex that subserves a different function has its own microscopic anatomy: the thickness of the six principal cell layers varies, as does the density of neurons in each layer. Areas that subserve sensation have a thick layer IV. This is the layer in which axons from most thalamic sensory neurons synapse. In contrast, the primary motor cortex has a thin layer IV and a thick layer V. Layer V contains the neurons that project to the spinal cord, via the corticospinal tract.
The spatial and temporal control of the excitability of cortical neurons is extremely precise. Specific connections have particular laminar targets. Synaptic input is spatially segregated onto postsynaptic cells; the laminar basis for connectional specificity. There are a variety of inputs onto the dendrite arbors of a pyramidal cell. Inputs from local circuits onto apical dendrites in layer I, non-specific thalamic input to dendritic segments in layer VI and specific thalamic input to dendritic segments in layer IV. The intrinsic connections with a layer provide for local feedback and feedforward loops. Local collaterals of pyramidal cell axons within layer V provide for intrinsic excitatory interactions.
There are interlaminar connections between different layers, projections from layer VI cells to layer IV cells in visual cortex, short, ipsilateral corticocortical connections within an area may provide further local processing. Facial cortiocortical projections connect area 17 with itself. Medium-length corticocortical projections link different areas within a modality, such connections link A1 with A2, S1 with S2 and area 17 with area 18. Long corticocortical projections allow for interactions between different modalities. Convergens of visual, auditory, and somatic sensory information in the superior temporal sulcus permits large-scale transfer of information across modalities and the comparison of results with expectations. Commissural projections may interconnect homo- or heterotypic cortical regions. Some cortical areas (for example area A1) have rich commissural connections, others have not while still others have intermedaite patterns.
Based primarily on differences in the thickness of cortical layers and on the sizes and shapes of neurons, the German anatomist Korbinian Brodmann identified over 50 divisions (now termed Brodmann's areas;) These divisions are based only on the neuronal architecture, or cytoarchitectrue, of the cortex, such as the size and shapes of neurons in the different laminae and their packing densities. It is remarkable that research on the functions of the cerebral cortex has shown that different functional areas of the cortex have a different cytoarchitecture. In humans, by noting the particular behavioral changes that follow discrete cortical lesions and using functional imaging approaches, such as positron emission tomography and functional MRI, we have gained some insight into the functions of most of the cytoarchitectonic divisions identified by Brodmann.
Callosal neurons are also located in layers II and III. they project their axons to the contralateral cortex via the corpus callosum
Descending projection neurons are separate classes of projection neurons whose axons descend to 1 the straiatum (the caudate nucleus and putamen) 2 the thalamus, 3 the brain stem or 4 the spinal cord. Descending projection neurons that terminate in the striatum brain stem, and spinal cord are found in layer V whereas those projecting to the thalamus are located in layer VI.