Intrinsic Circuits

A variety of techniques have proven useful in delineating the intrinsic organization of cortical circuits. In particular, the recent developments in histochemical methods coupled with intracellular recordings and dye labeling have enabled investigators to unravel the neuronal organization of the local circuitry of the cortex (Fig. 2). As indicated previously, the layers of the cortex can be characterized by the arrangement and density of stellate and pyramidal neurons. The

Figure 2 Intrinsic circuitry of the layers of the cerebral cortex. Indicated are inhibitory projections (dashed, heavy line) from aspiny inhibitory neurons (open cell with heavy lines), pyramidal neurons in gray, a stelate neuron in black and pyramidal neurons in adjacent columns (open cells with thin lines.).

Figure 2 Intrinsic circuitry of the layers of the cerebral cortex. Indicated are inhibitory projections (dashed, heavy line) from aspiny inhibitory neurons (open cell with heavy lines), pyramidal neurons in gray, a stelate neuron in black and pyramidal neurons in adjacent columns (open cells with thin lines.).

pyramidal neurons are responsible for the output of the cerebral cortex, are excitatory in nature, and utilize the excitatory amino acids, aspartate and glutamate, as neurotransmitters.

These projection neurons have apical dendrites that ascend to the outer layers of the cortex and often form apical tufts that receive connections within layers 1 and 2. Other dendrites project from the bases of pyramidal neurons. The stellate neurons, like the pyramidal cells, have spiny dendrites and are excitatory. These spiny stellate cells, primarily seen in layer 4, are local circuit neurons and the axons do not project out of the cortical layers.

Another type of cell that is abundant within the layers of the cerebral cortex is the local circuit neurons with aspiny dendrites. These smooth neurons are inhibitory in nature and have axons that in the neocortex are restricted to a discrete region of the cortical layers. Unlike the stellate and pyramidal neurons, the axons of these neurons can arise from any site on the cell body. The neurotransmitter of these neurons is GABA and these cells often colocalize neuropeptides such as somatostatin, substance P, vasoactive intestinal polypeptide, or cholecystokinin.

The primary corticopetal fibers to the cerebral cortex are from the sensory and motor nuclei of the thalamus. However, there are many other sources for cortical inputs. Basal forebrain nuclei contain acet-ylcholine neurons with extensive projections to the cerebral cortex. The reduction in this forebrain cholinergic innervation is an important contributor to Alzheimer's disease. The noradrenergic neurons of the locus coeruleus in the pons diffusely innervate the cerebral cortex and appear to play a role in modulating the signal-to-noise ratio of specific cortical inputs. The raphe nuclei of the brain stem are the source of a serotonergic input, which is primarily inhibitory. Finally, the cortex has extensive interconnections that can be from the opposite hemisphere, from adjacent gyri of the same lobe, and from other lobes.

A typical local circuitry in sensory cortex is shown in Fig. 2. The thalamocortical input primarily provides an excitatory termination on the intrinsic stellate neurons in layer 4 and to some extent in layer 6. In fact, in sensory cortex, small columns of neurons are formed such that the cells in the column selectively receive input from a specific sensory stimulus. Inputs from other regions of the cortex or other subcortical sites terminate on pyramidal neuron dendrites in layers 1-3. The stellate neurons in layer 4, receiving the thalamocortical input, terminate on apical dendrites of pyramidal cells in layers 2 and 3. These pyramidal neurons in layers 2 and 3 project back onto layer 5 pyramidal cells, which then project back to pyramidal cells of layers 6, which in turn project back to layer 4, completing a local circuit in the cortical column. In addition, in layer 4, the pyramidal neurons will excite aspiny inhibitory neurons that send an inhibitory impulse to adjacent columns of the cortex.

Unraveling Alzheimers Disease

Unraveling Alzheimers Disease

I leave absolutely nothing out! Everything that I learned about Alzheimer’s I share with you. This is the most comprehensive report on Alzheimer’s you will ever read. No stone is left unturned in this comprehensive report.

Get My Free Ebook


Post a comment