Entorhinal cortex in the context of Sleep

In neuroanatomy, a neural pathway is the connection formed by axons that project from neurons to make synapses onto neurons in another location, to enable neurotransmission (the sending of a signal from one region of the nervous system to another). Neurons are connected by a single axon, or by a bundle of axons known as a nerve tract, or fasciculus. Shorter neural pathways are found within grey matter in the brain, whereas longer projections, made up of myelinated axons, constitute white matter.

In the hippocampus, there are neural pathways involved in its circuitry including the perforant pathway, that provides a connectional route from the entorhinal cortex to all fields of the hippocampal formation, including the dentate gyrus, all CA fields (including CA1), and the subiculum.

In the brain, the perforant path or perforant pathway provides a connectional route from the entorhinal cortex to all fields of the hippocampal formation, including the dentate gyrus, all CA fields (including CA1), and the subiculum.

Though it arises mainly from entorhinal layers II and III, the perforant path comprises a smaller component that originates in deep layers V and VI.There is a major dichotomy with respect to the laminar origin and related terminal distribution: neurons in layer II (and possibly layer VI) project to the dentate gyrus and CA3, whereas layer III (and possibly layer V) cells project to CA1 and the subiculum via the temporoammonic pathway.

The hippocampal formation is a compound structure in the medial temporal lobe of the brain. It forms a c-shaped bulge on the floor of the inferior horn of the lateral ventricle. Typically, the hippocampal formation is said to included the dentate gyrus, the hippocampus, and the subiculum. The presubiculum, parasubiculum, and the entorhinal cortex may also be included. The hippocampal formation is thought to play a role in memory, spatial navigation and control of attention. The neural layout and pathways within the hippocampal formation are very similar in all mammals.

The subiculum (Latin for "support") also known as the subicular complex, or subicular cortex, is the most inferior component of the hippocampal formation. It lies between the entorhinal cortex and the CA1 hippocampal subfield.

The subicular complex comprises a set of four related structures including the prosubiculum, presubiculum, postsubiculum and parasubiculum.

Periallocortex is one of three subtypes of allocortex, the other two subtypes being paleocortex and archicortex. The periallocortex is formed at transition areas where any of the other two subtypes of allocortex borders with the neocortex (which is also called isocortex).

Thus, the periallocortex is also subdivided to two subtypes. One subtype is called peripaleocortex, which is formed at borders between paleocortex and neocortex. Areas considered to belong to peripaleocortex are for example anterior insular cortex. Another subtype of periallocortex is called periarchicortex. It is formed at borders between archicortex and neocortex. Areas considered to belong to periarchicortex include entorhinal cortex, perirhinal cortex, presubiculum, parasubiculum, retrosplenial cortex, subcallosal area and subgenual area.

The entorhinal cortex (EC) is a major part of the hippocampal formation of the brain, and is reciprocally connected with the hippocampus.

The hippocampal formation, which consists of the hippocampus, perirhinal cortex, the dentate gyrus, the subicular areas and the EC forms one of the most important parts of the limbic system. The entorhinal cortex is an infolding of the parahippocampal gyrus into the inferior (temporal) horn of the lateral ventricle.

In the rodent, the parasubiculum is a retrohippocampal isocortical structure, and a major component of the subicular complex. It receives numerous subcortical and cortical inputs, and sends major projections to the superficial layers of the entorhinal cortex (Amaral & Witter, 1995).

The parasubicular area is a transitional zone between the presubiculum and the entorhinal area in the mouse (Paxinos-2001), the rat (Swanson, 1998) and the primate (Zilles, 1990). Defined on the basis of cytoarchitecture, it is more similar to the presubiculum than to the entorhinal area (Zilles, 1990), however electrophysiological evidence suggests a similarity with the entorhinal cortex (Funahashi and Stewart, 1997; Glasgow & Chapman, 2007). To be specific, cells in this area are modulated by local theta rhythm, and display theta-frequency membrane potential oscillations (Glasgow & Chapman, 2007; Taube, 1995). Furthermore, cells in the parasubiculum, and neighboring presubiculum, fire in relation to the animal's location in space, suggesting properties similar to place cells. It is postulated that this area may play an integral role in spatial navigation and the integration of head-directional information (Chrobak & Buzsáki, 1994; Taube, 1995).

The trisynaptic circuit or trisynaptic loop is a relay of synaptic transmission in the hippocampus. The trisynaptic circuit is a neural circuit in the hippocampus, which is made up of three major cell groups: granule cells in the dentate gyrus, pyramidal neurons in CA3, and pyramidal neurons in CA1. The hippocampal relay involves three main regions within the hippocampus which are classified according to their cell type and projection fibers. The first projection of the hippocampus occurs between the entorhinal cortex (EC) and the dentate gyrus (DG). The entorhinal cortex transmits its signals from the parahippocampal gyrus to the dentate gyrus via granule cell fibers known collectively as the perforant path. The dentate gyrus then synapses on pyramidal cells in CA3 via mossy cell fibers. CA3 then fires to CA1 via Schaffer collaterals which synapse in the subiculum and are carried out through the fornix of the brain. Collectively the dentate gyrus, CA1, and CA3 of the hippocampus compose the trisynaptic loop.

EC → DG via the perforant path (synapse 1), DG → CA3 via mossy fibres (synapse 2), CA3 → CA1 via schaffer collaterals (synapse 3)

Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammalian central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli. Axons of the mitral cells transfer information to a number of areas in the brain, including the piriform cortex, entorhinal cortex, and amygdala. Mitral cells receive excitatory input from olfactory sensory neurons and external tufted cells on their primary dendrites, whereas inhibitory input arises either from granule cells onto their lateral dendrites and soma or from periglomerular cells onto their dendritic tuft. Mitral cells together with tufted cells form an obligatory relay for all olfactory information entering from the olfactory nerve. Mitral cell output is not a passive reflection of their input from the olfactory nerve. In mice, each mitral cell sends a single primary dendrite into a glomerulus receiving input from a population of olfactory sensory neurons expressing identical olfactory receptor proteins, yet the odor responsiveness of the 20-40 mitral cells connected to a single glomerulus (called sister mitral cells) is not identical to the tuning curve of the input cells, and also differs between sister mitral cells. Odorant response properties of individual neurons in an olfactory glomerular module. The exact type of processing that mitral cells perform with their inputs is still a matter of controversy. One prominent hypothesis is that mitral cells encode the strength of an olfactory input into their firing phases relative to the sniff cycle. A second hypothesis is that the olfactory bulb network acts as a dynamical system that decorrelates to differentiate between representations of highly similar odorants over time. Support for the second hypothesis comes primarily from research in zebrafish (where mitral and tufted cells cannot be distinguished).

The olfactory tract (olfactory peduncle or olfactory stalk) is a bilateral bundle of afferent nerve fibers from the mitral and tufted cells of the olfactory bulb that connects to several target regions in the brain, including the piriform cortex, amygdala, and entorhinal cortex. It is a narrow white band, triangular on coronal section, the apex being directed upward.

The term olfactory tract is a misnomer, as the olfactory peduncle is actually made up of the juxtaposition of two tracts, the medial olfactory tract (giving the medial and intermediate olfactory stria) and the lateral olfactory tract (giving the lateral and intermediate olfactory stria). However, the existence of the medial olfactory tract (and consequently the medial stria) is controversial in primates (including humans).

Periarchicortex is one of two subtypes of periallocortex, the other being peripaleocortex. It is formed at borders between archicortex (a subtype of allocortex) and isocortex and shows slow histological transition from the four-layered structure typical for archicortex to the six-layered structure typical for isocortex.

Cortical areas that are generally considered to belong to periarchicortex, include presubiculum, parasubiculum, entorhinal cortex, perirhinal cortex, retrosplenial cortex, periarchicortical part of cingulate cortex, posterior part of subcallosal area, and subgenual area.

The cingulate cortex is a part of the brain situated in the medial aspect of the cerebral cortex. The cingulate cortex includes the entire cingulate gyrus, which lies immediately above the corpus callosum, and the continuation of this in the cingulate sulcus. The cingulate cortex is usually considered part of the limbic lobe.

It receives inputs from the thalamus and the neocortex, and projects to the entorhinal cortex via the cingulum. It is an integral part of the limbic system, which is involved with emotion formation and processing, learning, and memory. The combination of these three functions makes the cingulate gyrus highly influential in linking motivational outcomes to behavior (e.g. a certain action induced a positive emotional response, which results in learning). This role makes the cingulate cortex highly important in disorders such as depression and schizophrenia. It also plays a role in executive function and respiratory control.

Brodmann area 30, also known as agranular retrolimbic area 30, is a subdivision of the cytoarchitecturally defined retrosplenial region of the cerebral cortex. In the human it is located in the isthmus of cingulate gyrus. Cytoarchitecturally it is bounded internally by the granular retrolimbic area 29, dorsally by the ventral posterior cingulate area 23 and ventrolaterally by the ectorhinal area 36 (Brodmann-1909).

In primates, Brodmann area 30 demonstrates projections to the mid-dorsolateral prefrontal cortex (Brodmann areas 46 and 9) and the thalamus. Additionally, approximately 20% of cortical inputs to the entorhinal cortex arise from the retrosplenial cortex.

The perirhinal cortex is a cortical region in the medial temporal lobe that is made up of Brodmann areas 35 and 36. It receives highly processed sensory information from all sensory regions, and is generally accepted to be an important region for memory. It is bordered caudally by postrhinal cortex or parahippocampal cortex (homologous regions in rodents and primates, respectively) and ventrally and medially by entorhinal cortex.

In the human brain, the entorhinal cortex appears as a longitudinal elevation anterior to the parahippocampal gyrus, with a corresponding internal furrow, the external rhinal sulcus (or rhinal fissure). The rhinal sulcus separates the parahippocampal uncus from the rest of the temporal lobe in the neocortex. The rhinal sulcus and the hippocampal sulcus were both present in early mammals.

It is analogous to the collateral fissure found further caudally in the inferior part of the temporal lobe.