The entorhinal cortex (EC) is an area of the brain's allocortex, located in the medial temporal lobe, whose functions include being a widespread network hub for memory, navigation, and the perception of time. The EC is the main interface between the hippocampus and neocortex. The EC-hippocampus system plays an important role in declarative (autobiographical/episodic/semantic) memories and in particular spatial memories including memory formation, memory consolidation, and memory optimization in sleep. The EC is also responsible for the pre-processing (familiarity) of the input signals in the reflex nictitating membrane response of classical trace conditioning; the association of impulses from the eye and the ear occurs in the entorhinal cortex.
The hippocampus is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, and plays important roles in the consolidation of information from short-term memory to long-term memory, and in spatial memory that enables navigation. The hippocampus is located in the allocortex, with neural projections into the neocortex in humans, as well as primates. The hippocampus, as the medial pallium, is a structure found in all vertebrates. In humans, it contains two main interlocking parts: the hippocampus proper and the dentate gyrus.
The dentate gyrus (DG) is part of the hippocampal formation in the temporal lobe of the brain that includes the hippocampus and the subiculum. The dentate gyrus is part of the hippocampal trisynaptic circuit and is thought to contribute to the formation of new episodic memories, the spontaneous exploration of novel environments and other functions.
The subiculum is the most inferior component of the hippocampal formation. It lies between the entorhinal cortex and the CA1 subfield of the hippocampus proper.
Head direction (HD) cells are neurons found in a number of brain regions that increase their firing rates above baseline levels only when the animal's head points in a specific direction. They have been reported in rats, monkeys, mice, chinchillas and bats, but are thought to be common to all mammals, perhaps all vertebrates and perhaps even some invertebrates, and to underlie the "sense of direction". When the animal's head is facing in the cell's "preferred firing direction" these neurons fire at a steady rate, but firing decreases back to baseline rates as the animal's head turns away from the preferred direction.
Theta waves generate the theta rhythm, a neural oscillation in the brain that underlies various aspects of cognition and behavior, including learning, memory, and spatial navigation in many animals. It can be recorded using various electrophysiological methods, such as electroencephalogram (EEG), recorded either from inside the brain or from electrodes attached to the scalp.
The hippocampal formation is a compound structure in the medial temporal lobe of the brain. There is no consensus concerning which brain regions are encompassed by the term, with some authors defining it as the dentate gyrus, the hippocampus proper and the subiculum; and others including also the presubiculum, parasubiculum, and entorhinal cortex. 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.
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, and the subiculum.
A grid cell is a type of neuron within the entorhinal cortex that fires at regular intervals as an animal navigates an open area, allowing it to understand its position in space by storing and integrating information about location, distance, and direction. Grid cells have been found in many animals, including rats, mice, bats, monkeys, and humans.
In the hippocampus, the mossy fiber pathway consists of unmyelinated axons projecting from granule cells in the dentate gyrus that terminate on modulatory hilar mossy cells and in Cornu Ammonis area 3 (CA3), a region involved in encoding short-term memory. These axons were first described as mossy fibers by Santiago Ramón y Cajal as they displayed varicosities along their lengths that gave them a mossy appearance. The axons that make up the pathway emerge from the basal portions of the granule cells and pass through the hilus of the dentate gyrus before entering the stratum lucidum of CA3. Granule cell synapses tend to be glutamatergic, though immunohistological data has indicated that some synapses contain neuropeptidergic elements including opiate peptides such as dynorphin and enkephalin. There is also evidence for co-localization of both GABAergic and glutamatergic neurotransmitters within mossy fiber terminals. GABAergic and glutamatergic co-localization in mossy fiber boutons has been observed primarily in the developing hippocampus, but in adulthood, evidence suggests that mossy fiber synapses may alternate which neurotransmitter is released through activity-dependent regulation.
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.
The trisynaptic circuit, or trisynaptic loop is a relay of synaptic transmission in the hippocampus. The circuit was initially described by the neuroanatomist Santiago Ramon y Cajal, in the early twentieth century, using the Golgi staining method. After the discovery of the trisynaptic circuit, a series of research has been conducted to determine the mechanisms driving this circuit. Today, research is focused on how this loop interacts with other parts of the brain, and how it influences human physiology and behaviour. For example, it has been shown that disruptions within the trisynaptic circuit leads to behavioural changes in rodent and feline models.
Hippocampus anatomy describes the physical aspects and properties of the hippocampus, a neural structure in the medial temporal lobe of the brain. It has a distinctive, curved shape that has been likened to the sea-horse monster of Greek mythology and the ram's horns of Amun in Egyptian mythology. This general layout holds across the full range of mammalian species, from hedgehog to human, although the details vary. For example, in the rat, the two hippocampi look similar to a pair of bananas, joined at the stems. In primate brains, including humans, the portion of the hippocampus near the base of the temporal lobe is much broader than the part at the top. Due to the three-dimensional curvature of this structure, two-dimensional sections such as shown are commonly seen. Neuroimaging pictures can show a number of different shapes, depending on the angle and location of the cut.
Boundary cells are neurons found in the hippocampal formation that respond to the presence of an environmental boundary at a particular distance and direction from an animal. The existence of cells with these firing characteristics were first predicted on the basis of properties of place cells. Boundary cells were subsequently discovered in several regions of the hippocampal formation: the subiculum, presubiculum and entorhinal cortex.
Edvard Ingjald Moser is a Norwegian professor of psychology and neuroscience at the Kavli Institute for Systems Neuroscience, at the Norwegian University of Science and Technology (NTNU) in Trondheim. In 2005, he and May-Britt Moser discovered grid cells in the brain's medial entorhinal cortex. Grid cells are specialized neurons that provide the brain with a coordinate system and a metric for space. In 2018 he discovered a neural network that expresses your sense of time in experiences and memories located in the brain's lateral entorhinal cortex. He shared the Nobel Prize in Physiology or Medicine in 2014 with long-term collaborator and then-wife May-Britt Moser, and previous mentor John O'Keefe for their work identifying the brain's positioning system. The two main components of the brain's GPS are; grid cells and place cells, a specialized type of neuron that respond to specific locations in space. Together with May-Britt Moser he established the Moser research environment, which they lead.
Large (amplitude) irregular activity (LIA), refers to one of two local field states that have been observed in the hippocampus. The other field state is that of the theta rhythm. The theta state is characterised by a steady slow oscillation of around 6–7 Hz. LIA has a predominantly lower oscillation frequency but contains some sharp spikes, called sharp waves of a higher frequency than that of theta. LIA accompanies the small irregular activity state to which the term LIA has been used to describe overall.
Sharp waves and ripples (SWRs) are oscillatory patterns in the mammalian brain hippocampus seen on an EEG during immobility and sleep. There are three major network oscillation patterns in the hippocampus: theta waves, SWRs and gamma waves. Gamma oscillations are found in all major brain structures, whereas theta and sharp waves are specific to the hippocampus and its neighbouring areas. SWRs are composed of large amplitude sharp waves in local field potential and associated fast field oscillations known as ripples. SWRs are shown to be involved in memory consolidation and the replay of wakefulness-acquired memory. These network oscillations are the most synchronous patterns in the brain, making them susceptible to pathological patterns such as epilepsy.
The hippocampus proper refers to the actual structure of the hippocampus which is made up of four regions or subfields. The subfields CA1, CA2, CA3, and CA4 use the initials of cornu Ammonis, an earlier name of the hippocampus.
Phase precession is a neurophysiological process in which the time of firing of action potentials by individual neurons occurs progressively earlier in relation to the phase of the local field potential oscillation with each successive cycle. In place cells, a type of neuron found in the hippocampal region of the brain, phase precession is believed to play a major role in the neural coding of information. John O'Keefe, who later shared the 2014 Nobel Prize in Physiology or Medicine for his discovery that place cells help form a "map" of the body's position in space, co-discovered phase precession with Michael Recce in 1993.
The supramammillary nucleus (SuM), or supramammillary area, is a thin layer of cells in the brain that lies above the mammillary bodies. It can be considered part of the hypothalamus and diencephalon. The nucleus can be divided into medial and lateral sections. The medial SuM, or SuMM, is made of smaller cells which release dopamine and give input to the lateral septal nucleus. The lateral SuM, or SuML, is made of larger cells that project to the hippocampus.
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