engineering

Trisynaptic circuit

Trisynaptic circuit is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Trisynaptic circuit rather than just read about it. In short: 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.

Key takeaways

Trisynaptic circuit belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Trisynaptic circuit to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Trisynaptic circuit from memory before moving on to harder problems.

Reference excerpt

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)

History 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 lead to behavioural changes in rodent and feline models.

Structures

Entorhinal cortex The entorhinal cortex (EC) is a structure in the brain located in the medial temporal lobe. The EC is composed of six distinct layers. The superficial (outer) layers, which include layers I through III, are mainly input layers that receive signals from other parts of the EC, but also project to hippocampal structures via the perforant path. Layer II of the entorhinal cortex projects mainly to the dentate gyrus and CA3, while layer III is thought to project mainly to the CA1 of the hippocampus. The deep (inner) layers, layers IV to VI, are the main output layers, and send signals to different parts of the EC and other cortical areas.

Dentate gyrus The dentate gyrus (DG) is the innermost section of the hippocampal formation. The dentate gyrus consists of three layers: molecular, granular, and polymorphic. Granule neurons, which are the most prominent type of DG cells, are mainly found in the granular layer. These granule cells are the major source of input of the hippocampal formation, receiving most of their information from layer II of the entorhinal cortex, via the perforant pathway. Information from the DG is directed to the pyramidal cells of CA3 through mossy fibres. Neurons within the DG are famous for being one of two nervous system areas capable of neurogenesis, the growth or development of nervous tissue.

Cornu ammonis 3 The CA3 is a portion of the hippocampal formation adjacent to the dentate gyrus. Input is received from the granule cells of the dentate gyrus through the mossy fibres. The CA3 is rich in pyramidal neurons (like those found throughout the neocortex), which project mainly to the CA1 pyramidal neurons via the Schaffer collateral pathway. The CA3 pyramidal neurons have been analogized as the "pacemaker" of the trisynaptic loop in the generation of the hippocampal theta rhythm. One study has found that the CA3 plays an essential role in the consolidation of memories when examining CA3 regions using the Morris water maze.

Cornu ammonis 1 The CA1 is the region within the hippocampus between the subiculum, the innermost area of the hippocampal formation, and region CA2. The CA1 is separated from the dentate gyrus by the hippocampal sulcus. Cells within the CA1 are mostly pyramidal cells, similar to those in CA3. The CA1 completes the circuit by feeding back to the deep layers, mainly layer V, of the entorhinal cortex.

Associated brain areas There are many brain structures that transmit information to, and from the trisynaptic circuit. The activity of these different structures can be directly or indirectly modulated by the activity of the trisynaptic loop.

Fornix The fornix is a C-shaped bundle of axons that begins in the hippocampal formation of both hemispheres, referred to as the fimbria, and extend through the crus of fornix, also known as the posterior pillars. The fimbria section of the fornix is directly connected to the alveus, which is a portion of the hippocampal formation that arises from the subiculum and the hippocampus (specifically the CA1). Both crura of the fornix form intimate connections with the underside of the corpus callosum and support the hippocampal commissure, a large bundle of axon that connects the left and right hippocampal formations. The fornix plays a key role in hippocampal outputs, specifically in connecting CA3 to a variety of subcortical structures, and connecting CA1 and the subiculum to a variety of parahippocampal regions, via the fimbria. The fornix is also essential for hippocampal information input and neuromodulatory input, specifically from the medial septum, diencephalic brain structures, and the brain stem.

Cingulate gyrus The cingulate gyrus plays a key role in the limbic system's emotion formation and processing. The cingulate cortex is separated into an anterior and a posterior region, which corresponds to areas 24, 32, 33 (anterior) and 23 (posterior) of the Brodmann areas. The anterior region receives information mainly from the mamillary bodies while the posterior cingulate receives information from the subiculum via the Papez circuit.

Mammillary bodies The mammillary bodies are two clusters of cell bodies found at the ends of the posterior fibres of the fornix within the diencephalon. The mammillary bodies relay information from the hippocampal formation (via the fornix) to the thalamus (via the mammillothalamic tract). The mammillary bodies are integral parts of the limbic system and have been shown to be important in recollective memory.

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Worked examples

Example 1 — a first encounter with Trisynaptic circuit

Start with the simplest possible case. Write down what Trisynaptic circuit claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Trisynaptic circuit before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Trisynaptic circuit ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Trisynaptic circuit

In research: Trisynaptic circuit appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Trisynaptic circuit in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Trisynaptic circuit is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cell signaling, Hippocampus (brain), so understanding it makes those chapters shorter.
In everyday life: Look for Trisynaptic circuit outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Trisynaptic circuit in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Trisynaptic circuit means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Trisynaptic circuit out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Trisynaptic circuit in simple terms?

Why does Trisynaptic circuit matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Trisynaptic circuit?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Trisynaptic circuit.