Neural Circuits & Neural network
Simple neural circuits
Definition
A neural circuit is a network in which brain regions or individual neurons are the nodes and axonal connections are represented by directed edges. Edges can encode many variables but minimally designate the type of neurotransmitter, which determines whether a cell will excite, inhibit or modulate its targets.
Introduction
Neurons never function in isolation; they are organized into ensembles or circuits that process specific kinds of information. Although the arrangement of neural circuits varies greatly according to the intended function, some features are characteristic of all such ensembles. The synaptic connections that define a circuit are typically made in a dense tangle of dendrites, axons terminals, and glial cell processes that together constitute neuropil (the suffix -pil comes from the Greek word pilos, meaning “felt”; see Figure 1.3).
Figure 1.3
{(A) Diagram of nerve cells and their component parts. (B) Axon initial segment (blue) enters a myelin sheath (gold). (C) Terminal boutons (blue) loaded with synaptic vesicles (arrowheads) form synapses (arrows) with a dendrite (purple). (D) The transverse section of axons (blue) is ensheathed by the processes of oligodendrocytes (gold). (E) Apical dendrites (purple) of cortical pyramidal cells. (F) Nerve cell bodies (purple) are occupied by large round nuclei. (G) The portion of a myelinated axon (blue) illustrates the intervals between adjacent segments of myelin (gold) is referred to as nodes of Ranvier (arrows). (Micrographs from Peters et al., 1991.)}Thus, the neuropil between nerve cell bodies is the region where most synaptic connectivity occurs. The direction of information flow in any particular circuit is essential to understanding its function. Nerve cells that carry information toward the central nervous system (or farther centrally within the spinal cord and brain) are called afferent neurons; nerve cells that carry information away from the brain or spinal cord (or away from the circuit in question) are called efferent neurons. Nerve cells that only participate in the local aspects of a circuit are called interneurons or local circuit neurons. These three classes—afferent neurons, efferent neurons, and interneurons—are the basic constituents of all neural circuits.
Neural circuits are both anatomical and functional entities. A simple example is a circuit that subserves the myotatic (or “knee-jerk”) spinal reflex (Figure 1.5).
Fig 1.5
{A simple reflex circuit, the knee-jerk response (more formally, the myotatic reflex), illustrates several points about the functional organization of neural circuits. Stimulation of peripheral sensors (a muscle stretch receptor in this case) initiates receptor potentials that trigger action potentials that travel centrally along the afferent axons of the sensory neurons. This information stimulates spinal motor neurons by means of synaptic contacts. The action potentials triggered by the synaptic potential in motor neurons travel peripherally in efferent axons, giving rise to muscle contraction and a behavioral response. One of the purposes of this particular reflex is to help maintain an upright posture in the face of unexpected changes.}
The afferent limb of the reflex is sensory neurons of the dorsal root ganglion in the periphery. These afferents target neurons in the spinal cord. The efferent limb comprises motor neurons in the ventral horn of the spinal cord with different peripheral targets: One efferent group projects to flexor muscles in the limb, and the other to extensor muscles. The third element of this circuit is the interneurons in the ventral horn of the spinal cord. The interneurons receive synaptic contacts from the sensory afferent neurons and make synapses on the efferent motor neurons that project to the flexor muscles. The synaptic connections between the sensory afferents and the extensor afferents are excitatory, causing the extensor muscles to contract; conversely, the interneurons activated by the afferents are inhibitory, and their activation by the afferents diminishes electrical activity in motor neurons and causes the flexor muscles to become less active (Figure 1.6). The result is a complementary activation and inactivation of the synergist and antagonist muscles that control the position of the leg.
A more detailed picture of the events underlying the myotatic or any other circuit can be obtained by electrophysiological recording (Figures 1.6 and 1.7). There are two basic approaches to measuring electrical activity: extracellular recording where an electrode is placed near the nerve cell of interest to detect activity, and intracellular recording where the electrode is placed inside the cell. Such recordings detect two basic types of signals. Extracellular recordings primarily detect action potentials, the all-or-nothing changes in the potential across nerve cell membranes that convey information from one point to another in the nervous system. Intracellular recordings can detect the smaller graded potential changes that serve to trigger action potentials. These graded triggering potentials can arise at either sensory receptors or synapses and are called receptor potentials or synaptic potentials, respectively. For the myotatic circuit, action potential activity can be measured from each element (afferents, efferents, and interneurons) before, during, and after a stimulus (see Figure 1.6). By comparing the onset, duration, and frequency of action potential activity in each cell, a functional picture of the circuit emerges. As a result of the stimulus, the sensory neuron is triggered to fire at a higher frequency (i.e., more action potentials per unit time). This increase triggers in turn a higher frequency of action potentials in both the extensor motor neurons and the interneurons. Concurrently, the inhibitory synapses made by the interneurons onto the flexor motor neurons cause the frequency of action potentials in these cells to decline. Using intracellular recording (see Chapter 2), it is possible to observe directly the potential changes underlying the synaptic connections of the myotatic reflex circuit, as illustrated in Figure 1.7.
Fig 1.7
{Intracellularly recorded responses underlying the myotatic reflex. (A) Action potential is measured in a sensory neuron. (B) Postsynaptic triggering potential is recorded in an extensor motor neuron. (C) Postsynaptic triggering potential in an interneuron. (D) Postsynaptic inhibitory potential in a flexor motor neuron. Such intracellular recordings are the basis for understanding the cellular mechanisms of action potential generation, and the sensory receptor and synaptic potentials that trigger these conducted signals.}Reference: (Neuroscience. 2nd edition.)
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Neural circuit diagrams
Use alignment and consistency to untangle complex circuit diagrams.
Neural circuit diagrams show connections between neurons or brain regions. They are similar to pathways1 but typically have more complex connections and more variables, and they can be generated at different scales. Unfortunately, diagrams of even simple circuits are often unnecessarily complex, making understanding brain connectivity maps difficult.
A neural circuit is a network in which brain regions or individual neurons are the nodes and axonal connections are represented by directed edges. Edges can encode many variables but minimally designate the type of neurotransmitter, which determines whether a cell will excite, inhibit or modulate its targets. We recommend limiting this encoding to line caps (Fig. 1a). Variables such as cell type, cell location (layer or brain region), cell morphology, the location of a synapse on a cell, the composition of neurotransmitter receptors on a cell and the strength or density of connections can be encoded using a combination of labels, node position, color and shape, carefully selected on the basis of their salience
Encoding several variables without sacrificing information, while still maintaining clarity, is a challenge. To do this, exclude extraneous variables—vary a graphical element only if it encodes something relevant2, and do not encode any variables twice.
We can apply strategies used to draw pathways1 to clarify the flow of information and highlight important features in circuits. If the physical locations of nodes are not important, nodes should be rearranged to clarify the circuit structure and limit the number of edge crossings. In Figure 2a, the position, color and labels all redundantly distinguish brain regions. By removing this redundancy and improving the layout, we make room for additional features to highlight circuit elements (Fig. 2b,c). Other strategies are shown in Supplementary Figure 1.
For neural circuits such as the brainstem auditory circuits, physical arrangement is a fundamental part of function. Another topology that is commonly necessary in neural circuit diagrams is the laminar organization of the cerebral cortex (Fig. 3). When some parts of a circuit diagram are anatomically correct, readers may assume all aspects of the figure are similarly correct. For example, if cells are in their appropriate layers, one may assume that the path that one axon travels to reach another cell is also accurate. Be careful not to portray misleading information—draw edges clearly within or between layers, and always clearly communicate any uncertainty in the circuit.
In some circuits, the location where an axon contacts a postsynaptic neuron can be important to circuit function. A synapse on a cell body may be different than a synapse on an apical dendrite, requiring some cell morphology to be encoded in the circuit diagram. However, to keep the diagram as clear as possible, do not show more morphological information than necessary. Just because the apical dendrite must be shown does not mean all dendrites should be included (Fig. 3). Likewise, just because cell bodies are drawn in the diagram does not mean that the size and shape differences across cells are necessary. The goal is to convey a circuit as clearly and accurately as possible; extraneous variables, such as cell size, will unnecessarily complicate the presentation of your circuit.
Neural circuits were able to see in real time.
To better understand how the brain works, neuroscientists need to see in real time the chains of neurons that are responsible for, for example, processing sensory information or forming new memories, and describing them in detail. A group of researchers from the California Institute of Technology talked about a technique that allows you to do this.
The team developed a microchip system that was implanted into the brains of mice. The circuit emits light and thus stimulates the genetically modified neurons around it. Cells are labeled with a fluorescent marker and glow when exposed to light waves, and the system registers their activity, showing functionality. The details are published in the journal Neuron.
The developed method was called "integrated neurophotonics". Right now it has only been used on mice, but in the future, it could tell a lot about how the human brain works, the researchers said. “Recording neural activity at great depth is the key to new research,” said Michael Roux, author of the paper and professor of applied physics and bioengineering at the California Institute of Technology. “We won’t be able to record all of the brain activity any time soon, but it’s possible to study some important structures — and this opportunity gives us the motivation to work further.”
Laurent Moreau, a senior research fellow at the California Institute of Technology and one of the authors of the paper, said there are serious physical limitations to current optogenetic brain research. Brain tissue scatters light, and it can only travel short distances in it, so it has not yet been possible to study neural circuits located in the brain at a depth of more than two millimeters.
Roux believes that integrated neurophotonics will solve this problem. Microchips can be implanted at any depth and placed next to chains of neurons: then they will record in real time what each cell in a particular area of the brain is doing.
“The resources to create such an approach have existed for many years, but until recently we did not have the right vision and funding to collect them and create a powerful new tool for neuroscience,” the authors summed up. And the new method, they say, will seriously advance research.
Molecules of joy: how our brain creates neural connections and forms habits and intelligence
Hormones influence the mechanisms of emotion formation and the action of various neurochemicals, and, as a result, are involved in the formation of stable habits. The author of the book “Hormones of Happiness”, Professor Emeritus of the University of California Loretta Graziano Breuning, suggests revising our behavior patterns and learning how to trigger the action of serotonin, dopamine, endorphin and oxytocin. T&P publishes a book chapter on how our brains self-adjust to experience and form appropriate neural connections.
Shifting neural pathways
Each person is born with many neurons, but very few connections between them. These connections are built as we interact with the world around us and ultimately make us who we are. But sometimes you have a desire to somewhat modify these formed connections. It would seem that this should be easy, because they developed with us without much effort on our part even in our youth. However, the formation of new neural pathways in adulthood is surprisingly complex. Old connections are so effective that letting go of them makes you feel like your survival is in danger. Any new neural circuits are very fragile compared to the old ones. When you can understand how difficult it is to create new neural pathways in the human brain,
Five Ways Your Brain Is Self-Tuning
We mammals are capable of making neural connections throughout our lives, unlike species with stable connections. These connections are created as the world around us affects our senses, which send the appropriate electrical impulses to the brain. These impulses lay out neural pathways that other impulses will run faster and easier in the future. The brain of each individual is tuned to an individual experience. Below are five ways experiences physically change your brain.
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Life experience isolate young neurons
A constantly working neuron is covered with a shell of a special substance called myelin over time. This substance significantly increases the efficiency of the neuron as a conductor of electrical impulses. This can be compared to the fact that insulated wires can withstand a much greater load than bare ones. Myelin-coated neurons work without undue effort, which is characteristic of slow, "open" neurons. Myelin sheathed neurons look more white than gray, which is why we divide our brain matter into "white" and "grey".
Most of the myelin coating of neurons is completed in a child by the age of two, as his body learns to move, see, and hear. When a mammal is born, a mental model of the world around it must form in its brain, which will provide it with the means to survive. Therefore, the production of myelin in a child is maximum at birth, and by the age of seven it decreases slightly. By this time, you no longer need to re-learn the truth that fire burns, and the gravity of the earth can make you fall.
With the achievement of puberty in a person, the formation of myelin in his body is activated again. This is because the mammal has to rewire its brain to find the best mate. Often during the mating season, animals migrate to new groups. Therefore, they have to get used to new places in search of food, as well as to new tribesmen. In search of a married couple, people are also often forced to move to new tribes or clans and learn new customs and culture. The increase in myelin production during puberty contributes to all this. Natural selection arranged the brain in such a way that it is during this period that it changes the mental model of the surrounding world.
Everything you do purposefully and consistently during your “myelinated heyday” creates powerful and branching neural pathways in your brain. That is why so often the genius of a person manifests itself precisely in childhood. That is why little skiers so famously fly past you on mountain slopes that you cannot master, no matter how hard you try. That is why it becomes so difficult to study foreign languages with the end of adolescence. As an adult, you can memorize foreign words, but more often than not, you cannot quickly pick them up to express your thoughts. This is because your verbal memory is concentrated in thin, unmyelinated neurons. Powerful myelinated neural connections are engaged in your high mental activity, therefore, new electrical impulses find it difficult to find free neurons.
Fluctuations in the activity of the body in the myelination of neurons can help you understand why people have certain problems at different periods of life. Remember that the human brain does not reach its maturity automatically. Therefore, it is often said that the brain of adolescents is not yet fully formed. The brain “myelinates” all of our life experiences. So if there are episodes in the life of a teenager when he receives an undeserved reward, then he remembers firmly that the reward can be received without effort. Some parents forgive teens for bad behavior by saying that "their brains aren't fully formed yet." That is why it is very important to purposefully control the life experience that they absorb. If you allow a teenager to avoid responsibility for his actions, then you can form a mind in him, who will expect the possibility of evading such responsibility in the future.
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Life experience increases the efficiency of the synapse
A synapse is a point of contact (a small gap) between two neurons. An electrical impulse in our brain can only travel if it reaches the end of a neuron with enough force to jump across that gap to the next neuron. These barriers help us filter out the really important incoming information from the irrelevant so-called "noise". The passage of an electrical impulse through synaptic spaces is a very complex natural mechanism. It can be imagined in such a way that a whole fleet of boats accumulates at the tip of one neuron, which transports the neural "spark" to special receiving docks that the adjacent neuron has. Each time, the boats are better able to handle transportation. This is why the experience we gain increases the chances of electrical signals being transmitted between neurons. There are over 100 trillion synaptic connections in the human brain. And our life experience plays an important role in conducting nerve impulses through them in a way that is consistent with the interests of survival.
On a conscious level, you cannot decide which synaptic connections you should develop. They are formed in two main ways:
1) Gradually, by repeated repetition.
2) Simultaneously, under the influence of strong emotions.
Synaptic connections are built on the basis of repetition or emotions experienced by you in the past. Your mind exists because your neurons have formed connections that reflect good and bad experiences. Some episodes from this experience were “pumped” into your brain thanks to “joy molecules” or “stress molecules”, others were fixed in it thanks to constant repetition. When the model of the world around you matches the information that is contained in your synaptic connections, electrical impulses run through them easily, and it seems to you that you are quite aware of the events taking place around you.
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Neural circuits are formed only due to active neurons
Those neurons that are not actively used by the brain begin to gradually weaken already in a two-year-old child. Oddly enough, this contributes to the development of his intellect. Reducing the number of active neurons allows the baby not to glide with an absent-minded gaze around everything around, which is characteristic of a newborn, but to rely on the neural pathways that he has already formed. A two-year-old baby is already able to independently concentrate on what gave him pleasant sensations in the past, such as a familiar face or a bottle of his favorite food. He may be wary of things that have caused him negative emotions in the past, such as a pugnacious playmate or a closed door. The young brain relies on its little life experience to meet needs and avoid potential threats.
No matter how the neural connections are built in the brain, you feel them as “truth”
Between the ages of two and seven years, the process of optimizing the child's brain continues. This forces him to relate new experiences to old ones, instead of accumulating new experiences in some separate block. Tightly intertwined neural connections and neural pathways form the basis of our intelligence. We create them by branching old neural trunks instead of creating new ones. Thus, by the age of seven, we usually clearly see what we have seen once and hear what we have heard once.
You may think that this is bad. However, consider the value of all this. Imagine that you lied to a six-year-old child. He believes you because his brain greedily absorbs everything that is offered to him. Now suppose that you have deceived an eight-year-old child. He is already questioning your words because he compares the incoming information with the information he already has, and not just “swallows” new information. At the age of eight, it is already more difficult for a child to form new neural connections, which pushes him to use the existing ones. Relying on old neural circuits allows him to recognize lies. This was of great survival value in a time when parents died young and children had to learn to take care of themselves from an early age. In our youth, we form certain neural connections, allowing others to gradually fade away. Some of them disappear as the wind blows away the autumn leaves. This helps to make the human thought process more efficient and focused. Of course, as you age, you gain more and more knowledge. However, this new information is concentrated in areas of the brain that already have active electrical pathways. For example, if our ancestors were born in hunting tribes, they quickly gained experience as a hunter, and if in the tribes of tillers - agricultural experience. Thus, the brain tuned in to survive in the world in which they really existed. However, this new information is concentrated in areas of the brain that already have active electrical pathways. For example, if our ancestors were born in hunting tribes, they quickly gained experience as a hunter, and if in the tribes of tillers - agricultural experience. Thus, the brain tuned in to survive in the world in which they really existed. However, this new information is concentrated in areas of the brain that already have active electrical pathways. For example, if our ancestors were born in hunting tribes, they quickly gained experience as a hunter, and if in the tribes of tillers - agricultural experience. Thus, the brain tuned in to survive in the world in which they really existed.
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New synaptic connections are formed between the neurons you actively use
Each neuron can have many synapses because it has many processes or dendrites. New processes in neurons are formed when it is actively stimulated by electrical impulses. As the dendrites grow towards points of electrical activity, they can get so close that electrical impulses from other neurons can bridge the distance between them. Thus, new synaptic connections are born. When this happens, at the level of consciousness you get a connection between two ideas, for example.
You cannot feel your own synaptic connections, but you can easily see it in others. A person who loves dogs looks at the whole world around him through the prism of this affection. A person who is fascinated by modern technologies associates everything in the world with them. A lover of politics evaluates the surrounding reality politically, and a religiously convinced person - from the standpoint of religion. One person sees the world positively, the other negatively. No matter how the neural connections are built in the brain, you do not feel them as numerous processes, similar to the tentacles of an octopus. You experience these connections as "truth."
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Emotion receptors develop or atrophy
In order for an electrical impulse to cross the synaptic cleft, the dendrite on one side must eject chemical molecules that are picked up by special receptors on the other neuron. Each of the neurochemicals produced by our brain has a complex structure that is perceived by only one specific receptor. It fits the receptor-like a key to a lock. When emotions overwhelm you, more neurochemicals are released than the receptor can pick up and process. You feel overwhelmed and disoriented until your brain creates more receptors. So you adapt to the fact that "something is happening around you."
When a neuron's receptor is inactive for a long time, it disappears, leaving room for other receptors that you may need to appear. Flexibility in nature means that the receptors on neurons must either be used or they can be lost. "Hormones of joy" are constantly present in the brain, searching for "their" receptors. This is how you “know” the reason for your positive feelings. The neuron “fires” because the right hormone molecules open the lock on its receptor. And then, based on this neuron, a whole neural circuit is created that tells you where to expect joy in the future.
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