The activity of the cerebral cortex is subject to a number of principles and laws. The main ones were first established by I.P. Pavlov. Currently, some provisions of Pavlov’s teaching have been clarified, developed, and some of them have been revised. However, to master the basics of modern neurophysiology, it is necessary to become familiar with the fundamental provisions of Pavlovian teaching.
Analytical-synthetic principle of higher nervous activity.
As established by I.P. Pavlov, the main fundamental principle of operation of the cerebral cortex is the analytical-synthetic principle. Orientation in the environment is associated with the isolation of its individual properties, aspects, features (analysis) and the unification, connection of these features with what is useful or harmful to the body (synthesis). Synthesis is the closure of connections, and analysis is an increasingly subtle separation of one stimulus from another.
The analytical and synthetic activity of the cerebral cortex is carried out by the interaction of two nervous processes: excitation and inhibition. These processes are subject to the following laws.
Law of excitation irradiation. Very strong (as well as very weak) stimuli with prolonged exposure to the body cause irradiation - the spread of excitation over a significant part of the cerebral cortex.
Only optimal stimuli of medium strength cause strictly localized foci of excitation, which is the most important condition for successful activity.
Law of concentration of excitation. Excitation that has spread from a certain point to other zones of the cortex, over time, is concentrated in the place of its primary occurrence.
This law underlies the main condition of our activity - attention (the concentration of consciousness on certain objects of activity).
When excitation is concentrated in certain areas of the cerebral cortex, its functional interaction with inhibition occurs, and this ensures normal analytical and synthetic activity.
Law of mutual induction of nervous processes. At the periphery of the focus of one nervous process, a process with the opposite sign always occurs.
If the process of excitation is concentrated in one area of the cortex, then the process of inhibition inductively arises around it. The more intense the concentrated excitation, the more intense and widespread the process of inhibition.
Along with simultaneous induction, there is sequential induction of nervous processes - a sequential change of nervous processes in the same areas of the brain.
Only a normal ratio of the processes of excitation and inhibition ensures behavior that is adequate (corresponding) to the environment.
An imbalance between these processes, the predominance of one of them causes significant disturbances in mental regulation.
Thus, the predominance of inhibition and its insufficient interaction with excitation leads to a decrease in the activity of the body. The predominance of excitement can be expressed in disordered chaotic activity, excessive fussiness, which reduces the effectiveness of activity. The process of inhibition is an active nervous process. It limits and directs the process of excitation in a certain direction, promotes concentration and concentration of excitation.
Inhibition can be external or internal. Thus, if an animal is suddenly affected by some new strong stimulus, then the animal’s previous activity will be inhibited at that moment. This is external (unconditional) inhibition. In this case, the emergence of a focus of excitation, according to the law of negative induction, causes inhibition of other areas of the cortex.
One of the types of internal or conditioned inhibition is the extinction of a conditioned reflex if it is not reinforced by an unconditioned stimulus (extinction inhibition). This type of inhibition causes the cessation of previously developed reactions if they become useless under new conditions.
Inhibition also occurs when the brain is overexcited. It protects nerve cells from exhaustion. This type of inhibition is called protective inhibition.
The analytical activity of the cerebral cortex, the ability to distinguish objects and phenomena that are similar in their properties, are also based on the internal type of inhibition. So, for example, when an animal develops a conditioned reflex to an ellipse, it first reacts to both the ellipse and the circle. Generalization occurs, the primary generalization of similar stimuli. But, if you constantly accompany the presentation of an ellipse with a food stimulus and do not reinforce the presentation of a circle, then the animal gradually begins to separate (differentiate) the ellipse from the circle (the reaction to the circle is inhibited). This type of inhibition, which underlies analysis and differentiation, is called differentiation inhibition. It clarifies the actions of the animal, making it more adapted to the environment.
When the circuit is closed and opened, the current does not set instantly. The retardation effect is determined by the inductance of the circuit. Let's find the dependence when opening and closing the circuit.
P 
When the circuit is opened, the current decreases from the value 
to zero and at the same time an emf arises. self-induction 
, counteracting the decrease in current. At each moment of time, the current in the circuit is determined by Ohm's law:
.
Integrating the equation from 
before 
, we get:
,
Where 
- a constant that has the dimension of time is called relaxation time.
The more 
, the slower the current decreases. During 
the current in the circuit decreases by 
times (approximately 3 times) (see dependence 1 in the figure).
.
Explore on your own.
The phenomenon of mutual induction. Mutual inductance. Mutual induction emf.
E 
If two electrical circuits are close, they can influence each other. Such contours are called inductively coupled. Let's consider two such circuits (see figure). If current is passed through the first circuit 
then the magnetic flux coupled to the second circuit will be proportional to the current 
, and will also depend on the relative orientation of the circuits, their geometric dimensions, the number of turns and the magnetic properties of the medium. You can write:
.
Here the coefficient 
called mutual inductance the second circuit depending on the first. Back if you pass current 
through the second circuit, then for the magnetic flux coupled to the first circuit, we can write:
.
For linear media the coefficients 
And 
are equal to each other:
.
Mutual inductance or inductance is measured in henry (H).
Mutual inductance 
is numerically equal to the magnetic flux coupled to one of the circuits with a unit current in the other circuit. Mutual inductance depends on the shape, size and mutual orientation of the circuits, and the magnetic permeability of the medium.
For example, the mutual inductance of two coils with a common core is:
,
Where 
– core volume, 
And 
- the number of turns per unit length of the core generatrix for the first and second coils.
D 
let's do it. Let current flow through the first coil 
(see picture). For a sufficiently long coil, neglecting edge effects, we will assume that the magnetic field in the core is uniform:
.
The magnetic flux coupled to the second coil will be equal to:
This is where the expression for 
, considering that 
- core length.
Note that the resulting relation for 
is approximate and can be represented differently:
,
Where 
And 
- inductance of the coils.
E 
If alternating current is passed through one of the circuits, an induced current will arise in the second in accordance with Faraday’s law.
For example, if in the first circuit 
, then the magnetic flux coupled to the second circuit will change over time 
and an emf will arise in it. induction.
.
At 
:
.
Obviously, 
.
The e.m.f. arising in the circuits is called e.m.f. mutual induction.
Direction of currents and emf. mutual induction is determined by Lenz's rule (see figure)
The resulting induced current in the second circuit 
its magnetic field prevents the growth of magnetic flux from the primary circuit.
The resulting induced current in the second circuit, with its magnetic field, prevents a decrease in the magnetic flux from the first circuit.
The change in currents in inductively coupled circuits in linear media is described by Ohm’s law:
Where 
- e.m.f. sources in circuits 1 and 2, 
- circuit inductance, 
– mutual inductance of the circuits.
Note that the action of transformers used to convert currents and voltages is based on the phenomenon of mutual induction.
R 
Let's look at the idle speed of the transformer. This is the case when the secondary winding of the transformer is not loaded (see figure). In this case, you can write:
.
because 
.
Neglecting the resistance of the primary winding of the transformer 
, let's estimate the voltage on the secondary winding:
.
Transformers are used to increase or decrease voltage. For currents in the transformer windings, an inversely proportional dependence on the number of turns is observed:
.
Justify it yourself.
What conditions are necessary for the development of a conditioned reflex?
How does reflex inhibition occur?
Repeated repetition and the emergence of a temporary connection
As a result of systematic non-reinforcement of actions
1. How does the nervous system regulate the functioning of organs?
In the neurons of the nervous system, two main oppositely directed processes operate: excitation inhibition Excitation stimulates an organ to work, as if including it in it, inhibition slows down or stops this work Thanks to these processes, the work of organs is regulated. This regulation is multi-level.
2. What is the essence of multi-level regulation? What significance did I.M.’s discovery have for its substantiation? Sechenov central braking?
As studies by I.M. have shown. Sechenov, lower centers work under the control of higher centers. They can inhibit many unconditioned reflexes (central inhibition) or strengthen them. It is the centers of the cerebral cortex that send inhibitory signals to the spinal cord, and we do not withdraw our hand when our blood is taken for analysis.
3. What types of inhibition were discovered by I.P. Pavlov?
Continuing the research of I.M. Sechenova, I.P. Pavlov showed that there is conditioned and unconditioned inhibition.
4. Give examples of unconditioned and conditioned inhibition.
Unconditional, or innate, inhibition. Imagine that you are doing something, for example reading a book, and you are called to dinner. You are presented with two stimuli, and the most important one is selected. If the book is very interesting, you may not hear the words addressed to you, since stimuli of little significance to you affect inhibited areas of the cortex. It will be a different choice if you are hungry and the book is boring. Then the previous activity will be inhibited and a new one will begin. Thanks to unconditional inhibition, a choice of activity is possible: with the beginning of one activity, another automatically stops (or does not begin). Conditioned, or acquired, inhibition. Conditioned inhibition includes, for example, the extinction of a conditioned reflex. If a conditioned signal is left without reinforcement, then the conditioned reflex will soon fade away, and with prolonged non-reinforcement it can turn into a negative (inhibitory) conditioned connection. Thanks to these inhibitory connections, animals and humans learn to distinguish between similar stimuli. If the dog is fed after one call and not given food after two, then salivation will begin to occur only after one call (it will not occur after two). Of course, this will not happen immediately. At first, saliva will be separated for both stimuli, and only after long training will the animal learn to correctly distinguish between signals.
5. In what cases is a negative (inhibitory) conditioned connection formed between a signal and behavior?
Conditioned inhibition is developed in cases where the conditioned reflex is not reinforced by the vital event about which the conditioned signal warned. Thanks to conditioned inhibition, it is possible to distinguish important signals from stimuli similar to them. I. P. Pavlov discovered the law of mutual induction: excitation in one center causes inhibition in a competing center, and vice versa. There is also sequential induction: excitation in one center after some time is replaced by inhibition, and vice versa.
6. What is a dominant and how does it manifest itself?
The behavior of animals and humans is regulated by needs. They retreat for a while after they are satisfied, then appear again. A.A. Ukhtomsky discovered the phenomenon of dominance: the emergence in the brain of a powerful temporary focus of excitation caused by some urgent need. Thanks to the dominant, the formation of a temporary connection between the future signal and the emerging need is facilitated, which favors the development of a conditioned reflex.
7. Give examples of the manifestation of the law of mutual induction of excitation and inhibition.
The light gray background around the black square appears white in contrast. There is no light irritation from the black square. In the corresponding cortical cells of the visual analyzer, an inhibitory process occurs, which, by induction, enhances the excitation process that arose in neighboring cells from the perception of a light gray background. This creates the illusion of brighter illumination of this background than it actually is. Second example. The monotonous, quiet speech of the teacher during the lesson, not accompanied by the demonstration of visual aids or experiments and not containing vivid descriptions, very quickly tires schoolchildren, especially young children. Their attention becomes distracted. In the tired nerve cells of the speech-auditory area of the cortex, a process of inhibition occurs, which, by induction, increases the excitation of neighboring nerve cells of the visual, auditory and motor analyzers, caused by the action of weak stimuli: the child now notices the occasional creaking of a desk, the rustling of paper from behind, coughing; looks at his hands and objects lying on the desk of the students sitting in front of him; rummages through some familiar things in his pockets or desk, etc. Orienting reflexes to extraneous weak stimuli are enhanced precisely because the main stimulus - the teacher's voice - caused persistent inhibition in the speech-auditory area of the cortex. This is simultaneous positive induction. As an example of consistent positive induction, we can cite the same fact with a boring lesson: after a long forced sitting in the classroom, even disciplined children and adolescents spend rather noisy breaks. Long-term inhibition of motor reactions was replaced by increased motor activity. Inductive relationships of basic nervous processes also exist between the cortex and the immediate subcortex. With strong emotions (anger, fear, despair), the excited subcortex causes induction inhibition of cortical nerve connections. This explains the lack of rationality of some actions of an emotionally excited person. The opposite is also possible.