Nervous system of fish divided by peripheral And central. central nervous system consists of the brain and spinal cord, and peripheral- from nerve fibers and nerve cells.

The brain of fish.

fish brain consists of three main parts: forebrain, midbrain and hindbrain. forebrain consists of the telencephalon ( telencephalon) and diencephalon - diencephalon. At the anterior end of the telencephalon are bulbs responsible for the sense of smell. They receive signals from olfactory receptors.

Schematic of the olfactory chain in fish can be described as follows: in the olfactory lobes of the brain there are neurons that are part of the olfactory nerve or a pair of nerves. Neurons join the olfactory tracts of the telencephalon, which are also called the olfactory lobes. Olfactory bulbs are particularly prominent in fish that use the senses, such as sharks, which survive on scent.

Diencephalon consists of three parts: epithalamus, thalamus And hypothalamus and performs the functions of a regulator of the internal environment of the fish body. The epithalamus contains the pineal organ, which in turn consists of neurons and photoreceptors. pineal organ located at the end of the epiphysis and in many fish species it can be sensitive to light due to the transparency of the skull bones. Due to this, the pineal organ can act as a regulator of activity cycles and their change.

The midbrain of fish contains visual lobes And tegmentum or a tire - both are used to process optical signals. The optic nerve of fish is very branched and has many fibers extending from the visual lobes. As with the olfactory lobes, enlarged visual lobes can be found in fish that rely on vision to survive.

The tegmentum in fish controls the internal muscles of the eye and thus ensures its focus on the object. Also, the tegmentum can act as a regulator of active control functions - it is here that the locomotor region of the midbrain is located, which is responsible for rhythmic swimming movements.

The hindbrain of fish is made up of cerebellum, elongated brain And bridge. The cerebellum is an unpaired organ that performs the function of maintaining balance and controlling the position of the body of the fish in the environment. The medulla oblongata and the pons together make up brain stem, to which a large number of cranial nerves that carry sensory information stretch. Most of all nerves communicate with and enter the brain through the brainstem and hindbrain.

Spinal cord.

Spinal cord is located inside the neural arches of the vertebrae of the fish spine. The spine has segmentation. In each segment, neurons connect to the spinal cord via dorsal roots, and agile neurons exit them via ventral roots. Within the central nervous system are also interneurons that provide communication between agile and sensory neurons.

Brain bony fish consists of five sections typical of most vertebrates.

Rhomboid brain(rhombencephalon) includes the medulla oblongata and cerebellum.

medulla oblongata the anterior section goes under the cerebellum, and behind without visible borders passes into the spinal cord. To view the anterior medulla oblongata, it is necessary to turn the body of the cerebellum forward (in some fish, the cerebellum is small and the anterior medulla oblongata is clearly visible). The roof in this part of the brain is represented by the choroid plexus. Underneath is a large rhomboid fossa (fossa rhomboidea), expanded at the anterior end and passing behind into a narrow medial gap, it is a cavity fourth cerebral ventricle (ventriculus quartus). The medulla oblongata serves as the origin of most of the brain nerves, as well as a pathway that connects the various centers of the anterior sections of the brain with the spinal cord. However, the layer of white matter covering the medulla oblongata is rather thin in fish, since the body and tail are largely autonomous - they carry out most of the movements reflexively, without correlating with the brain. In the bottom of the medulla oblongata in fish and tailed amphibians lies a pair of giant mauthner cells, associated with acoustic-lateral centers. Their thick axons extend along the entire spinal cord. Locomotion in fish is carried out mainly due to the rhythmic bending of the body, which, apparently, is controlled mainly by local spinal reflexes. However, the overall control of these movements is carried out by Mauthner cells. At the bottom of the medulla oblongata lies the respiratory center.

Viewing the brain from below, one can distinguish the places where some nerves originate. Three round roots extend from the lateral side of the anterior part of the medulla oblongata. The first, lying most cranial, belongs to V and VII nerves, middle root - only VII nerve, and finally, the third root, lying caudally, is VIII nerve. Behind them, also from the lateral surface of the medulla oblongata, the IX and X pairs depart together in several roots. The rest of the nerves are thin and are usually cut off during preparation.

Cerebellum rather well developed, round or elongated, it lies above the anterior part of the medulla oblongata directly behind the visual lobes. With its posterior edge, it covers the medulla oblongata. The raised part is the body of the cerebellum (corpus cerebelli). The cerebellum is the center of fine regulation of all motor innervations associated with swimming and grasping food.

midbrain(mesencephalon) - the part of the brain stem that is permeated by the cerebral aqueduct. It consists of large, longitudinally elongated visual lobes (they are visible from above).

Visual lobes, or visual roof (lobis opticus s. Tectum opticus) - paired formations separated from each other by a deep longitudinal furrow. The visual lobes are the primary visual centers that perceive excitation. They terminate the fibers of the optic nerve. In fish, this part of the brain is of paramount importance, it is the center that has the main influence on the activity of the body. The gray matter covering the visual lobes has a complex layered structure, reminiscent of the structure of the cerebellar cortex or hemispheres.

From the ventral surface of the visual lobes depart thick optic nerves, crossing under the surface of the diencephalon.

If you open the visual lobes of the midbrain, you can see that in their cavity a fold is separated from the cerebellum, which is called cerebellar valve (valvule cerebellis). On the sides of it in the bottom of the cavity of the midbrain, two bean-shaped elevations are distinguished, called semilunar bodies (tori semicircularis) and being additional centers of the statoacoustic organ.

forebrain(prosencephalon) less developed than the middle one, it consists of the terminal and diencephalon.

Parts intermediate brain (diencephalon) lie around a vertical slot third cerebral ventricle (ventriculus tertius). Lateral walls of the ventricle visual tubercles or thalamus ( thalamus) in fish and amphibians are of secondary importance (as coordinating sensory and motor centers). The roof of the third cerebral ventricle - the epithalamus or epithalamus - does not contain neurons. It contains the anterior vascular plexus (the vascular tegmentum of the third ventricle) and the superior brain gland - epiphysis. The bottom of the third cerebral ventricle - the hypothalamus or hypothalamus in fish forms paired swellings - lower lobes (lobus inferior). In front of them lies the lower brain gland - the pituitary gland. In many fish, this gland fits snugly into a special recess in the bottom of the skull and usually breaks off during preparation; then clearly visible funnel (infundibulum). Ahead, on the border between the bottom of the final and intermediate parts of the brain is optic chiasm (chiasma nervorum opticorum).

telencephalon (telencephalon) in bony fish, compared with other parts of the brain, it is very small. Most fish (except for lungfish and crossopterygians) are distinguished by an everted (inverted) structure of the hemispheres of the telencephalon. They seem to be "turned out" ventro-laterally. The roof of the forebrain does not contain nerve cells, it consists of a thin epithelial membrane (pallium), which during preparation is usually removed along with the meninges. In this case, the bottom of the first ventricle is visible on the preparation, divided by a deep longitudinal groove into two striped bodies. Striped bodies (corpora striatum1) consist of two sections, which can be seen when considering the brain from the side. In fact, these massive structures contain striatal and crustal material of a rather complex structure.

Olfactory bulbs (bulbus olfactorius) adjacent to the anterior margin of the telencephalon. From them go forward olfactory nerves. In some fish (for example, cod), the olfactory bulbs are carried far forward, in which case they are connected to the brain olfactory tracts.

The brain of fish is very small, making up thousandths of % of body weight in sharks, hundredths of % in teleosts and sturgeons. In small fish, the mass of the brain reaches about 1%.

The brain of fish consists of 5 sections: anterior, intermediate, middle, cerebellum and medulla oblongata. The development of individual parts of the brain depends on the way of life of fish and their ecology. So, in good swimmers (mainly pelagic fish), the cerebellum and visual lobes are well developed. In fish with a well-developed sense of smell, the forebrain is enlarged. In fish with well-developed vision (predators) - the midbrain. Sedentary fish have a well-developed medulla oblongata.

The medulla oblongata is a continuation of the spinal cord. Together with the midbrain and diencephalon, it forms the brainstem. In the medulla oblongata, compared with the spinal cord, there is no clear distribution of gray and white matter. The medulla oblongata performs the following functions: conduction and reflex.

The conduction function is to conduct nerve impulses between the spinal cord and other parts of the brain. Through the medulla oblongata pass ascending paths from the spinal cord to the brain and descending paths connecting the brain with the spinal cord.

Reflex function of the medulla oblongata. In the medulla oblongata there are centers of both relatively simple and complex reflexes. Due to the activity of the medulla oblongata, the following reflex reactions are carried out:

1) regulation of breathing;

2) regulation of cardiac activity and blood vessels;

3) regulation of digestion;

4) regulation of the work of taste organs;

5) regulation of the work of chromatophores;

6) regulation of the work of electrical organs;

7) regulation of the centers of movement of the fins;

8) regulation of the spinal cord.

The medulla oblongata contains the nuclei of six pairs of cranial nerves (V-X).

V pair - the trigeminal nerve is divided into 3 branches: the ophthalmic nerve innervates the anterior part of the head, the maxillary innervates the skin of the anterior part of the head and palate, and the mandibular innervates the mucous membrane oral cavity and mandibular muscles.

VI pair - the opening nerve innervates the muscles of the eyes.

VII pair - the facial nerve is divided into 2 lines: the first innervates the lateral line of the head, the second - the mucous membrane of the palate, the hyoid region, the taste buds of the oral cavity and the muscles of the gill cover.

VIII pair - auditory or sensory nerve - innervates the inner ear and labyrinth.

IX pair - glossopharyngeal nerve - innervates the mucous membrane of the palate and the muscles of the first branchial arch.

X pair - the vagus nerve is divided into two branching branches: the lateral nerve innervates the organs of the lateral line in the trunk, the nerve of the gill cover innervates the gill apparatus and other internal organs.

The midbrain of fish is represented by two sections: the visual roof (tectum) - located horizontally and the tegmentum - located vertically.

The tectum or visual roof of the midbrain is swollen in the form of paired visual lobes, which are well developed in fish with a high degree of development of the organs of vision and poorly developed in blind deep-sea and cave fish. On the inner side of the tectum there is a longitudinal torus. It is associated with vision. In the tegmentum of the midbrain, the highest visual center of fish is located. The fibers of the second pair of optic nerves terminate in the tectum.

The midbrain performs the following functions:

1) The function of the visual analyzer, as evidenced by the following experiments. After the removal of the textum on one side of the eye of the fish, the one lying on the opposite side becomes blind. When the entire tectum is removed, complete blindness occurs. The tectum also houses the center of the visual grasping reflex, which consists in the fact that the movements of the eyes, head, and torso are directed in such a way as to maximize the fixation of the food object in the region of greatest visual acuity, i.e. in the center of the retina. In the tectum there are centers of the III and IV pairs of nerves that innervate the muscles of the eyes, as well as muscles that change the width of the pupil, i.e. performing accommodation, allowing you to clearly see objects at different distances due to the movement of the lens.

2) Participates in the regulation of fish coloration. So, after the removal of the tectum, the body of the fish brightens, while when the eyes are removed, the opposite phenomenon is observed - darkening of the body.

3) In addition, the tectum is closely connected with the cerebellum, hypothalamus, and through them with the forebrain. Therefore, the tectum coordinates the functions of the somatosensory (balance, posture), olfactory, and visual systems.

4) The tectum is connected with the VIII pair of nerves, which perform acoustic and receptor functions, and with the V pair of nerves, i.e. trigeminal nerves.

5) Afferent fibers from the lateral line organs, from the auditory and trigeminal nerves approach the midbrain.

6) In the tectum there are afferent fibers from the olfactory and taste receptors.

7) In the midbrain of fish, there are centers for regulating movement and muscle tone.

8) The midbrain has an inhibitory effect on the centers of the medulla oblongata and spinal cord.

Thus, the midbrain regulates a number of vegetative functions of the body. Due to the midbrain, the reflex activity of the organism becomes diverse (orienting reflexes to sound and visual stimuli appear).

Intermediate brain. The main formation of the diencephalon is the visual tubercles - the thalamus. Under the visual tubercles is the hypothalamic region - the epithalamus, and under the thalamus is the hypothalamic region - the hypothalamus. The diencephalon in fish is partially covered by the roof of the midbrain.

The epithalamus consists of the epiphysis, a rudiment of the parietal eye, which functions as an endocrine gland. The second element of the epithalamus is the frenulum (gabenula), which is located between the forebrain and the roof of the midbrain. The frenulum is a link between the epiphysis and the olfactory fibers of the forebrain, i.e. participates in the performance of the function of light perception and smell. The epithalamus is connected to the midbrain through efferent nerves.

The thalamus (visual tubercles) in fish is located in the central part of the diencephalon. In the visual tubercles, especially in the dorsal part, many nuclear formations were found. The nuclei receive information from receptors, process it and transmit it to certain areas of the brain, where the corresponding sensations arise (visual, auditory, olfactory, etc.). Thus, the thalamus is an organ of integration and regulation of the body's sensitivity, and also takes part in the implementation of the body's motor reactions.

If the visual tubercles are damaged, there is a decrease in sensitivity, hearing, vision, which causes impaired coordination.

The hypothalamus consists of an unpaired hollow protrusion - a funnel that forms a vascular sac. The vascular sac responds to pressure changes and is well developed in deep sea pelagic fish. The vascular sac is involved in the regulation of buoyancy, and through its connection with the cerebellum, it is involved in the regulation of balance and muscle tone.

The hypothalamus is the main center for receiving information from the forebrain. The hypothalamus receives afferent fibers from taste endings and from the acoustic system. Efferent nerves from the hypothalamus go to the forebrain, to the dorsal thalamus, tectum, cerebellum and neurohypophysis, i.e. regulates their activities and influences their work.

The cerebellum is an unpaired formation, it is located in the back of the brain and partially covers the medulla oblongata. Distinguish between the body of the cerebellum (middle part) and the ears of the cerebellum (i.e., two lateral sections). The anterior end of the cerebellum forms a flap.

Leading fish sedentary image life (for example, in bottom fish, such as scorpions, gobies, anglerfish), the cerebellum is underdeveloped in comparison with fish that lead an active lifestyle (pelagic, such as mackerel, herring or predators - pike perch, tuna, pike).

Functions of the cerebellum. With the complete removal of the cerebellum in moving fish, a drop in muscle tone (atony) and impaired coordination of movements are observed. This was expressed in the circular swimming of fish. In addition, the reaction to pain stimuli weakens in fish, sensory disturbances occur, and tactile sensitivity. Approximately, after three to four weeks, the lost functions are restored due to the regulatory processes of other parts of the brain.

After removal of the body of the cerebellum, bony fish show motor disturbances in the form of body swaying from side to side. After removal of the body and the valve of the cerebellum, motor activity is completely disrupted, and trophic disorders develop. This indicates that the cerebellum also regulates metabolism in the brain.

It should be noted that the auricles of the cerebellum reach large sizes in fish with a well-developed lateral line. Thus, the cerebellum is the site of closure of conditioned reflexes coming from the lateral line organs.

Thus, the main functions of the cerebellum are the coordination of movement, the normal distribution of muscle tone and the regulation of autonomic functions. The cerebellum realizes its influence through the nuclear formations of the middle and medulla oblongata, as well as the motor neurons of the spinal cord.

The forebrain of fish consists of two parts: the mantle or cloak and the striatum. The mantle, or the so-called cloak, lies dorsally, i.e. from above and from the sides in the form of a thin epithelial plate over the striatum. In the anterior wall of the forebrain are the olfactory lobes, which are often differentiated into the main part, stalk and olfactory bulb. Secondary olfactory fibers from the olfactory bulb enter the mantle.

Functions of the forebrain. The forebrain of fish performs an olfactory function. This, in particular, is evidenced by the following experiments. When the forebrain is removed, fish lose the developed conditioned reflexes to olfactory stimuli. In addition, the removal of the forebrain of fish leads to a decrease in their motor activity and to a decrease in schooling conditioned reflexes. The forebrain also plays an important role in the sexual behavior of fish (when it is removed, sexual desire disappears).

Thus, the forebrain is involved in the protective-defensive reaction, the ability to swim in schools, the ability to take care of offspring, etc. It has a general stimulating effect on other parts of the brain.

7. Principles of the reflex theory I.P. Pavlova

Pavlov's theory is based on the basic principles of the conditioned reflex activity of the brain of animals, including fish:

1. The principle of structure.

2. The principle of determinism.

3. The principle of analysis and synthesis.

The principle of structurality is as follows: each morphological structure corresponds to a certain function. The principle of determinism is that reflex reactions have a strict causality, i.e. they are determined. For the manifestation of any reflex, a reason, a push, an impact from the outside world or the internal environment of the body is necessary. The analytical and synthetic activity of the central nervous system is carried out due to the complex relationship between the processes of excitation and inhibition.

According to Pavlov's theory, the activity of the central nervous system is based on a reflex. A reflex is a causally determined (deterministic) reaction of the body to changes in the external or internal environment, carried out with the obligatory participation of the central nervous system in response to irritation of the receptors. This is how the emergence, change or cessation of any activity of the body occurs.

Pavlov divided all the reflex reactions of the body into two main groups: unconditioned reflexes and conditioned reflexes. Unconditioned reflexes are congenital, inherited reflex reactions. Unconditioned reflexes appear in the presence of a stimulus without special, special conditions (swallowing, breathing, salivation). Unconditioned reflexes have ready-made reflex arcs. Unconditioned reflexes are divided into various groups according to a number of characteristics. On a biological basis, food (search, intake and processing of food), defensive (defensive reaction), sexual (animal behavior), indicative (orientation in space), positional (taking a characteristic posture), locomotor (motor reactions) are distinguished.

Depending on the location of the irritated receptor, exteroceptive reflexes are isolated, i.e. reflexes that occur when the outer surface of the body (skin, mucous membranes) is irritated, interoreceptive reflexes, i.e. reflexes that occur when irritated by internal organs, proprioceptive reflexes that occur when receptors of skeletal muscles, joints, and ligaments are irritated.

Depending on the part of the brain that is involved in the reflex reaction, the following reflexes are distinguished: spinal (spinal) - centers of the spinal cord participate, bulbar - centers of the medulla oblongata, mesencephalic - centers of the midbrain, diencephalic - centers of the diencephalon.

In addition, reactions are divided according to the organ that is involved in the response: motor or motor (muscle participates), secretory (internal or external secretion gland participates), vasomotor (vessel participates), etc.

Unconditioned reflexes - specific reactions. They are common to all representatives of this species. Unconditioned reflexes are relatively constant reflex reactions, stereotyped, little changeable, inert. As a result of this, it is impossible to adapt to the changing conditions of existence only due to unconditioned reflexes.

Conditioned reflexes - a temporary nervous connection of the body with some stimulus of the external or internal environment of the body. Conditioned reflexes are acquired during the individual life of the organism. They are not the same in different representatives of this species. Conditioned reflexes do not have ready-made reflex arcs, they are formed under certain conditions. Conditioned reflexes are changeable, easily arise and also easily disappear, depending on the conditions in which the given organism is located. Conditioned reflexes are formed on the basis of unconditioned reflexes under certain conditions.

For the formation of a conditioned reflex, it is necessary to combine two stimuli in time: an indifferent (indifferent) for a given type of activity, which will later become a conditioned signal (knocking on glass) and an unconditioned stimulus that causes a certain unconditioned reflex (food). The conditioned signal always precedes the action of the unconditioned stimulus. Reinforcement of the conditioned signal with an unconditioned stimulus must be repeated. It is necessary that the conditioned and unconditioned stimuli meet the following requirements: the unconditioned stimulus must be biologically strong (food), the conditioned stimulus must have a moderate optimal strength (knock).

8. Behavior of fish

The behavior of fish becomes more complicated in the course of their development, i.e. ontogeny. The simplest reaction of the body of a fish in response to an irritant is kinesis. Kinesis is an increase in motor activity in response to adverse effects. Kinesis is observed already at the last stages of the embryonic development of fish, when the oxygen content in the environment decreases. An increase in the movement of larvae in eggs or in water in this case improves gas exchange. Kinesis promotes the movement of larvae from poor living conditions to better ones. Another example of kinesis is the erratic movement of schooling fish (verkhovka, uklya, etc.) when a predator appears. This confuses him and prevents him from focusing on one fish. This can be considered a defensive reaction of schooling fish.

A more complex form of fish behavior is taxis - this is a directed movement of fish in response to a stimulus. A distinction is made between positive taxis (attraction) and negative taxis (avoidance). An example is phototaxis, i.e. reaction of fish to the light factor. Thus, anchovy and big-eyed kilka have positive phototaxis, i.e. are well attracted to the light, forming clusters, which makes it possible to use this property in the fishery of these fish. In contrast to the Caspian sprat, the mullet exhibits negative phototaxis. Representatives of this species of fish tend to get out of the illuminated background. This property is also used by humans when fishing for this fish.

An example of negative phototaxis is the behavior of salmon larvae. During the day, they hide among stones, in gravel, which allows them to avoid meeting with predators. And in the larvae of cyprinids, positive phototaxis is observed, which allows them to avoid deadly deep-sea areas and find more food.

The direction of taxis may undergo age-related changes. Thus, fry of salmon at the stage of the pestryanka are typical benthic sedentary fish that protect their territory from their own kind. They avoid light, live among stones, easily change color according to color. environment, when frightened, they are able to hide. As they grow in front of a slope in the sea, they change color to non-silver, gather in flocks, lose their aggressiveness. When frightened, they quickly swim away, are not afraid of light, and vice versa, stay near the surface of the water. As you can see, the behavior of juveniles of this species changes to the opposite with age.

In fish, unlike higher vertebrates, there is no cerebral cortex, which plays a leading role in the development of conditioned reflexes. However, fish are able to produce them without it, for example, a conditioned reflex to sound (Frolov's experiment). After the action of a sound stimulus, a current was switched on in a few seconds, to which the fish reacted by moving its body. After a certain number of repetitions, the fish, without waiting for the electric current, reacted to the sound, i.e. reacted with body movements. In this case, the conditioned stimulus is the sound, and the unconditioned stimulus is the induction current.

In contrast to higher animals, fish develop reflexes worse, they are unstable and difficult to develop. Fish are less able than higher animals to differentiate, i.e. distinguish between conditioned stimuli or changes in the external environment. It should be noted that in bony fish conditioned reflexes are developed faster and they are more persistent than in others.

There are works in the literature that show rather persistent conditioned reflexes, where the unconditioned stimuli are a triangle, a circle, a square, various letters, etc. If a feeder is placed in a pond that gives a portion of food in response to pressing a lever, pulling a bead or other devices, then the fish master this device quickly enough and receive food.

Those who are engaged in aquarium fish farming, they have observed that when approaching the aquarium, the fish gather at the feeding place in anticipation of food. This is also a conditioned reflex, and in this case, you are the conditioned stimulus, and knocking on the glass of the aquarium can also serve as a conditioned stimulus.

In fish farms, the fish are usually fed at certain times of the day, so they often gather at certain places at the time to feed. The fish also quickly get used to the type of food, the way food is distributed, etc.

big practical value may have the development of conditioned reflexes to a predator in the conditions of fish hatcheries and NVH in juveniles of commercial fish, which are then released into natural reservoirs. This is due to the fact that in the conditions of fish hatcheries and NVH, juveniles do not have the experience of communicating with enemies and at the first stages become the prey of predators until they get an individual and spectacular experience.

Using conditioned reflexes, various aspects of the biology of various fish are studied, such as the spectral sensitivity of the eye, the ability to distinguish silhouettes, the effect of various toxicants, the hearing of fish by the strength and frequency of sound, the thresholds of taste sensitivity, the role of various parts of the nervous system.

In the natural environment, the behavior of fish depends on the lifestyle. Schooling fish have the ability to coordinate maneuvers when feeding, at the sight of a predator, etc. Thus, the appearance of a predator or food organisms at one edge of the flock causes the entire flock to react accordingly, including individuals that did not see the stimulus. The reaction can be very diverse. So at the sight of a predator, the flock instantly scatters. You can observe this in the spring time in the coastal zone of our reservoirs, fry of many fish concentrate in flocks. This is one kind of imitation. Another example of imitation is following the leader, i.e. for an individual in whose behavior there are no elements of oscillation. The leader is most often individuals who have great individual experience. Sometimes even a fish of a different species can serve as such a leader. So, carps learn to take food on the fly faster if they are planted with trout or carp individuals that can do this.

When fish live in groups, a “social” organization can arise with dominant and subordinate fish. So, in a flock of Mozambian tilapia, the most intensely colored male is the main one, the next in the hierarchy are lighter. Males, which do not differ from females in color, are subordinate and do not participate in spawning at all.

The sexual behavior of fish is very diverse, this includes elements of courtship and rivalry, building nests, etc. Complex spawning and parental behavior is typical for fish with low individual fecundity. Some fish take care of eggs, larvae and even fry (protect the nest, aerate the water (zander, smelt, catfish)). Juveniles of some fish species feed near their parents (for example, discus even feed their juveniles with their mucus). Juveniles of some fish species hide with their parents in the oral and gill cavities (tilapia). Thus, the plasticity of fish behavior is very diverse, as can be seen from the above materials.

Questions for self-control:

1. Features of the structure and function of nerves and synapses.

2. Parabiosis as a special kind of localized excitation.

3. Scheme of the structure of the nervous system of fish.

4. Structure and functions of the peripheral nervous system.

5. Features of the structure and function of the brain.

6. Principles and essence of the reflex theory.

7. Features of the behavior of fish.

Representatives of this class have variations in the structure of the brain, but, nevertheless, common characteristic features can be distinguished for them. Their brain has a relatively primitive structure and in general small size.

The forebrain, or terminal, in most fish consists of one hemisphere (some sharks that lead a benthic lifestyle have two) and one ventricle. The roof does not contain nerve elements and is formed by the epithelium, and only in shark nerve cells rise from the base of the brain to the sides and partly to the roof. The bottom of the brain is represented by two clusters of neurons - these are striatal bodies (corpora striata).

Anterior to the brain are two olfactory lobes (bulbs) connected by olfactory nerves to the olfactory organ located in the nostrils.

In lower vertebrates, the forebrain is a part of the nervous system that serves only the olfactory analyzer. It is the highest olfactory center.

The diencephalon consists of the epithalamus, thalamus, and hypothalamus, which are common to all vertebrates, although their degree varies. The thalamus plays a special role in the evolution of the diencephalon, in which the ventral and dorsal parts are distinguished. Later, in vertebrates, in the course of evolution, the size of the ventral part of the thalamus decreases, while the dorsal part increases. The lower vertebrates are characterized by the predominance of the ventral thalamus. Here are the nuclei that act as an integrator between the midbrain and the olfactory system of the forebrain, in addition, in lower vertebrates, the thalamus is one of the main motor centers.

Below the ventral thalamus is the hypothalamus. From below, it forms a hollow stalk - a funnel, which passes into the neurohypophysis, connected to the adenohypophysis. The hypothalamus plays a major role in the hormonal regulation of the body.

The epithalamus is located in the dorsal part of the diencephalon. It does not contain neurons and is associated with the pineal gland. The epithalamus, together with the pineal gland, constitutes a system of neurohormonal regulation of the daily and seasonal activity of animals.

Rice. 6. The brain of a perch (view from the dorsal side).

1 - nasal capsule.
2 - olfactory nerves.
3 - olfactory lobes.
4 - forebrain.
5 - midbrain.
6 - cerebellum.
7 - medulla oblongata.
8 - spinal cord.
9 - diamond-shaped fossa.

The midbrain of fish is relatively large. It distinguishes the dorsal part - the roof (tekum), which looks like a colliculus, and the ventral part, which is called the tegment and is a continuation of the motor centers of the brain stem.

The midbrain developed as a primary visual and seismosensory center. It contains visual and auditory centers. In addition, it is the highest integrative and coordinating center of the brain, approaching in its value to big hemispheres forebrain of higher vertebrates. This type of brain, where the midbrain is the highest integrative center, is called ichthyopsid.

The cerebellum is formed from the posterior cerebral bladder and is laid in the form of a fold. Its size and shape vary considerably. In most fish, it consists of the middle part - the body of the cerebellum and of the lateral ears - the auricles. Bony fish are characterized by anterior growth - a flap. The latter in some species takes on such a large size that it can hide part of the forebrain. In sharks and bony fish, the cerebellum has a folded surface, due to which its area can reach a considerable size.

Through ascending and descending nerve fibers, the cerebellum is connected to the middle, medulla oblongata and spinal cord. Its main function is the regulation of coordination of movements, in connection with which, in fish with a high motor activity it is large and can make up to 15% of the total mass of the brain.

The medulla oblongata is a continuation of the spinal cord and generally repeats its structure. The border between the medulla oblongata and the spinal cord is considered to be the place where the central canal of the spinal cord on cross section takes the form of a circle. In this case, the cavity of the central canal expands, forming the ventricle. The side walls of the latter grow strongly to the sides, and the roof is formed by an epithelial plate, in which the choroid plexus is located with numerous folds facing the cavity of the ventricle. In the side walls there are nerve fibers that provide innervation to the visceral apparatus, the organs of the lateral line and hearing. In the dorsal parts of the lateral walls there are gray matter nuclei, in which the switching of nerve impulses occurs, coming along the ascending pathways from the spinal cord to the cerebellum, midbrain and to the neurons of the striatal bodies of the forebrain. In addition, there is also a switch of nerve impulses to descending pathways that connect the brain with the motor neurons of the spinal cord.

The reflex activity of the medulla oblongata is very diverse. It contains: the respiratory center, the center for the regulation of cardiovascular activity, through the nuclei of the vagus nerve, the regulation of the digestive organs and other organs is carried out.

From the brain stem (medium, medulla oblongata and pons) in fish, 10 pairs of cranial nerves depart.