Anatomical structure and biological functions of the brain of teleost fish

The diencephalon includes the epithalamus, thalamus, and hypothalamus. The epithalamus consists of the paired habenular ganglion and the pineal gland. The thalamus forms the base of the brain, limiting the third ventricle with its lower and lateral walls. The hypothalamus includes the pituitary gland and the saccus vasculosus. Since the diencephalon is a center for switching excitations, numerous brain nuclei connected to all divisions are located in the thalamus and hypothalamus.

The midbrain (tectum) consists of a massive base, which is a continuation of the thalamus, and a roof divided into two hemispheres—the optic lobes. Inside the midbrain is the third ventricle, at the bottom of which are two protrusions of the cerebellar peduncles extending into it. The optic lobes, which are the precursor to the cerebral hemispheres, are structured more simply than in mammals. They consist of 4–5 rows of nerve cells. The midbrain is the center of visual perception. The optic, oculomotor, and trochlear nerves originate from it. The tectum is the main coordination center that replaces the mammalian cerebral hemispheres in fish.

The cerebellum lies behind the optic lobes above the medulla oblongata. A body and two peduncles are distinguished in it. The body consists of three layers: the outer molecular, the middle ganglionic, and the inner granular. The molecular layer has a neurofibrillar structure with single small neurons. The ganglionic layer is represented by one row of pear-shaped neurons—Purkinje cells; the granular layer includes numerous small cells—microglia.

The cerebellum in fish also performs coordination functions, regulates swimming and body balance, and ensures the proportionality of motor stimuli by sending signals to the midbrain, from where they enter the motor centers of the brain.

The medulla oblongata is a continuation of the midbrain base and transitions into the spinal cord without visible boundaries. On its dorsal side is the fourth ventricle, along the lateral sides of which are thickenings containing the nuclei of the vagus nerve. In cyprinids, a powerful medial eminence protrudes from the bottom of the ventricle between these thickenings.

The medulla oblongata contains a conduction system that connects the brain with the spinal cord. It is the site of origin for the 5th to 10th pairs of cranial nerves. It houses the respiratory, vasomotor, and other vital centers. Here, they correspond to brain nuclei consisting of large multipolar neurons.

In bony fish, unlike higher vertebrates, there are 10 pairs of cranial nerves. The nerve corresponding to the accessory nerve (11th pair) is not separated from the vagus, and the hypoglossal nerve is absent. The homolog of the 12th pair is the first spinal nerve.

The spinal cord, in the form of a cylindrical body, lies in the neural arches of the vertebrae along the entire spine. Inside the spinal cord runs the central canal, around which multipolar neurons are located. The grey matter is inside the spinal cord, forming ventral, lateral, and dorsal horns. Spinal nerves exit metamerically from the spinal cord, the visceral branches of which participate in the formation of the sympathetic nerve trunk.

Externally, the brain is covered by three membranes: the inner pia mater, the middle arachnoid, and the outer fibrous dura mater.

Autonomic nervous system and fish reproduction

The sympathetic nervous system is represented by two sympathetic border tracts running along the vertebral column. Corresponding to each spinal nerve, ganglia are located in these tracts, from which branches extend to internal organs, blood vessels, and the heart.

The basis of the parasympathetic nervous system is the vagus nerve. Fish also have anterior (brachial) and posterior (pelvic) plexuses and intramural ganglia in the intestine and other organs. In most fish organs, scientists have established dual—sympathetic and parasympathetic—innervation.

Reproductive system. Fish are generally dioecious. However, some fish are hermaphroditic. There are cases of sex reversal. The male gonads are represented by paired testes (milt) containing a huge number of spermatozoa during the spawning period. Ducts lead from the testes and open into the genital pore.

The female gonads consist of paired (exceptionally single) ovaries that transition into short efferent ducts opening into the genital pore. The ovaries contain eggs. The number of eggs depends on:

• age and size of the fish (the older and larger the fish, the more eggs);
• their biological characteristics (as a rule, the less a given fish species provides parental care, the more eggs the females spawn).

Egg structure and initial stages of development

Most eggs are very small, round, and rich in yolk. The structure of the egg (ovum) of bony fish is quite complex. The egg is covered by two membranes:

• the outer part can be gelatinous and sticky;
• the thick inner membrane sometimes consists of two layers—one denser and one less dense.

The shells have an opening – the micropyle, through which spermatozoa penetrate into the egg. In addition to two shells, there is a germinal disc consisting of protoplasm with a nucleus. A reserve of protein substance is concentrated in the yolk, in a specific area of which one or many fat droplets are located.

Already at the larva stage, intensive fish growth begins. During this period, growth and development occur mainly due to the resorption of the yolk sac. Upon transition to active feeding, growth continues at the expense of food.