Types of vertebrate brains

1. Ichthyopsid type (fish and amphibians)

This type is characteristic of lower vertebrates. The integrating center is the midbrain (mesencephalon). Animals possess only the first signaling system, and their thinking is limited to visual-figurative and visual-active levels.

Features in fish:

• The forebrain functions primarily as an olfactory center (chemical sense is critical for survival).
• There is no division into hemispheres; only a superficial sulcus is present.
• Gray matter forms basal ganglia (striatum) on the floor of the brain and lines the ventricular cavity as the paleocortex (paleocortex).
• There are practically no neurons in the roof of the brain (or very few of them).

Features in amphibians:

• Two full-fledged hemispheres appear, each with its own ventricle.
• The mass of the basal ganglia increases.
• Beneath the white matter of the roof, the primary cerebral fornix is formed — the primordium of the archicortex (archicortex).
• The paired hippocampus begins to develop from the archicortex primordium (this first occurs in lungfish and Gymnophiona).

2. Sauropsid type (reptiles and birds)

The leading center moves higher — this function is now performed by the floor of the forebrain (basal ganglia). The signaling system and types of thinking remain the same as in the ichthyopsid type.

Development in reptiles:

• The forebrain becomes the largest division.
• The striatum migrates from the floor to the center, significantly increasing the mass of the hemispheres.
• In the roof of the forebrain (old cortex), neurons are arranged in layers. Medial islands (the future hippocampus, responsible for olfaction, aggression, sex, and learning) and lateral islands (the primordium of the neocortex — neocortex) are formed.

Development in birds:

• The hemispheres are clearly separated and have large dimensions due to migrating basal ganglia.
• The old cortex is represented only by medial islands.
• A unique feature of birds is the presence of the hyperstriatum (special areas of gray matter).

3. Mammalian type (mammals and humans)

The main feature of the mammalian brain is the transfer of all higher nervous functions to the roof of the forebrain (cerebral cortex). In higher mammals, the hemispheres are connected by a complex commissure — the corpus callosum (corpus callosum).

Evolution of cortical structures:

1. The paleocortex (paleocortex) in highly organized animals is reduced to the pyriform (olfactory) lobe.
2. The archicortex (archicortex) folds on the medial surface and forms the hippocampus (part of the limbic system).
3. The neocortex (neocortex) rapidly expands from the lateral islands. In higher mammals, its area increases due to folds — sulci and gyri.

With the advent of humans, the second signaling system (speech and abstraction) is added to the first, and abstract-logical thinking is formed. Functional asymmetry arises: the right hemisphere is responsible for images (biological essence), and the left for logic and speech (social essence).

Class and exam questions

What else to study on this topic

• Histological layered organization of the cortex in mammals
• Connection of the hippocampus with the hypothalamus and its functions in the limbic system
• Evolution and reduction of the paleocortex
• Development of the first and second signaling systems

Other topics in the section "Phylogeny of the vertebrate nervous system"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.