Phylogenesis of the Nervous System

Phylogenesis of the Nervous System


  • 1 The evolution of the nervous system
  • 2 Animals without nervous system
  • 3 Nervous system of invertebrates
  • 4 Vertebral Nervous System

The evolution of the nervous system

The efforts to understand our nervous system They have led us to study the SN of other animals. In fact, we share many biological and behavioral characteristics with all animals and, therefore, understanding how your SN works can help us understand how ours works. Keep in mind that, despite the similarities, there are aspects in which your SN is simpler, and even aspects that differ significantly from ours.

Many researchers have considered comparative studies as a part of evolutionary history, that is, of the phylogeny of humans. Thus, comparisons between different animals, but also the data obtained from fossil remains, give us an idea about the history of the SN and, therefore, how the human SN has been formed.

Here is a brief description of the evolutionary study of the SN, that is, as the SN has developed along the phylogenetic scale until it reaches the shape of humans (mammals).

Animals without nervous system

The development of the nervous system of the invertebrates, before the evolutionary appearance of the spinal cord, the neuronal organization depends on the ectoderm and the sensory surface.

The single cell animals Like amoebas, still they have no nervous system, and communication with the environment is carried out by means of liquids entering and leaving an organism as a form of self-regulation.

There's also multicellular animals without nervous system, such as sponges, anemones, jellyfish, mollusks, worms, arthropods and others.

Invertebrate nervous system

Most animals on Earth are invertebrates, that is, no spine. The fact that the SN of these animals is very simple, and at the same time they can present a great variety of behavioral adaptations, including forms of learning and memory, has stimulated the study.

The phylogenetic evolution of SN in invertebrates goes through the following stages:

Initially there is a reticular nervous system, in the form of a broadcast network. In these animals cells form tissues, but not organs.

For example, jellyfish or sea anemones, a type of celenterae, have a nervous system in the form of a random and widespread network. This type of organization does not imply connections with a centrally located structure of the type of a brain.

From the worms we find a growing tendency towards centralization, so that the SN is organized as a ganglionic nervous system:

  • The SN is ventral.
  • There is a longitudinal axis with head and tail at the ends.
  • At the end of the head there is a cluster of nerve cells (ganglia) with a tendency to specialization (smell, sight, etc.). The nodes are the precursors of the CNS.
  • Start of segmentation (Metam). Each segmental ganglion receives information from sensory cells in the skin and sends impulses to that part of the body.

For example, in the body of annelids, such as earthworms or leeches, we can already see differentiated segments, each one controlled by a local group of nerve cells arranged in elaborate nodes. In the case of the leech, its CNS includes a chain of twenty one ganglia joined at one end by a cephalic ganglion and on the other to the tail ganglion.

Vertebral Nervous System

The vertebrate nervous system has two main divisions: the Central Nervous System, which consists of the brain and spinal cord, and the peripheral nervous system, which in humans includes 12 pairs of cranial nerves, 31 pairs of spinal nerves and the system autonomic or involuntary nervous.

In principle, all vertebrates share these characteristics, as they descend from a common ancestor. It seems that most have the same main subdivisions, although differences are found between the species in the relative and absolute sizes of these.

For example, if we compare the human and the rat's brain, we will see that the differences are mainly quantitative, that is, they refer to the absolute and relative measurement of the entire brain, brain regions and brain cells. We can also observe differences in the degree of development or complexity of the nervous systems.

Evolution of mammalian bark

All mammals have, in addition to paleocortex (which already appears in reptiles), neocortex that is formed by six layers.

In more advanced mammals, the neocortex It represents more than half the volume of the brain. In addition, in many mammals, such as humans, the neocortex is fully folded, covering the rest of the brain.

In more advanced mammals, the bark It is primarily responsible for many complex functions, such as the perception of objects. The regions of the brain that were responsible for the perceptual functions in non-evolved animals, in more modern mammals become centers of control of reflex behaviors or passageways in the projection path to the cortex.

Brain size evolution

We might think that the percentage of brain weight / body weight in humans is higher. This is only when we compare them with animals of high body weight, but not when we do it with small animals, such as mice for example.

It has been seen that the weight of the brain does not increase in the same proportion as the size of the body, but is proportional to 2/3 of the body weight and dependsIn addition, a constant value (the encephalization factor) that varies between animal species. This constant is higher in animals that have evolved more recently, and is especially high in humans.

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