Human Anatomy - H. I. Koliadenko 2009

Sense Organs (Analyzers)
Early Stages of Phylogeny of Analyzers and the Nervous System

The Evolution of the Structure and Functions of The Nervous system begins in lower animals, specifically invertebrates. Using them as an example, one can trace how receptors and Nerve Cells developed and complexified, and how the architecture of the nervous system progressed from lower animals to highly organized ones.

Nervous Tissue is characterized by such fundamental properties as irritability, excitation, and conductivity.

The formation of nervous tissue occurred gradually, moving from the undifferentiated protoplasm of the amoeba to the remarkably complex nervous system of mammals. For instance, an amoeba responds to stimuli through the excitation of the entire Cell, whereas in Multicellular Organisms, excitation is transmitted from the superficial cell layer to deeper-lying cells.

A multicellular Organism possesses already differentiated tissues. Alongside other tissues, nervous tissue emerges to perform its uniquely specialized functions.

It is believed that the initial stage in The Development of nervous tissue is represented by epithelial-motor cells in hydras—essentially epithelial cells featuring protoplasmic extensions capable of contraction. Further complexification of the nervous system proceeds through the formation of neuromotor cells, combining a superficial receptor with an elongated conductor of excitation.

In polyps and jellyfish, the nervous system is diffuse, taking the form of a nerve net composed of receptor and motor cells interconnected by protoplasmic processes. Due to this network structure, excitation spreads across a vast number of cells, causing them to contract and eliciting the contraction of all body Muscles. The subsequent stage of nervous system evolution involves the formation of clusters known as ganglia, arranged in chain-like series. Thus, in segmented worms (Annelids), each body segment contains a pair of ganglia linked together by a commissure. These ganglia connect with receptors distributed across the body surface as well as with muscles. Such a nervous system is referred to as a ganglion chain or ladder-type system. Its anterior segment features a dominant cephalic ganglion, which concentrates nerve cells alongside numerous receptors, alongside the elementary structural unit of the nervous system—the reflex arc. The Emergence of the reflex arc during phylogenetic development was of paramount importance for the evolution of the nervous system. The number of elements within each reflex arc increases, forming intricate complexes. The nervous system achieves its ultimate development in humans.

Under the Influence of the environment, the nervous system undergoes a series of modifications that enable humans to adapt optimally to living conditions. A general refinement of all elements within the reflex arc takes place. Receptors undergo particularly significant changes, showing clear specialization of receptor cells and their adaptation to specific stimuli: chemical, mechanical, thermal, auditory, visual, and others. Receptor cells concentrate in specific regions to form complex receptive Organs known as analyzers (the eye, ear, Olfactory Organ, etc.). The specific stimulus to which a given nerve cell is adapted is termed its adequate stimulus.

In a chain ganglion nervous system, the anteriorly positioned leading nerve ganglion increases significantly in size and, over the course of evolution, assumes the coordination of movements across all body parts.

In Chordates, the nervous system takes the form of a dorsal nerve tube; the Brain is in a rudimentary stage, lacking distant reception. Thus, the transitional form found in invertebrates (represented by amphioxus) exhibits a primitive nervous system structure compared to vertebrates, which feature developing cerebral hemispheres and pre-existing specialized Sense Organs.

The brain performs complex integrative and reflex functions, coordinating The activity of all organs and physiological systems within the organism.

Evolution of the Spinal Cord and brain in vertebrates. In the shark's spinal cord, the Gray matter features anterior horns and weakly defined posterior horns. The nerve fibers are myelinated. The Medulla Oblongata possesses well-developed nuclei that innervate Internal Organs. The shark's Cerebellum is relatively large, consisting of a central body and two lateral lobes, and is responsible for swift locomotion and maneuvering in Water. The Midbrain includes the optic tectum (bilobate body), which acts as a regulatory center connected to all sensory modalities. The Diencephalon consists of a small thalamus and a well-developed Hypothalamus. The Telencephalon is small, featuring an unpaired ventricle, a Pallium, and subcortical ganglia. Anteriorly, the telencephalon transitions into paired olfactory bulbs. The rudimentary structures of the paleocortex are visible on the ventral surface of the telencephalon.

The transition of animals to a terrestrial lifestyle triggered notable changes in brain structure. For example, in reptiles, the telencephalon and its cerebral hemispheres are considerably more developed. The spinal cord also becomes more advanced. The gray matter, organized into anterior and posterior horns, divides the White matter into three pairs of funiculi: anterior, lateral, and posterior. Progress is also evident in the Development of the cerebellum, which is substantially larger than that of fish and is divided by transverse sulci into anterior, middle, and posterior lobes. The medulla oblongata features thin and cuneate tubercles of ascending pathways, along with the functioning sensory Nucleus of the Trigeminal nerve. In the midbrain of certain reptiles, a posterior colliculus appears caudal to the anterior colliculus. The tegmentum of the cerebral peduncles contains the red nucleus. The diencephalon comprises a relatively large thalamus and hypothalamus. The thalamus is connected via nerve fibers to the telencephalon, wherein the paleocortex and archicortex of the cerebral hemispheres develop, alongside subcortical nuclei. The archicortex occupies a relatively large surface area; while in amphibians it is confined to the medial surface, in reptiles it extends to the dorsal surface. On the lateral surface of the hemispheres, the neocortex emerges, taking over the organism's integrative coordination functions.

In mammals, the spinal cord terminates in a slender filum terminale extending from the thickened conus medullaris. A distinctive feature of the lateral funiculi is the presence of a high density of fibers belonging to the anterior and lateral Pyramidal Tracts. A direct correlation has been established between the Abundance of pyramidal tract fibers and an animal's position on the evolutionary ladder. Specifically, pyramidal fibers account for 7% of the total WHITE MATTER OF the spinal cord in dogs, 20% in monkeys, and 30% in humans. The medulla oblongata in dogs is compact. Its ventral surface bears the pyramids, flanked by the olives and the inferior cerebellar peduncles. Rostral to the medulla oblongata, the basis pontis appears, connecting with the cerebral peduncles of the midbrain, which features a well-developed tectal plate. The complex Processing previously handled by the optic tectum is partially delegated to the lateral geniculate bodies, while the intricate auditory Functions of the posterior colliculi shift to the medial geniculate bodies of the diencephalon. The mammalian diencephalon undergoes progressive development, with the thalami concentrating all sensory modalities before relaying them to the Cerebral Cortex. The telencephalon features a well-developed pallium that expands into the frontal, temporal, and occipital lobes. However, not all mammals possess a highly convoluted pallium; in rodents, insectivores, and marsupials, the cerebral hemispheres are small and possess a smooth (lissencephalic) surface. In carnivores and ungulates, sulci emerge, which significantly increase the cortical surface area. In primates, the hemispheres are richly covered with sulci and gyri. They attain their highest degree of development in humans. The neocortex, which almost entirely covers the hemispheres, plays the primary role in regulating all physiological functions. It maintains direct connections with all analyzers and receptors, thereby executing complex integrative and reflex regulatory functions for the entire organism. The paleocortex and archicortex are displaced by the neocortex toward the ventral and medial surfaces of the cerebral hemispheres. In humans, the neocortex accounts for 96% of the total surface area of the cerebral hemispheres, compared to 84% in dogs, 55% in rabbits, and 32% in hedgehogs. This Progressive development of the cortex and the increasing structural complexity of the brain are closely linked to both the labor-related and social factors that accompanied Human Evolution.