Human Anatomy - Kotsan I. Y. 2009

Sense Organs

Through their Sense Organs, humans receive comprehensive information about the external world, study and explore it, and form objective concepts of the objects and phenomena surrounding us. Thanks to the sense organs, a person navigates the environment, develops an attitude toward external influences, and responds to them through various actions.

Sense organs (organa sensoria) are complexes of anatomical structures that perceive stimuli from the external and internal environment, transform them into nerve impulses, and transmit these impulses to specific PARTS OF THE Brain to ensure a specific analysis of neural excitation, resulting in a sensation. This entire apparatus, necessary for The Emergence of a sensation, was called an analyzer by I. P. Pavlov.

Each analyzer consists of three parts:

1) the peripheral (receptor) part, which directly perceives the stimulus and converts its energy into a Nerve Impulse;

2) the conduction pathway (conductor), along which the nerve impulse travels to the corresponding nerve center;

3) The Nerve Center (the cortical end of the analyzer), located in the corresponding area of the Cerebral Cortex, where the higher analysis of neural excitation takes place and sensation arises.

Two groups of sensations are distinguished:

1. Sensations that reflect The properties of objects and phenomena of the surrounding material world (sensations of Touch, pressure, pain, Temperature, as well as auditory, visual, gustatory, olfactory, and gravity sensations).

2. Sensations that reflect the movements of individual body parts and the state of Internal Organs (motor sensations, body equilibrium sensations, visceral sensations).

Accordingly, all sense organs are also divided into two groups:

1. Organs of external Senses that receive nerve impulses from the exteroceptive field — exteroceptors. There are six of them: the organs of cutaneous sensation, gravity, Hearing, Vision, taste, and smell.

2. Organs of internal senses:

a) organs that receive nerve impulses from the interoceptive field (internal organs and Blood Vessels) — interoceptors;

b) organs that receive impulses from the proprioceptive field (muscular-articular sense, closely related to the sense of gravity) — proprioceptors.

Sensations originating from internal organs under normal conditions do not reach consciousness, affecting only the "general well-being." In general, all internal processes regulated by the Autonomic Nervous system occur without our conscious awareness and only manifest through pathological disorders, usually accompanied by more or less severe pain. Therefore, we will not discuss interoceptors in this chapter, especially since everything concerning the innervation of internal organs and vessels has already been covered in the chapter on the autonomic nervous system.

Among the excitations originating from the proprioceptive field, mention should be made of the muscular-articular sense, through which THE POSITION OF body parts is perceived and movements are coordinated. On the one hand, this sense is combined with cutaneous sensitivity (stereognosis), and on the other hand, it is connected with The Organ of gravity, providing orientation relative to the gravitational field, and can also be considered as a statokinetic apparatus ensuring body balance.

Let us examine the organs that perceive sensations coming from the external world — the exteroceptors.

The General structural plan of stimulus-perceiving formations is similar in all animals, despite subsequent significant complications in details. The primary element, with the exception of cutaneous sense organs, in terrestrial animals consists of special sensory Cells (type I cells) that develop from the epithelium of the outer germ layer (ectoderm), which by its very position is in contact with the external environment. Each such Cell has a small rod or sensory hairs on the end facing the external surface, and a process on the other end (in the olfactory and visual organs) that connects with the processes of Nerve Cells in the conduction pathways.

In other organs (taste, hearing), the sensory cell does not have a central process; instead, it is entwined by the terminal branches of an approaching afferent nerve (type II cells). The first type of sensory cells should be considered primary compared to the second. In aquatic animals, this form of receptor elements is also found in the integument, where these elements are moistened by the surrounding fluid. In the Skin of terrestrial animals, sensory cells are absent, and receptor nerve fibers either terminate freely between the Cells of the epithelial cover or possess special end-corpuscles at their tips.

The mesoderm also participates in The formation of sense organs, but only secondarily, by forming protective, supporting, and auxiliary devices for them. These devices, which grow around and Complement the sensory cells, form the peripheral parts of the sense organs together with them: the skin, ear, eye, Tongue, Nose. For example, the visual receptor consists of the sensory cells of the retina (rods and cones), whereas the peripheral part is the entire eye.