The senses and the brain are the same

Nervous system, sensory perceptions and locomotion

Publication: 1.6.2002
Origin: La main à la pâte, Paris

"I consider the brain to be the most powerful organ in the human body [...] the eyes, ears, tongue, hands and legs act solely on the command of the brain [...] I claim that the brain is the interpreter of our consciousness "(Hippocrates, 460-377 BC).

The nervous system

The nervous system is a wired communication system that is characteristic of animals. It consists of nerve cells, the neurons, which communicate via sometimes very long processes that serve to transmit chemical and electrical stimuli. The number of neurons in the nervous system is estimated to be 100 billion in humans.

Fig. 1: Some neurons in the brain (light microscope with 600x magnification; Golgi staining). Neurons are cells that consist of a cell body from which protrusions extend that transmit the stimuli.

The nervous system makes it possible to absorb stimuli from the environment and from inside the body, to pass them on, to process them and to trigger appropriate physiological and behavioral reactions.

As in all other vertebrates, the human central nervous system consists of the brain, all of the nerve centers in the cranial cavity - the cerebrum, cerebellum and brain stem - and the spinal cord, which is enclosed in the spine.

Fig. 2: The human brain comprises all nerve centers located in the cranial cavity. It processes all motor and sensory information.

All organs are connected to the central nervous system by nerves. Sensory nerves transport sensory stimuli from the sensory organs to the brain, while motor nerves transmit the motor stimuli triggered by the brain to the muscles.

The brain ceaselessly processes the flood of information given off by the sensory receptors and is also the seat of memory, logic, arithmetic, impressions, feelings and language.

The five human senses

The senses enable us to perceive different physical (radiation, acoustic vibrations, etc.) and chemical (molecules) characteristics of our environment. The stimuli from the environment are picked up by the special receptors of the sensory organs and converted into nerve stimuli. These are then passed on via the sensory nerves to certain regions of the brain, where the stimuli are analyzed and a mental image of the environment is derived from it, a so-called "mental image". It must be noted, however, that the different sense organs do not act independently of one another. The regions of the brain that decipher the sensory stimuli are all interconnected and the information from other sensory organs is used by the brain to constantly interpret the various sensory stimuli.

In humans, a general distinction is made between 5 senses: smell, sight, hearing, taste and touch, but in reality we have many more senses. Our brain records the position of our body in space, perceives accelerations, the movements of the limbs, internal injuries and numerous other internal and external stimuli. Every excessive stimulus is perceived as pain (depending on the sensitivity, also called a nociceptive threshold), which represents an alarm signal for the organism.

The sense of sight

What is perceived

The human eye reacts to the electromagnetic radiation of light. We can see things when they emit or reflect the visible rays of light and when they hit the retina (the light-sensitive tissue that lines the back of the eye). Daylight emitted by the sun consists of various radiations that differ in their wavelengths. While the spectrum of solar radiation ranges from ultraviolet to infrared, visible light only corresponds to a narrow range of wavelengths between 380 nm (violet) and 700 nm (red), because the light-sensitive receptors in the human eye cannot have wavelengths below 380 nm (ultraviolet light ) can still perceive over 700 nm (infrared light). The wavelengths of visible light correspond to the colors of the rainbow.

What is happening there?

The eye is a complex organ that develops from the developing brain in the embryo. In particular, it has optical devices that guide light to the sensory receptors of the retina and whose movements are made possible by special muscles. The retina is a veritable nerve center through which the visual information is processed for the first time. The resulting nerve stimuli are passed on to the brain via two optic nerves. In humans, 40% of the sensory stimuli can be traced back to the sense of sight.

The light emitted by an object enters the eye through the pupil. The pupil is the opening on the front of the eyeball and has a diaphragm, the iris, the diameter of which is automatically adjusted by muscles depending on the intensity of the light hitting the eye (analogous to the diaphragm on cameras).

Fig. 3: The iris (the colored part of the eye) has muscles that regulate the diameter of the pupil (the opening through which light enters the eye).
The diameter of the pupil increases in weak light.
With strong incidence of light, the diameter of the pupil decreases.

The reaction time for this reflex means that when we change from a very bright environment to a dark one, we can no longer perceive our environment for a few seconds. Our pupils need this time to open sufficiently and thus allow more light to penetrate into the eye. If, on the other hand, you change from a dark to a lighter environment, you are initially blinded until the pupil has closed so far that the amount of light entering the eye is reduced. In addition to this system for regulating the amount of light penetrating, the eye has a convergent lens that throws an inverted image of the objects on the retina. The objects are still not perceived the other way around, as the brain reconstructs these images so that we can see them the right way round. The refractive power of the lens is variable and is also controlled by reflexes. In order for an object to be seen clearly, its image must be reproduced exactly on the retina. The image appears blurred if the lens focuses the image just in front of the retina, as is the case with nearsightedness, or just behind it with farsightedness (hypermetropia). The task of the lens is to focus the light rays emanating from the object on the retina in such a way that a sharp image is created. There are muscles around the lens that allow the refractive power to change by changing the radius of curvature of the lens. This control mechanism of the nervous system is called accommodation.


If the eyes are exposed to sunlight for too long, irreversible damage can occur. For this reason, wearing sunglasses, even for children, is essential in strong sunlight. On the one hand, the retinal receptors are damaged by excessive UV rays (for this reason you should never look directly into the sun) and, on the other hand, the UV radiation also affects the lens, which can be clouded as a result (cataract / cataract ). Furthermore, this effect is cumulative and explains why it is mostly older people who are affected by cataracts.

The sense of hearing

What is perceived

The hearing perceives tones, i.e. air vibrations with a frequency between 20 Hz and 20 kHz. These vibrations are caused by objects that emit their own vibrations into the air, such as B. Musical instrument strings or the skin of a drum or the vocal cords of a person speaking. The vibrations are transmitted to the inner ear and perceived there.

What is happening there?

The sound waves reach the auricle, whose shape in the form of an ear tube amplifies certain tones. You then get into the external ear canal, at the end of which is the eardrum. The eardrum consists of a thin membrane that begins to vibrate due to sound. The vibrations of the eardrum are transmitted to the inner ear through a chain of three bones. These ossicles set a fluid in motion, which is located in the sensitive part of the inner ear, in the cochlea. The sensory cells that react to sound are also located here. The stimulated sensory cells send signals to the brain via the auditory nerve. These signals provide information about the frequency and intensity of the sound.


If the hearing system is exposed to noise for too long, it can be damaged. In children and adolescents, the greatest risk is the sound pressure level from Walkmans. A sound pressure level of over 90 dB can cause irreversible damage. In France, a law limits the sound pressure level of Walkmans to a maximum of 100 dB and stipulates that the following warning must be given: "At full power and prolonged use, the user's ear can be damaged"

The sense of taste

What is perceived

We can recognize flavors through the sense of taste. However, this perception is only possible when the substances are dissolved in saliva or, more generally, in water, because the taste receptors only react in an aqueous environment. The sensations triggered by food do not only depend on their taste, but also on other properties such as consistency, temperature or aroma.

What is happening there?

The taste receptors are called taste buds. These are nerve endings that open into the taste papillae on the surface of the tongue. The corresponding nerve fibers are bundled in pairs to form taste buds and these transmit signals from the taste buds to the brain.

A distinction is traditionally made between 4 basic flavors: bitter, sweet, sour and salty, to which the flavor sodium glutamate (umami) is still added today. Each basic taste corresponds to specific receptors that have their designated place on the tongue. Once the taste has been identified, this information is passed on to the brain and interpreted there. The taste perception goes far beyond the simple combination of elementary flavors. First of all, there is an individual variety in taste perception. For this reason, some people describe some substances as tasteless, while others find the taste very unpleasant. Furthermore, the sensations during chewing of the food do not depend on its taste alone. Another decisive factor is the consistency of the food (floury, melting, soft, crispy, etc.), its degree of dissolution in saliva or in drinks, its temperature (cold affects the sensitivity of the taste buds) and the duration of chewing. In addition, the scent of the food absorbed via the sense of smell has a very strong influence on the taste. This is why the food with a cold, in which the olfactory abilities are severely restricted, usually has a different taste than normal or even none at all. How long a taste lasts and how you perceive a taste are two things that influence each other. For this reason, it is important to rinse your mouth every now and then when trying different foods.

It must also be added that the impressions evoked by food also have a significant cultural background, which is particularly shaped by education and tradition. As a result, some dishes that are perceived as very tasty by one cultural group are inedible by another.


The taste of a substance alone does not provide sufficient information about its possible toxicity. Even if many poisonous substances in nature have a bitter taste, such as most alkaloids of plants, this is not a universal property of poisonous substances. For this reason, children should not use the sense of taste to discover unknown substances, unless the latter has been specially prepared by the teacher for this purpose.

The sense of smell

What is perceived

The olfactory system detects certain molecules floating in the air. These are small molecules that must be sufficiently volatile and in sufficient concentration to get into the nasal cavity. As with all other sensory organs, the stimulation of the specific receptors transmits signals to the brain and decodes them there. However, the olfactory system stands out from the other sensory systems due to the large number of different receptors.

What is happening there?

The olfactory receptors are nerve cells located in the upper part of the nasal cavity. They are embedded in a mucous membrane and only the molecules that dissolve in this nasal mucous membrane can be perceived by the sense of smell. Despite the large variety of receptors that react to molecules with a wide variety of chemical structures, not a single receptor seems to be specifically tied to a molecule and thus to a particular smell. The recognition of smells seems to set various mechanisms in motion. If certain receptors specifically recognize a small number of molecules, then the recognition of an odor appears, at least in most cases, to be based on the combination of different receptors. The signals resulting from the stimulation of this combination of receptors are first sent to the olfactory bulb (Olfactory bulb) and then to the olfactory cortex (olfactory brain), which "derives" the corresponding odor from it. In addition, the olfactory system is closely related to memory and emotions.


Since certain chemical substances, such as ammonia or some solvents, can destroy the lining of the nose, it can be dangerous to sniff on an unknown substance. Other substances can have long-term effects. Accordingly, the sense of smell should not be used to investigate an unknown substance.

The sense of touch

What is perceived

The ability of our organism to perceive mechanical stimuli through the skin is known as the sense of touch. However, the skin is able to detect significantly more information, the sense of touch is just one of the sensory impressions that neurobiologists summarize under the term somatosensitivity (haptic perception). This includes not only the sense of touch, but also the perception of pressure, vibrations and temperature. The skin has different types of receptors and only those that react to skin contact are actually assigned to the sense of touch (also tactile perception or surface sensitivity). Other receptors play a role in the general sensory perception of our body, such as the receptors in the muscles, joints and intestines, but we do not want to go into these here.

What is happening there?

There are different tactile receptors. Some form the nerve endings at the hairline, others are pressure receptors located under the surface of the skin or deep under the skin, and still others are vibration detectors. Thus, the former are stimulated by the changed hair position, the latter by the pressure on the skin and the third by rapid changes in pressure on the skin. Once stimulated, these receptors send signals to the brain via the sensory nerves. By combining the responses sent out by the various receptors, the brain can distinguish, say, between a caress and a scratch.

The density of receptors varies considerably depending on the region of the body. It will come as no surprise that the skin on the fingertips is the region of the body with the highest density of tactile receptors. There are also many receptors in the skin of the face, especially the lips. These differences in receptor density lead to significant differences in sensitivity in different parts of the body. Due to the large number of receptors in the fingertips, it is possible for blind people to read Braille, i.e. they use their fingers to feel the raised dot characters in the paper, the different combinations of which correspond to letters, numbers, etc.

The skin's receptors adapt quickly to chronic stimulation. For this reason, after a while we no longer feel the contact of clothing on our skin.


The skin is a sensitive organ that reacts very sensitively to the effects of irritation, such as numerous chemical substances such as acids or bases, solar radiation and contact with objects that are too hot.If the skin is exposed to the sun for too long, it can not only lead to sunburn, but also to skin cancer. For this reason, children should learn to protect their skin (clothing, headgear) at an early stage, as already mentioned under the subject of eyes. However, it should not be forgotten that the UV rays of the sun are essential for the biosynthesis of vitamin D, which is important for the structure of our skeleton, which takes place in the skin.

Furthermore, you should also protect yourself from burns, because the. even if only kueze. Contact with a very hot object can lead to extremely serious injuries.

The locomotion

Locomotion is the function by which one can get from one place to another. In animals with a rigid skeleton, such as vertebrates and thus also in humans, it is the movements of the limbs coordinated by the nervous system that enable locomotion. The limbs are divided into segments that are movably connected to one another by joints. Movement is ensured by the contraction of the muscles connected to the bones by tendons.

The skeleton

The structure of the skeleton of all vertebrates is essentially the same, which goes back to their common evolutionary origin, and is divided into axial skeleton and limb skeleton. The axial part of the skeleton includes the spine, which consists of vertebrae lying on top of one another, and the skull. The axial skeleton contains and protects the central nervous system, the spinal cord, and the brain. Two groups of bones, the so-called girdles (shoulder and pelvic girdles), connect the bones of the upper (arms) and lower limbs (legs) with the axial skeleton. The human skeleton is made up of around 200 bones. The bone is a living tissue that grows with the development phase and is constantly renewed.

The muscles

Muscles are motor organs. The human body has almost 700 different muscles. As they move, their contraction activates the skeletal segments to which they are connected by tendons. The elastic muscles contract when they contract, and when they relax, they return to their original length. The contraction of these muscles, the skeletal muscles, is controlled. It is triggered by nerve stimuli that are sent out by the brain and transmitted by the spinal cord. These nerve stimuli are passed on to the muscles through the motor nerves. Every muscle contraction consumes energy, much of which is lost in the form of heat during the contraction. The muscles are supplied with oxygen and nutrients from the blood vessels, which are needed to produce chemical energy, which in turn is consumed during contraction.

The movements

The basic movements made possible by the muscles between the skeletal segments connected by movable joints are flexion and extension movements. If the distance between two bones is shortened, one speaks of flexion or flexion; if it increases, one speaks of stretching or extension. In humans, for example, the forearm is bent by the contraction of the biceps, one of which is attached to the bone of the upper arm and the other to those of the forearm. Since the bones of the upper arm are connected to those of the forearm at the level of the elbow by a flexible joint, the contraction of the biceps causes the forearm to bend towards the upper arm. This muscle is called the flexor muscle.

Each movable joint is moved by antagonistic muscles; H. they cause opposite movements. For example, the antagonistic muscle of the biceps is the triceps. Its ends are also connected to the bones of the upper arm and forearm by tendons (but on the side opposite the attachment point of the biceps tendon). Because of this, flexing is only possible when the triceps are relaxed. Conversely, when the triceps contracts, the forearm is pivoted outward and stretched. The triceps is an extensor muscle.

Fig. 4: By contraction of the antagonistic muscles, the same skeletal segments can perform opposite movements.

The combination of these elementary joint movements enables complex movements such as locomotion. Man is a biped, i. H. he is able to stand upright and move on his hind limbs, the legs. Walking combines a series of movements that involve the joints of the feet, knees and hips in sequence, not to mention the joints of the upper limbs, which maintain balance when walking. If we look at the individual steps of the movements of the legs while walking, we can observe the following sequence of movements (the joints concerned are indicated in brackets): stretching of the right foot in relation to the leg (ankle); Flexion of the leg in relation to the thigh (knee); Flexion of the thigh in relation to the trunk (hip); Extension of the right leg in relation to the thigh (knee); Extension of the left foot in relation to the leg (ankle); Flexion of the left leg in relation to the thigh (knee); Flexion of the left thigh in relation to the trunk (hip); Extension of the left leg in relation to the thigh (knee), etc. This combination of elementary movements enables us to walk and run.

Last update: 13.9.2018

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