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The sense organs

The sense organs

Organs that detect signals of the environment or of the body and transmit them to the brain as nerve impulses.



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  • - It is the eye’s aperture; the iris acts like a shutter that controls the amount of light that falls on the retina. In strong light the iris is contracted by its smooth muscles, in weak light it dilates. The pupillary light reflex is an unconditioned reflex, its center is located in the brain stem. Abnormal operation of the pupillary reflex therefore indicates the injury of the brain stem.
  • optical center - The center is located in the cortex of the occipital lobe.

The amount of light entering the eyes is regulated by the pupillary light reflex. In strong light the pupil is contracted by the smooth muscles of the iris, while in low light it is dilated. The pupillary light reflex is an unconditioned reflex, its center is located in the brain stem. Abnormal operation of the pupillary reflex therefore is an indication of injury to the brain stem.
The eyeballs are moved by the extraocular muscles. These are striated muscles under voluntary control.

The vitreous chamber forms the main mass of the eye. The cross section of the eye shows three main layers.
The outermost is the sclera, a very durable layer of connective tissue, which continues in the transparent cornea. This is where light entering the eye is refracted by the greatest angle.
The second layer is the choroid, which contains blood vessels that supply the eye. Its continuation at the front of the eye is the ciliary body and the iris. The smooth muscles of the iris are responsible for the pupillary light reflex. The iris contains pigments which lend human eyes their color.

The smooth muscles of the ciliary body ensure the accommodation of the eye lens to the distance of the object viewed by changing its curvature.
The lens is connected to the ciliary body through the ciliary zonules. The ciliary body is also responsible for producing the aqueous humor, the liquid that fills the anterior chamber. If the drainage of the aqueous humor is insufficient, the pressure increases in the eye, which causes glaucoma. In serious cases it may lead to blindness.
The innermost layer is the retina. This is where an inverted miniature image is formed of the object viewed; this is created by the lens. Its receptors are called rod cells and cone cells. The area of the retina responsible for visual acuity is called the macula lutea: in its center there are only cone cells, while around the edge there are more rod cells. The blind spot is the place where the optic nerve passes through the retina. There are no receptor cells here. The impulses produced by the receptors in the retina are transmitted to the brain by the nerve fibers in the optic nerve.

Sound is the vibration of air perceived by our ears. Healthy ears can perceive sound waves of frequencies from about 20 to 20,000 Hz. This range will become narrower due to aging or noise exposure.

Sound waves create signals in the inner ear, which are transmitted to the auditory cortex by the cochlear nerve and auditory pathway. The sense of sound is produced in the auditory cortex.

Sound waves are directed into the external auditory canal by the auricle. Sound waves cause the eardrum which closes the auditory canal to vibrate. The vibration of the eardrum is transmitted to the cochlea by the ossicles - the hammer, the anvil and the stirrup.

The base of the stirrup fits snugly into the oval window of the cochlea. The basilar membrane is located inside the cochlea. It runs along the tip of the cochlea, where it turns back and continues in Reissner's membrane. The membranes divide the cochlea longitudinally into three cavities: the scala tympani, the scala media and the scala vestibuli.

The chochlea is filled with a fluid, which is vibrated by the stirrup. Higher frequency sounds cause vibrations of higher frequency in the liquid, which are absorbed in the initial section of the membrane. Lower frequency vibrations generated by deep sounds enter the cochlea and become absorbed closer to the tip. When a vibration is absorbed, an electrical signal is produced which is transmitted into the brain. The pitch of the sound is encoded by the site of absorption: this is called tonotopy.

Electrical signals are generated in the organ of Corti. Vibrations spreading inside the cochlea push the tectorial membrane against the cells of the hairs on the basilar membrane and bends them, generating a signal in the cells. Thus the organ of Corti transforms vibrations into electrical signals, which are transmitted into the brain by the cochlear nerve, and then into the auditory cortex by the auditory pathway. Finally, the sense of sound is produced in the cerebral cortex.

  • - Incoming fibers form the olfactory nerve and transmit impulses produced by the receptors. Outgoing fibers form the olfactory tract and transmit impulses to the brain.
  • - It connects the nasal cavity with the middle ear. The pressure between the middle ear and the atmosphere are equalized through it.
  • perception of odors

Many of the particles found in the air can stimulate olfactory receptors in the nose, establishing a sense of smell in the cerebral cortex. Smelling plays an important role in choosing food that is safe to eat, or recognizing the characteristic smell of another animal – and therefore in social relationships. Humans and other primates have a relatively weak sense of smell compared to that of other mammals, but even humans can distinguish thousands of smells.

The protruding part of the nose is supported by the nasal bone and the cartilage attached to it. The nasal cavity is separated from the mouth cavity by the palate and connected to the middle ear by the Eustachian tube. The Eustachian tube equalizes pressure between the middle ear and the atmosphere. The nasal cavity is bordered on top by the cribriform plate, a thin, perforated part of the ethmoid bone where the olfactory nerve (or cranial nerve I) crosses the nasal epithelium and reaches the olfactory bulb.

  • taste center

Our sense of taste plays an important role in choosing food that is safe to eat and in detecting and avoiding dangerous, poisonous substances. Taste receptors found on the tongue are sensitive to various chemical substances, each of which produce a different taste sensation. Impulses produced by taste receptors are transmitted to the brain through cranial nerves VII, IX and X, where they are switched by the thalamus and finally reach the cortical taste center. Taste sensation is generated in the cerebral cortex.

  • - Its different types detect different stimuli (heat, cold, pain, pressure, vibration).


The human sense organs are the eyes, the ears, the nose, the tongue and the skin. These organs serve the detect signals of the outside world by means of the five basic senses, that is, vision, hearing, smell, taste and touch. The skin is also responsible for the detecting heat, pain and pressure.

The human eyeballs are housed in a bony cavity. They are protected by the eyelids, lacrimal glands and conjunctiva. Light passes through the eye through a transparent medium until it reaches the retina, i.e. the eye’s innermost layer. The light reaching the eye generates impulses in the receptors of the retina, which are transmitted to the cerebral cortex via the optic nerve.

The outer, middle and inner ear all play a role in the perception of sound. The auricle helps determine the direction of sound and is responsible for transmitting it. The sound reaches the middle ear via the external auditory canal, where it causes the auditory ossicles to vibrate. The vibration of the sound finally spreads to the inner ear and generates electric impulses in the receptors located on the basilar membrane in the cochlea. These impulses are transmitted to the auditory cortex via the fibers of the cochlear nerve. It is only the inner ear that is responsible for the sense of balance. The receptors of the semicircular canals located in the inner ear are responsible for detecting the position of the head and its movement.

During the detection of odors, the particles in the air enter the nasal cavity and stimulate the receptors of the nasal epithelium located there. Different receptors are sensitive to different odors. The impulses produced in the receptors reach the olfactory bulb and are transmitted to the cerebral cortex via the fibers of the olfactory tract.

The taste receptors found on the surface of the tongue and on the mucous membrane of the oral cavity detect the flavors of food and convert them into impulses. On the surface of the tongue there are papillae which are surrounded by trenches filled with saliva. flavors dissolve into this saliva and diffuse to the taste receptors. The impulses produced in the receptors are transmitted to the cortical taste center through the cranial nerves VII, IX and X.

The skin covering the outer surface of the human body is our largest organ, comprising about 15-20% of the average body mass. The skin consists of several layers, some of which contain receptors that detect heat, cold and mechanical and chemical stimuli. Receptors are not evenly distributed in the body. The skin of fingertips, for example, contains a large number of receptors for fine touch. The sole, however, has a high density of receptors detecting stronger pressure. The impulses formed in the receptors of the skin are transmitted to the brain stem or to the spinal cord via sensory nerves. The information is processed in the sensory cortex of the cerebrum.

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The eye is one of our most important sense organs. When stimulated by light, electric impulses are produced by its receptors.

The sense of balance

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