SENSE ORGANS

(The Senses)

SENSE ORGAN

A bodily structure that receives a stimulus and is affected in such a manner as to initiate excitation of associated sensory nerve fibers which convey specific impulses to the central nervous system where they are interpreted as corresponding sensations: receptor

On the basis of the energy to which the sense organs respond, they are classified as (1) chemoreceptors, sensitive to changes in the chemical environment; include senses of taste and smell, (2) mechanoreceptors, responsive to mechanical stimuli such as stretching or compression; include the senses of touch, pain, proprioception, equilibrium, and lateral line sense of fishes & amphibians, & hearing, (3) photoreceptors, sensitive to light waves, (4) thermoreceptors, sensitive to temperature, & (5) electroreceptors, in a few fishes, to electric signals.

CHEMORECEPTORS

Chemical changes in their environment are perceived by chemoreceptors that are present over all the body in amphibians and fishes and many other aquatic animals and in the human mouth and nasal passages.

Smell or olfaction in humans is possible through neurons that have directly exposed tips that lie in the mucuos membrane high in the nasal cavity. The olfactory neuron receptor is enlarged, somewhat rod-shaped, and contains up to 20 motile cilialike filaments that are bathed by mucus on the surface of the nasal epithelium; its axon passes directly to the brain.

The nose is the organ responsible for the sense of smell. The cavity of the nose is lined with mucous membranes that have smell receptors connected to the olfactory nerve. The smells themselves consist of vapors of various substances. The smell receptors interact with the molecules of these vapors and transmit the sensations to the brain.

Taste. The chemoreceptors for the perception of taste are the taste buds.

Human tastes- sweet, salty, acidic, and bitter (alkaloidal)

MECHANOREPTORSSENSE OF TOUCH (SKIN)

Cells called mechanoreptors respond to mechanical deformation, which causes receptor potentials in one of two ways: by stretching the membrane receptor cell (as in receptors for touch and pressure) or by bending “hairs” that project from the receptor cell membrane. Receptors for sound, motion, and gravity bear hairlike structures and are called hair cells.

Four kinds of touch sensations can be identified: cold, heat, contact, and pain.

Pacinian corpuscle- which responds to rapid changes in pressure such as those produced by vibrations.

Free nerve endings- produce sensations of itching and tickling.

Proprioceptors- allow one to walk without watching one’s feet or eat without watching the fork on its way to the mouth.

In vertebrates, the organs that detect sound, gravity, and movement almost certainly evolved from the lateral line organ.

HEARING (EAR) The human ear has an external sound-collecting

appendage, the pinna, around a tubular external auditory canal. At the end of the canal, sound waves act to set the eardrum (tympanum) into vibration which is amplified and transmitted by three auditory ossicles- malleus, incus, and stapes-to the oval membrane, produces vibrations in the fluid filling the spiral cochlea of the inner ear.

The inner ear, or cochlea, is a spiral-shaped chamber covered internally by nerve fibers that react to the vibrations and transmit impulses to the brain via the auditory nerve. The brain combines the input of our two ears to determine the direction and distance of sounds.

HEARING(EAR)

External Auditory Canal

Pinna

The inner ear also allows us to perceive loudness and pitch. A weak sound causes small vibrations, which bend the hairs only slightly. This produces small receptor potentials in the hair cells and a low frequency of action potentials in the auditory nerve axons. A loud sound causes large vibrations, which cause greater bending of the hairs and a larger receptor potential. This leads to a high frequency of action potentials in axons of the auditory nerve. Loud sounds sustained for a long time can actually damage the hairs, resulting in hearing loss.

The human ear can perceive frequencies from 16 cycles per second, which is a very deep bass, to 28,000 cycles per second, which is a very high pitch. Bats and dolphins can detect frequencies higher than 100,000 cycles per second. The human ear can detect pitch changes as small as 3 hundredths of one percent of the original frequency in some frequency ranges. Some people have "perfect pitch", which is the ability to map a tone precisely on the musical scale without reference to an external standard. It is estimated that less than one in ten thousand people have perfect pitch

PHOTORECEPTORS

Photoreceptors are sensitive to light. They are present in earthworms and are called eyespots on various cnidarians and some mollusks.

Vertebrate eye is similar in structure to that of a camera. It consists of three basic parts: a light sensitive layer (retina), a (lens) for focusing light, and a set of (muscles) for adjusting focus by moving or changing the shape of the lens.

Choroid

Vitreous humor

Acqueous humor

VISION (EYE) Incoming light first encounters the cornea. Cornea- transparent covering over the front

of the eyeball. Aqueous humor- chamber filled with watery

fluid behind the cornea, which provides nourishment for the lens.

Iris-adjust the amount of light entering the eye.

Lens- structure resembling a flattened sphere and composed of transparent proteinaceous fibers. Focuses light on the retina.

Vitreous humor- helps maintain the shape of the eye.

Optic nerve- carries the impulses to the appropriate regions of the brain. The brain interprets and provides information about the external world.

After passing through the vitreous humor, light reaches the retina, a multilayered nervous tissue where the light energy is converted into electrical nerve impulses that are transmitted to the brain.

The rod cells are not sensitive to color, but have greater sensitivity to light than the cone cells. These cells are located around the fovea and are responsible for peripheral vision and night vision. The eye is connected to the brain through the optic nerve. The point of this connection is called the "blind spot" because it is insensitive to light. Experiments have shown that the back of the brain maps the visual input from the eyes.

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