Sense Organ
A sense organ is any collection of cells which serve to receive sensory information from the environment either external or internal to a particular organism. That sensory information is then sent to the organism's brain, where it is processed. Some types of sensory information is consciously perceived by the organism; other types of sensory information may help the organism regulate various bodily functions, but may not actually be consciously perceived.
Sense organs require some type of receptor. The receptor is a collection of nerve cells which receive the information. Other neurons carry that information to the appropriate location for processing. Although we usually think of humans as enjoying five basic senses (hearing, vision, smelling, tasting, and touch), in fact more specific categories of receptors exist.
Mechanoreceptors are those receptors which gather information about touch, pressure, gravity, stretch, or motion. These receptors help the body maintain its position in relation to gravity (for example, most animals prefer a head up, feet down position), as well helping the various parts of the body stay in position relative to other parts of the body. Mechanoreceptors include tactile receptors, which can exist on the skin, or at the base of a hair or bristle. When the hair or bristle is bumped, the tactile receptor at its base sends information about this sensation to the brain. Different types of tactile receptors exist to give information about such attributes as pressure, pain, temperature.
Some invertebrates have organs called statocysts which help them maintain their bodies in an appropriate position with regard to gravity. Fish have organs called lateral line organs which helps them sense obstacles in the water around them, which could represent either danger or the presence of prey.
Organs of proprioception are sense organs which provide an organism with continuous information with regard to the position of its muscles and joints. This allows an individual to locate his/her nose with the tip of a finger, even with eyes closed.
The sense of balance is maintained through the interaction of a number of different sense organs, including those of vision and proprioception. Organs within the inner ear also send vital information to the brain to allow an individual to maintain a sense of equlibrium, and therefore to allow that individual to stay balanced when walking or even when sitting.
Auditory receptors collect information from sound wave vibrations, and turn it into electrical impulses which go to the brain to provide information on sound in the environment. Some animals use their sense of sound to locate prey or enemies in their environment. Bats, for example, send high-frequency sound waves out into their environment. These sound wave bounce back off of anything which is in the bat's vicinity. The bat can use this information to very accurately track prey and avoid enemies.
Chemoreceptors are receptors which are stimulated by the presence of chemical substances in the environment. These include receptors which provide information for the senses of olfaction (smelling) and gustation (tasting). Taste receptors have only a four categories of taste which they are able to discern: sweet, sour, salty, and bitter. Of course, the mouth is also full of receptors which provide information on the mechanical attributes of the item being tasted, including texture, temperature, etc. The olfactory receptors can sense as many as 50 different types of smells, and can do so based on unbelievably tiny amounts of the material being smelled. A manufactured product which imitates the smell of a violet, for example, can be sensed by an individual at a concentration of only one unit per 30 billion units of air.
Thermoreceptors are those receptors which respond to the presence of heat. Thermoreceptors, for example, help mosquitoes find their next blood meal, and help snakes (who have notoriously poor vision)locate its prey and position itself to strike effectively. Mammals have thermoreceptors which sense temperature in the external environment, as well as specialized thermoreceptors to sense and regulate their own internal body temperature.
Photoreceptors are receptors which respond to light (the units of which are referred to as photons). Photoreceptors can be quite primitive, allowing some animals to sense light, but not distinguish actual objects. Other photoreceptors (as in the human eye) are quite sophisticated, providing information about light, objects, textures, colors, and depth perception. Insects have compound eyes, which are covered with multiple facets. These eyes do not sense form particulary well, but they are extremely acute at perceiving flicker (much more than are human eyes). This ability to perceive flicker allows flies, for example to sense the tiniest, beginning motion of a hand reaching for a fly swatter.
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