In order to detect changes in the environment, animals must necessarily have mechanisms to sense these changes.
The sense organs are receptive structures, located in the head and throughout the skin, muscles, and internal organs, that are each able to detect changes in specific sensory modalities, including pressure, light, sound, and stretch.
Some sensory receptors are part of the nervous system proper—the receptors in the skin and throughout the body that detect noxious stimuli, for example—but most receptors are composed of both neural and nonneural tissue. Stretch receptors located in muscle, known as muscle spindles, are composed largely of specialized muscle cells. The eye is a highly complex structure of which only the retina is of neural origin. Regardless of its composition, each receptor is connected with a sensory neuron that transmits information about the activity of the receptor, in the form of a series of action potentials, to the central nervous system.Many of the senses are conscious, meaning that the animal is aware of what it has registered. However, there are sensory systems associated with muscle and viscera of which the animal is less aware and through which it is in touch with the "internal environment" of its own body.
The sense organs are generally classified into those of special sense—eye, ear, smell, taste—and the more general senses of touch, pressure, and pain. The organs of special sense are described first.
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