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Sense Organs

General Senses

General senses have simple receptors, which are distributed throughout the body to detect internal and external environmental conditions.

General senses differ from special senses in two major ways. First, general sensory receptors are located throughout the entire body and are not limited to special sensory organs in the head, like the special senses. Second, general senses have a more simplistic structure and stimulus-detection mechanism. General sensory neurons detect stimuli using their dendritic nerve endings. These dendritic nerve endings are classified as either encapsulated or nonencapsulated. An encapsulated sensory neuron has nerve endings surrounded by connective tissue or cells, which are sensitive to touch or vibration. Mechanoreceptor nerve endings are primarily encapsulated. A nonencapsulated sensory neuron does not have a sheath of connective tissue; this category includes nociceptors, thermoreceptors, and some mechanoreceptors. Nonencapsulated nerve endings respond to painful or thermal stimuli and may be more sensitive than encapsulated sensory neurons.

There are many different forms of encapsulated nerve endings, including tactile corpuscles, end bulbs, bulbous corpuscles, lamellar corpuscles, muscle spindles, and tendon organs.

  • A tactile corpuscle detects light touch and texture and is found in the dermis of the skin.
  • An end bulb is an encapsulated nerve ending that detects light touch in mucous membranes, such as the inside of the nose, mouth, and eyelid.
  • A bulbous corpuscle is an encapsulated nerve ending that detects sustained touch, pressure, and stretch in the dermis of the skin, subcutaneous fat, and joints.
  • A lamellar corpuscle detects vibrations and is responsible for the tickle sensation. They are associated with Schwann cells that surround the nerve ending and are covered by connective tissues. Muscle spindles and tendon organs are proprioceptors in muscles and tendons, respectively.

Nonencapsulated receptors are found in three different forms: free nerve endings, tactile discs, and hair receptors.

  • A free nerve ending is a nonencapsulated nerve ending of a sensory receptor that is bare and not associated with another cell or tissue. It is a dendritic extension that penetrates epithelial tissue of the skin's epidermis and connective tissue.
  • A receptor in the skin that detects gentle touch has nerve endings that form a disc called a tactile disc. A tactile disc is adjacent to specialized cells in the skin and releases a chemical when changing shape after compression. Tactile discs respond to this chemical by sending electrical signals to the central nervous system.
  • A hair follicle receptor is a receptor that wraps around hair follicles and detects movement in the hair.

General Sensory Receptors in the Skin

General sensory receptors found in the skin can detect various stimuli such as touch, temperature, pain, and pressure. Tactile discs and free nerve endings are found in the epidermis. Lamellar corpuscles, bulbous corpuscles, hair receptors, end bulbs, and tactile corpuscles are found in the dermis.
A nociceptor, which is a nonencapsulated free nerve ending, detects different kinds of pain. Sharp pain signals are transmitted by myelinated neurons, which have axons encased in layers of Schwann cells, or membrane-enveloped sheaths. This structure produces faster transmission of pain messages to the brain. Dull pain signals are transmitted by unmyelinated neurons, with slower transmission speeds. The receptor ends are nonencapsulated free nerve endings, which penetrate the epithelial layer by situating between cells.

Generalized Sensory Receptors

There are several kinds of sensory receptors found in the body. These include nociceptors, which are responsive to pain; thermoreceptors, which detect changes in temperature; chemoreceptors, which respond to dissolved chemicals; and mechanoreceptors, which activate when the position of a body part is moved or there is added pressure to that part