16.2 The General Senses

root hair plexus

are specialized dendritic endings of sensory neurons that form a weblike sheath around hair follicles in the reticular layer (deeper layer) of the dermis. Any movement or displacement of the hair changes the arrangement of these dendritic endings, initiating nerve signals. These phasic receptors quickly adapt; thus, although we feel the initial contact of a long-sleeved shirt on our arm hairs when we put on the garment, our conscious awareness subsides immediately until we move and the root hair plexuses are restimulated.

free nerve endings

are the least complex of the tactile receptors and reside closest to the surface of the skin, usually in the papillary layer (superficial layer) of the dermis (see section 6.1b). Often, some branches extend into the deepest epidermal strata and terminate between the epithelial cells. Free nerve endings are also located in mucous membranes. These tactile receptors primarily detect temperature and pain stimuli, but some also detect light touch and pressure. Free nerve endings can be either tonic receptors (adapt slowly) or phasic receptors (adapt quickly).

tactile receptor

are the most numerous type of sensory receptor. They are mechanoreceptors located in the skin and mucous membranes. The dendritic endings that compose these sensory receptors are either unencapsulated or encapsulated

unencapsulated tactile receptors

dendritic endings of sensory neurons with no protective covering. The three types of unencapsulated receptors are free nerve endings, root hair plexuses, and tactile discs.

phantom pain

is a sensation associated with a body part that has been removed. Following the amputation of an appendage, a patient often continues to experience pain from what is perceived as the removed part. For example, a patient may feel pain that seems localized in a foot that is no longer there

end bulbs

or Krause bulbs, are dendritic endings of sensory neurons ensheathed in connective tissue. They are located both in the dermis of the skin and in the mucous membranes of the Page 617 oral cavity, nasal cavity, vagina, and anal canal. End bulbs are tonic receptors that detect light pressure stimuli and low-frequency vibration.

bulbous corpuscles

or Ruffini corpuscles, are dendritic endings of sensory neurons ensheathed within connective tissue that are housed within the dermis and subcutaneous layer. They are tonic receptors that detect both continuous deep pressure and distortion in the skin.

referred pain

pain sensed, not from the organ, but from an unrelated region of the body. occurs when sensory nerve signals from certain viscera are perceived as originating not from the organ, but from somatic sensory receptors within the skin and skeletal muscle. Numerous somatic sensory neurons and visceral sensory neurons conduct nerve signals on the same ascending tracts within the spinal cord

tactile corpuscles

previously called Meissner corpuscles, are receptors formed from highly intertwined dendritic endings of sensory neurons enclosed by modified neurolemmocytes, which are then covered with dense irregular connective tissue. They are housed within the dermal papillae (which are projections of the dermis; see section 6.1b), especially in the lips, palms, eyelids, nipples, and genitals. Tactile corpuscles are phasic receptors for discriminative touch to distinguish texture and shape of an object and for detecting light touch.

tactile discs

previously called Merkel discs, are flattened dendritic endings of sensory neurons that extend to tactile cells (Merkel cells), which are specialized epithelial cells located in the stratum basale (deepest layer) of the epidermis. These discs function as tonic receptors for light touch. (Note that tactile cells are the only specialized tactile receptor cells; the remaining tactile receptors are simply the dendritic endings of sensory neurons.)

lamellated corpuscles

previously called Pacinian corpuscles, are large, leaf-shaped tactile receptors composed of several dendritic endings ensheathed with an inner core of neurolemmocytes and outer concentric layers of connective tissue. They are phasic receptors found deep within the reticular layer of the dermis of the skin; in the hypodermis of the palms of the hands, soles of the feet, breasts, and external genitalia; and in the walls of some organs. The structure and location of lamellated corpuscles allow them to function in coarse touch, and sensing continuous deep-pressure and high-frequency vibration stimuli.