FIGURE 12.23 • Electron micrograph of a peripheral nerve and its surrounding perineurium. a. Electron micrograph of unmyelinated nerve fibers and a single myelinated fiber (MF). The perineurium (P), consisting of several cell layers, is seen at the left of the micrograph. Perineurial cell processes (arrowheads) have also extended into the nerve to surround a group of axons (A) and their Schwann cell as well as a small blood vessel (BV ). The enclosure of this group of axons represents the root of a small nerve branch that is joining or leaving the larger fascicle. 10,000. The area within the circle encompassing the endothelium of the vessel and the adjacent perineurial cytoplasm is shown in the inset at higher magnification. Note the basal (external) laminae of the vessel and the perineurial cell (arrows). The junction between endothelial cells of the blood vessel is also apparent (arrowheads). 46,000. b. Electron micrograph showing the perineurium of a nerve. Four cellular layers of the perineurium are present. Each layer has a basal (external) lamina (BL) associated with it on both surfaces. Other features of the perineurial cell include an extensive population of actin microfilaments (MF), pinocytotic vesicles (arrows), and cytoplasmic densities (CD). These features are characteristic of smooth muscle cells. The innermost perineurial cell layer (right) exhibits tight junctions (asterisks) where one cell is overlapping a second cell in forming the sheath. Other features seen in the cytoplasm are mitochondria (M), rough-surfaced endoplasmic reticulum (rER), and free ribosomes (R). 27,000.
The epineurium forms the outermost tissue of the peripheral nerve. It is a typical dense connective tissue that surrounds the fascicles formed by the perineurium (Plate 28, page 392). Adipose tissue is often associated with the epineurium in larger nerves.
The blood vessels that supply the nerves travel in the epineurium, and their branches penetrate into the nerve and travel within the perineurium. Tissue at the level of the endoneurium is poorly vascularized; metabolic exchange of substrates and wastes in this tissue depends on diffusion from and to the blood vessels through the perineurial sheath (see Fig. 12.23).
Afferent receptors are specialized structures located at the distal tips of the peripheral processes of sensory neurons.
Although receptors may have many different structures, they have one basic characteristic in common: They can initiate a nerve impulse in response to a stimulus. Receptors may be classified as the following.
 Exteroceptors react to stimuli from the external environment—for example, temperature, touch, smell, sound, and vision.
 Enteroceptors react to stimuli from within the body— for example, the degree of filling or stretch of the alimen tary canal, bladder, and blood vessels.  Proprioceptors, which also react to stimuli from within the body, provide sensation of body position and muscle tone and movement.
The simplest receptor is a bare axon called a nonencapsulated (free) nerve ending. This ending is found in epithelia, in connective tissue, and in close association with hair follicles.
Most sensory nerve endings acquire connective tissue capsules or sheaths of varying complexity.