Chapter 19

Explain the four types of mechanoreceptors responsible for the sense of touch in the human hand
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The terminals of myelinated sensory nerves innervating the hand are surrounded by specialized structures that detect contact on the skin. The receptors differ in morphology, innervation patterns, location in the skin, receptive field size, and physiological responses to touch.

Refer to Figure...

A. The superficial and deep layers of the glabrous (hairless) skin of the hand each contain distinct types of mechanoreceptors. The superficial layers contain small receptor cells: Meissner corpuscles (RA1, rapidly adapting type 1) and Merkel cells (SA1, slowly adapting type 1). The sensory nerve fibers that innervate these receptors have branching terminals that innervate multiple receptors of one type. The deep layers of the skin and subcutaneous tissue contain large receptors: Pacinian corpuscles (RA2, rapidly adapting type 2) and Ruffini endings (SA2, slowly adapting type 2). Each of these receptors is innervated by a single nerve fiber, and each fiber innervates only one receptor.

B. The receptive field of a mechanoreceptor reflects the location and distribution of its terminals in the skin. Touch receptors in the superficial layers of the skin have smaller receptive fields than those in the deep layers.

C. The nerve fibers innervating each type of mechanoreceptor respond differently when activated. The schematic spike trains show responses of each type of nerve when its receptor is activated by slowly increasing and constant pressure against the skin. The rapidly adapting fibers respond to motion at the onset and end of a pressure stimulus and adapt rapidly to constant stimulation, whereas the slowly adapting fibers respond to both steady pressure and motion and adapt slowly.
terminate in clusters of small receptor organs (Meissner corpuscles or Merkel cells) in the superficial layers of the skin at the margin between the dermis and epidermis.

RA1 fibers are the most numerous tactile afferents in the hand, reaching a density of approximately 150 per cm2 at the fingertip in man and monkey

SA1 fibers are also widely distributed in the hand, at densities of 70 per cm2 in the fingertips.
innervate the skin sparsely and terminate in single large receptors (Pacinian corpuscles and Ruffini endings) located in the dermis or in subcutaneous tissue. These receptors are larger and less numerous than the receptor organs of the type 1 fibers. The large size of type 2 receptors allows them to sense mechanical displacement of the skin at some distance from the sensory nerve endings.

The density of RA2 fibers in human fingers is only 21 per cm2 ;

SA2 fibers are the least abundant, providing only 9 fibers per cm2
A cross section of the glabrous skin shows the principal receptors for touch in the human hand. All of these receptors are innervated by large-diameter Aβ myelinated fibers. The Meissner corpuscles and Merkel cells lie in the superficial layers of the skin at the base of the epidermis, 0.5 to 1.0 mm below the skin surface. The Meissner corpuscles are located in the dermal papillae that border the edges of each papillary ridge. The Merkel cells form dense bands below the intermediate ridge surrounding the sweat gland ducts along the center of the papillary ridges. The RA1 and SA1 fibers that innervate these receptors branch at their terminals so that each fiber innervates several nearby receptor organs. The Pacinian and Ruffini corpuscles lie within the dermis (2-3 mm thick) and in deeper tissues. The RA2 and SA2 fibers that innervate these receptors each innervate only one receptor organ. (Abbreviations: RA1, rapidly adapting type 1; RA2, rapidly adapting type 2; SA1, slowly adapting type 1; SA2, slowly adapting type 2.)

Figure may help on this one**
Somatosensory information from the limbs and trunk is conveyed to the thalamus and cerebral cortex by two ascending pathways.

Dorsal column—medial lemniscal system
Touch and limb proprioception signals are conveyed to the spinal cord and brain stem by large-diameter myelinated nerve fibers and transmitted to the thalamus in this system. In the spinal cord, the fibers for touch and proprioception divide, one branch going to the ipsilateral spinal gray matter and the other ascending in the ipsilateral dorsal column to the medulla. The secondorder fibers from neurons in the dorsal column nuclei cross the midline in the medulla and ascend in the contralateral medial lemniscus toward the thalamus, where they terminate in the lateral and medial ventral posterior nuclei. Thalamic neurons in these nuclei convey tactile and propri

Anterolateral system
Pain, itch, temperature, and visceral information is conveyed to the spinal cord by smalldiameter myelinated and unmyelinated fibers that terminate in the ipsilateral dorsal horn. This information is conveyed across the midline by neurons within the spinal cord and transmitted to the brain stem and the thalamus in the contralateral anterolateral system. Anterolateral fibers terminating in the brain stem compose the spinoreticular and spinomesencephalic tracts; the remaining anterolateral fibers form the spinothalamic tract.