Chapter 13 : Touch.
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Touch: The skin-based receptor system. The entire surface of the body on which there is living tissue (skin) is a potential receptive surface for the touch system. However, the most active and sensitive part of this receptive field are the hands. In a sense, the two hands are to the touch system, what the two foveas are to the retinas of the visual system.
Haptics : active exploratory touch strategies for acquiring information from an object. Haptics includes not only touch information but also kinesthesis (information about the movement and location of the limbs and digits)
Von Frey hairs: small hairs, like those found on a paint brush, of various diameter are pressed against different parts of the body to see if it is felt. The thicker the diameter hair needed to get a response, the less sensitive the area.
Not surprisingly different parts of the body vary in sensitivity to touch, lips and hands are highly sensitive, back and buttocks, much less so. Furthermore, for any area of body, females tend to be more sensitive than males.
Two point thresholds: a little different procedure, but same general results. A compass-type instrument is used which has two adjustable points, points can be set a different distances from each other. Sensitivity is measured by determining how far apart points must be sent before sub. can detect that there are two points stimulating skin, not one.
Skin receptors: at various depths under the skin are the mechanoreceptors, which start the process of analyzing skin sensations by responding to indentation or pressure on the skin.
In order of depth, nearest to surface to deepest:
a) Meisnner Corpuscles: give strongest response to transient stimulation such as a finger rubbing over a surface. RA-P
b) Merkel Disks: give strongest response to steady pressure by small object. SA-P
c) Ruffini Endings: give greatest response to fairly strong, steady pressure. Are also quite sensitive to movements which result in stretching of skin. SA-D
d) Pacinian Corpuscles: respond best to initiation and termination of diffused pressure against skin. RA-D
Nerve Fibers: afferent fibers travelling from skin receptors to spin and (for some) eventually brain. These fibers are of four distinct categories.
a) Slowly Adapting: fibers which carry messages about steady pressure on skin. Not surprisingly these fibers are connected to Mekel disks and Ruffini Endings in skin.
b) Rapidly Adapting: carry message about transient pressure changes on skin. connected to Miessner and Pacinian Corpuscles.
c) Punctate fibers: ones with distinct receptive fields (connect to Miessner & Merkel)
d) Diffuse fibers: ones with less disctict receptive fields. (connect to Ruffini & Pacinian).
Combination of these four types produce four types of nerve fibers.
Touch pathway runs up dorsal (back) of spinal column. Some connect with interneurons and motor neurons and mediate reflexive arcs. 2 main pathways: Lemniscal (red; newer) more sophisticated aspects of touch. Spinothalamic (older; blue) pain and temperature.
Major touch processing area in cortex
4 maps on S-I (primary somatosensory cortex)
“middle maps” (3b and 1) respond greatest to light touch. Receive inputs from superficial punctate (SA and RA) fibers. Outer maps (3a and 2) respond best to movements of joints, tendons, and muscles (kinesthetic). Inputs from deep diffuse fibers.
Pain serves an important adaptive function – it alerts the organism to potential tissue damage and compels withdraw of affected area from pain source.
Chronic pain, however, often makes life miserable for those afflicted.
Nociceptors: free nerve endings that signal pain. Two locations: skin surface – temperature; Subcutaneous fat layer: punctures.
A_delta: myelinated; fast responding; sharp, acute; thermal pain
C-type: slow responding;
building pain; mechanical; thermal; chemical
Same basic design as all touch pathways. Up spinal column to thalamus; then Sensory cortex; but note presence of descending pathway running along same route.
T cells send pain message. When only fast (A-beta; A-alpha) fibers active; no pain. Inhibitory message send from SG to T cells. When both fast and slow (C-fibers) respond, SG cells are inhibited from sending their inhibitory message to T cells, T cells fire and pain message is sent.
Note also: T’s can be inhibited by top-down messages from cortex. “Meaning” of pain relevant.
It is not uncommon for amputees to claim that they experience pain in the missing limb, often this seems associated with cortical re-organization in somatosensory cortex after loss of limb