User:Hangue Park/sandbox

Definition
Tactile feedback is the sensation produced by physical interaction between skin and foreign object (In broader term, the tactile feedback may include thermoception and/or pain).

Anatomy
Tactile feedback is generated by Mechanoreceptors located in the skin, such as Meissner’s corpuscle, Merkel cells, Pacinian corpuscle, Ruffini endings, Hair plexus, and Free nerve endings. Each receptors responds to different tactile sensation: Meissner’s corpuscle responds to skin motion and detects slipping objects, Merkel cells can discriminate fine shapes and textures, Pacinian corpuscle responds to vibration, Ruffini endings responds to stretch of the skin, Hair plexus responds to skin movement, and Free nerve endings responds to either pressure or stretch as well as temperature and pain. The mechanoreceptors can be classified by several physiological properties, such as rate of adaptation, encapsulation, depth from the skin surface, and receptive field. For example, Meissner’s corpuscle, Merkel cells, and Hair plexus are rapidly adapting receptors, Pacinian corpuscle and Ruffini endings are slow adapting receptors, and Free nerve endings contains both rapidly adapting receptors and slow adapting receptors. The type of tactile feedback can be defined subjectively but includes pressure, texture, vibration, slipping, skin stretch, skin movement, itchness, and tickling. For example, pressure is a sustained sensation over a larger area of the skin. It is interesting to know that tickling is the only sensation that one cannot elicit by himself or herself. Tactile feedback is known to be delivered from the skin to cortex through Medial lemniscys pathway, along with proprioception. The acuity of tactile feedback can be measured by Two-point discrimination test.

Modulation of tactile feedback in locomotion
Modulation of tactile feedback includes both suppression and enhancement of tactile feedback. To suppress the tactile feedback, either denervation or local skin anesthesia has been used. Denervation means any loss of nerve supply regardless of the cause. If this is done for the experiment, surgical neurectomy of nerves are performed. Denervation deactivates a specific sensory nerve working as a pathway from cutanoues sensory receptors in the skin to the central nervous system. Denervation has been used in many animal studies for the modulation of tactile feedback, because it is the most clear way to remove the tactile feedback. To suppress the tactile feedback from the sole of the foot, Tibial nerve and/or Sural nerve have been denervated because they cover most of the area of the sole of the foot. In some experiments, Saphenous nerve has been denervated too. However, denervation causes a permanent damage in the nerve and can be used only for the animal study except very special cases. Local skin anesthesia is an alternative way to suppress the tactile feedback. Local skin anesthesia deactivates the cutanoues sensory receptors in the skin. Lidocaine is the most widely used drug for the local skin anesthesia, because it shows a rapid onset of action and is available as a generic medication with reasonable price. However, it only lasts about 30 minutes. To suppress the tactile feedback from the sole of the foot, 1-2 % Lidocaine is injected directly to the sole of the foot. Injecting the Lidocaine to the sole of the foot presents many difficulties because the sensitive and thick skin is hard to be penetrated with a needle. Also, the underlying soft tissue of the sole of the foot is tightly bound to the fascia and not easily separated. Using topical anesthetic before the injection of Lidocaine can be helpful. To enhance or generate the tactile feedback, electrical stimulation has been used in many experiments. The underlying assumption in using the electrical stimulation is that the electrical stimulation may generate a compound action potential, similar to, even though not the same as, the action potential delivering the tactile feedback to the central nervous system. To enhance the tactile feedback from the sole of the foot, electrical stimulus has been applied either to the sole of the foot via surface electrode or to the cutaneous nerves innervating to the sole of the foot via cuff electrode or penetrating needle electrode.

Role of tactile feedback in locomotion
Since Charles Scott Sherrington and Thomas Graham Brown introduced the spinal locomotion and the concept of the central pattern generator, the effect of sensory feedback to the locomotion has been actively explored. Thanks to the effort of researchers as Sten Grillner, Anders Lundberg, Keir Gordon Pearson, and David A McCrea, it is generally believed that the sensory feedback plays an important role in the control of locomotion. However, the understanding for the role of sensory feedback on locomotion is still limited. For example, the role of tactile feedback from the sole of the foot on locomotion is not clear yet. Several studies have been done in human and cat to explore the role of tactile feedback on locomotion.In human experiments, E. Paul Zehr found that the tactile feedback from the foot functionally stabilizes human gait, through a electrical stimulation of the nerves innervating either the dorsum or the sole of the foot. The superficial peronial nerve stimulation causes reflexes including dorsiflexion to correct the stumbling response and the tibial nerve stimulation causes kinematic responses including plantarflexion to prevent tripping of the swing leg and to assist placing and weight acceptance at the beginning of stance. Also, he found that the effect of stimulation depends upon both the part of the step cycle in which the nerve was stimulated and the intensity of stimulation. Angela Höhne tested the reduced tactile sensation from the sole of the foot using the intradermal injections of an anaesthetic solution, and found that the reduced tactile sensation from the sole of the foot modifies gait dynamics, lower-limb kinematics, and muscle activity during locomotion. The tactile feedback from the sole of the foot also plays an important role in regulating the balance of the body during the locomotion. It helped people to properly react to the perturbation of the ground surface through the alterations in rigidity of surface.

In cat experiments, Laurent Bouyer and Serge Rossignol showed that the loss of tactile feedback from the hindpaw caused only small deficiency in level treadmill walking and the deficits were rapidly compensated, through the denervation of the some or all cutaneous sensory nerves innervating the hindpaw of cats. However, in the slope walking or the ladder walking, cats showed a considerable change in the kinematics of walking. Also, cats could not recover the original kinematics of walking, even though they showed compensatory adaptation during slope and ladder walking after several weeks following denervation. By the experimental results, they concluded that the locomotor system of the adult cat cannot regain fine control of the movement in the absence of tactile feedback from the hindpaw, although it is possible to compensate the removal of tactile feedback from hindpaw. They also concluded that cutaneous inputs contribute more to demanding situations such as walking on a ladder or on inclines than to level walking. Dave Bolton showed that the loss of tactile sensation from the hindlimbs leads to instability for the cats during walking. After denervation of five cutaneous nerves in both hindlimbs, all of the cats changed their step width, lowered their pelvis, and spent more time with the hindlegs in double-support when walking across the walkway. Also, cats lost the corrective responses to medial-lateral perturbation after the denervation. The paw placements were markedly displaced by the medial-lateral perturbation, instead of returning back to the original walking path.

Impairment of tactile feedback in locomotion
Tactile feedback can be impaired by either physical nerve damage or complications from disease. The potential reason to lose part or all of the tactile feedback from the sole of the foot includes Amputation, Diabetic neuropathy, Tarsal Tunnel Syndrome, Peripheral artery disease, Herniated disc, Multiple sclerosis, Fibromyalgia.