The special senses - Touch

The special senses - Touch

1. The Special Senses Touch Dr. Binu Babu Asso . Professor Mrs. Jincy Ealias Assi . Professor

2. Introduction Touch is a perception causing from the activation of neural receptors in the skin, hair follicles, tongue, throat, and mucosa. A variety of pressure receptors respond to variations in pressure. Tactile anesthesia: is the loss or impairment of the ability to feel touch Paresthesia : it is a sensation of tingling, pricking, or numbness of the skin that may result from nerve damage and may be permanent or temporary.

3. 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. The most active and sensitive part of this receptive field are the hands. Haptics : active exploratory touch strategies for acquiring information from an object. Haptics includes not only touch information but also kinesthesis

4. Types of sensory nerves

5. Classifications of touch sensations Pain or nociception: Signals nerve and other tissue damage. Balance or equilibrioception : Allows the sensing of body movement, direction, and acceleration, and to attain and maintain postural equilibrium and balance. Body awareness or proprioception: Provides the parietal cortex of the brain with information on the relative positions of the parts of the body .

6. Temperature sensing or thermoception : The sensation of heat and the absence of heat. Interoceptive senses: if also considered, sensation can be expanded to include stretch oxygen and carbon dioxide sensing, pH sensing, and etc.

7. Measuring touch sensitivity 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. 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.

8. Measuring touch sensitivity 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.

9. Physiology of touch 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: Meisnner Corpuscles: give strongest response to transient stimulation such as a finger rubbing over a surface. RA-P Merkel Disks: give strongest response to steady pressure by small object. SA-P Ruffini Endings: give greatest response to fairly strong, steady pressure. Are also quite sensitive to movements which result in stretching of skin. SA-D. Pacinian Corpuscles: respond best to initiation and termination of diffused pressure against skin. RA-D

10. Physiology of touch Nerve Fibers : afferent fibers travelling from skin receptors to spin and (for some) eventually brain. These fibers are of four distinct categories . Slow Adapting: fibers which carry messages about steady pressure on skin. Not surprisingly these fibers are connected to Mekel disks and Ruffini Endings in skin. Rapid Adapting: carry message about transient pressure changes on skin. connected to Miessner and Pacinian Corpuscles. Punctate fibers: ones with distinct receptive fields (connect to Miessner & Merkel) Diffuse fibers: ones with less disctict receptive fields. (connect to Ruffini & Pacinian ). Combination of these four types produce four types of nerve fibers.

11. Touch pathways 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.

12. Somatosensory cortex The somatosensory cortex is a region of the brain which is responsible for receiving and processing sensory information from across the body, such as touch, temperature, and pain.

13. Pain perception 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.

14. Nerve fibers

15. Pain fibers A delta : myelinated ; fast responding; sharp, acute; thermal pain C-type : slow responding; building pain; mechanical; thermal; chemical

16. Gate control theory of pain 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.

17. Pain pathway The stages of pain pathway Transduction Conduction Transmission Modulation Perception

18. TRANSDUCTION   Transduction begins when peripheral terminals of nociceptive (sensory receptors of pain) C fibers A-delta ( Aδ ) fibers Nociceptive fibers are depolarized by noxious mechanical, thermal, or chemical energy. The membranes of these terminals contain proteins and voltage-gated ion channels that convert thermal, mechanical, or chemical energy into an action potential (AP). Nociceptor terminals are spread densely throughout the skin. They are found less on periosteum , joints, tendons, muscles, and least on the surface of organs.

19. CONDUCTION Conduction of an Action Potential is the second phase of nociception. An AP generated in nociceptor terminals is conducted across the peripheral process to the central process were it depolarizes the presynaptic terminal. The presynaptic terminal interfaces with a network of interneurons and second order neurons in the dorsal horn. Interneurons can facilitate or inhibit transmission to second order neurons

20. These nociceptors projects from the trigeminal ganglion and enter the CNS at the level of the pons. Trigeminal Að and C fibers pass down through the pons into the medulla where they synapse on second order neurons which then rise to decussate within the pons and pass to the thalamus.

21. MODULATION Modulation of nociceptive transmission is an adaptive process involving both excitory and inhibitory mechanisms. The responses of second order neurons can be suppressed or facilitated dependent on importance of the event.

22. PERCEPTION   Perception of nociceptive pain is dependant upon neural processing in the spinal cord and several brain regions. Pain becomes more than a pattern of nociceptive action potentials when they reach the brain. Action potentials ascending the spinothalamic tract are decoded by the thalamus, sensorimotor cortex, insular cortex and the anterior cingulate to be perceived as an unpleasant sensation that can be localized to a specific region of the body. Action potentials ascending the spinobulbar tract are decoded by the amygdala and hypothalamus to generate a sense of urgency and intensity. It is the intergration of sensations, emotions and cognition that result in our perception of pain.

23. References Acree , T. E., & Beidler , L. M. (1971). Taste,. Berlin, New York, Springer- Verlag . Per Brodal . (2010). The central nervous system : structure and function. Oxford University Press, Cop . Nieuwenhuys, R., J Voogd, Voogd, J., & Christiaan Van Huijzen. (2008).  The Human Central Nervous System . Springer.

24. Thank You