Monday, November 26, 2007

PSC 129 Chapter 12 Touch

- Touch is the mechanical displacements of the skin; kinesthesis: internal sensations that inform us of the psoitions and movements of our limbs

-Somatosensation: sensory signals from the body

-Proprioception: perception by kinesthetic and vestibular receptors.


I. Touch Physiology

1. The sense organ and receptors for touch

-Touch is in skin

-Touch receptors are in both the outer layer (epidermis) and the underlying layer (dermis)

-Multiple types of touch receptors; receptors form the basis for multiple "channels" that contribute to the overall sense of touch; shape, temperature, texture

-Each touch receptor has three attributes:

a. Type of stimulation the receptor responds to (pressure, vibration, or temperature changes)

b. Size of the receptive field: the extent of the body area to which teh receptor will respond.

c. Rate of adaptation

- A fast-adapting receptor responds with lots of action potentials when its preferred stimulus is first applied and when it is removed but not in between.

- A slow-adapting receptor remains active throughout the period when the stimulus is in contact with its receptive field.


2. Tactile receptors

-Four types of receptors: mechanoreceptors because they respond to mechanical stimulation or pressure

a. Meissner corpuscles, Merkel cell neurite complexes, Pacinian corpuscles, and Ruffini endings

b. Meissner and Merkel receptors are located at the junction of the epidermis and dermis; enlarged endings of nerve fibers that have smaller receptive fields than Pacinian and Ruffini which are in the dermis and subcuticle

c. The four types of receptors can be characterizzed by their adaptation rates and sizes of receptive fields (look at table in 289)

d. Each receptor has a different range of responsiveness and is responsible for perceiving a different feature of mechanical stimulation

e. SAI respond to fine spatial details; important in texture and pattern perception

f. SAII respond to sustained downward pressure; lateral skin stretch when you grasp an object.

g. FAI respond to low-freq vibrations; feel the motion of coffee cup slipping acorss your fingers

h. FAII detect high-freq vibrations when object first makes contact with skin; object in hand contacts another object

3. Kinesthetic receptors

-Another type of mechanoreceptors that lie within msucles, tendons and joints; play rorle in our sense of where our limbs are and what kinds of movement they are making

-Spindles: muscle receptors that perceive the angle formed by a limb; convey the rate that the muscle fibers are changing in length

- Receptors in the tendons provide signals about hte tension in muscles attached to the tendons and receptors directly in the joints themselves come into play when a joint is bents to an extreme angle

4. Thermoreceptors
- Located in both the epidermis and dermis; changes in skin temperature
- Warmth fibers fire when the temp of skin surrounding the fibers rises; cold fibers (outnumbers warmth fibers) fire in response to decrease in temp
- Fire when you touch something that is warmer or colder than your skin

5. Nociceptors
- Signals pain; are touch receptors that have bare nerve endings and that respond to various forms of tissue damage
- Two types: A-delta fibers respond primarily to strong pressure or heat and are myelinated which allows them to conduct signals rapidly; C fibers are unmyelinated and respond to intense stimulation such as pressure, temp, or poisonous chemicals; both are smaller in diameter
- Pain in two stages: quick sharp burst of pain followed by throbbing sensation; A-delta fibers respond and then C fibers

6. From Skin to Brain

- Axons of various tactile receptors are combined into single nerve trunks in the same way that retinal ganglion axons converge in the optic nerve and cochlear hair cells converge in the auditory nerve.

- A number of somatosensory nerve trunks; axons in the older nerve trunks synapse first in the spinal cord.

- In spinal cord, touch information proceed upward toward the brain via two pathways

a. Spinothalamic pathway is the slower of the two and carries most of the information from thermoreceptors and nociceptors; includes a number of synapses within the spinal cord, thus slowing conduction while providing a mechanism for inhibiting pain perception

b. The dorsal-column-medial-lemniscal (DCML) pathway: wider-diameter axons and fewer synases and therefore conveys information more quickly to brain; planning and executing rapid movements; carries signals from skin, muscles, tendons and joints

c. Neurons in DCML make first synapse in the medulla; then passed to neurons that synapse in the ventral posterior nucleus of the thalamus à somatosensory area 1 (S1) à somatosensory area 2 (S2)

- Mapped somatotopically in correspondence to the skin

- Somatosensory cortex is organized into sensory homunculus which is a spatial map of the layout of the skin; twin homunculi; the left-hemisphere S1 receives information from the right side of the body

7. Pain
- Analgesia and gate control theory: damping of pain without losing consciousness- In soldiers caused by endogenous opiates: chemicals that block the release or uptake of NT to transmit pain
- Gate control theory: pain sensations can be blocked by a feedback circuit located in an area called substantia gelatinosa of the dorsal horn of the spinal cord.
- Inhibitory signals from the gate neurons cancel transmission to the brain.
- Gate neurons activated by expreme pressure, or cold.

8. Pain sensitization
- Nociceptive pain: ongoing damage to the body's tissue; once damaged, the site becomes more sensitive, triggers pain more readily than before --> hyperalgesia = heightened response
- Resulting pain is inflammatory
- Neuropathic pain: damage to nervous system

9. Cognitive aspects of pain
- the sensation of pain and the emotion that accompanies it

III. Tactile Sensitivity and Acuity
1. How finely can we resolve spatial details?
- Two-point touch threshold: the smallest separation at which you can tell that you are being touched by two points and not just one
- Low two-point threshold only when the density of receptors is relatively high, the receptive fields are small and cortical convergence does not occur

Summary:
1. The sense of touch produces a number of distinct sensory experiences, each mediated by its own sensory receptor system (s). Touch sensors are responsive not only to pressure, but also to vibration, temperature, and noxious stimulation. The kinesthetic system, which also contributes to our sense of touch, is further involved in sensing limbs in space.

2. The skin is the largest sensory organ, covering the entire exterior surface of the body. Four classes of pressure-sensitive (mechano-) receptors have been found within the skin. The organs used to sense limb position and movement (namely, our muscles, tendons, and joints) ar emore deeply situated within the body. Thermoreceptors respond to changes in skin temperature that occur, for example, when we contact objects that are warmer or cooler than our bodies. Nociceptors signal tissue damage (or its potential) and give rise to sensations of pain.

3. The pathways from touch receptors to the brain are complex. Two major pathways have been identified: a fast pathway that carries information from mechanoreceptors, and a slower one that carries thermal and nociceptive information. Only the second pathway synapses when it first enters the spinal cord. These pathways project to the thalamus and from there to the primary somatosensory area, located in the parietal lobe just behind the central sulcus. This area contains several somatotopically organized subregions, in which adjacent areas of the body project to adjacent areas of the brain.

4. Downward pathways from the brain play an important role int he perception of pain. According to the gate control theory, signals along these pathways interact at the spinal cord with those from the periphery of the body. Such interactions can block the pain signals that would otherwise be sent forward to the brain. The sensation of pain is further moderated by areas in the cortex.

5. Investigators have measured sensitivity to mechanical pressure by applying nylon hairs of different diameters to the skin. They determine spatial acuity of the skin by measuring the two-point touch threshold, and more precisely by discriminating the orientation of gratings applied to the skin. Tactile pressure sensitivity and spatial acuity vary with body site, because of varying concentrations of different types of mechanoreceptors. The minimum depression of the skin needed to feel a stimulus vibrating at a particular rate provides a measure of vibration sensitivity.

6. The sense of touch is intimately related to our ability to perform actions. Signals from the mechanoreceptors are necessary for simple actions such as grasping and lifting an object. Conversely, our own movements determine how touch receptors respond and, hence, what properties of the concrete world we can feel. Touch is better adapted to feeling the material properties of objects than it is to feeling their shapes, particularly when an object is large enough to extend bejond the fingertip.

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