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Sensation and Perception, Chapter 12 (300-316)

Terms in this set (61)

Touch
A term used to refer to the sensations caused by mechanical displacements of the skin. These displacements occur when you are poked by your four-year-old nephew, licked by your dog, or kissed by your significant other. They occur any time you grasp, wield, or otherwise make contact with an object.
Tactile
This is the adjective form of touch. It refers to these mechanical interactions and will expand the definition of touch to subsume the perception of temperature changes (thermal sensation), the sensation of pain, which takes place when our body tissues are damaged (or potentially damaged) in some way, itchiness, and the internal sensations that inform us of the positions and movements of our limbs in space.
Kinesthesis
This refers to the perception of the position and movement of our limbs in space. The internal sensations arise from muscles, tendons, and joints.
Proprioception
This refers to perception mediated by kinesthetic and vestibular receptors.
Somatosensation
This is a collective, technical term for sensory signals from the body.
Where is our sensing equipment for touch?
The human sense of touch is housed in the largest and heaviest of sense organs - the skin. Touch receptors are embedded all over the body, in both hairless and hairy skin, as well as within our mouths and our muscles, tendons, and joints.
What are the substructures of the skin?
The receptor units for touch are embedded in both the outer layer, called the epidermis, and the underlying layer, known as the dermis. The dermis is comprised of nutritive and connective tissues, within which lie the mechanoreceptors. These receptors form the basis for multiple "channels," specialized information processing subsystems that each contribute to the overall sense of touch (e.g. if you wrap your fingers around an ice cube, different channels convey information about its temperature, its shape, and its texture).
Mechanoreceptors
These are sensory receptors that respond to mechanical stimulation (pressure, vibration, and movement).
Tactile Receptor
Tacticle receptors are called mechanoreceptors because they respond to mechanical stimulation or pressure. It consists of a "nerve fiber" and an associated expanded ending. All tactile nerve fibers fall into a class called A-beta fibers, which have somewhat wide diameters that allow for very fast neural conduction.
Specialized Nerve Ending #1: Meissner corpuscle
Associated with fast-adapting (FA I) fibers that have small receptive fields. Located at the junction of the epidermis and dermis.
Specialized Nerve Ending #2: Merkel cell neurite complex
Associated with slow-adapting (SA I) fibers that have small receptive fields. Located at the junction of the epidermis and dermis.
Specialized Nerve Ending #3: Pacinian corpuscle
Associated with fast-adapting (FA II) fibers that have large receptive fields. Embedded more deeply in the dermis and underlying subcutaneous tissue.
Specialized Nerve Ending #4: Ruffini ending
Associated with slow-adapting (SA II) fibers that have large receptive fields. Embedded more deeply in the dermis and underlying subcutaneous tissue.
SA I Fiber
Respond best to steady downward pressure, fine spatial details, and very low frequency vibrations of less than roughly 5 Hz. Especially important for texture and pattern perception. Activities such as reading Braille and figuring out the location and orientation of the slot on the head of a screw depend particularly on this touch channel. When this fiber is stimulated, people report feeling "pressure." Terminate in Merkel cell neurite complexes.
SA II Fiber
In the skin and fibrous tissues of the body. Respond to sustained downward pressure and, in particular, to lateral skin stretch which happens (e.g. when you grasp an object). When you reach out for your coffee cup, this fiber will help you figure out when your fingers are shaped properly for picking up the cup. Assumed to terminate in the Ruffini endings.
FA I Fiber
Respond best to low-frequency vibrations from about 5 to 50 Hz. If your coffee cup is heavier than you expected and begins to slip across your fingers, then this motion across the skin will lead to such vibrations, and your FA I fibers will help you correct your grip before your coffee spills all over you. When a single FA I fiber is stimulated, people will report a very localized sensation that they describe as "wobble" or "flutter." Terminate in Meissner corpuscles.
FA II Fiber
Respond best to high-frequency vibrations from about 50 to 700 Hz (highest frequency tested to date). Occur when an object first makes contact with the skin (e.g. mosquito landing on your arm). Also created when an object that you are holding makes contact with another object (e.g. how hard you're tapping pencil on desk as you cram info into brain). When a lone FA II fiber is stimulated, people report a more diffuse sensation in the skin that matches "buzz." Terminate in Pacinian corpuscles.
Kinesthetic
Perception involving sensory mechanoreceptors in the muscles, the tendons, and the joints. Kinesthetic receptors play a crucial role in sensing where our limbs are as well as what types of movements we are making. Whereas receptors in the tendons will give us signals about tension in muscles that are attached to the tendons, receptors in the joints will be crucial when a joint is being bent to an extreme angle.
Muscle Spindle
Sensory receptor situated in a muscle that senses its tension. It conveys the rate at which the muscle fibers are changing in length. The angle formed by a limb at a joint is perceived mainly through it.
Thermoreceptors
Sensory receptors that signal info about changes in skin temperature. Located in both the epidermal and dermal layers of the skin.
Thermoreceptor #1: Warmth Fiber
Sensory nerve fiber that fires when the skin temperature increases. Does not respond much when skin temperature is within the range of 30 Degrees Celsius and 36 Degrees Celsius (86 Degrees Fareinheit and 96 Degrees Fareinheit), which are normal skin conditions. If you bundle up in your long underwear and snowsuit but then sit inside of the fire, your skin temperature will probably rise above 36 Degrees Celsius, and your warmth fibers begin to fire.
Thermoreceptor #2: Cold Fiber
Sensory nerve fiber that fires when the skin temperature decreases. Does not respond much when skin temperature is within the range of 30 Degrees Celsius and 36 Degrees Celsius (86 Degrees Fareinheit and 96 Degrees Fareinheit). If you take the snowsuit off and walk out into the snow, your skin temperature will quickly begin to fall and as soon as it goes below 30 Degrees Celsius, your cold receptors will begin firing.
What is the typical temperature of objects in the environment?
Objects in the environment are usually cooler than 30 Degrees Celsius, so it is usually the cold fibers that tell us about the object. Steel conducts heat more efficiently than stone. Cold fibers will fire less quickly and for a shorter period of time when you touch a steel object than when you touch a stone object. This is because the steel object will warm more quickly to match your skin temperature.
Nociceptors
These are touch receptors that have bare nerve endings and respond to numerous forms of tissue damage or to stimuli that could potentially damage tissue (subsuming extreme skin temperatures lower than 15 Degrees Celsius or higher than 45 Degrees Celsius). Without nociceptors, we would not be able to sense dangerously sharp or hot objects. Diseases such as Hansen's disease (leprosy) and diabetes are marked by the loss of pain sensation.
Nociceptor #1: A-Delta Fibers
Intermediate size. Myelinated, so they can conduct signals very rapidly. Respond mainly to strong pressure or heat. Smaller in diameter than those coming from non-nociceptive mechanoreceptors.
Nociceptor #2: C Fibers
Narrow diameter. Unmyelinated. Respond to intense stimulation of various types: pressure, heat or cold, or noxious chemicals. Smaller in diameter than those coming from non-nociceptive mechanoreceptors.
"A-Beta" Fibers
These are the wider-diameter fibers. Come from non-nociceptive mechanoreceptors.
What are the two stages of painful events?
The first stage is a quick, sharp burst of pain and the next stage follows the first stage with a throbbing sensation. The two stages may possibly be a reflection of the onset of signals first from the A-delta fibers and then from the C fibers.
Describe the somatosensory pathways that deliver information from the skin to the brain.
Touch messages must travel as far as 2 meters to get from the skin and muscles of the feet to the brain. In order to cross this distance, the info must move up through the spinal cord. At first, the axons of numerous tactile receptors are combined into single nerve trunks.
What are the differences between the visual and auditory pathways and the pathways for touch?
Although there are only two optic nerves and two auditory nerves, there are a number of somatosensory nerve trunks, arising in the hands, arms, feet, legs, and other areas of the skin. Axons in the optic and auditory nerves go directly to the brain whereas axons in the older nerve trunks synapse in the spinal cord first.
Spinothalamic Pathway
The slower of the two pathways. Carries most of the information from thermoreceptors and nociceptors. Includes a number of synapses within the spinal cord, therefore slowing conduction while giving a mechanism for inhibiting pain perception whenever it is necessary.
Dorsal Column-Medial Lemniscal (DCML) Pathway
Includes wider-diameter axons and fewer synapses and thus conveys info more quickly to the brain. Tactile and kinesthetic info carried along this pathway is used for planning and implementing quick movements, where quick feedback is a must. Neurons in the DCML pathway first synapse in the cuneate and gracile nuclei near the base of the brain. Activity is then passed on to neurons that synapse in the ventral posterior nucleus of the thalamus.
Somatosensory Area 1 (S1)
Located in parietal lobe, just behind the post central gyrus. Primary receiving area for touch in the cortex. Analogous to V1 in vision.
Somatosensory Area 2 (S2)
Lies in upper bank of the lateral sulcus and with other cortical areas. Secondary receiving area for touch in the cortex. Motor areas of cortex, which regulate body parts' movements, are situated just in front of central sulcus. This enhances communication between somatosensory and motor control systems.
Somatotopic
Touch sensations that are spatially mapped in the somatosensory cortex, corresponding to spatial events on the skin. Analogous to the topographic spatial representation of events on the retina found in vision.
Homunculus
This is a representation of regions of the body in the brain that resembles a map. As a result of adjacent areas on the skin being connected to adjacent areas in the brain due to somatotopy, the somatosensory cortex is organized into the homunculus. We all have twin homunculi, one in each hemisphere of the brain. The left hemisphere S1 receives info from the right side of the body and vice versa.
Wilder Penfield
Charted the somatotopic map with help from patients undergoing brain surgery to alleviate epilepsy. Since there are no pain receptors in the brain, the patients did not need anesthesia and could thus remain responsive. During the operation, he systematically stimulated different parts of the patient's somatosensory cortex with an electrode. As he moved the electrode from one location in S1 to another, the patient said that he felt sensations in the arms, legs, face, etc. The correspondence that took place between the stimulation and the sensation produced a map of the body in the brain.
Distortions of Penfield's somatotopic map in S1
The thumb grabs a big piece of real estate relative to its size. Sensations from the leg are processed in a relatively small portion of S1. A larger chunk of S1 is committed to processing info from the lips and the fingers than from the neck because tactile receptors are more heavily concentrated in the lips than they are in the neck.
Phantom Limb
This is a sensation perceived from a physically amputated limb of the body. At times, patients may perceive their phantom limbs to be in uncomfortable positions, leading to persistent - and very real - pain. May appear on the face and stump subsequent to amputation. The area responding to the face in the homunculus is located adjacent to the area responding to the hand and arm. The hand and arm areas of S1 are, to some degree, "invaded" by neurons carrying info from touch receptors in the face. However, other parts of the brain listening to the hand and arm areas are not entirely aware of these altered connections and thus they attribute activity in these areas to stimulation from the missing limb.
Neural Plasticity
When neural circuits are able to undergo changes in their function or organization because of previous activity.
At how many different levels does pain arise?
Three: sensory, emotional, and cognitive. Pain is in fact a very subjective state with distinguishable components. The three levels interact with one another to form a conscious experience.
Substantia Gelatinosa
Jellylike region of interconnecting neurons in the dorsal horn of the spinal cord. Neurons carrying nociceptive signals arrive at the spinal cord here.
Dorsal Horn
Region at rear of spinal cord that gets inputs from receptors in the skin.
Gate Control Theory (Melzack and Wall)
States that bottom-up signals from the nociceptors can be blocked through a feedback circuit that is located in the dorsal horn. Once these gate neurons send excitatory signals, the sensory info can go through but inhibitory signals from gate neurons cancel transmission to the brain. The results of these interactions at spinal cord are transmitted to somatosensory areas S1 and S2.
Anterior Cingulate Cortex (ACC)
This is a region of the brain that is associated with the perceived unpleasantness of a pain sensation.
Secondary Pain Affect
Emotional response associated with long-term suffering that takes place when painful events are imagined or remembered. For example, cancer patients who must undergo a second round of chemo may remember the first and dread what is coming next. This component of pain is associated with the prefrontal cortex.
Prefrontal Cortex
This is a region of the brain that is concerned with cognition and executive control.
The role of pain in tickling and itchiness
Some of the response people have to tickling seems to rely on nociceptors. Signals from the brain appear to come into play when we try to tickle ourselves. In fact, when we tickle ourselves, not only does it engender less laughter but it also produces less activity in the somatosensory cortex due to canceling signals (most likely mediated by endogenous opiates) from other brain areas that know where the tickling stimulation came from. As for itchiness, although research suggests that pain and itch may be mediated by the same neural systems, it is unclear how itch works or how to relieve it.
Analgesia
Damping of pain sensations without losing consciousness. Responses to noxious stimulation can be influenced by analgesic drugs but apparently anticipation, religious belief, prior experience, and excitement can have attenuating effects as well. Interpersonal and broader social influences can have a fundamental impact on the emotional component of pain.
Counter Irritation / Diffuse Noxious Inhibitory Control
Activate gate neurons that attenuate pain at the level of the spinal cord. Extreme pressure, cold, or other noxious stimulation applied to another site distant from the source of the pain. Pain from electrically stimulating a tooth, for example, can be reduced by noxious stimulation of the hand. Ascending signals from the counter irritation appear to reach the brain stem and set off a new set of signals that are sent back down to the pain-blocking gate in the spinal cord.
Counterstimulation
Even if your mother tells you not to scratch a mosquito bite, gate control theory states that rubbing the skin near the bit can, in fact, provide some relief. This is a product of the stimulation of fibers other than the nociceptors and can be produced by interactions between neurons within the spinal cord.
Endogenous Opiates
These are chemicals released by the body that block the release or uptake of neurotransmitters necessary to transmit pain sensations to the brain. Differences between individuals concerning pain responsiveness (pain thresholds) may reflect differences in their baseline levels of these substances. Substances produced externally such as morphine, heroin, and codeine are similar in chemical structure to these opiates and therefore have similar analgesic effects. Drugs like acetaminophen and ibuprofen alleviate pain at its source by counteracting chemicals that would otherwise start the nociceptors firing.
Hyperalgesia
This refers to an increased or heightened response to a normally painful stimulus. In other words, once the damage has occurred to the body's tissue, the site can become more sensitive and trigger the feeling of pain more readily than before. The resulting pain is "inflammatory" pain and the heightened pain sensitivity usually disappears once the tissue heals.
"Neuropathic" Pain
Pain that results from the absence of immediate trauma due to damage to or dysfunction of the nervous system. Some neuropathic pain reflects changes in the sensory fibers at the skin that don't normally produce pain but now become pain inducers (allodynia). Other neuropathic pain arises from changes in the dorsal horn of the spinal cord. Changes at the level of the skin are called peripheral changes and changes at the level of the spinal cord are called central changes. Mechanisms by which neuropathic pain arises are understood more and more at the cellular and molecular levels.
Is there a single medication that will alleviate all types of pain?
Different underlying mechanisms for nociceptive, inflammatory, and neuropathic pain (peripheral or central) call for different analgesics.
"Discriminative Touch"
Tactile, thermal, pain, and itch experiences collectively.
"Pleasant"/"Emotional" Touch
A previously unknown fifth component discovered by scientists in just the last few years. These scientists argue that the emotional properties of non-painful bodily touch are mediated in large part by a class of unmyelinated and therefore slower peripheral C fibers known as "C tactile afferents" (or "CT afferents," for short) that are unrelated to pain and itch. The pleasant qualities of the mechanical stimulation activates an area of the frontal lobes known to be involved in emotion - the orbitofrontal cortex.
Orbitofrontal Cortex
Activated by both painful and pleasant touch. Suggests that this brain region represents the emotional features of cutaneous stimulation, both rewarding and punitive. Pleasant bodily contacts promote endorphin and oxytocin responses, contributing to feelings of well-being, confidence, and calmness.
Max von Frey Approach
Used carefully calibrated stimuli, including horse and human hairs. Touch different parts of your skin with a hair from your head and a bristle from a hairbrush to reveal the relative skin sensitivity to these two different forces. The thinner hair is felt on the more sensitive areas (lips, some parts of your hand) than on your thigh or upper arm. The bristle demonstrates that your skin is sensitive to mechanical pressure all over but uniformly so.
Absolute Vibratory Threshold
The minimum amount that a vibrating stimulus displaces the skin in order to be detected. People could detect the presence of vibrations from below roughly 5 Hz (Hz; 5 cycles in 1 second) up to about 400 Hz, the highest frequency that was tested. Other studies have confirmed that people can detect vibrations up to 700 Hz, the highest frequency tested to date.
Two-Point Touch Threshold
The smallest separation at which we can tell that we are being touched by two points and not just one. Results from a systematic study of two-point thresholds in females as a function of body site show that the extremities - fingertips, face, and toes - show the highest acuity. The pattern for males is very similar. The two-point touch threshold is low only when the density of receptors is somewhat high, the receptive fields are small, and cortical convergence doesn't take place.
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