Lectures 12 and 13
Lectures 12 and 13 NSC 3361
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This 10 page Class Notes was uploaded by Rachael Couch on Tuesday March 1, 2016. The Class Notes belongs to NSC 3361 at University of Texas at Dallas taught by Van S Miller in Summer 2015. Since its upload, it has received 34 views. For similar materials see Behavioral Neuroscience in Neuroscience at University of Texas at Dallas.
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Date Created: 03/01/16
Lectures 12 and 13: Sensation: Sensory Processing, Touch, and Pain All sensory processing begins in receptor cells A stimulus to the receptor produces a graded potential (EPSP) o Harder push = more response of receptor If the potential is big enough, the receptor generates an action potential If not enough to cause an action potential – you still feel it even though the sensory neuron did not fire to the brain sensory receptor feels it even though the brain didn’t How can intensity of a stimulus known? How can you tell between a light touch and a punch? o Both signal an action potential – all action potentials are the same intensity o Coding – patterns of action potentials in a sensory system 3 strategies used: o 1) A single neuron changes the frequency of action potentials with intensity o 2) Multiple neurons fire together As the stimulus strengthens, more neurons are recruited o 3) D ifferent neurons respond to different ranges Range fractionation different cells have different thresholds for firing, over a range of stimulus intensities “specialized neurons” One set of neurons fires action potential when lightly stimulated Different set of neurons fire only with medium intensity stimulus Other set have highthreshold for firing Neurons limited in intensity of signaling because of refractory periods At about 250 impulses/sec it can tell that low threshold neurons are firing at about half the frequency (150 impulses/sec – their max firing rate) and the mediumthreshold are firing at 250/sec but stimulus is not intense enough to trigger highthreshold Somatosensory system detects touch and pain Pain is NOT intense touch – pain and touch are different and independent systems Four touch receptor classes: pain, touch, vibration, stretch o Pain and touch are in the epidermis Pain closer to surface – want to detect pain as soon as something penetrates o Vibration and stretch receptors are deeper – don’t need to be on top because anything that vibrates (stretches) the outside will also vibrate (stretch) the inside Five touch receptors o Free nerve endings – sample the extracellular fluid of the tissue Pain and temperature o Merkel’s disc and Meissner’s corpuscle – touch Very precise o Pacinian corpuscle (vibration) and ruffini’s ending (stretch) Less precise Pain pathway Temperature receptors use the pain pathway o Free nerve endings – each class responds to a different temperature o Cold receptor – CMR1 o Warm receptor – TRPV1 o Hot receptor – TRP2 Pain uses two types of neurons o Small unmyelinated C fibers (dull pain) Most common temp/pain fibers Dull because not myelinated – conducts slowly Used in cold and warm Capsaicin the “hot” in chili pepper Activates C fibers via TRVP1 and is neurotoxic o Kills fibers over time – need more spicy to get the same effect May prevent the growth of some cancer o Larger myelinated Adelta fibers (sharp pain) Used in hot touch o Injury is a “2 phase” response: Adelta fibers respond and then C fibers o Drugs can block one type of pain and not the other Vibration perception First receptors – evolutionarily oldest/simplest Unipolar cell Vibration stretches the skin and stretches open the sodium channels – sodium comes in and fires vibration system Dorsal root ganglion – has cell bodies o Chronic pain (and all sensation) can be stopped from cutting it out Two parallel touch systems 2 system involved in early or later touch With stimulus, first a burst of a lot of firing up to half a second (phasic) and then stop then tonic receptors keep firing Phasic receptors – adapt quickly by decreased frequency o Brief but intense o “Shout out” – grabs the brains attention Tonic receptors – slow or no decline in action potential frequency o Fire from onset Basic organizing principle of the sensory cortex Receptive field The part of the world a sensory neuron responds to o The area that, when stimulated, the neuron will respond to o If touched on finger, hearing cells do not respond because touch is out of their receptive field and toe cells don’t fire because it’s too far away (out of their receptive field) Each single sensory neuron has a receptive field Differ in size, shape – some overlap, some don’t People can detect touches as distinct until they are 2 mm apart on the tongue, 4 mm on the finger, and 64 mm on the back because there are more sensory receptors involved in detecting touches on the tongue and hand than the back Electrode in brain – poke different areas, what poke makes this cell fire? Find it and then move around close Smaller sensory fields = increased precision of input (ex: hands, tongue) Touch receptive fields “On center, off surround” Target shaped Touch in center – maximal (increased) firing, excitatory o Fires a lot initially (phasic) and then slows down (tonic) Touch in surrounding area (outer circle of target)– decreased firing, inhibitory o In receptive field because it changes firing (responds) when touched Touch faraway no effect on firing – outside of receptive field Levels of touch processing Sensory input from anywhere goes to the thalamus and then to the primary sensory cortex Primary sensory cortex talks to nonprimary (on both sides) to help process the signals Two tactile systems Dorsal for touch and vibration o Don’t use free nerve endings – use other 4 types of neurons Spinothalamic for pain, temperature o Use free nerve endings Both go through the thalamus Somatosensory Cortex the homunculus All sensory info (touch and pain) ends up in the somatosensory cortex (specifically the postcentral sulcus/gyrus) where it is analyzed The postcentral sulcus is divided into different regions – one for each specific part of the body o The cortical map represents the innervation of a body region o Devotes different amount of area to different parts Ex: little finger has same area as genitals even though the genitals are more sensitive because the little finger has to be more precise –it’s used to “map the world” o Smaller receptor field = bigger area in cortex because one neuron for each area = more neurons and requires more processing o The extremities are located laterally with the mouth areas CQ: a stroke just medial to your ear would cause loss of sensation of the contralateral face Central sulcus (“landmark”) is the divider between the sensory cortex and motor cortex The somatosensory cortex reorganizes after amputation of a hand Area that was previously devoted to hand is now shared between other areas (other areas expand and take over) Takes time to develop (a few months) No sensory input for that area after amputation so those neurons have nothing to do – some die but most find something else to do those close to the face neurons start to respond to the face Brain reorganizes itself – compare to stroke, brain tries to remake connections and makes the wrong connections Pain The multifaceted character of pain Pain is an experience (sensation + other stuff; multisystem reaction) not just a sensation Peripheral input break wrist Input stimulates free nerve endings (pain receptors) that fire an action potential to the spinal cord The spinal cord transmits this information to the brain = pain perception 3 parts of pain perception o Motivationalaffective system I’m hurting but I’m in such an intense situation that I didn’t notice in the middle of a game Leads to motor response (grabbing wrist) Feedback – motivationalaffective system affects spinal cord processing Can block pain signals Ex: broke your wrist and didn’t even notice the pain until later o Cognitive system putting a towel over face because “men don’t cry” cultural/learned – if men don’t cry their pain eases Involves the frontal cortex o Sensorydiscriminative system brain identification of pain characteristics Ex: “my wrist hurts on this side at this point and it’s getting worse/better” Involves somatosensory cortex Peripheral mediation of pain Trauma – defined as tissue injury (measured by swelling, chemical release etc.) Trauma causes chemical release o Histamine release swelling o Substance P (pain/peptide) is a NT that transmits the feeling of pain o Causes others to release (ex: serotonin) that signal to the free nerve ending of C fibers to fire an action potential Free nerve endings in the skin have specialized receptors to respond to temperature change, chemicals, and tissue injury o Free nerve endings sample the extracellular environment looking for tissue injury C fibers receive chemicals and send an action potential from the spinal cord through the dorsal to the thalamus Pain disorders Congenital insensitivity to pain – inherited mutated sodium channel in pain fibers in dorsal root ganglia (cell bodies of the sensory neurons that go all over body) o Periphery receptors are stimulated normally but don’t signal o Ex: baby bit off own fingers when teething because it didn’t hurt Man on Fire syndrome – congenital hypersensitivity to pain o Severe burning pain o Activation of C fibers by mild stimuli (warmth, sunlight) o Caused by gain of function mutation in sodium channel Exaggeration of normal pain pathways Pain is lifesaving Low levels of pain during everyday tasks tell us when a certain movement or prolonged posture is harmful to our body Even during sleep, mild pain makes us toss and turn enough to prevent bedsores or skeletal strain o Bedsores can lead to severe infection that causes death in those who can’t feel the are In cases where someone can’t feel a lack of blood flow to an area they can end up losing feeling permanently Spinothalamic system transmits pain and temperature Skin spinal cord PAG thalamus cingulate cortex and somatosensory cortex Periaqueductal gray (PAG) o In the midbrain o Involved in pain perception o Has opioid receptors Endogenous opioid endorphins o System to block pain transmission because blocking pain may be helpful in intense situations where you have to keep going (childbirth etc.) o In these situations a surge of endorphins blocks the pain Cingulate cortex (aka cingulate gyrus) o Small offshoot from the pathway from skin to somatosensory cortex o Part of limbic system (emotional);involved in empathy o Causes emotional memory Ex: So you don’t burn your hand again that way o CQ: A part of the pathway that contributes emotion to pain sensation is cingulate gyrus After giving a patient opioids they feel the pain but it doesn’t bother them as much o Takes away the emotional quality of the pain, not the actual pain Descending pathways can reduce (or increase) pain Increasing pain: Spinal cord thalamus Decreasing pain: Limbic system and hypothalamus PAG Pain can also be blocked in the spinal cord (periphery) = gate control theory of pain “Squeezing the thumb you just hit blocks the pain” Touch/nonpainful sensation (deep pressure) activates Abeta fibers o The gate (from c fibers spinal cord) can be closed by increased activity in Abeta fibers Transcutaneous electrical nerve stimulation (TENS) delivers electrical pulses to the skin o Enough impulse to active deep pressure fibers but not enough to stimulate pain fibers Activates all A fibers o Stimulate an inhibitory neuron o Supports gate control idea Other (descending) factors influence the gate: attention, emotional/cognitive factors Other pain treatments o Meditation/prayer, acupuncture, massage, physical activity, placebo effects Chronic pain Chronic pain persists even after an injury has healed Pain signals remain active in CNS for months to years Pain is linked to emotion wide range effects Pain causes lack of sleep and sleep disorders cause more pain and depression o = pain cycle/chronic pain Emotional effects include depression, anger, fear of reinjury Neurogenic pain (pain resulting from damage to nerves) Psychogenic pain (pain not due to disease, injury or any visible damage) Neurogenic pain: reflex sympathetic dystrophy Stage I (normal) o Onset of severe pain limited to the site of injury o Increased sensitivity of skin to touch Nonpainful stimuli causes pain o Localized swelling o Usually lasts a few weeks then subsides Stage II (abnormal – chronic pain) o Pain becomes more severe and spreads o Swelling tends to spread o Hair and nails become brittle; muscle wasting begins Mechanism of RSD Broken wrist causes pain which releases chemicals to signal to CNS Signals trigger the sympathetic nervous system which releases the same chemicals back to the area to instigate an inflammatory response and causes more pain This causes increased signaling to the sympathetic nervous system more chemicals, etc. Occurs over months More likely to happen in young females (higher activation of sympathetic nervous system) Treatments o Antidepressants – increase mood o Cut sympathetic nerves o Use drugs that block the sympathetic nervous system o Use an antiinflammatory that blocks tissue swelling to block chemical release o CQ: What surgical treatment would help a patient with RSD? Cutting the sympathetic nerve to the injured area Cutting spinal cord above the site of injury would remove all pain but would also paralyze – remove all feeling Chronic pain remodels the spinal cord as well as the brain The spinal cord learns to be in pain (= innapropriate signaling of pain by neurons) o This causes neurogenic pain 1) Postsynaptic spinal cord neurons take up substance P (released during pain) and remodel their dendrites o Nonpain and painful dendrites become crosswired so that things that stimulate the touch (nonpain) receptors such as a light touch also stimulate the pain receptors 2) Overactivity of sympathetic input o Dorsal horn neurons (light touch) become hyperexcitable o Increased spontaneous activity and response to all touch Somatosensory cortex is rewired in chronic pain o Patients in chronic pain have an overactive somatosensory cortex at rest What makes it go away? o Over a long time the spinal cord realizes it’s made a mistake and remodels itself (the brain also remodels along with it) Pain becomes an emotion in the cingulate cortex Increased pain increased activity in the somatosensory cortex until it reaches a max activity so it stops o Then, the cingulate gyrus starts activating Sensory integration disorder Most common in children Continuum – all people have it – many children have it and they grow out of it Normal amount – have to cut out tags in the back of the shirt, etc. 1) Hypersensitivity to touch (stimulus avoiders) o Common o Light touch activates the pain pathway o Distressed by having face washed or hair or nails cut o Picky eater only eats certain textures o May refuse to walk barefoot on grass or sand o Walks on toes (to minimize amount of foot touching the ground) o SID is part of autism but not enough for a diagnosis of autism 2) Hyposensitivity to touch (stimulus seekers) o Less common o Mild – tapping foot, drumming fingers o Correlated with hyperactivity o Craves touch, touches everything and everyone o Hurts other children or pets while playing o May pinch, bite, or bang own head Association brain areas mix inputs of different modalities Synesthesia a stimulus in one modality creates a sensation in another o “This food tastes blue” 4% of population according to some studies Crosslinkage between different sensory systems (senses) o All sensory parts of the brain are very close anatomically Cross between ANY 2 systems – not all o Not taste and smell – those are the same pathway Causes of synesthesia: LSD, posterior temporal lobe seizure, result of blindness or deafness, occurs spontaneously Case Studies Case: Ian At 19, Ian became sick with a virus His speech become slurred and a few days later he could neither walk nor sit Things progressed until he could not control his own body movement because he could not feel his limbs at all If he did not look at them, he could not tell where his arms or legs were – lost the ability to detect where his limbs were in space – had to use visual feedback to move If the lights were out, he could not move purposefully o Not weak, could feel his arms if touched, but couldn’t move his arms unless he was looking at them Case: pain A Canadian woman born with pain insensitivity no other sensory or mental deficits Set alarm every 2 hours to wake up and turn Developed progressive degeneration of her joints and spinal vertebrae, causing skeletal deformation, degeneration, infection (from bed sores – bacteria in blood stream) Died at age 28 Case: synesthesia Woman had a small stroke in right thalamus Certain sounds induced intense unpleasant tingling in her left hand and arm Overlap of auditory and touch cortex o Normal – overlap but don’t interact Area where overlap occurs – tried to rewire o decreased response to touch and increased response to sound
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