PSB2000 EXAM 3 LECTURE 14 TOUCH PAIN Somatosensation Touch Touch receptors may be Simple bare neuron ending warm cold and pain Encapsulated receptors touch elaborated neuron ending Stimulation opens sodium channels to trigger an action potential Sensors The receptors are mechanoreceptors they are activated by physical force Stretching or displacement of the skin opens channels to activate them When activated they send APs up the axon toward the spinal cord The sensory neuron cell body is located in the DRG These are afferent fibers they go toward the CNS Shallow layers Meissner s corpuscle and Merkel disks Fine detail Movement if you stroke an object to explore its shape Deep layers Pacinian corpuscle and Ruffini endings Stretch Perception of the shape of a grasped object Receptive Fields Different parts of the body have different sensitivities to touch A group of sensory neurons that converge on the same neuron in the spinal cord form a receptive field The more closely packed these fields are the more sensitive the area is to touch stimuli Receptive fields overlap to some extent This lets us feel touch over our entire bodies Receptors that respond to light touch ex gentle stroking of the skin Women have better detail touch than men Merkel disks Men and women generally have the same number of Merkel disks but women tend to have smaller fingers Results in the Merkel disks compacted into a smaller area More sensitive to feeling the distances between grooves Touch receptors to brain Information from touch receptors in the head enters the CNS through the cranial nerves Information from receptors below the head enters the spinal cord and travel through the 31 spinal nerves to the brain Each spinal nerve has a sensory component and a motor component and connects to a limited area of the body Dermatomes A dermatome skin area connected to or innervated by a single sensory spinal nerve Sensory information entering the spinal cord travel in well defined and distinct pathways They overlap Example touch pathway is distinct from pain pathway Somatosensory Cortex Various aspects of body sensation remain separate all the way to the cortex Various areas of the somatosensory thalamus send impulses to different areas of the somatosensory cortex located in the parietal lobe Different sub areas of the somatosensory cortex respond to different areas of the body Damage to the somatosensory cortex can result in the impairment of the body Whiskers The homunculus corresponds to the area of greatest innervation in humans Different animals have different relative sensitivity Rats investigate with their nose mouth more than anything else Each whicker has a large field cortical representation These are called whisker barrels Capsaicin a chemical found in hot pepper stimulates these receptors Pain Pain is the experience evoked by a harmful stimulus directs our attention and holds it Pain sensation begins with the least specialized of all receptors bare nerve endings Some pain receptors also respond to acids heat or cold Axons carrying pain info have little or no myelin impulses travel slowly Mild pain triggers the release of glutamate in the spinal cord and stronger pain triggers the release of glutamate and substance P Pain is pure perception the stimulus is tissue damage Nociceptors responds to AND release chemical stimuli However brain processes pain information rapidly and motor responses are fast Substance P results in the increased intensity of pain Pain Receptors Mechanical pinching and cutting Thermal hot and cold Polymodal all mechanical and thermal plus chemicals released when tissue is injured The stimulus for pain is IN YOU Pain Pathways Pain pathways cross to a tract ascending the contralateral side of the spinal cord Pain sensitive cells in the spinal cord relay information to several areas of the brain Somatosensory cortex responds to painful stimuli memories of pain and signals that warn of impending pain Central nuclei of the thalamus amygdala hippocampus prefrontal cortex and cingulate cortex are associated with emotional associations Pain Controls Opioid mechanisms are systems that are sensitive to opioid drugs and similar chemicals Activation opiate receptors blocks the release of substance P in the spinal cord and in the periaqueductal gray area of the midbrain Capsaicin dumps substance P so you have to remake it OR can damage the actual pain receptor Cannabinoids are chemicals related to marijuana that also block certain kinds of pain Act mainly in the periphery of the body Gate Theory The spinal cord areas that receive messages from pain receptors also receive input from touch receptors and from axons descending from the brain These other areas that provide input can close the gates and decrease pain perception Non pain stimuli around it can modify the intensity of the pain Pain signals ascend the pain pathway and reach conscious awareness in the thalamus cortex Descending pathways can keep the CNS from receiving pain signals The CNS activates neurons that release endorphins They close the gate to pain This blocks the release of substance P This keeps pain signals from getting in Placebo A placebo is a drug or other procedure with no pharmacological effect Decreases pain perception by decreasing the brain s emotional response to pain perception not the sensation itself Decreases response in cingulate cortex but not in the somatosensory cortex Pain Sensitivity Mechanisms of the body to increase sensitivity to pain include Damages or inflamed tissue releases histamine nerve growth factor and other chemicals that increase the responses of nearby pain receptors Non steroidal anti inflammatory drugs ibuprofen decrease chemicals that pester pain receptors Chronic Pain Certain receptors become potentiated after an intense barrage of painful stimuli Leads to increased sensitivity or chronic pain later Emotional Pain Emotional pain resembles physical pain in many regards Increases activity in the cingulate cortex when someone felt left out of an activity People taking acetaminophen Tylenol reported less incidences of hurt feelings and social pain Itch Itch is NOT pain The release of histamines by the skin produce itching sensations Activates a distinct pathway in the spinal cord to the brain Impulses travel slowly along this pathway half a meter per second Pain and itch have an inhibitory relationship Opiates increase itch while antihistamines decrease itch Tickling Poorly understood Maybe
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