Touch and Pain January 29 2016 Somatosensory System Relays information about the body Touch Temperature Body position proprioception Organic senses heart burn Pain nociception Labeled Line System Receptors neurons that transduce or change a physical stimulus into neural events In the skin each receptors has a specialized ending that responds to a specific attribute of the stimulus Labeled line different receptors for different qualities and have different lines to the brain Labeled Line System Receptor Potential A Pacinian corpuscle responds to pressure When deformed skin pressed sodium channels are opened which depolarizes the ending Receptor or generator potential is the potential measured in the specialized ending If ending depolarized enough produce an action potential Adaptation Adaptation loss of sensitivity to continuous presence of stimulus Receptive Fields Each receptor has a receptive field the region of the receptor surface that excites or inhibits a sensory neuron Size of Receptive Fields For each type of receptor the receptive fields are smallest in the fingertips larger in the hand even larger in the arm Receptive field size varies with use The Spinal Cord Somatosensory Pathways Somatosensory Cortex Cell body in dorsal root ganglion Information ascends in the dorsal columns to the medulla then to thalamus and then to somatosensory cortex Relating the Somatosensory and Motor Systems Spinal Segments and Dermatomes Each spinal segment corresponds to a region of body surface called a dermatome Dermatomes Somatosensory Pathways Thalamus Thalamus to Cortex Somatosensory Cortex Dorsal Anterior Posterior Ventral Somatosensory cortex is just posterior to the central sulcus Somatotopic Map Modulation of Somatosensory Representation Two Types of Pain Receptors 1 Thermal or mechanical receptors 2 Sharp prickling pain Axons A fibers are myelinated Polymodal receptors Searing diffuse pain Axons C fibers are unmyelinated Peripheral Mediation of Pain Ascending Pain Pathways Pain fibers synapse in the dorsal horn Spinothalamic tract carries the pain fibers to the thalamus Ascending Pain Pathways Left Spinothalamic tract blue carries the pain fibers to the thalamus Spinoparabrachial pathway red innervates areas of the brain concerned with affect Right Descending pathway begins in periaqueductal gray PAG Descending Pain Pathways Midbrain periaqueductal gray EPSP Medulla Raphe nucleus Spino thalamic tract Pain fiber EPSP IPSP Dorsal horn neuron Referred Pain Spino thalamic tract From skin or muscle From internal organ EPSPs Dorsal horn neuron Pain fiber from skin or muscle and pain fiber from an internal organ e g heart synapse on the same dorsal horn neuron Pain in internal organ feels like pain in muscle or skin Modulation of Pain Analgesic Blocks the sensation of pain but does not produce unconsciousness Anesthetic Analgesia plus unconsciousness Placebo Phony drug that appears to alleviate pain 30 of the time Endorphins Endogenous drugs that produce pain relief by activating the descending pathway Opiates Known to have medicinal properties 1500 B C Reduce pain without unconsciousness but with relaxation and sleep Produce a sense of well being and euphoria Reduces coughing Relief from diarrhea and dysentery Used in tonics syrups and wines Laudanum wine something to be praised Morphine Common analgesic Derived from opium Opiate Receptors in the Rat Brain Red shows the highest concentration followed by orange yellow etc