18 The Somatosensory System II: Touch, Thermal Sense, and Pain,Suhail Abdulla AlRukn18-3-2008,Out-line,ObjectiveTestMain Sensory PathwaySensory dysfunctionAnterolateral SystemPeripheral Sensitization and Central SensitizationSpinal Trigeminal Pathway,Objectives,Define the main pathway for the Anterolateral pathwaySpinal Trigeminal PathwayThe mechanism of peripheral Vs centrat sensitisation,Test,What are the main sensory pathways (body and face), what function, and where it cross?,Main Sensory Pathway,Anterolateral system (ALS):Crude touchThermal sanitationPainAnterior Trigeminothalamic pathway (Spinal Trigeminal Pathway): Crude touchThermal sanitationPain,Trunk, limbs, back of the head,Face and from of the head,Anterolateral system and anterior trigeminothalamic Pathway,ALS Vs post. Colum pathways:,(1) Generalized feeling of being touched but do not give precise localization, (2) Receptive fields are larger, (3) Fibres are smaller in diameter and more slowly conducting.,Peripheral Sensory and Motor Fibers: Groups, Diameters, and Conduction Velocities,Sensory dysfunction,Hypesthesia: reduced sensibility, Paresthesia: numbness, tingling, and prickling, Anesthesia loss of sensibility. Allodynia: an innocuous stimulus will result in a perception of pain in the absence of a proper pain stimulus.Hyperalgesia: is a heightened sensitivity to painful stimuli,Anterolateral System,spinothalamic fibersspinomesencephalic fibersspinoreticular fibersspinobulbar fibersspinohypothalamic fibers,Spinothalamic fibers project directly from the spinal cord to the ventral posterolateral (VPL) nucleus Spinomesencephalic axons project to the periaqueductal area and to the tectum; the latter are spinotectal fibers. Play role in central modulaton of pain.Spinoreticular tract: carried the emotional and arousal aspect of pain.,Anterolateral System,Figure 18-1 Summary of anterolateral system and anterior trigeminothalamic tract fibers conveying nondiscriminative tactile, thermal, and nociceptive inputs to the contralateral somatosensory cortex.,Figure 18-1 Summary of anterolateral system and anterior trigeminothalamic tract fibers conveying nondiscriminative tactile, thermal, and nociceptive inputs to the contralateral somatosensory cortex.,To summarize,If you step on a sharp object with your left foot, your spinothalamic tract enables you to realize “something sharp is puncturing the sole of my left foot”. Your spinothalamic intralaminar projections and spinoreticular tract cause you to feel “ouch, that hurts!”; And your spinomesencephalic tract leads to pain modulation, allowing you eventually to think “aah, that feels better”.,Peripheral Sensitization and Primary Hyperalgesia,Following an insult, pain receptors become more sensitive lower pain threshold increases in firing rate to noxious stimulation. So there will be increase in spontaneous activity in the A and C fibers.Although the mechanisms responsible for receptor sensitization are not completely known, chemicals released by the damaged skin or by products from plasma, or both, are thought to contribute to this phenomenon.,As a result of this heightened sensitivity, the affected area is super-sensitive to painful stimuli and patients experience hyperalgesia. Primary hyperalgesia: occurs in the region of damaged skin and is probably the result of receptor sensitization. An example of primary hyperalgesia is the extreme sensitivity of sunburned skin, which results from sensitization of the skin pain endings by local tissue products from the burn-perhaps histamine, prostaglandins, and others,Peripheral Sensitization and Primary Hyperalgesia,Central Sensitization and Secondary Hyperalgesia,Secondary hyperalgesia occurs in the skin bordering the damaged tissue. Although receptor sensitization may contribute to secondary hyperalgesia, there is likely to be a central (e.g., spinal) component as well. There is hyper-activation of the cell in the posterior horn.,This could be explain by:increase in the receptive field size of the posterior horn neuron an increased response of the cells to the application of suprathreshold stimuli, This phenomenon is known as central sensitization, and it represents a potentiated state in which the system has been shifted from one functional level (normal) to another (sensitized) by a change in transcription.,Central Sensitization and Secondary Hyperalgesia,Pain Receptors in Muscles, Joints, and Viscera,In addition to the cutaneous pain receptors, pain receptors in muscles, joints, and viscera have also been identified, which are also carried by III and IV afferent fibers type.A pinprick exam usually activates A fibers. Dull, persistent ache that follows a muscle pull results from activation of C fibers.,Dermatom of the body and face,Central Pathways,A and C fibers enter the spinal cord via the lateral division of the posterior root entry zone. The fibers enter the posterolateral fasciculus (Lissauer tract) and bifurcate into ascending and descending branches,