Specialized receptors for pain and temperature Parallel pathways for pain - the pain message always gets through Because of multiple pathways, it makes it more difficult to get rid of painful sensations Pain part of mechanisms for homeostasis. Must be aware of what's happening in your body and make adjustments. Pain vs. anguish and suffering Receptors Simple in sense that we don't really have histologically specialized receptors like for mechanoreception. Most are free nerve endings of dorsal root ganglion cells. Examining them histologically, they don't look like much. 2 general categories for receptors on small fibers: innocuous to noxious Cutaneous mechanoreceptors Pinch, rub, stretch, squeeze Cutaneous thermoreceptors innocuous changes of hot and cold Mechanical Nociceptors - sensitive for noxious stimulation only. Higher threshold. Thermal nociceptors - noxious thermal stimuli Polymodal nociceptors Specific Channels in Pain Transduction Transient receptor potential (TRP) ion channels Vanilloid receptor (VR1) --capsaicin/heat <--remember. Sensitive to heat. Capsaicin is in chili peppers. Cation selective channels Acid sensing ion channel (ASIC) Purinergic receptor - extracellular ATP (ATP released from cell when it's damaged) GPCR Histamine (itch) Prostaglandin Neuropeptides Brady kinin This is important area which will be developing rapidly during our careers. Fast and Slow Pain A? - small, myelinated C - small, unmyelinated fibers Consequence of having these 2 types is fast and slow pain. First pain is sharper. Second comes in later, and is more of a dull pain. First pain is Ad and second is C. A lot of the receptors can be on either type of fiber. Research says that primates have developed specialized pain pathways that are not fully developed in the same way in all species. Only discovered this on monkeys and humans. Points to 2 pathways in this pain system that are doing different things. Anterolateral System from deep dorsal horn-integrated, intensity-related system: A different interpretation Function Integrated sensorimotor, intensity-related activity - Crude touch, polymodal. Detection of harmful or potentially harmful stimuli Tells you that a painful sensation is happening. Not a lot of detail about what it is though. The other system provides us with the details. This is the "whoops" system. Dorsal Root Ganglion Cell - Neuron 1 Origin: A? and C fibers from dorsal root ganglion (DRG) Mechanoreceptors, thermoceptors, and nociceptors Course: Enter dorsolateral funiculus (Lissauer's tract) Uncrossed Termination: Deeper Layers of dorsal horn Only nociceptors go to layer 5 (numbers of the layers are not critical info). Layer 5 is perhaps the bottom most layer of dorsal horn. Convergence of many inputs Somatotopic organization: Organized by the spinal segments where they enter. Spinothalamic Pathway - Neuron 2 Origin: Deep layers of the dorsal horn Function: Crude touch and crude pain detection Crude mechanical stimuli. Very large receptive fields. Modality ambiguous - respond to mechanical, thermal, and noxious stimuli. Wide dynamic range. Can detect stimulus when light, but as it gets heavier it can detect more and more. Graded response. Can tell how intense the stimulus is. Larger receptive fields - entire limbs. Course: Axons CROSS spinal cord Ventral (anterior) spinothalamic tract Termination of Neuron 2 Ventral posterolateral nucleus (VPL) of the thalamus - continuation of somatosensory pathway for touch Cerebellum (ventral spinocerebellar tract) - e.g. Spinocerebellar tract. Realtime sensory information about movements. Brainstem reticular formation - sensorimotor coordination, behavioral state, posture. Regulates gamma neurons so important for muscle tone and posture. Thalamocortical Pathway - Neuron 3 Origin - VPL Course - Posterior limb of internal capsule. Uncrossed Termination - Primary somatosensory cortex (S1) probably area 3b. Note: lesions in this cortical area usually do not alter pain sensation, but they do alter tactile discrimination Anterolateral system from superficial dorsal Horn: Affective-Motivational Pain Pathways Function Interoceptive info Highly distinct, specific sensations Temperature First and second pain Itch Muscle and visceral sensations Sensual tooth. Paired with hunger and thirst info from viscera (from the gustatory system). Dorsal Root Ganglion Cell - Neuron 1 Origin: Ad and C fibers - nociceptors Course: Enter the spinal cord at the dorsolateral funiculus ((Lissauer's tract). Uncrossed Termination: layer 1 of posterior (dorsal) horn of the spinal cord. Main distinguishing feature between these two systems Layer 1 Spinothalamic Pathway - Neuron 2 in the pathway Origin: neurons in later 1 in the dorsal horn Function: Small receptive fields Separate neurons for: Sharp (fast) pain - nociceptive specific Burning (second) pain - C fiber Polymodal HPC (heat, pinch, cold), C fiber - Also second pain Cool Warm Histamine (itch) Course: Axons cross the spinal cord near their origin and form the lateral part of the spinothalamic tract in the anterolateral white matter of spinal cord. Crossed Termination of Neuron 2 VPL and MD in thalamus VPL region continuous with area receiving homeostatic inputs from the viscera and taste systems MDvc (ventrocaudal) Sympathetic cell columns in thoracic spinal cord - -regulate sympathetic function Spino-reticular pathways - homeostatic integration centers in the brain stem Intralaminar nuclei of thalamus Thalamocortical Pathways Pain cortex is in dorsa posterior insular Also in cingulate cortex (part of limbic system) responsible for anguish and suffering. How you feel about the pain. Frontal lobotomy severs the connection to cingulate cortex . Frontal lobotomies only performed now in terminally ill patients who are in lots of pain that will not go away. Pain is still going to be there, but they won't care. Perception of pain is subjective Depends on context Pain sensations after somatosensory cortex lesions Phantom limb pain - takes some time for your body map to change after amputation Central pain Pathology/cutting the nociceptive pathways Sprouting or abnormal excitation of axons often produces new sensations of pain. Often get false sense of pain
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