Ch. 8 The somatic sensory system The somatic sensory system has two major components: (1) a...
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Transcript of Ch. 8 The somatic sensory system The somatic sensory system has two major components: (1) a...
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Ch. 8 The somatic sensory system
• The somatic sensory system has two major components: (1) a subsystem for the detection of mechanical stimuli (2) a subsystem for the detection of painful stimuli and temperature.
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Cutaneous & subcutaneous somatic sensory receptors
• Based on function: mechanoreceptors, nociceptors & thermoceptors
• Based on morphology: free nerve endings & encapsulated
• the first step: sensory transduction
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• Quality of stimulus is determined by the properties of receptors & the location of their central targets
• Strength of stimulus is determined by the rate of AP
• 可更明確表示 quality of stimulus: (1) rapidly adapting (phasic, convey information of the change) (2) slowly adapting (tonic, convey information about persistence ) (table 8-1)
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Mechanoreceptors specialized to receive tactile information
low thresholdmylinated
rapidly adapt40%30-50 Hz
比 Mei 更快更低250-350 Hz, 10-15%
slowly adapt pressure, 25%
stretch, 20%
共同特徵: (1) low threshold (2) A
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Differences in mechanosensory discrimination across the body surface
• two-point discrimination
• 決定因子: (1) receptor density (2) receptive field (3) 由 CNS 控制的其他因素,如會影響 threshold 的因子 , 如 practice, fatigue, & stress 及刺激本身的重要性
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Mechanoreceptors specialized for proprioception (本體感覺)
• Detect mechanical forces arising from the body itself, provide information about the position of the limbs and other body parts in space
• muscle spindles, Golgi tendon organs & joint receptors
Provide information about muscle length
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Active tactile exploration
• Relative movement between the skin and a surface
• For humans, this is the single most important requirement for accurate discrimination of texture
• haptics, active touching by facial whiskers in rodents
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Dermatomes 皮板:由單一脊神經感覺纖維所支配的皮膚區域
• Vary among individuals
• dermatomal overlap is more extensive for mechanoreceptors than for pain & temp.
• test for pain provides a precise assessment of a segmental nerve injury
• proprioceptors does not follow the dermatomal map
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The major mechanosensory pathway: the dorsal column-medial lemniscus system
• 1st neurons in sensory ganglia, 2nd neurons in lower medulla, 3rd in thalamus VPL nucleus
• axons in medulla are topographically organized
• dorsal column lesions 只有中等程度影響,因為有部分神經走 spinothalamic pathway 楔形
VPL
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The trigeminal portion of the mechanosensory system
Information from face andhead2nd neurons in principalnucleus of brainstem3rd neurons in VPM nucleus of thalamus
VPM
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The somatic sensory components of the thalamus VP of the thalamus: VPL & VPM
Brodmann’s areas
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The somatic sensory cortex
Human :所佔腦部面積與實際器官大小所成的比例並不一致
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Somatotopic map in the owl monkey
receptive fields in 3bare simple, 給 area 1 的刺激須有方向性,area 2 的刺激須多樣
lesions to area 3 b 將失去對刺激性質及形狀的判斷, to area 1 影響對刺激性質的判斷, to area 2 失去動作的協調及對形狀大小的判斷
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Higher-order cortical representations
SI SII limbic system
integration of sensory & motor information is association regions
descending projections: from sensory cortex thalamusspinal cord, 數目甚至多過 ascending pathways, 功能未定
In topographical cortical maps: modules形式各異,如圖,只有某些種類有, inmammalian brain, 也只有某些區域有,只知其內為神經網路,並無一致的功能
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Ch. 9 Pain
• Alerting the brain to the dangers nociception
• Informing the brain to dangers somatic sensory
• Nociceptors, free endings, cell body in dorsal root ganglia, receptor potential action potential
• Table 8-1, A, myelinated, 20 m/s, fast pain pathway, intense mechanical/ mechanothermal stimuli, receptive field: clusters of sensitive spots
• C fiber, 2 m/s, unmylinated, slow pathway, polymodal (thermal, mechanicaal & chemical)
• All pain receptive fields are large, particularly in the thalamus & cortex
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• Nociceptive & non-nociceptive thermoreceptors• Electrically stimulate somatic sensory fiber at
any frequency does not produce pain• Electrically stimulate A & C fiber produce pain
刺激在同一點上
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The perception of pain
• Sharp first pain & slow second pain
Long lasting Drugs selectively block A or C fiber
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Hyperalgesia & sensitization
• Hyperalgesia: enhanced sensitivity and responsivity to stimulation of the area around damaged tissue
• Damaged tissue release various substances (bradykinin, histamine, prostaglandins) which induce sensitization of nociceptors
• Electrical activity in the nociceptors themselves release substance P will increase blood flow to promote healing
• Aspirin, inhibit cyclooxygenase which can synthesize porstaglandins
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Central pain pathways: the spinothalamic tract
• Axon dorsal horn ascending descending
• axon lamina I & II
• lamina IV, V, VI, then cross
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• Spinal trigeminal tract: trigeminal ganglion cells descend to medulla
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• Dissociated sensory loss
• T10, left side
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Referred pain
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Phantom limbs & phantom pain
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The nociceptive components of the thalamus and cortex
• Thalamus: VP (VPL: other regions of the body; VPM: face), 與 mechanosensory 一樣的原因是,如此才好 locate a pain & judge its intensity
• primary somatic sensory cortex, remains something of a mystery
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Central regulation of pain perception
• 心理對痛覺的影響,軍人在戰場上受傷,常常不覺得痛
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The Plaecbo effect
• 經由給予非藥理醫學而得到的止痛反應,例如acupuncture 針灸; hypnosis 催眠
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The physiological basis of pain modulation
• descending pain-modulation pathways
• discovery of endogenous opioids (1960-1970)