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Ch. 8 The somatic sensory system

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1 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.

2 Cutaneous & subcutaneous somatic sensory receptors
Based on function: mechanoreceptors, nociceptors & thermoceptors Based on morphology: free nerve endings & encapsulated the first step: sensory transduction

3 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)

4 Mechanoreceptors specialized to receive tactile information
共同特徵:(1) low threshold (2) A low threshold mylinated rapidly adapt 40% 30-50 Hz slowly adapt pressure, 25% 比 Mei 更快更低 Hz, 10-15% stretch, 20%

5 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

6 Mechanoreceptors specialized for proprioception (本體感覺)
Provide information about muscle length 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

7 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

8 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

9 The major mechanosensory pathway: the dorsal column-medial lemniscus system
VPL 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

10 The trigeminal portion of the mechanosensory system
VPM Information from face and head 2nd neurons in principal nucleus of brainstem 3rd neurons in VPM nucleus of thalamus

11 The somatic sensory components of the thalamus
VP of the thalamus: VPL & VPM Brodmann’s areas

12 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

13 Somatotopic map in the owl monkey receptive fields in 3b are 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

14 Higher-order cortical representations
SI SII limbic system integration of sensory & motor information is association regions descending projections: from sensory cortex thalamus spinal 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,in mammalian 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

15 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

16 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

17 The perception of pain Sharp first pain & slow second pain
Drugs selectively block A or C fiber Long lasting

18 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

19 Central pain pathways: the spinothalamic tract
Axon dorsal horn ascending descending axon lamina I & II lamina IV, V, VI, then cross

20 Spinal trigeminal tract: trigeminal ganglion cells descend to medulla

21 Dissociated sensory loss
T10, left side

22 Referred pain

23 Phantom limbs & phantom pain

24 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

25 Central regulation of pain perception

26 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

27 The physiological basis of pain modulation
descending pain-modulation pathways discovery of endogenous opioids ( )

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