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.

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.

Cutaneous & subcutaneous somatic sensory receptors

• Based on function: mechanoreceptors, nociceptors & thermoceptors

• Based on morphology: free nerve endings & encapsulated

• the first step: sensory transduction

• 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)

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

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

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

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

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

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

The trigeminal portion of the mechanosensory system

Information from face andhead2nd neurons in principalnucleus of brainstem3rd neurons in VPM nucleus of thalamus

VPM

The somatic sensory components of the thalamus VP of the thalamus: VPL & VPM

Brodmann’s areas

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

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

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

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

• 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

The perception of pain

• Sharp first pain & slow second pain

Long lasting Drugs selectively block A or C fiber

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

Central pain pathways: the spinothalamic tract

• Axon dorsal horn ascending descending

• axon lamina I & II

• lamina IV, V, VI, then cross

• Spinal trigeminal tract: trigeminal ganglion cells descend to medulla

• Dissociated sensory loss

• T10, left side

Referred pain

Phantom limbs & phantom pain

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

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

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

The physiological basis of pain modulation

• descending pain-modulation pathways

• discovery of endogenous opioids (1960-1970)