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Somatic Sensation (MCB160 Lecture by Mu-ming Poo, Friday March 9, 2007)
• Introduction – Adrian’s work on sensory coding – Spinal cord and dorsal root ganglia– Four somatic sense modalities
• Touch– Mechanoreceptors and signal transduction– Receptive fields of DRG neurons
• Proprioception• Nociception• Thermal Sensation• Central Projection and Somatotopic Maps
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Weak pressure
Higher pressure
very high pressure
Some of Adrian’s first recordings from a very small number of
nerve fibers in the sensory nerves of cat’s toe.
Adrian’s Findings:
1. The nerve impulse (action potential) is “all-or-none”
2. The strength of stimulus is coded by the firing frequency
3. There is adaptation of neuronal firing after stimulus onset
Origin of Sensory Physiology and Neuronal Information Coding
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Weber-Fechner’s LawI = k Log (S/So) where I= intensity of sensation (freq of firing)
k, So (threshold intensity) are constants
Power law: I = k (S-So)n , where n is a constant
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(ventral horn)
Spinal Cord and Dorsal Root Ganglia (DRG)
Dorsal root
Ventral root
Spinal nerve
Dorsal root ganglia - spinal nerve - limb and trunk
Trigeminal ganglia - cranial nerve - head and face
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Stimuli: mechanical force, temperature change, tissue damage, or chemical action; mediated specifically by different receptors
Nociceptors
Thermoceptors
Mechanoceptors
Mechanoceptors
Nociceptors, thermoceptors – bare endings, small diameter unmyelinated axonMechanoreceptors: encapsulated (wrapped) endings, large diameter myelinated axon
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Transduction by mechanoreceptors
• Stress-gated ion channels open when membrane is deformed
• Na+ influx• Depolarization
(generator potential)• Initiation of action
potential
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Receptive Field
• The area in the periphery within which sensory stimulus can modulate the firing of the sensory neuron.
• Spatial resolution of the RF:
Size: smaller RF - higher resolution
Density: higher density – higher resolution
“Two-point discrimination test”
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RFs of mechanoreceptor neurons
RF size: 2-10 mm Each DRG axon receives convergent inputs from 10-25 corpuscles
RF size: several cm Each DRG axon receives input from single corpuscle or ending
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Proprioception (Sense of position and movement of limb and body)
Mechanoreceptors in muscle and joints
1. Muscle spindle receptors – detect the extent and rate of muscle contraction, endings in parallel with muscle fibers
2. Golgi tendon organs – detect tension exerted by the muscle, ending in series with muscle fibers
3. Joint capsule receptors – detect flexion or extension of joints
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Nociception (Pain Sensation)
Nociceptors-- Respond to noxious (mechano-, thermo) stimuli
directly or chemical released by damaged cells-- Chemicals: histamine, bradykinin, substance P,
ATP, serotonin, acetylcholine, acids, high K+
Three types:1. Mechano-sensitive nociceptors (high threshold)2. Thermo-sensitive nociceptors (high threshold) 3. Polymodal (chemo-sensitive) nociceptors
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Thermal Sensation
Thermoreceptors -- Ending of unmyelinated C-fibers -- Spontaneous firing at low-freq at skin T of 34oC, respond
only within innocuous range.
Two Types: 1. Cold receptors – fire when T decreases from 34oC, maximal
firing at 25oC2 . Warmth receptors – fire when T increases from 34oC,
maximal firing at 45oC
Hot sensation – noxious stimulus detected by nociceptors, not thermal receptors
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Projection to CNS (you should know these pathways, although not covered in the lecture, see textbook for detail)
Dorsal Column-Medial Lemniscal System (Touch and proprioception)-- afferent to spinal cord laminae III-VI-- ascend ipsilaterally (dorsal column)-- cross midline in medulla-- ascend to midbrain via medial lemnicus-- to thalamus and somatosensory cortex
Anterolateral System (Pain and Thermal Sensation) -- afferent to spinal cord laminae I & II-- cross midline to contralateral anterolateral
column-- ascend via direct (spinothalamic) and indirect pathways to thalamus -- to somatosensory cortex
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Sensory homunculus: This model shows what a man's body would look like if each part grew in proportion to the area of the cortex of the brain concerned with its sensory perception.
Somatotopic map in the cortex