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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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Organization of the Spinal Cord

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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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Peripheral endings of DRG neurons

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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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Four types of mechanoreceptors

glaborous (hairless) skin

1.

2. 3. 4.

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Superficial

SubcutaneousPacinian

Meissner

Raffini

Merkeldisk

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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